The Effects of Medicinal Plants and Bioactive Natural Compounds on Homocysteine

Background: Among non-communicable diseases, cardiovascular diseases (CVDs) are the leading cause of mortality and morbidity in global communities. By 2030, CVD-related deaths are projected to reach a global rise of 25 million. Obesity, smoking, alcohol, hyperlipidemia, hypertension, and hyperhomocysteinemia are several known risk factors for CVDs. Elevated homocysteine is tightly related to CVDs through multiple mechanisms, including inflammation of the vascular endothelium. The strategies for appropriate management of CVDs are constantly evolving; medicinal plants have received remarkable attention in recent researches, since these natural products have promising effects on the prevention and treatment of various chronic diseases. The effects of nutraceuticals and herbal products on CVD/dyslipidemia have been previously studied. However, to our knowledge, the association between herbal bioactive compounds and homocysteine has not been reviewed in details. Thus, the main objective of this study is to review the efficacy of bioactive natural compounds on homocysteine levels according to clinical trials and animal studies. Results: Based on animal studies, black and green tea, cinnamon, resveratrol, curcumin, garlic extract, ginger, and soy significantly reduced the homocysteine levels. According to the clinical trials, curcumin and resveratrol showed favorable effects on serum homocysteine. In conclusion, this review highlighted the beneficial effects of medicinal plants as natural, inexpensive, and accessible agents on homocysteine levels based on animal studies. Nevertheless, the results of the clinical trials were not uniform, suggesting that more well-designed trials are warranted.


Introduction
In recent decades, we have experienced a shift in the disease burden towards noncommunicable diseases, which is likely due to the improved efficacy of treatments and lifestyle changes [1,2]. According to the World Health Organization (WHO), non-communicable diseases account for 75% of global deaths in 2020; of these, cardiovascular diseases (CVDs) are the leading cause of mortality and morbidity in communities [3][4][5]. By 2030, CVD-related [55,56]. Hence, the recent focus is on natural or low-risk supplements, such as herbal medicines or functional foods [57]. The protective effects of nutraceuticals on CVDs have been previously identified [58,59]. The term nutraceutical was first coined in 1989 by Stephen Deflis. Nutraceuticals are defined as "foods or parts of a food that provide medical or health benefits including prevention or treatment of a disease" [60]. Nutraceuticals are substances that can be used instead of a medication as dietary supplements to control, prevent, or even treat diseases, such as CVDs [61]. These are known as food drugs and they are particularly helpful in patients who do not have access to a medication or for treatment of chronic diseases, such as osteoporosis, heart diseases, among others [62]. Recently, medicinal plants are becoming the focus of several researches, since these natural products have shown promising effects on the prevention and treatment of various diseases, particularly CVDs [63][64][65][66], diabetes mellitus [67][68][69], hypertension [70][71][72], and non-alcoholic fatty liver disease (NAFLD) [56]. The beneficial effects of herbal bioactive compounds on 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMG-CoA reductase) [73], Apolipoprotein B (Apo B) [74], and small dense LDL [74] have recently been reviewed. These are three important factors with unfavorable effects on CVDs. The effects of nutraceuticals and herbal products on CVDs/dyslipidemia have been previously studied. However, to our knowledge, the association between herbal bioactive compounds and homocysteine has not been reviewed in detail [63][64][65][66][75][76][77][78][79]. Thus, the main objective of this study is to address the efficacy of bioactive natural compounds on homocysteine levels, which was assessed through clinical trials and animal studies ( Figure 1).

Figure 1.
A schematic summary of pathways depicting the beneficial effects of herbal medicine on serum homocysteine (based on animal studies) and the possible mechanisms occurred after reduction in homocysteine level resulted in favorable outcomes; reduction in cardiovascular diseases (CVD).

Black Tea
Tea is widely consumed around the world, and all the popular types of tea, like black tea, are manufactured from the leaves of Camellia sinensis (L.) Kuntze from Theaceae family. Several studies have shown that tea and its bioactive polyphenolic constituents have numerous beneficial effects on the prevention of diseases, like cancer, diabetes, arthritis, CVDs, stroke, and obesity [80][81][82][83]. These effects are due to antioxidative, anti-inflammatory, antihypertensive, cholesterol-lowering, antimicrobial, anticarcinogenic, neuroprotective, and thermogenic properties of the tea [83]. The favorable effects of tea on CVDs have been demonstrated in epidemiological studies and clinical trials [83]. Its effect on homocysteine level is one proposed mechanism. In one experiment, the rats were assigned into three groups: (1) Vehicle (saline), (2) angiotensin (Ang) II (50 ng/kg/min.) to induce hypertension, and (3) Ang II + Black tea extract (BT) in which animals were given a 15 mg/kg/day of black tea extract (starting from Day 1 after Ang II pump insertion) for two weeks. Initially, angiotensin II infusion enhanced the plasma homocysteine level and it resulted in endoplasmic reticulum (ER) and oxidative stress, which, in turn, triggered endothelial dysfunction. However, black tea extract dramatically decreased the blood pressure and plasma homocysteine and, consequently, protected arteries of hypertensive rats from ER stress and endothelial dysfunction [84]. According to a clinical trial conducted by Hodgson et al.,20 adults with CAD were randomly assigned to four groups: (I) water and no meal, (II) black tea and no meal, (III) meal with water, or (IV) meal with black tea. The participants were asked to drink a cup of black tea (contained 2.2 g of tea leaves at times 0, 1.5 h, and 3 h) or three cups of hot water with and without a meal (comprised a sausage, egg, bacon, McMuffins, and two hash browns). The total homocysteine was measured at baseline and 3.5 h after drinking black tea or hot water with and without a meal. In the end, an acute increase in homocysteine was observed after drinking black tea. Although the meal caused an acute decline in homocysteine level, it did not alter the homocysteine-raising effect of tea [85]. Similarly, in a randomized crossover study, 22 subjects were divided into two groups to drink 1250 mL black tea/d (five cups each containing 2 g tea leaves in 250 mL boiled water) or 1250 mL hot water/d for four weeks. For the next four weeks, the participants consumed the alternate drink. The findings demonstrated that black tea did not significantly alter the mean homocysteine concentrations [86]. Twenty healthy subjects in another crossover study received a diet low in polyphenols and they were randomly assigned into four groups of supplemented regimens: (I) 2 g chlorogenic acid (a compound found in coffee and black tea), (II) 4 g black tea solids (III) 440 mg quercetin-3-rutinoside, or (IV) 0.5 g citric acid as a placebo. The duration of each trial was seven days (a four-week trial). The results showed that, after 4-5 h of supplementation, chlorogenic acid and black tea both raised total homocysteine concentrations in plasma when compared to the placebo. Quercetin-3-rutinoside exerted no effect on plasma homocysteine [87]. The different results between the animal and clinical studies might be due to the fact that, in a previous animal study, black tea was administered in experimentally hypertensive rats. It was suggested that homocysteine levels are reduced after the intake of black tea polyphenols in hypertension, which might be mediated and attributed to the promotion of homocysteine metabolism [84]. On the other hand, in the clinical trials, a small sample size of healthy subjects without hypertension were recruited, which might be a potential explanation for reporting the different results between animal and human studies [86,87]. In addition, it is suggested that tea and coffee, and their major constituents, such as polyphenols and caffeine, might increase homocysteine by acting as acceptors of methyl groups during the metabolism of methionine to homocysteine [86][87][88][89]. When considering these controversial findings and the fact that several studies suggest that tea consumption has a protective role against CVDs [90][91][92], there have been calls to conduct more studies in the future related to both the mechanistical and clinical aspects to explore the role of black tea on homocysteine.

Green Tea
Green tea is a non-fermented tea [93] that is traditionally used as a natural medicine. It is a rich source of polyphenols, mainly epigallocatechin-3-gallate (EGCG) (Figure 2) [94,95], which is proved to have favorable effects on neurological diseases, cancer, inflammation [94,[96][97][98][99], and homocysteine-induced cerebrovascular injury [100]. In a study on adult male Wister rats, six groups were randomly assigned: (1st) normal laboratory diet, (2nd) 2.5 mg/kg body weight EGCG, (3rd) 5 mg/kg body weight EGCG, (4th) exposure to 4 Gy of γ radiation, (5th) 2.5 mg/kg body weight EGCG + exposure to 4 Gy of γ radiation, and (6th) 5 mg/kg body weight EGCG + exposure to 4 Gy of γ radiation. The intervention was performed for three consecutive days in the 2nd and 3rd groups, and for two days in 5th and 6th groups. The third dose was administered for 30 min. before irradiation. In rats that were pretreated with EGCG at a dose of 2.5 and 5 mg/kg, plasma homocysteine was significantly decreased when compared to the first group. The homocysteine levels were significantly decreased in the second and third groups as compared to the first group [101].
Green tea is a non-fermented tea [93] that is traditionally used as a natural medicine. It is a rich source of polyphenols, mainly epigallocatechin-3-gallate (EGCG) (Figure 2) [94,95], which is proved to have favorable effects on neurological diseases, cancer, inflammation [94,[96][97][98][99], and homocysteine-induced cerebrovascular injury [100]. In a study on adult male Wister rats, six groups were randomly assigned: (1st) normal laboratory diet, (2nd) 2.5 mg/kg body weight EGCG, (3rd) 5 mg/kg body weight EGCG, (4th) exposure to 4 Gy of γ radiation, (5th) 2.5 mg/kg body weight EGCG + exposure to 4 Gy of γ radiation, and (6th) 5 mg/kg body weight EGCG + exposure to 4 Gy of γ radiation. The intervention was performed for three consecutive days in the 2nd and 3rd groups, and for two days in 5th and 6th groups. The third dose was administered for 30 min. before irradiation. In rats that were pretreated with EGCG at a dose of 2.5 and 5 mg/kg, plasma homocysteine was significantly decreased when compared to the first group. The homocysteine levels were significantly decreased in the second and third groups as compared to the first group [101].

Cinnamon
Cinnamon (Cinnamomum verum J.Presl), a plant from the Lauraceae family, is mostly used as a spice [102]. It is a herbal medicine used for conditions, such as flatulence, amenorrhea, diarrhea, toothache, fever, leukorrhea, common cold, and headache [103,104]. Cinnamon and its main component, Cinnamaldehyde (Figure 3), have insulin sensitizer, antioxidant, and anti-inflammatory properties [105,106]. This herbal medicine was also traditionally recommended for the treatment of impotency, frigidity, dyspnea, eye inflammation, vaginitis, cough, rheumatism, neuralgia, and CVDs [107]. A two-phase clinical trial conducted by Amin et al. was conducted on 48 male albino rats for 5-8 weeks. Initially, hypercholesterolemia (the addition of 1% cholesterol powder, 0.25% bile salts, and beef tallow in a percentage of 4% to basal normal diet for 15 days) was induced. Subsequently, during the treatment period (starting from the third week and continued for six weeks), the hypercholesterolemic rats were divided into three subgroups (12 rats per group) according to the type of treatment. These regimens included HCD, HCD + Atorvastatin (0.2 mg/kg body weight), and HCD + cinnamon (C. zeylanicum Blume) (20 mg/day/rat). It was showed that cinnamon extract could reduce hypercholesterolemia and modulate oxidative stress and homocysteine [108].

Cinnamon
Cinnamon (Cinnamomum verum J.Presl), a plant from the Lauraceae family, is mostly used as a spice [102]. It is a herbal medicine used for conditions, such as flatulence, amenorrhea, diarrhea, toothache, fever, leukorrhea, common cold, and headache [103,104]. Cinnamon and its main component, Cinnamaldehyde (Figure 3), have insulin sensitizer, antioxidant, and anti-inflammatory properties [105,106]. This herbal medicine was also traditionally recommended for the treatment of impotency, frigidity, dyspnea, eye inflammation, vaginitis, cough, rheumatism, neuralgia, and CVDs [107]. A two-phase clinical trial conducted by Amin et al. was conducted on 48 male albino rats for 5-8 weeks. Initially, hypercholesterolemia (the addition of 1% cholesterol powder, 0.25% bile salts, and beef tallow in a percentage of 4% to basal normal diet for 15 days) was induced. Subsequently, during the treatment period (starting from the third week and continued for six weeks), the hypercholesterolemic rats were divided into three subgroups (12 rats per group) according to the type of treatment. These regimens included HCD, HCD + Atorvastatin (0.2 mg/kg body weight), and HCD + cinnamon (C. zeylanicum Blume) (20 mg/day/rat). It was showed that cinnamon extract could reduce hypercholesterolemia and modulate oxidative stress and homocysteine [108].

Anthocyanin
The word anthocyanin is derived from two Greek words, plant (Anthos) and blue (kianos), which are the most important pigments in vascular plants [109]. Like chlorophyll, they are natural pigments that are non-toxic, water-soluble, and widely present in plant cells [110]. Anthocyanins are the most colorful compounds among the flavonoids that are responsible for different colors found in many fruits, vegetables, and flowers [111,112]. They play essential roles in the management of CADs due to their high antioxidant effect [113,114]. In one clinical trial, 20 healthy female volunteers were randomly assigned into two groups to receive a placebo beverage or cranberry juice (750 mL/day (3 × 250 mL), which contained 2.80 mg/L anthocyanins) for two weeks. In the end, the plasma total homocysteine remained unchanged [115].

Anthocyanin
The word anthocyanin is derived from two Greek words, plant (Anthos) and blue (kianos), which are the most important pigments in vascular plants [109]. Like chlorophyll, they are natural pigments that are non-toxic, water-soluble, and widely present in plant cells [110]. Anthocyanins are the most colorful compounds among the flavonoids that are responsible for different colors found in many fruits, vegetables, and flowers [111,112].
They play essential roles in the management of CADs due to their high antioxidant effect [113,114]. In one clinical trial, 20 healthy female volunteers were randomly assigned into two groups to receive a placebo beverage or cranberry juice (750 mL/day (3 × 250 mL), which contained 2.80 mg/L anthocyanins) for two weeks. In the end, the plasma total homocysteine remained unchanged [115].

Garlic Extract
Garlic (Allium sativum L. Liliaceae) is a well-recognized medicinal plant. Several pharmacological implications of A. sativum and its organosulfur compounds, especially Allicin (Figure 4), include antibacterial, antiviral, antifungal, antiparasitic, anticancer, antiinflammatory, and cardiovascular protective properties [116][117][118][119][120][121][122][123]. It has beneficial roles on dyslipidemia, which lowers the total cholesterol concentrations by approximately 10% and favorably alters HDL/LDL ratios. Additionally, it acts as a mild anti-hypertensive that reduces blood pressure by 5-7% [124]. A group of 60 subjects were randomly assigned to two groups: (I) intervention (received a daily capsule of aged garlic-extract (AGE) (250 mg) plus vitamin-B12 (100 µg), folic-acid (300 µg), vitamin-B6 (12.5 mg), and L-arginine (100 mg) for 12 months) and (II) placebo. Finally, a reduced level of homocysteine was observed in the intervention group [125]. In a study conducted by Budoff et al., 65 patients with an intermediate risk for CVDs were randomly allocated to two groups: (I) the daily administration of a capsule containing AGE (250 mg) plus Vitamin B12 (100 µg), folic acid (300 µg), Vitamin B6 (12.5 mg), and L-arginine (100 mg) or (II) placebo. After one year of treatment, the homocysteine level was decreased in the AGE group [126]. In another experiment, 40 rats were employed to receive one of the following diets for six weeks (four groups of 10 each): (I) AIN-93G folic-acid sufficient (2 mg/kg of diet); (II) AIN-93G folic-acid deficient; (III) AIN-93G folic-acid sufficient that was supplemented with AGE (4% of diet, wt:wt); and, (IV) AIN-93G folic-acid deficient supplemented with AGE. The results showed that the addition of AGE to the severely folate-deficient diet decreased the total plasma homocysteine concentration by 30% [127]. Based on a similar study, 23 patients with known CAD were randomly assigned to two groups: (I) received AGE (4 mL) or (II) received placebo for one year. Following intervention, the homocysteine levels showed no significant improvement [128]. In another randomized controlled trial (RCT), 30 postmenopausal women were randomly assigned to four groups: (1) placebo, (2) consumption of AGE (5 × 65 mg per week), (3) exercise (60% of maximum heart rate) and placebo, and (4) exercise and aged garlic extract. After 12 weeks, homocysteine was significantly decreased with intervention in the second and fourth groups [129]. Ried et al., conducted an RCT on 30 pediatric (aged 8 to 18 years old) patients who were administered 300 mg AGE (three capsules/ daily) or placebo. After eight weeks of treatment, no significant difference was observed in homocysteine levels between groups [130]. In a similar RCT, 88 uncontrolled hypertensive patients were given 1.2 gr of AGE powder+ 1.2 mg S-allylcysteine each day or placebo for 12 weeks. Following intervention, no significant change was detected in the homocysteine level in both groups [131].
Molecules 2021, 26, x FOR PEER REVIEW 7 of 28 each day or placebo for 12 weeks. Following intervention, no significant change was detected in the homocysteine level in both groups [131].

Ginseng
Ginseng (Panax ginseng C.A.Mey., Araliaceae) is a medicinal plant with favorable pharmacological effects in cancer, diabetes, and CVDs, which also improves the immune system and central nervous system (CNS) function, relieves stress, and possesses antioxidant properties, and these benefits are mainly attributed to the presence of ginsenosides ( Figure 5) [132][133][134][135]. A group of 40 wistar male rats were randomly assigned to one of the following four groups: (1st) control group (tap water), (2nd) Methionine (Met) (1 g/kg per day) and succinyl sulfathiazole (SSL) (0.5 g/kg per day), (3rd) ginsenosides total saponins (GTS) (Korean ginseng) (50 mg/kg every 12 h), and (4th) Met + GTS + SSL. The homocysteine levels were measured within 30 and 60 days of the intervention, and they were found to be reduced in the fourth group as compared to the second group, whereas the third

Ginseng
Ginseng (Panax ginseng C.A.Mey., Araliaceae) is a medicinal plant with favorable pharmacological effects in cancer, diabetes, and CVDs, which also improves the immune system and central nervous system (CNS) function, relieves stress, and possesses antioxidant properties, and these benefits are mainly attributed to the presence of ginsenosides ( Figure 5) [132][133][134][135]. A group of 40 wistar male rats were randomly assigned to one of the following four groups: (1st) control group (tap water), (2nd) Methionine (Met) (1 g/kg per day) and succinyl sulfathiazole (SSL) (0.5 g/kg per day), (3rd) ginsenosides total saponins (GTS) (Korean ginseng) (50 mg/kg every 12 h), and (4th) Met + GTS + SSL. The homocysteine levels were measured within 30 and 60 days of the intervention, and they were found to be reduced in the fourth group as compared to the second group, whereas the third group had no significant change when compared to the first group [136].

Ginseng
Ginseng (Panax ginseng C.A.Mey., Araliaceae) is a medicinal plant with favorable pharmacological effects in cancer, diabetes, and CVDs, which also improves the immune system and central nervous system (CNS) function, relieves stress, and possesses antioxidant properties, and these benefits are mainly attributed to the presence of ginsenosides ( Figure 5) [132][133][134][135]. A group of 40 wistar male rats were randomly assigned to one of the following four groups: (1st) control group (tap water), (2nd) Methionine (Met) (1 g/kg per day) and succinyl sulfathiazole (SSL) (0.5 g/kg per day), (3rd) ginsenosides total saponins (GTS) (Korean ginseng) (50 mg/kg every 12 h), and (4th) Met + GTS + SSL. The homocysteine levels were measured within 30 and 60 days of the intervention, and they were found to be reduced in the fourth group as compared to the second group, whereas the third group had no significant change when compared to the first group [136].

Chlorella
Chlorella is a genus of approximately thirteen species of single-celled green algae belonging to the division Chlorophyta [137]. Chlorella pyrenoidosa, a single cell alga that is found in freshwater, has a rich content of chlorophyll and high concentration of nucleic acids, minerals, amino acids, dietary fiber, and vitamins. It has a strong cell wall that can only be digested by humans during a Dyno-Mill process. Studies showed that injection or oral intake of Chlorella following its cell wall destruction improves the immune function against infection and anti-cancer activity [138][139][140][141][142][143]. In study conducted by Merchant et al., 17 vegetarian/vegan (aged 26-57 years old) with a vitamin B12 deficiency were asked to add 9 gr/day of C. pyrenoidosa to their routine diet for 60 days. The findings reveled that the homocysteine level was reduced by an average of 10% [144].

Chlorella
Chlorella is a genus of approximately thirteen species of single-celled green algae belonging to the division Chlorophyta [137]. Chlorella pyrenoidosa, a single cell alga that is found in freshwater, has a rich content of chlorophyll and high concentration of nucleic acids, minerals, amino acids, dietary fiber, and vitamins. It has a strong cell wall that can only be digested by humans during a Dyno-Mill process. Studies showed that injection or oral intake of Chlorella following its cell wall destruction improves the immune function against infection and anti-cancer activity [138][139][140][141][142][143]. In study conducted by Merchant et al., 17 vegetarian/vegan (aged 26-57 years old) with a vitamin B12 deficiency were asked to add 9 gr/day of C. pyrenoidosa to their routine diet for 60 days. The findings reveled that the homocysteine level was reduced by an average of 10% [144].

Ginger
Ginger (Zingiber Officinale Roscoe, Zingiberaceae) is a part of a family of plants, including cardamom and turmeric. It has a strong aroma due to the presence of gingerols (pungent ketones) ( Figure 6). The rhizome part of the plant is traditionally used in Asia and tropical areas for fever, common cold, digestive problems, among others. Ginger is an appetite stimulant, and it is categorized by the U.S. FDA as a food additive [145]. Its beneficial effects on nausea/vomiting and arthritis have been previously reported [146][147][148][149][150][151][152][153]. In one experiment, 24 male Wistar rats were randomized into three groups: (1) non-diabetic control (tap water), (2) non-treated diabetic (tap water), and (3) ginger (Z. Officinale) extract treated diabetic (50 mg/kg of hydroalcoholic ginger + tap water). After six weeks, the homocysteine level was notably enhanced in the second group when compared to the first group, whereas it was significantly declined in the third group as compared to the second group [154]. In another experiment by Akbari et al., 28 adult male Sprague Dawley rats were randomized into four groups: (1) control (2 mL/day corn oil), (2) ginger (Z. Officinale) (1 g/kg body weight daily), (3) ethanol (4 g/kg body weight daily), and (4) ethanol-ginger (Z. Officinale). After 28 days, the homocysteine level was significantly increased in the third group when compared to the first group, whereas it was significantly decreased in the fourth group as compared to the third group [155]. the second group [154]. In another experiment by Akbari et al., 28 adult male Sprague Dawley rats were randomized into four groups: (1) control (2 mL/day corn oil), (2) ginger (Z. Officinale) (1 g/kg body weight daily), (3) ethanol (4 g/kg body weight daily), and (4) ethanol-ginger (Z. Officinale). After 28 days, the homocysteine level was significantly increased in the third group when compared to the first group, whereas it was significantly decreased in the fourth group as compared to the third group [155].

Soy
Soybean (Glycine max (L.) Merr., Fabaceae) is a traditional plant that is native to East Asia, which is a good source of phytochemicals (e.g., isoflavones: daidzein, and genistein, Figure 7), fiber, and plant sterols [156][157][158]. Various studies have revealed its beneficial effects on blood lipids, CVDs, fertility, and menopause [159][160][161]. The result of a systematic review and meta-analysis (2016), which reviewed 19 randomized controlled studies, showed that Soy or isoflavones had no effect on homocysteine levels [162]. In a randomized cohort study, 87 healthy postmenopausal women were assigned to one of these groups: (1) 1200 Kcal diet + exercise or (2) 1200 Kcal diet + exercise + 200mg of Glycine max (80 mg of soy isoflavone: 60.8 mg of genistein, 16 mg of daidzein, and 3.2 mg of glicitein) for six months. Following intervention, the homocysteine level remained unchanged in both of the groups [163]. A group of 117 patients with hypercholesterolemia were also divided into three groups to receive: 15 gr/day soy protein (containing SuproSoy from Solae, Saint Louis, MO, USA) and 25 gr/day soy protein or placebo. No change in homocysteine level as compared to the baseline was found in all groups [164]. Similarly, in a cross over study on 34 postmenopausal women, the participants received 26 ± 5 g/day of isolated soy protein (containing 44 ± 8 mg isoflavones per day) or 26 ± 5 g/day of milk protein isolate for six weeks each and two weeks washout. The homocysteine level was not different between the two groups [165]. Forty patients on peritoneal dialysis were studied in a clinical trial; two groups were, as follows: received 28 gr/day of textured soy flour (contained 14 g of soy protein) or followed their regular diet for eight weeks. The concentration of homocysteine had no significant difference between the two groups [166]. In another randomized crossover study on forty-one hyperlipidemic men and postmenopausal women, three groups were assigned to receive one of these regimens: (1) a low-fat dairy control diet, (2) low-isoflavone soy food diet (10 mg isoflavones/day), and

Soy
Soybean (Glycine max (L.) Merr., Fabaceae) is a traditional plant that is native to East Asia, which is a good source of phytochemicals (e.g., isoflavones: daidzein, and genistein, Figure 7), fiber, and plant sterols [156][157][158]. Various studies have revealed its beneficial effects on blood lipids, CVDs, fertility, and menopause [159][160][161]. The result of a systematic review and meta-analysis (2016), which reviewed 19 randomized controlled studies, showed that Soy or isoflavones had no effect on homocysteine levels [162]. In a randomized cohort study, 87 healthy postmenopausal women were assigned to one of these groups: (1) 1200 Kcal diet + exercise or (2) 1200 Kcal diet + exercise + 200 mg of Glycine max (80 mg of soy isoflavone: 60.8 mg of genistein, 16 mg of daidzein, and 3.2 mg of glicitein) for six months. Following intervention, the homocysteine level remained unchanged in both of the groups [163]. A group of 117 patients with hypercholesterolemia were also divided into three groups to receive: 15 gr/day soy protein (containing SuproSoy from Solae, Saint Louis, MO, USA) and 25 gr/day soy protein or placebo. No change in homocysteine level as compared to the baseline was found in all groups [164]. Similarly, in a cross over study on 34 postmenopausal women, the participants received 26 ± 5 g/day of isolated soy protein (containing 44 ± 8 mg isoflavones per day) or 26 ± 5 g/day of milk protein isolate for six weeks each and two weeks washout. The homocysteine level was not different between the two groups [165]. Forty patients on peritoneal dialysis were studied in a clinical trial; two groups were, as follows: received 28 gr/day of textured soy flour (contained 14 g of soy protein) or followed their regular diet for eight weeks. The concentration of homocysteine had no significant difference between the two groups [166]. In another randomized crossover study on forty-one hyperlipidemic men and postmenopausal women, three groups were assigned to receive one of these regimens: (1) a low-fat dairy control diet, (2) low-isoflavone soy food diet (10 mg isoflavones/day), and (3) high-isoflavone soy food diet (73 mg isoflavones/day). Intervention was conducted for 3* one months, allowing a two-week washout period between interventions. It was showed that the homocysteine level was lower in both isoflavones groups when compared to the control group [167]. A total of 55 postmenopausal women (aged 42-72 years) in a double blind clinical trial were randomly recruited to receive one of the four soy protein isolate treatments (40 g/d): (1st) normal phytate and isoflavone (Phytate: 0.78 g and Aglycone isoflavones: 84.6 mg), (2nd) normal phytate and low isoflavone (Phytate: 0.64 g and Aglycone isoflavones: 1.2 mg), (3rd) low phytate and normal isoflavone (Phytate: 0.22 g and Aglycone isoflavones: 85.5 mg), and (4th) low phytate and isoflavone (Phytate: 0.22 g and Aglycone isoflavones: 1.2 mg). After six weeks, the homocysteine levels were significantly reduced in soy protein normal phytate groups, whilst no significant change was detected in the soy protein normal isoflavone groups [168]. In another recent randomized crossover study, 89 postmenopausal women were randomly assigned to consume (1) two fruit cereal bar,s each including 25 mg soy isoflavons (genistein:daidzein ratio of 2:1) or (2) only two fruit cereal bars with no isoflavones. The intervention was conducted for eight weeks with an eight-week washout. The results showed that homocysteine did not change in both groups [169]. In a recent double-blind RCT, 38 postmenopausal women complaining of insomnia were asked to either consume 80 mg/day of isoflavones (60.8 mg of genistein, 16 mg of daidzein, and 3.2 mg of glicitein) daily or placebo for four months. Following intervention, the homocysteine showed no significant change in both groups [170]. Simi-larly, 30 female wild-type mice were randomly allocated to three groups: (I) the control group (modified standard maintenance chow), (II) soy group (55% total energy), or (III) casein group (55% total energy). After 12 weeks, the homocysteine level was higher in casein group when compared to the control group, while the homocysteine level remained the same in the soy and control groups [171]. A total of 24 adult female Sprague Dawley rats were also randomized into four groups: (1) 2% cholesterol diet (2) 2% cholesterol diet + fresh soy oil, (3) 2% cholesterol diet + one-heated soy oil, and (4) 2% cholesterol diet + five-time-heated soy oil. After four months, 2% cholesterol diet + fresh soy oil significantly reduced the homocysteine level when compared to the others [172].
fruit cereal bar,s each including 25 mg soy isoflavons (genistein:daidzein ratio of 2:1) or (2) only two fruit cereal bars with no isoflavones. The intervention was conducted for eight weeks with an eight-week washout. The results showed that homocysteine did not change in both groups [169]. In a recent double-blind RCT, 38 postmenopausal women complaining of insomnia were asked to either consume 80 mg/day of isoflavones (60.8 mg of genistein, 16 mg of daidzein, and 3.2 mg of glicitein) daily or placebo for four months. Following intervention, the homocysteine showed no significant change in both groups [170]. Similarly, 30 female wild-type mice were randomly allocated to three groups: (I) the control group (modified standard maintenance chow), (II) soy group (55% total energy), or (III) casein group (55% total energy). After 12 weeks, the homocysteine level was higher in casein group when compared to the control group, while the homocysteine level remained the same in the soy and control groups [171]. A total of 24 adult female Sprague Dawley rats were also randomized into four groups: (1) 2% cholesterol diet (2) 2% cholesterol diet + fresh soy oil, (3) 2% cholesterol diet + one-heated soy oil, and (4) 2% cholesterol diet + five-time-heated soy oil. After four months, 2% cholesterol diet + fresh soy oil significantly reduced the homocysteine level when compared to the others [172].

Nuts
Nuts have scant amounts of bioactive compounds (e.g., phytosterols, unsaturated FAs, fiber, protein, vitamins, calcium, magnesium, sodium, and potassium) and they are a good source of antioxidants [192][193][194]. In a study on 15 hypercholesterolemic (serum cholesterol level > 200 mg/dl) adult males (aged 33-59 years), the patients received hazelnut (Corylus avellana L., Betulaceae) enriched diet (control diet (low-fat, low-cholesterol, and highcarbohydrate) + 40 g/d hazelnut) for four weeks directly after four weeks of a control diet. Following intervention, no change in homocysteine level was detected between the groups [195]. In a two-phase single blind crossover study on 67 patients (serum total cholesterol > 5.2 mmol/L), the participants first consumed low-fat, low-cholesterol diet for six weeks. On the second phase, two groups were randomly selected to either consume 64 g/d walnut (Juglans regia L., Juglandaceae) with their diet or continue the same diet for six weeks. Finally, all of the patients were crossed over into the opposite treatment arm for another six weeks. The findings reveled no statistically significant change on the homocysteine levels [196].
effects [176,[187][188][189]. In a recent double-blind RCT, 98 patients affected with dyslipidemia were asked to either consume 500 mg capsule of Amla extract (from Indian gooseberry) each day or placebo for 12 weeks. Following intervention, the homocysteine level did not change significantly in both groups [190]. Similarly, 17 uremic patients were selected to receive Amla extract tablets (300 mg, 50% dextrin + 50% amla extract) four times a day. Based on the findings, the homocysteine level did not change after four months of intervention [191].

Nuts
Nuts have scant amounts of bioactive compounds (e.g., phytosterols, unsaturated FAs, fiber, protein, vitamins, calcium, magnesium, sodium, and potassium) and they are a good source of antioxidants [192][193][194]. In a study on 15 hypercholesterolemic (serum cholesterol level > 200 mg/dl) adult males (aged 33-59 years), the patients received hazelnut (Corylus avellana L., Betulaceae) enriched diet (control diet (low-fat, low-cholesterol, and high-carbohydrate) + 40 g/d hazelnut) for four weeks directly after four weeks of a control diet. Following intervention, no change in homocysteine level was detected between the groups [195]. In a two-phase single blind crossover study on 67 patients (serum total cholesterol > 5.2 mmol/L), the participants first consumed low-fat, low-cholesterol diet for six weeks. On the second phase, two groups were randomly selected to either consume 64 g/d walnut (Juglans regia L., Juglandaceae) with their diet or continue the same diet for six weeks. Finally, all of the patients were crossed over into the opposite treatment arm for another six weeks. The findings reveled no statistically significant change on the homocysteine levels [196].

Berberine
Berberine, a benzylisoquinoline alkaloid (Figure 9), is an active constituent in numerous medicinal plants with many pharmacological properties. It has been largely used in Ayurvedic and Chinese medicine for its antimicrobial, antiprotozoal, antidiarrheal, and antitrachoma activities. Several clinical and preclinical studies have indicated the promising effects of berberine on metabolic, neurological, and cardiological disorders [207][208][209][210][211][212]. In a study, healthy male rats (weighing 190-210 g) randomly received a standard diet or a high-fat diet (HFD) for 24 weeks. After eight weeks of feeding, rats that were fed with HFD were randomly assigned into two groups: (I) berberine (extracted from Coptis chinensis Franch.) (200 mg/kg/day) or (II) vehicle by gavage for 16 weeks (n = eight per each group). The results showed that the berberine consumption led to a significant reduction in serum homocysteine by about 60% when compared to the vehicle in rats fed with a HFD [213]. In a double-blind RCT, 31 diabetic patients were randomly assigned into two groups to receive Berberis vulgaris L. fruit extract (3 g/d) (which contains the berberine alkaloid) or placebo for three months. No significant change was observed in the serum homocysteine level between the two groups [214]. group). The results showed that the berberine consumption led to a significant reduction in serum homocysteine by about 60% when compared to the vehicle in rats fed with a HFD [213]. In a double-blind RCT, 31 diabetic patients were randomly assigned into two groups to receive Berberis vulgaris L. fruit extract (3 g/d) (which contains the berberine alkaloid) or placebo for three months. No significant change was observed in the serum homocysteine level between the two groups [214].

Curcumin
Curcumin, a turmeric-derived polyphenol (Curcuma longa L., Zingiberaceae) (Figure 10), is known for its safety and medicinal properties against a variety of diseases [215][216][217][218][219][220][221][222][223][224]. It also has beneficial effects in metabolic syndrome and obesity [225,226]. In a recent doubleblind RCT, 22 obese men were administered a 500 mg curcumin supplement (193 mg of curcuminoids in the form of 81.8% curcumin, 15.3% demethoxycurcumin (C20H18O5) (Figure 11), and 2.8% bisdemethoxycurcumin(C19H16O4) ( Figure 12) were infused into 60% soluble fiber from fenugreek to improve bioavailability) or placebo supplement (comprised of equal parts of soluble fiber from fenugreek) every day for 12 weeks. After intervention, the plasma homocysteine concentration was significantly reduced in the intervention group when compared to the placebo group [227]. In a clinical trial, female Wistar-Furth rats were randomly divided into low ω-3 PUFA (LFO, n = 12) and high ω-3 PUFA (HFO, n = 12) groups and they were further divided into three subgroups: LFO or HFO (controls), LFOE or HFOE (LFO or HFO plus ethanol, 35% of dietary calories derived from ethanol), and LFOEC or HFOEC (LFOE or HFOE supplemented with curcumin 150 mg/kg body weight/day). All of these groups were pair-fed for eight weeks. Curcumin caused a significant increase in homocysteine thiolactonase activities as compared to the high ω-3 PUFA and ethanol groups [228]. In a group of fifty healthy men who were randomly selected, a two month consumption of biscuits with a bioactive complex, such as organic selenium (115 mg), quercetin (6 g dried selenized onion), curcumin (1.3 g curcuma), and catechins (2 g green tea), was associated with decreased homocysteine levels [229]. In a clinical trial on 32 adult male Wister rats, four groups were selected: (1) control group (no injection), (2) vehicle of homocysteine (2 µmol/µL), (3) vehicle of curcumin (50 mg/kg), and (4) homocysteine-curcumin group. Curcumin was injected intraperitoneally once daily for 10 days, beginning five days prior to homocysteine intracerebroventricular injection. In the end, curcumin significantly reversed the behavioral and biochemical changes caused by exposure of homocysteine in the control mice. Similar to homocysteine, curcumin could be considered to be a therapeutic agent in preventing the progression of neurotoxicity [230]. A total of 50 female Sprague-Dawley rats (weighing 220-250 gr) were randomly divided to the following groups: (1) control (0.3 mL of the vehicle), (2) sham (0.3 mL of the vehicle + surgery stress), (3) danazol treatment (7.2 mg/kg BW), (4) curcumin treatment (48 mg/kg BW), and (5) test (0.3 mL of the vehicle). After four weeks of treatment, no significant difference was observed between all groups [231]. traperitoneally once daily for 10 days, beginning five days prior to homocysteine intracerebroventricular injection. In the end, curcumin significantly reversed the behavioral and biochemical changes caused by exposure of homocysteine in the control mice. Similar to homocysteine, curcumin could be considered to be a therapeutic agent in preventing the progression of neurotoxicity [230]. A total of 50 female Sprague-Dawley rats (weighing 220-250 gr) were randomly divided to the following groups: (1)

Resveratrol
Resveratrol (3,5,4′-trihydroxy-trans-stilbene) ( Figure 13), a natural polyphenol that is found in numerous fruits and vegetables, has several properties, including anti-aging, anticancer, anti-inflammatory, and antidiabetic. Although the positive effects of resveratrol against selected cardiovascular risk factors have been a subject of debate over recent years [232,233], there are numerous effects against oxidative stress, apoptosis, mitochondrial dysfunction, endothelial dysfunction, and angiogenesis that might still support the potential role of this phytochemical in the prevention of CVDs [234][235][236]. A total of 24 rats were randomly assigned to three groups: (I) control group (received standard rat food), (II) homocysteine group (received 1 g/kg bodyweight/day methionine in drinking water), and (III) homocysteine + resveratrol group (received same amount of methionine+ 20 mg/kg/day resveratrol intraperitoneally). After 30 days, the results indicated that the plasma homocysteine level was significantly reduced in a group that was treated with resveratrol [237]. In a study conducted by Noll et al., the mice were randomly divided into four groups and maintained on the following diets for three months: (I) control group (received standard rodent diet), (II) high methionine (received standard diet plus 0.5% Lmethionine (36 mg/day) in drinking water), (III) methionine/resveratrol (received highmethionine diet with 0.001% trans-resveratrol (50 μg/day) in the last month, and (IV) resveratrol (received standard diet with 0.001% trans-resveratrol in the last month). The findings showed that plasma total homocysteine concentration in high methionine group

Resveratrol
Resveratrol (3,5,4′-trihydroxy-trans-stilbene) ( Figure 13), a natural polyphenol that is found in numerous fruits and vegetables, has several properties, including anti-aging, anticancer, anti-inflammatory, and antidiabetic. Although the positive effects of resveratrol against selected cardiovascular risk factors have been a subject of debate over recent years [232,233], there are numerous effects against oxidative stress, apoptosis, mitochondrial dysfunction, endothelial dysfunction, and angiogenesis that might still support the potential role of this phytochemical in the prevention of CVDs [234][235][236]. A total of 24 rats were randomly assigned to three groups: (I) control group (received standard rat food), (II) homocysteine group (received 1 g/kg bodyweight/day methionine in drinking water), and (III) homocysteine + resveratrol group (received same amount of methionine+ 20 mg/kg/day resveratrol intraperitoneally). After 30 days, the results indicated that the plasma homocysteine level was significantly reduced in a group that was treated with resveratrol [237]. In a study conducted by Noll et al., the mice were randomly divided into four groups and maintained on the following diets for three months: (I) control group (received standard rodent diet), (II) high methionine (received standard diet plus 0.5% Lmethionine (36 mg/day) in drinking water), (III) methionine/resveratrol (received highmethionine diet with 0.001% trans-resveratrol (50 μg/day) in the last month, and (IV) resveratrol (received standard diet with 0.001% trans-resveratrol in the last month). The findings showed that plasma total homocysteine concentration in high methionine group

Resveratrol
Resveratrol (3,5,4 -trihydroxy-trans-stilbene) ( Figure 13), a natural polyphenol that is found in numerous fruits and vegetables, has several properties, including anti-aging, anticancer, anti-inflammatory, and antidiabetic. Although the positive effects of resveratrol against selected cardiovascular risk factors have been a subject of debate over recent years [232,233], there are numerous effects against oxidative stress, apoptosis, mitochondrial dysfunction, endothelial dysfunction, and angiogenesis that might still support the potential role of this phytochemical in the prevention of CVDs [234][235][236]. A total of 24 rats were randomly assigned to three groups: (I) control group (received standard rat food), (II) homocysteine group (received 1 g/kg bodyweight/day methionine in drinking water), and (III) homocysteine + resveratrol group (received same amount of methionine + 20 mg/kg/day resveratrol intraperitoneally). After 30 days, the results indicated that the plasma homocysteine level was significantly reduced in a group that was treated with resveratrol [237]. In a study conducted by Noll et al., the mice were randomly divided into four groups and maintained on the following diets for three months: (I) control group (received standard rodent diet), (II) high methionine (received standard diet plus 0.5% L-methionine (36 mg/day) in drinking water), (III) methionine/resveratrol (received high-methionine diet with 0.001% trans-resveratrol (50 µg/day) in the last month, and (IV) resveratrol (received standard diet with 0.001% trans-resveratrol in the last month). The findings showed that plasma total homocysteine concentration in high methionine group was four times higher than the control group. In the methionine/resveratrol group, plasma total homocysteine levels were non-significantly increased when compared to the methionine group. However, in methionine/resveratrol group plasma homocysteine levels were 1.3 times higher as compared to the methionine group. Additionally, in the resveratrol group, the homocysteine levels were significantly increased by 1.7 (uM) when compared to the control group. Based on these results, resveratrol had detrimental effects on homocysteine levels [238]. Similarly, an experiment was performed on 30 female rats that were randomly allocated to three groups (I) control group, (II) potassium bromate group (KBrO 3 80 mg/kg), and (III) resveratrol (33 mg/kg four times a week) + KBrO 3 (80 mg/kg). These treatments were continued for five weeks. The findings demonstrated that the serum homocysteine levels in the resveratrol+ KBrO3 group were significantly lower than the control group [239]. According to a study conducted by Schroecksnadel et al., peripheral blood mononuclear cells (PBMC) were isolated from healthy volunteer blood donors by density centrifugation. In order to examine the effects of resveratrol on PBMCs, the cells were either pre-incubated with 10-100 mM resveratrol or stimulated with mitogens after 30 min., or resveratrol was added 2 h after stimulation. The cells were incubated at 37 • C in 5% CO 2 for 72 h and supernatants were harvested by centrifugation and then frozen at -208 • C until measurement. The results showed that unstimulated PBMCs produced small amounts of homocysteine and pretreatment of unstimulated cells with 10-100 mM resveratrol only slightly decreased the homocysteine production of resting cells [240]. eral blood mononuclear cells (PBMC) were isolated from healthy volunteer blood donors by density centrifugation. In order to examine the effects of resveratrol on PBMCs, the cells were either pre-incubated with 10-100 mM resveratrol or stimulated with mitogens after 30 min., or resveratrol was added 2 h after stimulation. The cells were incubated at 37°C in 5% CO2 for 72 h and supernatants were harvested by centrifugation and then frozen at -208 °C until measurement. The results showed that unstimulated PBMCs produced small amounts of homocysteine and pretreatment of unstimulated cells with 10-100 mM resveratrol only slightly decreased the homocysteine production of resting cells [240].

Soluble Fiber
Water-insoluble fibers enhance the stool bulk and improve bowel movements. The viscous or gel-forming fibers (e.g., gums, mucilages, pectins, algal polysaccharides, some hemicelluloses, and some storage polysaccharides) are water soluble. Good sources of water-soluble fiber include oats, dried beans, barley, some vegetables, and fruits [241,242]. The positive effects of soluble fibers on cardiac diseases, hyperlipidemia, diabetes mellitus, insulin resistance, and obesity are well known [243]. In a double-blind randomized parallel controlled study, 29 overweight men (aged 20-69 years, BMI: 25-35 kg/m 2 ) were asked to consume 3 g soluble fiber every day or placebo for 12 weeks along with a carbohydrate restricted diet and standard daily multivitamins. In the end, the homocysteine level did not significantly increase in the fiber group as compared to the placebo group [244]. A total of 119 subjects were randomly recruited to two groups: (I) multivitamin and 4g soluble fiber or (II) placebo. After eight weeks of intervention, the homocysteine levels were significantly reduced in the fiber blend group when compared to the placebo group [245].

Conclusions and Future Perspective
This review demonstrated that medicinal plants and herbal bioactive compounds have promising effects on reducing the homocysteine levels. According to the clinical trials, resveratrol and curcumin had favorable effects on serum homocysteine levels, whilst the results regarding other compounds were inconclusive (Table 1). In some of the previ-

Other Soluble Fiber
Water-insoluble fibers enhance the stool bulk and improve bowel movements. The viscous or gel-forming fibers (e.g., gums, mucilages, pectins, algal polysaccharides, some hemicelluloses, and some storage polysaccharides) are water soluble. Good sources of water-soluble fiber include oats, dried beans, barley, some vegetables, and fruits [241,242]. The positive effects of soluble fibers on cardiac diseases, hyperlipidemia, diabetes mellitus, insulin resistance, and obesity are well known [243]. In a double-blind randomized parallel controlled study, 29 overweight men (aged 20-69 years, BMI: 25-35 kg/m 2 ) were asked to consume 3 g soluble fiber every day or placebo for 12 weeks along with a carbohydrate restricted diet and standard daily multivitamins. In the end, the homocysteine level did not significantly increase in the fiber group as compared to the placebo group [244]. A total of 119 subjects were randomly recruited to two groups: (I) multivitamin and 4g soluble fiber or (II) placebo. After eight weeks of intervention, the homocysteine levels were significantly reduced in the fiber blend group when compared to the placebo group [245].

Conclusions and Future Perspective
This review demonstrated that medicinal plants and herbal bioactive compounds have promising effects on reducing the homocysteine levels. According to the clinical trials, resveratrol and curcumin had favorable effects on serum homocysteine levels, whilst the results regarding other compounds were inconclusive (Table 1). In some of the previous studies, homocysteine was evaluated as a secondary outcome, and its effects on treatment or prevention of diseases were not elucidated. Moreover, factors, which included small sample size, diverse population in terms of age, being healthy or having underlying chronic diseases, different herbs with dissimilar dosages, and different duration of interventions, made it difficult to draw a strong evidence-based conclusion. Based on almost all animal studies, medicinal plants showed promising effects on homocysteine. Black and green tea, cinnamon, resveratrol, curcumin, garlic extract, ginger, and soy significantly reduced the homocysteine concentrations (Table 2). Altogether, this review highlighted the beneficial effects of medicinal plants as natural, inexpensive, and accessible agents without any considerable adverse effects on homocysteine levels. Nevertheless, the results of the clinical trials were not uniform, which suggested that more well-designed comprehensive clinical trials are warranted.