Natural Products as Protective Agents for Male Fertility

: The male reproductive system is highly susceptible to noxious influences, such as oxidative stress, inflammation, drugs, and even diseases that can induce germ cell damage and alterations in spermatogenesis. All of these factors, which are caused by actions at the testicular level and/or at the excurrent ducts and accessory glands, significantly affect sperm parameters and male fertility. For this reason, it is of major importance to investigate possible ways to protect the male reproductive system since males are exposed to these toxic factors constantly. For centuries, natural products have been used by humans in folk medicine as therapeutic agents, and because of their beneficial properties for human health, plenty of them have been introduced to the pharmaceutical market as supplementary therapies. The present review aims to compile available information regarding different natural exogenous factors that demonstrate potential useful activity in the male reproductive system. The studies presented here reopen the perspective of using natural products as protective agents and eventually as new supplementary therapeutic options for the recovery of hampered spermatogenesis and/or


Introduction
In mammals, adult testicles participate in the biosynthesis and secretion of sex steroid hormones and the production of testicular fluid and spermatozoa.These endocrine and exocrine functions are crucial for male fertility [1].The production of male gametes is a complex, dynamic, and continuous process that occurs inside the seminiferous tubules (SeT), the functional unit of the testis.Spermatogenesis encompasses three different sequential processes: (i) mitotic division of spermatogonia (spermatocytogenesis); (ii) meiosis; and (iii) spermiogenesis, the differentiation of spermatids into spermatozoa, which are released in the lumen of the SeT (spermiation) [1,2].
Several factors have been shown to affect the normal progression of spermatogenesis, which may lead to a significant decrease in the production and/or quality of spermatozoa, causing infertility.Oxidative stress is one of the most harmful factors and a major cause of male infertility, with relevant damaging effects on the development of germ cells and sperm function [3].The endogenous cellular antioxidant defense system protects the cells against reactive oxygen species (ROS), which can be generated via endogenous processes or in response to exogenous agents.When ROS production exceeds the capacity of the endogenous cellular antioxidant defense system, oxidative stress occurs [4].ROS can damage biological macromolecules such as proteins, lipids, and nucleic acids.These ROS actions will disturb the structure and function of the sperm membrane and DNA, declining the normal sperm parameters (e.g., motility, counts and viability) as well as the capacity to fertilize the oocyte [5,6].Even with poor intratesticular vascularization and low oxygen tension, spermatogenesis and steroidogenesis are very susceptible to damage due to the high levels of ROS production in the testis [7].In addition, it is well established that increased levels of ROS are associated with an inflammatory response, which may be triggered by several conditions, such as varicocele, tobacco, alcohol, and metabolic syndromes [8].Finally, the testis is very sensitive to exogenous agents, such as several pharmacological, treatments that can induce adverse effects on spermatogenesis, sperm parameters, and sexual function [9].
For centuries, humans have used natural products in folk medicine as therapeutic agents and drugs.However, the use of these natural products has increased in recent decades and have been gradually introduced into the pharmaceutical market [10].Using laboratory animals, several studies have demonstrated that certain natural products and derivatives might play a protective role against the damaging effects associated with oxidative stress, inflammation, and drugs side effects [11,12].Moreover, many natural products have already been described as being useful for the treatment of male disorders such as sexual impotence [13,14].
Various natural products have a set of properties that can be beneficial for the functioning of the male reproductive system, both in animals and in humans.These natural products can be an isolated bioactive compound or a product containing a mixture of several bioactive compounds in their composition.This review provides an overview of relevant bioactive compounds, such as curcumin, ellagic acid, vitamin C, and vitamin E, and natural products that contain bioactive compounds in their composition, such as garlic, ginger, grape, green tea, microalgae and algae, and propolis.All of them have been identified as protective agents and/or have beneficial effects on animal models for male reproductive function due to their biological activity.This review mainly focuses on reports of animal experiments because due to the high complexity of the male reproductive system, they are more realistic models.

Curcumin
Curcumin or [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadi-ene-3,5-dione] (chemical structure in the Supplementary Materials) is a bioactive substance that is present in the rhizome of the turmeric plant (Curcuma longa) and is the major constituent of turmeric powder [15][16][17].It has been used in Asian folk medicine for several purposes and is also described in Hindu texts [18].The most important properties of curcumin rely on its therapeutic benefits, which have been described in the treatment of several diseases, with the absence of toxic side effects [19][20][21].
In the testis, it has been shown that curcumin acts as a protective factor against oxidative stress-inducers such as cadmium.Curcumin prevents the histopathological damage caused by the presence of cadmium chloride [32,34], reversing the effects on the mean diameter of the SeT and lumen [32].Additionally, curcumin increases the levels of GSH, glutathione peroxidase (GSH-Px), and superoxide dismutase (SOD) [32,34]; testosterone levels, transcription factor NF-E2-related factor 2 (Nrf2), and γ-glutamylcysteine synthetase [34]; catalase (CAT) and total thiols levels [32]; and decreased malondialdehyde (MDA) and hydrogen peroxide content [32,34].Considering sperm parameters, the coadministration of curcumin with cadmium chloride increased sperm motility and concentration and decreased the morphologic defects of sperm compared to cadmium chloride alone [34].
Several drugs, such as cisplatin and artesunate, can induce damage in the male reproductive system.Cisplatin is a highly effective anti-neoplastic drug used in several types of solid tumors, and artesunate is used in the treatment of malaria; both induce severe damage to the reproductive system.Curcumin cotreatment with cisplatin or artesunate (low and high-dose) showed beneficial effects, preventing histopathologic changes [31,33,35] by increasing the mean SeT diameter and germ cell layer number [35] and by presenting a slight expression of nuclear factor kappa B (NF-kB) [31,35], iNOS, p38-MAPK [35], caspase-3, and 8-deoxyguanosine (8-OHdG) [31].Additionally, in the group treated with cisplatin plus curcumin, it was observed that the SeT displayed myoepithelial, spermatogonia, and spermatocytes cells with normal morphology [31].Curcumin coadministration with one of these drugs increased GSH, GSH-Px [33,35], testicular weight, testosterone levels [35], SOD, and glutathione reductase (GSH-Rx) levels [33] and decreased MDA levels [33,35], glutathione-S-transferase (GST) [33], and nitric oxide (NO) levels [35].
Aflatoxins are toxic fungal metabolites that are produced by Aspergillus flavus and Aspergillus parasiticus, which contaminate human food.Mathuria et al. showed that the effects of curcumin on minimizing aflatoxins damage were dose-dependent, with the maximum amelioration of a decrease in the thiobarbituric acid reactive substance (TBARS) levels in mice testis being observed at 200 mg/mL [36].Additionally, curcumin treatment plus aflatoxins (low-and high-dose) increased DNA, RNA, protein, and the activity of the 3β-hydroxysteroid dehydrogenase (3β-HSD) and 17β-hydroxysteroid dehydrogenase (17β-HSD), both of which are enzymes involved in steroidogenesis, and decreased cholesterol.These effects were more effective when low doses of aflatoxins were used [37].
Other events/incidents inducing testicular damage may include chronic variable stress, scrotal heat stress, ischemia-reperfusion injury, and ageing.Curcumin also seems to improve histopathological testicular changes in these situations [15,16,38] by increasing the Johnson's Testicular Score [15,17], germinal epithelium height [15], the number of germ cell layers, and the mean of the SeT diameter [17].Additionally, curcumin seems to decrease the number of apoptotic cells in the SeT [16] and oxidative stress, as indicated by the diminished MDA levels [17,38].
Chronic variable stress is a major problem that can lead to changes in endocrine and reproductive functions.When chronic variable stress was induced, curcumin treatment increased testosterone, follicle stimulating hormone (FSH), and luteinizing hormone (LH) levels compared to the non-treated group [16].
Testicular temperature is of significant importance to maintain the optimum environment for normal spermatogenesis, and consequently, scrotal heat stress is a primary factor associated with male infertility [38].The different doses of curcumin in all of the experimental groups showed that this bioactive compound increased testis weight, testosterone levels, SOD activity, cytoplasmic copper/zinc SOD, mitochondrial manganese SOD, 3β-HSD, and phospholipid hydroperoxide glutathione peroxidase in a dose-dependent manner.Curcumin treatment increased the expression of the anti-apoptotic protein, B-cell lymphoma-extra large while decreasing expression of the apoptosis marker, caspase-3, which is indicative of its anti-apoptotic effect.Finally, heat shock protein factor 1, a major regulator of the stress-inducible response (anti-apoptotic), showed increased levels, whereas the levels of transforming growth factor-β1, a secreted protein involved in many cellular functions (e.g., cell growth, proliferation, differentiation, and apoptosis), were decreased in all of the curcumin-treated groups, indicating the anti-apoptotic effects of this bioactive compound once again [38].
Testicular torsion is a urologic emergency that leads to a surgical detorsion of the spermatic cord.The primary effect of testicular torsion-detorsion is an ischemia-reperfusion injury that increases the production of ROS.Curcumin administration repairs the noxious effects of testicular torsion by increasing heme oxygenase-1 (HO-1) protein expression levels and by decreasing xanthine oxidase (XO) activity when compared to the control and torsion-distortion groups [17].
Ageing is associated with infertility caused by decreased steroidogenesis, spermatogenesis, and sexual dysfunction.The mechanism is not totally understood, but oxidative stress and apoptosis seem to be the two significant factors that play an essential role in the ageing process.In aged testicular tissue, curcumin treatment resulted in increased GSH levels compared to the aged control group [15].Despite the notorious beneficial effects in male reproduction in animal models, some studies claim that curcumin is not a therapeutic compound for male reproduction and other diseases [39,40].They consider that curcumin has a PAINS (pan-assay interference compounds) and could be an IMPS (invalid metabolic panaceas) candidate.
In Table 1, it is summarized the beneficial effects in male reproductive system.
Ellagic acid has been shown to protect the male reproductive system from several drugs that can induce damage [43,54,55].Ellagic acid can ameliorate cisplatin's toxicity, preventing negative effects from occurring during histopathological analysis and in sperm parameters [54].In addition, ellagic acid increases the weights of the epididymis, seminal vesicles, and prostate, and the CAT activity in plasma, sperm, and testicular tissue.On the other hand, it decreased the MDA levels in plasma, sperm, and testicular tissue as well as the GSH-Px activity [54].Similar results were observed by another research group, who noticed that ellagic acid ameliorated the toxicity of cisplatin [55].Additionally, ellagic acid treatment revealed similar results concomitantly with cyclosporine A, an immunosuppressor agent [43].
Several studies have used ellagic acid as a protective factor against several compounds that induce-reproductive damage, such as phthalates, arsenic, and polychlorinated biphenyl (Aroclor 1254), which are environmental pollutants that are present in a wide variety of daily products.Quite a few studies have shown that ellagic acid avoids histopathologic damage [56][57][58], preventing the decrease of SeT diameter, germinal cell layer thickness [56,58], and the Johnsen's testicular score [58].Additionally, it increases the levels of antioxidant defense, GSH [56][57][58], SOD [56,58], and CAT [57,58], total antioxidant capacity (TAC) [57], and GSH-Px [58] and decreased TBARS levels [56,58] as well as MDA and protein carbonyl (PC) content [57].Ellagic acid improves sperm parameters [57,58], functional membrane integrity, mitochondrial membrane potential, and sperm kinematics (progressive motility, rapid and fast progressive motility) [57].Additionally, arsenic up-regulates the Ppargc1a (peroxisome proliferative activated receptor, gamma, coactivator 1 alpha) gene and down-regulates the Nrf2 and StAR (steroidogenic acute regulatory protein) genes [57].Ppargc1a is a gene that is related to energy metabolism and mitochondrial biogenesis, which is upregulated in response to cell stress.Nrf2 is a regulator of the antioxidant defense system, and low expression of this gene is related to decreased TAC in the testes and increased sperm abnormalities.StAR is a gene that regulates steroid hormone synthesis, and low StAR expression of is linked with low testosterone production and sperm quality [57].These variations were prevented with ellagic acid treatment, demonstrating the protective effect of ellagic acid against the toxic effects of arsenic-induced toxicity [57].
Ellagic acid also seems to protect against monosodium glutamate, a food additive and flavor enhancer.The presence of low and high doses of monosodium glutamate ellagic acid improved histopathological findings, increased the antioxidant enzymes SOD, CAT, and GSH-Rx as well as thiol, testosterone, and inhibin B levels and decreased myeloperoxidase (MPO) XO, PC, 8-OHDG, MDA, and GSH-Px levels [59].
In Table 1, it is summarized the beneficial effects in male reproductive system.

Vitamin C
Vitamin C or ascorbic acid (chemical structure at Supplementary File) is a water-soluble vitamin found in berries, citrus fruits, green leafy vegetables, tomatoes, and potatoes [60].Vitamin C can only be obtained from the diet and is mainly absorbed by the intestine and is then distributed throughout the body [61].It functions as an enzymatic cofactor in the biosynthesis of collagen, carnitine, and catecholamines; enzyme complement; co-substrate; and a potent antioxidant [60,61].The antioxidant properties of Vitamin C are due to its ability to scavenge reactive oxygen and nitrogen species and its ability to regenerate other small antioxidant molecules such as α-tocopherol (Vitamin E), GSH, urate, and βcarotene [62].
Vitamin C plays an important role in the male reproductive system, protecting spermatogenesis, preventing sperm agglutination, and increasing testosterone synthesis [63,64].Several studies have shown the beneficial effects of vitamin C administration against the reproductive toxicity of environmental pollutants such as aluminum, mercury, carbon tetrachloride, and lead.The effects of vitamin C against aluminum chloride toxicity include ameliorated histopathological and sperm parameters, namely counts, concentration, morphology, and motility (increased fast progressive motility and decreased slow progressive motility).Additionally, vitamin C increased SOD activity while decreasing NO activity [65].Vitamin C co-administration against mercuric chloride toxicity improved histopathological findings, daily sperm production parameters, and final body weight compared to the group treated for mercuric chloride only.Moreover, it increased the total protein, high-density lipoprotein (HDL) cholesterol, testosterone and the antioxidant enzymes peroxidase, and GST.On the other hand, decreased levels of low-density lipoprotein (LDL) cholesterol and lipid peroxidation (ROS and TBARS levels) were observed [66].Additionally, vitamin C treatment against carbon tetrachloride toxicity improved the histopathological findings in testis sperm parameters (motility, counts and morphology).It increased FSH and LH hormone levels and the expression of antioxidant enzymes, CAT, and SOD.Additionally, vitamin C decreased GSH-Px expression and TBARS levels compared to the carbon tetrachloride group [67].Finally, vitamin C treatment can recover histopathological parameters to the control levels [68].
Vitamin C co-treatment against ethanol toxicity ameliorated testicular histopathological analysis, decreasing (i) abnormal tubules, (ii) immature germ cells in the lumen, (iii) acidophilic cells, and (iv) vacuolization.The sperm number in the testis and daily sperm production showed an increase, and MDA levels decreased [63].
Vitamin C treatment along testicular torsion induction prevented alterations in spermatogenesis and the SeT diameter and decreased serum MDA levels compared to non-treated rats [72].Although vitamin C has recently been receiving attention from the scientific community due to its beneficial effects in several systems, some studies have shown contradictory effects regarding its action or have even shown no effects [73,74].Additionally, it is not clear what the recommended dose of vitamin C required in humans could be to be able to have results that are similar to those obtained in animal models [75].

Vitamin E
Vitamin E (chemical structure at Supplementary File) is a lipo-soluble vitamin that is obtained exclusively through the diet [76] and is widely present in olive and sunflower oils, avocados, green leafy vegetables, nuts, soybeans, and wheat [77].Vitamin E presents eight different isoforms that include alpha, beta, gamma, and delta-tocopherol and alpha, beta, gamma, and delta-tocotrienol [77].
The main biological activity of vitamin E is its antioxidant properties [78], protecting cell membrane components such as polyunsaturated fatty acids (PUFA) and LDL from oxidative damage mediated by free radicals [76].It has been described that vitamin E deficiency is associated with defects in spermatogenesis, decreased testosterone production [79], and testicular degeneration [80].Several studies that have used vitamin E supplementation against environmental pollutants such as arsenic, para-nonylphenol, and 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) have shown the beneficial effects of vitamin E, mainly in terms of several histological parameters [81,82].Vitamin E also increases the epididymal sperm number [81], the number of spermatogonia A, spermatogonia B, spermatocytes, spermatids and Sertoli cells [82], and daily sperm production [83].Vitamin E treatment has been used against TCDD-induced toxicity; has been shown to increase body, testis, epididymis, seminal vesicles, and ventral prostate weights; antioxidant enzymes (SOD, CAT, GSH-Rx and GSH-Px); and has been shown to decrease hydrogen peroxide generation and TBARS levels [83].
Vitamin E also seems to have a protective effect against agricultural products that induce reproductive toxicity, such as diazinon (or o,o-diethyl-o-[2-isopropyl-6-methyl-4pyrimidinyl] phosphorothioate), an organophosphate pesticide.In the presence of this pesticide, vitamin E increased GSH levels and decreased MDA levels [84].Additionally, vitamin E seems to have a protective effect against carbendazim [85] (or methyl-2-benzimidazole carbamate), which is a fungicide that is often used as a pesticide and herbicide.Vitamin E co-administration ameliorated histopathological changes by increasing the SeT diameter and the SeT lumen diameter and decreased the carbendazim concentration in the testis and in serum [85].
Moreover, vitamin E seems capable of improving the effect of some drugs that can induce reproductive toxicity in males.Vitamin E counteracted the damage associated with methotrexate, improving histopathological parameters, increasing SOD levels, and decreasing MDA levels [86].Increasing evidence of the beneficial actions of vitamin E against several diseases prompted have its use as a dietary supplement.However, it should be taken into account that excessive doses of vitamin E supplementation induce toxicity and can lead to hazardous complications, such as bleeding, thyroid problems, weakness, emotional disorders, and gastrointestinal derangement, among others [87].
In Table 1, it is summarized the beneficial effects in male reproductive system.

Garlic
Garlic has been used as a therapeutic and medicinal agent in several cultures for centuries, specifically in Egyptian Cordex Ebers and in ancient Greece, Rome, India, China, and Japan [88,89].Garlic is mainly constituted by water, but it also contains carbohydrates, sulfur compounds, proteins, fibers, and free amino acids.In addition, it contains high levels of saponins, phosphorus, potassium, sulfur, zinc; moderate levels of selenium and vitamins A and C; and low levels of calcium, magnesium, sodium, iron, manganese, and Bcomplex vitamins [90].Furthermore, a substantial amount of evidence has shown that some of the compounds present in garlic have antioxidant properties with radical-scavenging functions and antioxidant enzyme modulation [91,92].
Several garlic preparations, such as raw garlic homogenate, garlic powder, aged garlic extract, and garlic oil have been used in scientific studies to determine the effects of garlic.These may explain the diversity of the results concerning the effect of garlic in the male reproductive system.For example, Oi et al. used garlic powder to evaluate its effect on testosterone production in rats on a casein-based diet, a high protein level diet [93].After 28 days of supplementation, the testosterone levels were increased in the rats fed 40 and 25% casein diets, indicating that garlic enhanced testosterone production in high protein level diets [93].
On the other hand, the effect of garlic extract on sperm characteristics and testicular oxidative damage after cadmium exposure showed that garlic extract improved epididymal sperm parameters (concentration, motility and counts) and reduced the number of abnormal sperm.It increased GSH, SOD, CAT, and alkaline phosphate (ALP) and decreased MDA and GST levels, improving antioxidant status after cadmium exposure [94].Concerning other environmental contaminants, Furan is a contaminant found in a wide variety of foods that have been heat-treated via thermal degradation that can induce damage in the testis.Garlic oil has been shown to ameliorate the effect of furan on histopathological alterations and the reduction of testosterone levels.Garlic oil also increased SOD, CAT, and GSH levels and decreased the MDA, CYP2E1, and caspase-3 levels [95].
The antibiotic Adriamycin has been shown to induce reproductive toxicity.Aged garlic extract attenuates the toxic effects of Adriamycin, improving all sperm parameters (count, motility, viability and morphology) as well as the histological, biochemical, and ultrastructural findings in testicular sections.Moreover, it increases the levels of antioxidant defenses (GSH, GSH-Px, CAT, and SOD) and decreased MDA levels [96].Although garlic has several beneficial effects and is considered safe, care should be taken in terms of its therapeutic use, as some studies have shown its potential toxicity in the male reproductive system [88,97].In addition, a study showed that garlic might cause gastric irritation, nausea, vomiting, flushing, tachycardia, headache, insomnia, sweating, and dizziness, among other symptoms [97].
In Table 2, it is summarized the beneficial effects in male reproductive system.

Ginger
Ginger or Zingiber officinale is a grayish-white rhizome with pale brown rings that is used worldwide as a cooking spice and can be used as a powder or fresh root [98].For generations, it has been used as a medicinal plant to treat several medical conditions, such as constipation, fever, indigestion, infectious diseases, muscle aches, nausea, rheumatoid arthritis, sore throat [99], and disorders of the male reproductive system [100].In medieval Persia, medical texts such as Qanoonfel-teb (The Canon) by Ebn-e-Sina (980-1037) described ginger as a very effective herb for erectile dysfunction [101].In Western Uganda, men drink ginger tea to treat sexual impotence and erectile dysfunction [102].Additionally, hot ginger remedies are used to increase sexual energy and semen volume [100].
In the testis, ginger is a protective factor against oxidative stress-inducers, such as carbon tetrachloride, an industrial chemical that, when it is decomposed, creates sulfite salts (e.g., sodium metabisulfite).These sulfite salts are used as preservatives and disinfectants in foods and pharmaceutical agents, as they release sulfur dioxide that can cause oxidative stress.Ginger treatment in the presence of carbon tetrachloride or sodium metabisulfite improved histopathological findings [113] and prevented the toxic effects in the SeT, epididymal tissue [99], and in the sperm parameters (morphology and motility) [113].Additionally, ginger increased the enzymes GHS-Px, [99,113], TAC, SOD, and CAT [99]; GSH-Rx levels; and testosterone [113] and decreased MDA levels [99,113].
Alcohol consumption has been a cause of great concern in terms of male fertility since excessive consumption is associated with increased oxidative stress levels [112,114].Ginger can attenuate the effects of ethanol by increasing SOD, GSH-Px, and CAT activity and by decreasing MDA and total homocysteine concentrations.Moreover, ginger administration increased the levels of testosterone, sex hormone-binding globulin, and dehydroepiandrosterone sulfate [112,115].
Other studies have shown that ginger also protects the male reproductive system from drugs that induce damage.Three different doses of ginger (50, 100, and 150 mg/kg) were used to attenuate the toxicity of busulfan, a drug used to treat chronic myelogenous leukemia and blood disorders.Sperm counts and motility were increased in all ginger extract doses, with the sperm motility increase being dose-dependent.At a dose of 50 and 100 mg/kg, ginger extract increased the SeT volume, and at a dose of 100 and 150 mg/kg, the testosterone levels increased [116].Additionally, two different ginger extract doses (300 and 600 mg/kg) were used to evaluate protective effects against the toxic effects of cyclophosphamide, an anti-neoplastic agent.Both doses led to histological improvement and increased germ cells counts and antioxidant levels, but only the dose of 300 mg/kg increased the epithelium thickness, and the 600 mg/kg dose increased testosterone levels [117].
The co-administration of ginger with gentamicin, an antibiotic, and cisplatin decreased the number of apoptotic cells compared to the non-treated group [12,98] and restored normal testicular morphology, spermatogenesis, and sperm parameters (counts, motility and morphology) [118].In addition, the MDA levels decreased in response to treatment with ginger [12].
Several metabolic syndromes can induce damage in the testis, such as high-fructose diet-induced metabolic syndrome and diabetes.Fructose is a carbohydrate that is widely used as a food additive because of its sweetness.However, it is one of the main factors that is responsible for the progression to metabolic syndrome, leading to oxidative stress [119].In rats fed with a high-fructose diet, ginger decreased the total body weight but increased testicular weight.Additionally, it improved histopathological findings, increasing the epithelial height and SeT perimeter and proliferative cell nuclear antigen (PCNA) and Beclin 1 immunoreactivity.PCNA is considered to be a proliferation marker that can be used to analyse spermatogenesis, and Becilin 1 is a protein involved in autophagic pathways and is considered to be an important regulatory mechanism in spermatogenesis and steroidogenesis [119].The increased immunoreactivity of PCNA and Beclin 1 indicate an improvement in spermatogenesis and steroidogenesis in the presence of ginger treatment.Ginger increased the FSH, LH, testosterone, HDL, and SOD levels and decreased the triglycerides, LDL, MDA, and serum levels of glucose, insulin as well as the subsequent diminution of the homeostasis model assessment for insulin resistance (HOMA-IR) [119].
Diabetes is a chronic metabolic disease that is associated with infertility [120].Ginger treatment in induced diabetic rats with streptozotocin improved histopathological findings; increased the TAC, androgen receptor, and the PCNA levels; and decreased the blood glucose level and caspase-3 expression [120].Additionally, a different study showed that in alloxan-induced diabetic rats, a ginger supplemented diet improved sperm parameters (counts and motility) and increased testis, epididymis, prostate, and seminal vesicles weights.Moreover, a ginger-supplemented diet in diabetic rats increased (i) the hormonal serum levels of testosterone, FSH and LH and (ii) the antioxidant enzymes SOD, CAT and GSH-Px levels and decreased (i) plasma glucose; (ii) the metabolic enzymes aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), and ALP; and (iii) MDA levels in the testis [121].
In Table 2, it is summarized the beneficial effects in male reproductive system.

Grape
Grape (Vitis vinifera) is one of the most farmed and largely produced fruits in the world [122] and has a history of use in Europe for traditional treatments [123].Dried grape seeds contain 35% fiber, 29% extractable components such as phenolic compounds, 11% proteins, 7% water, and 3% of minerals [124] and can be used to produce grape seed extract.The phenolic compounds found in grape include non-flavonoids such as resveratrol and flavonoids (e.g., catechin, epicatechin, quercetin, anthocyanin, and pro-anthocyanidins) [114,125].
Grape seed extract presents several biological activities, namely anti-apoptotic, antinecrotic, cardiovascular [126], anti-cancer [127], anti-inflammatory [128], and antioxidant [129] effects.Its capacity to scavenge oxidants and free radicals is due to the presence of the phenolic compounds, especially the pro-anthocyanidins [130], which present a higher antioxidant capacity than vitamin C or E [131].Both grape seed extract and grape seed pro-anthocyanidin extract present protective effects against male reproductive toxic inducers.
Several studies have identified grape seed extract, grape seed pro-anthocyanidin extract, and grape juice concentrate as protective factors against cadmium toxicity.Grape seed extract and grape seed pro-anthocyanidin extract improved histopathological findings [132][133][134][135], increasing the diameter and normal SeT [132,135], testicular weight [132], and Johnsen's mean testicular biopsy score and decreasing the apoptotic index [135].It was also observed to increase PCNA immunoreactivity [132] and total antioxidant status (TAS) [133] in the enzymes of the antioxidant defense system and genes associated with steroidogenesis and Ki-67 expression [134], and it decreased MDA levels [133,134] and the immunoreactivity of the apoptotic regulator Bax [134].Grape juice concentrate treatment against cadmium toxicity demonstrated that co-administration restored the testis, epididymis, and ventral prostate weight [136] to normal levels, ameliorating tissue architecture [125,136,137], epididymis epithelium height (caput and cauda regions) [125] and improving sperm parameters such as production, counts, transit time [125], and morphology [136].Additionally, it increased testosterone and GSH levels and decreased cadmium accumulation, MDA levels [125], and SOD and mitochondrial SOD activity [136].
Grape seed extract can also protect the testis against the toxic effects of drugs such as dexamethasone, an immunosuppressive and anti-inflammatory glucocorticoid drug [138].Using two doses (200 or 400 mg/kg body weight) of grape seed extract administrated with dexamethasone, it was possible to observe histopathological improvement, especially at a dose of 400 mg/kg, which increased body, testis weight, and serum testosterone.It also increased thyroid hormones, (free T3, T4 and thyroid-stimulating hormone); antioxidant defenses (CAT and GSH); total protein content; acid phosphatase (ACP), a specific marker for spermatogenesis; and glucose-6-phosphate dehydrogenase (G-6-PDH).Thyroid hormones are essential in the regulation of the reproductive system, and G-6-PDH is associated with GSH synthesis that when decreased can indicate testicular degeneration [138].
Additionally, grape seed extract seems to attenuate the effects of ethanol by increasing the weight of the testis, epididymis and accessory sex organs, sperm parameters (counts, motility, and morphology), testosterone, and GSH levels and decreasing the MDA levels in the testis [114].
Plant growth regulators, namely gibberellic acid and indoleacetic acid, are chemicals that are used worldwide in agriculture that can induce toxic effects on the male reproductive system [122].Grape seed pro-anthocyanidin extract co-treatment with different plant growth regulators improved histopathological architecture and the spermatogenesis process and decreased apoptotic cells [122].
Several events/incidents can induce damage in the testis, such as varicocele and testicular torsion.Varicocele is a dilatation of the pampiniform plexus, draining the testes and inducing male infertility [139].The administration of grape seed pro-anthocyanidin extract in a varicocele model rats improved histological and sperm parameters (concentration and motility).In addition, an increase of the antioxidant enzymes (SOD and GSH-Px), the apoptotic regulator's ratio Bax/Bcl-2, Nrf2 and HO-1 was observed as were decreased levels of MDA, apoptotic cells, and caspase-3 [139].Grape seed pro-anthocyanidin extract was administrated 7 days prior to the surgical procedure for induced testicular torsion in rats for 2 h followed by administration 2 h after detorsion.Grape seed proanthocyanidin extract ameliorated histopathological damage, increasing Johnsen's mean testicular biopsy score and decreasing apoptotic cells, iNOS immunoreactivity, and MDA levels [140].
Trans-Resveratrol (trans-3,4,5-trihydroxystilbene) is a natural antioxidant present in grapes.A study with rats treated with trans-resveratrol, which was detected in plasma after 24 h of treatment, showed that it did not have adverse effects.The testicular weight was similar to the control group.However, the trans-resveratrol groups showed a reduced SeT diameter but an increased SeT length, increasing the tubular density comparatively to the control group.In addition, it increased the sperm count and the hormonal levels of LH, FSH, and testosterone [141].
In Table 2, it is summarized the beneficial effects in male reproductive system.
Green tea consumption is a common practice that is well accepted by modern society, and its beneficial effects are well recognized in our day to day lives.During a 52-day study period, the administration of green tea extract (2 and 5%) in male Wistar rats resulted in the body and reproductive organs weight remaining unchanged compared to control group.Liver weight also remained unchanged, but decreased levels of AST and ALT were observed, suggesting hepatoprotective properties.The testosterone level was similar to the control group as was the weight of the testosterone-dependent organs.Normal histological sections (except an increase in diameter of SeT) were observed between the green tea consumption group and control group.Concerning sperm parameters, green tea increased sperm concentration and viability, but the sperm motility and velocity functions remain similar to the control.However, a significant increase in spontaneous acrosome reactions was observed.Finally, the antioxidant defenses (CAT, GSH, MDA, and SOD levels) remain unchanged, indicating the safety of green tea consumption and a balance of the oxidative stress status of the tissues [158].
Several studies identify green tea as a protective factor against environmental pollutants that are widely used in industry and that can induce oxidative damage.These includes para-nonylphenol, deltamethrin (a synthetic insecticide), cadmium, and nicotine (a volatile alkaloid that is the primary toxic component of cigarette smoking).Green tea extract ameliorated histopathologic damage [156,[159][160][161][162] by increasing the SeT diameter [156,161], epithelium height, Johnson's score [156], and the volume of the SeT and testis [159].Additionally, green tea extract increased testis, body, and reproductive organ weight [159,161,162].In terms of the sperm parameters (concentration, motility, viability, morphology, production and counts), green tea extract co-administration with these environmental pollutants seemed to induce a beneficial effect [156,[159][160][161][162]. Green tea extract increased testosterone levels [156,161,162] and antioxidant defenses (SOD, CAT, and GSH) [161,162], and decreased caspase-3 [156,161] and cholesterol levels [161].Regarding the lipid peroxidation, green tea extract decreased MDA [156,159,160] and TBARS levels [162] compared to the environmental pollutants groups.
Green tea can also protect against the toxic effects of some drugs, such as doxorubicin, an anthracycline antibiotic [163].The administrations of two different green tea extract concentrations (200 mg/kg and 500 mg/kg) against doxorubicin-induced toxicity seemed to ameliorate the histological findings and increased sperm parameters (concentration and mobility), the Sertoli cell index (ratio between germ cells number and Sertoli cells number), and telomerase activity in both concentrations [163].Several studies have indicated the beneficial effects of green tea, though there are also reports of relevant side effects, such as hepatotoxicity and gastrointestinal disorders as well as drug interactions [164].
In Table 2, it is summarized the beneficial effects in male reproductive system.

Microalgae and Algae
Microalgae are a natural source of various bioactive compounds, such as phycobilins, carotenoids, PUFA, polysaccharides, sterols, and vitamins, which have many applications from animal feed to human nutrition [165].PUFA are abundant in microalgae, and they are one of the major compounds responsible for its bioactivities [166].Therefore, their effects on male infertility have been explored because oxidative stress is a major contributor to defective spermatogenesis [167].Moreover, elevated oxidative stress within semen is also associated with infection/inflammation of the male reproductive system, which compromises sperm quality [168].After several studies indicating its safety and beneficial effects for human health, namely antitoxic, antigenotoxic, antioxidant effects, and reporting no reproductive and teratogenic toxicity, the microalgae Chlorella vulgaris and Spirulina platensis were classified as Generally Regarded as Safe (GRAS) substances by the United States Food and Drug Administration (FDA) [169,170].Additionally, some studies have reported the protective properties of C. vulgaris against various noxious stimuli [171][172][173], and it has been suggested that C. vulgaris might be helpful in diseases where the maintenance of antioxidant status is crucial [172].
In deltamethrin-intoxicated rats, C. vulgaris administered orally for 8 weeks was able to re-establish the serum testosterone concentration, sperm counts, sperm viability, sperm motility, and sperm abnormalities as well as the testicular levels of SOD and CAT antioxidant enzymes and the lipid peroxidation marker MDA to levels similar to the control group [174].More recently, Eissa et al., demonstrated that pre-and co-treatment with C. vulgaris was helpful against sodium nitrite-induced reproductive dysfunction via the partial prevention of negative changes in sperm quality, serum testosterone and FSH levels, testicular oxidant/antioxidant balance, and testicular histological architecture [175].
S. platensis was also pointed as a promising protective agent to ameliorate diseases and toxicities involving oxidative stress [176], having recognized antioxidant, anti-diabetic, anti-inflammatory and anti-apoptotic properties [177].At the reproductive level, S. platensis has been shown to be beneficial in improving spermatogenesis and steroidogenesis after cadmium intoxication [178] and counteracted furan toxicity in rat testis [179].Furthermore, S. platensis reduced the deterioration induced by mercuric chloride in the sperm quality and testis of rats through the decrease of (i) lipid peroxidation, (ii) mercury accumulation in the testis, (iii) testicular histopathological changes, and iv) sperm abnormalities [180].S. platensis also reverted the mercuric chloride-induced inhibition in the activity of key antioxidant enzymes (SOD, CAT, and GSH-Px) back to normal levels [180].Moreover, the in vivo protective action of S. platensis against bifenthrin-induced reprotoxicity has been documented.In that study, S. platensis administrated before bifenthrin partially re-established the oxidant/antioxidant equilibrium, testosterone production, testicular mRNA, and microRNA levels of the genes involved in spermatogenesis as well as sperm motility and viability [181].
Recently, it was highlighted that C. vulgaris or S. platensis can have positive effects on testicular dysfunction provoked by lead acetate due to their antioxidant activity, immunomodulatory ability, and anti-apoptotic activity [182].
Spirulina maxima has been shown to improve spermatogenesis and steroidogenesis in diabetic rat testis [183] and also mitigated benzo[alpha]pyrene-and cyclophosphamideinduced injury to male mouse germ cells [184,185].
Although fewer works that compare the effects of microalgae are available, some algae also seem to have garnered interest in terms of reproductive purposes.Such is the case of Halopteris scoparia, a brown alga that is generally consumed as a salad in Far East countries, which ha sbeen reported to protect against cadmium chloride-induced testicular damage in mice [186].In another study, fucoxanthin extract from brown algae Laminaria japonica ameliorated male reproductive function in diabetic rats by (i) decreasing the glucose level, (ii) restoring sperm motility, (iii) reducing sperm abnormalities, (iv) enhancing enzymatic antioxidant activity, (v) reducing proinflammatory cytokine levels, and (vi) recovering LH and testosterone levels [187].Indeed, fucoxanthin is a natural agent with the potential to be considered an anti-diabetic candidate and a functional food to improve male fertility affected by diabetes mellitus [187].Furthermore, fucoxanthin extract from brown algae Sargassum glaucescens increased sperm count, decreased sperm abnormalities, and improved the morphology of SeT as well as improved testosterone levels in hamsters treated with the chemotherapeutic drug cisplatin [188].
In Table 2, it is summarized the beneficial effects in male reproductive system.

Propolis
Propolis or bee glue is a natural resinous mixture produced by honeybees that is used to seal holes in their honeycombs, smooth out the internal walls, and protect the entrance against intruders [189].
Propolis is a complex mixture of several compounds derived from plants and is processed by the salivary enzymes of bees [190], and for this reason, propolis content can vary depending on the plant source used by the bees [191].Usually, the propolis of Northern temperate climates contains approximately 50% resins and vegetable balsams, 30% waxes, 10% essential oils, 5% pollen, and 5% other organic substances [192,193].This composition can also change depending on the hive, district, and season [193].Due to this variability, propolis can have more than 300 compounds with pharmacological activity, such as polyphenolic compounds (e.g., flavonoids), terpenoids, various steroids, amino acids, glucose, fructose, vitamins such as vitamin B1, B2, C, and E, and essential elements such as magnesium, calcium, nickel, iron, and zinc [189,194].
In fact, it has been used in folk medicine [195], and no side effects have been described after propolis administration in humans and mice [196].Several biological activities with pharmaceutical interest have been described for propolis, such as anti-hyperglycemic, antioxidant, anti-inflammatory, anti-apoptotic [196,197], and anti-microbial [196,198] effects.
Phenolic compounds, such as flavonoids, are the main bioactive components responsible for the biological activity of propolis, specifically antioxidant activity.Flavonoids are capable of scavenging free radicals associated with several diseases, ageing, and toxic substances [199,200].Additionally, propolis antioxidant activity includes the capacity to activate antioxidant enzymes such as CAT [201] and SOD [202].Moreover, propolis can inhibit the generation of superoxide anions and can reverse the consumption of glutathione, an enzyme with radical scavenging activity [199,203].
In testis, several studies have described the use of propolis as a protective factor against many different oxidative stress inducers, showing significant improvements in male fertility [204].In addition, it has been found to have a protective role against oxidative stress-inducers such as copper, cadmium, and aluminum in the reproductive tissues of rats [191,199,205].Propolis co-treatment improved histopathological changes [191,199,205] by decreasing the number of apoptotic cells [191] and degenerative changes in the tubular epithelium [205] and increasing Johnsen's testicular score [191].Additionally, propolis coadministration induced an increase in CAT and GSH levels [191,199], SOD [191], testosterone, 17-ketosteroid reductase, and GST levels [199], and it decreased MDA [191,205] and TBARS levels [199] and immunoreactivity of testicular HIF-1α [205].In terms of sperm parameters, propolis co-treatment increased sperm concentration and motility and decreased abnormal sperm [191,199] and sperm death [199].Finally, it also increased the weight of the testis [199,205] and epididymis [199].
Diabetes mellitus has been associated with male infertility [192].Propolis may regulate the testicular and epididymal oxidative stress levels in induced-streptozotocin diabetic rats.Improved histopathologic changes have been found, namely increased SeT diameter and seminiferous epithelial height and decreased germ cells loss and epididymal epithelial height.Propolis co-administration increased the weight of the testes, epididymis, prostate, and seminal vesicles.Additionally, it demonstrated an increase of Nrf2, SOD, CAT, GSH-Px, GSH-Rx, GST, and TAC levels, and it decreased NO and TBARS levels, supporting the antioxidant properties of propolis.Concerning the pro-inflammatory mediators and apoptosis-related genes, propolis increased the protein levels of NF-kB, tumour necrosis factor-α, interleukin (IL)-1β, and IL-10 and decreased testicular levels of Bax/Bcl-2 ratio, p53, caspase-8, caspase-9, and caspase-3 [192], demonstrating the anti-inflammatory and anti-apoptotic properties of propolis.Recently, it has also been shown that propolis treatment in diabetic rats ameliorated sperm parameters (counts, motility, viability and morphology) and diminished non-motile spermatozoa and sperm DNA fragmentation.Additionally, it improved steroidogenesis through the increase of testosterone levels, StAR, CYP11A1, CYP17A1, 3β-HSD, and 17β-HSD.Relative to the metabolic pathways, propolis increased glucose transporter 3 (GLUT3), MCT2, MCT4, and decreased LDH, intra-testicular glucose, and lactate levels [206].
In the presence of some drugs (paclitaxel, methotrexate and doxorubicin), propolis supplementation seems to a have a beneficial protective effects on the male reproductive system, improving histopathological changes by increasing the Johnsen testicular biopsy score [207] and the diameter of SeT [207,208]; sperm motility, viability [209], and counts [208,209]; and decreasing sperm abnormalities [209].Moreover, propolis co-administration increases GSH levels [208,209], LDH, sorbitol dehydrogenase, ACP, ALP, G-6-PDH, testosterone, FSH, LH, IL-4, 3β-HSD, 17β-HSD, and StAR [208] and decreases MDA [208,209], 8-hydroxy-2-deoxyguanosine (8-OHdG) [209], ALT, AST, MPO, tumor necrosis factor-alpha (TNF-α), Fas-L, and caspase-3 levels [208].Propolis also increases the ATP level in testis homogenate and decreases ADP and AMP levels [209].The existing data show that propolis presents beneficial effects against several diseases.However, it should be considered that there is no standardized propolis extraction method, which causes problems in terms of establishing its safe use.There have also been reports of its toxicity as well.Additionally, propolis has been identified as an allergen with immunological stimulating properties, which may affect human health [189].
In Table 2, it is summarized the beneficial effects in male reproductive system.

Conclusions and Future Perspectives
Various studies in animal models have demonstrated the beneficial effects of distinct natural compounds and bioactive compounds on male fertility when administered in isolation or in the presence of a noxious stimulus.Globally, they were capable of suppressing oxidative stress, inflammation, and damaging drug effects and also counteracted pathology-associated alterations.Several of these exogenous compounds have already been described as having high antioxidant and anti-inflammatory properties, indicating the possibility of beneficial effects on the male reproductive system.However, the mechanisms that lead to these beneficial effects need further understanding.The presented outcomes using animal models highlighted the usefulness of these natural agents in the protection of male animal fertility against damaging factors.Preclinical and clinical studies are needed to evaluate their effectiveness as supplementation in the prevention as well as the adjuvant treatment of diseases associated with the human male reproductive system.

Table 1 .
Bioactive compounds with beneficial effects in male reproductive system.

Table 2 .
Bioactive compounds present in natural products with beneficial effects in male reproductive system.