1. Introduction
Hypertension (HT) has become a major cause of mortality worldwide. An estimated 691 million people worldwide suffer from high blood pressure [
1]. Of the 15 million deaths caused by circulatory diseases, 7.2 million are due to coronary heart disease and 4.6 million to brain vascular disease [
2]. The prevalence of HT was 31.1% in adults worldwide in 2010 [
3]. In the United States, an estimated 50 million patients have HT [
4], and some 60,000 deaths annually are caused directly by HT [
5].
The World Health Organization estimated that 54% of strokes and 47% of cases of ischemic heart illness are connected with high blood pressure. This is due to the main risk factors for cardiovascular morbidity and mortality as well as HT [
6]. The decrease of strokes in recent years is a consequence of the reduction in blood pressure [
7]. However, while the connection between high blood pressure and cardiovascular morbidity and mortality is very clear [
5], blood pressure is frequently inadequately controlled. Either the blood pressure is not measured, the physician fails to react in the face of elevated blood pressure values [
8], treatment is not provided optimally, or the patient fails to take the necessary medication regularly [
9].
HT is a multifactorial disease that is strongly associated with lifestyle and especially with the consumption of alcohol, foods high in salt and fat, a sedentary lifestyle, and obesity among others [
5]. Tobacco, despite not constituting a risk factor for hypertension, can acutely increase the heart rate and blood pressure, aggravating cardiovascular problems in hypertensive patients. HT can be prevented by modifying these risk factors from an early age, and it is necessary to take actions that reduce exposure to these factors, thus, reducing the incidence of cardiovascular diseases, including HT [
5].
Polyphenols are natural extracts that have been extensively studied over recent years due to their antioxidant, antihypertensive, and anti-inflammatory capacities, in addition to their possible role in the prevention and management of several diseases, such as cardiovascular diseases, HT, diabetes, cancer, and neurodegenerative diseases [
10,
11,
12]. Recently,
Hibiscus sabdariffa (HS) and
Lippia citriodora (LC) have attracted scientific interest for their potential use in the treatment of metabolic syndrome [
13]. In particular, HS and LC exert a potent antihypertensive capacity [
14,
15].
Previous studies using a product called MetabolAid
®, comprised of HS and LC extracts highly purified in anthocyanins and verbascoside polyphenols, respectively, previously reported a significant hypotensive effect, albeit using single blood pressure measurements in placebo-controlled clinical trials. Overweight and obesity are associated with higher blood pressure figures. Several studies, including the Framinghan study, have shown the high prevalence of hypertension in the obese population. Similarly, research demonstrated that weight loss with a hypocaloric diet in obese patients was associated with reductions in blood pressure levels and reductions in the pharmacological therapy necessary to keep it under control [
16,
17].
The basis of the present work is the use of MetabolAid
® as part of antihypertensive treatment, due to the limited number of clinical studies linking LC and HS polyphenols with hypertension. Both plants that contain the ingredient are recognized for their high content of polyphenols, powerful antioxidant molecules that may be useful in many diseases such as HT, oxidative stress, dyslipidemia, lipid mobilization, and endothelial stiffness [
10]. The aim of the present study is to determine if supplementation with HS-LC extract is useful for the treatment of pre-hypertension and type 1 hypertensive, non-medicated populations.
3. Discussion
The consumption of HS-LC extract significantly decreased the SBP compared to the control group, especially during the daily measurements. Habitually, the calyxes of HS have been used as a hypertension treatment [
20]. In fact, animal and human studies have reported a reduction of SBP and DBP [
20,
21]. Physical exercise stimulates skeletal muscle adaptations as increased mitochondrial density, increased oxygen availability, increased blood flow, and distended arteries, which results in a lower resting heart rate and blood pressure and increased BMR. Studies hypothesized that bioactive compounds from the HS-LC extract could simulate the effects of physical exercise on the heart rate and blood pressure [
17,
20].
A study with 54 moderate hypertensive patients (similar to the present study) treated with sour HS tea revealed a noticeable reduction in blood pressure (systolic by 11.2% and diastolic by 10.7%). However, the length of the treatment (12 days) was not sufficient to draw firm conclusions [
22]. A study from 2004 with 75 subjects reported that the treatment with an HS aqueous extract was sufficient to reduce the blood pressure by more than 10%. HS was reported to be safe, compared with 25 mg of captopril and 10 mg of lisinopril [
23,
24]. However, the comparison of the mentioned studies is difficult due to the poor standardization of the products used in previous clinical interventions.
Natural products, such as plant derivates are characterized by the presence of high content of polyphenols [
25] with proven antihypertensive properties. However, the mechanism of action has not yet been accurately described [
26,
27]. The classic explanation of hypertensive properties of polyphenols was related to their antioxidant capacity, which reduces the formation of atheroma plaques.
Scientific evidence has shown that certain plant-derived extracts, such as HS and LC, can modulate different metabolic pathways and activate the AMPK pathway favoring lipolysis and, therefore, fat loss [
17], which could explain a decrease in blood pressure. HS and LC have shown therapeutic effects in oxidative stress treatments, lipid profile regulation, and the reduction of high blood pressure or atherosclerosis. That capacity may be attributed to their polyphenolic content and their capacity to inhibit low-density lipoprotein oxidation and decrease the atherosclerotic process [
28].
The SBP was reduced, which may be explained by the vasodilator capacity of polyphenols [
29]. However, the higher presence of a certain type of polyphenol and the differences in their chemical structure appeared to influence their bioavailability and bioactivity, which can, therefore, determine the final hypotensive effect. For example, the anthocyanin ponidine and certain flavonoids, such as cyanidin, catechin, and epicatechin, were able to generate apocynin-analogous compounds via catechol-O-methyltransferase, which is a known vasoactive compound [
30].
Due to their synergistic action, certain combinations of polyphenol metabolites exert greater antihypertensive effects compared with individually. Quercetin derivates, such as 3,4-dihydroxyphenylacetic acid (DHPA), 4-methylcatechol (4MC), and 3-(3-hydroxyphenyl) propionic acid (3HPPA), show different hypotensive effects if they act together [
31]. Despite the low availability observed for some polyphenols, vasodilator action is shown to occur through their metabolites at the colonical level. Through vasodilator effects on aortic vascular smooth muscle, 3HPPA is the compound with the highest efficacy, in whose mechanism of action research observed how the endothelium and nitric oxide (NO) played a role in vascular relaxation. [
32].
Therefore, the synergistic effect may be attributed to the different mechanisms of action of each metabolite [
33]. The mechanism of action of the polyphenols presented in the nutraceutical used in our study (catechins and epicatechins) could be an increase in endothelial nitric oxide synthase (ONSe), resulting in enhanced endothelium-dependent vasodilation [
34]. Another mechanism of action that may explain the decrease in blood pressure is the possible inhibition of the angiotensin-converting enzyme and, thus, the reduction of angiotensin II and its hypertensive mechanism [
35].
In addition, research showed that
H. sabdariffa has a compound that causes nitric oxide release from the vascular endothelium followed by kidney filtration increase, a mechanism that clears its diuretic effect on the blood pressure [
36,
37]. Another study reported a significant decrease in the blood pressure of patients treated with HS, reporting differences between both the control group and the experimental group. However, a limitation of the study was that all subjects were encouraged to lose weight, as well as to control their sodium/potassium intake ratio, which could be partially responsible for the results observed [
37].
In addition, researchers reported that the polyphenols derived from HS-LC extract might be able to increase the adiponectin gene expression and PPAR-α, while reducing the NF-kB protein, which regulates the genes that mediate immune and inflammatory responses [
38,
39]. Adiponectin has an anti-inflammatory capacity, leading to the activation of AMPK in hypertrophic adipocytes [
39]. Together, these mechanisms may explain weight control and related pathologies as hypertension.
Finally, the morning has also been associated with a greater risk for cardiovascular events compared with the rest of the day, due to fluctuations in the blood pressure flow and hemostatic changes [
40,
41].
4. Materials and Methods
The study consisted of a double-blind, randomized, placebo-controlled clinical trial, with two parallel branches to study depending on the extract consumed (experimental or placebo) and single-center. A total of 80 pre-hypertensive or type 1 hypertensive subjects of both sexes were included in the study after matching all the including criteria (age between 18–65 years, systolic blood pressure higher than 120 mmHg, diastolic blood pressure higher than 80 mmHg, and no pharmacological treatment for hypertension) and none of the exclusion criteria (illness, other pharmacological treatment, toxicological habits, or allergies).
After full disclosure of the implications and restrictions of the protocol, the subjects were required to sign an informed consent. The determination of blood pressure was done following the ESC/ESH criteria [
5,
42]. The study was conducted in accordance with the Declaration of Helsinki (randomized trial registration number (Clinicaltrials): NCT04105192).
The product under study (MetabolAid
®) consisted of a capsule, including a mixture of 500 mg of HS-LC extracts (175 mg of HS and 325 mg of LC), leading to high quantities of verbascoside and anthocyanins, respectively [
43]. The placebo capsules contained 500 mg of crystalline microcellulose, maintaining the same aspects as the product under study. MetabolAid
® was provided by Monteloeder S.L. (Alicante, Spain) (Patent application number WO2019058011A1). The polyphenolic composition of the product was quantified and reported in previous studies [
17,
43] that showed two main polyphenolic families (phenylpropanoids and anthocyanins).
The total polyphenolic composition was represented by phenylpropanoids (16% of the total dry weight) and anthocyanins (3.5% of the total dry weight). Four main phenolic compounds were identified; two phenylpropanoids (verbascoside and isoverbascoside) and two anthocyanins (delphinidin-3-O-sambubioside and cyanidin-3-O-sambubioside). Regarding the phenylpropanoids, 15% corresponded to verbascoside (93.75% of the total of phenylpropanoids) and 1% to isoverascoside (representing 6.25%). For the anthocyanins, 2.27% corresponded to delphinidin-3-O-sambubioside and 1.23% to cyanidin-3-O-sambubioside representing 65% and 35% of the total anthocyanin content, respectively.
The length of the present study was 84 days, during which the subjects consumed the botanical mixture or the placebo daily depending on the previous randomization. Therefore, 40 subjects were distributed in the placebo group, and the other 40 subjects were allocated in the HS-LC group. Each subject that joined the study was assigned a code (generated by a number software generator (Epidat v4.1)) assigning them to one of the two study groups. Both the researchers and the participants themselves did not know the group they belonged to.
As displayed in
Figure 5, the subjects attended the research center at the beginning of the study and at the end. At baseline, blood sampling was obtained from the cubital vein of subjects from both groups. After blood collection, and after explaining the operation of the study, the subjects received a treatment based on the prior randomization. To determine the physical activity, every subject was equipped with an accelerometer (ActiGraph wGT3X-BT) prior to the beginning of the study. After 84 days of intake of the HS-LC extract or the placebo, the same protocol was repeated.
The blood pressure was determined using an oscillometer sphygmomanometer to determine the systolic and diastolic blood pressures, necessary for the inclusion criteria of the study [
44]. However, the main limitations of this procedure are that it only offers information of the value at a specific time and that it has numerous biases [
40]. In 2011, the British guides of the National Institute for Health and Clinical Excellence 9 and, subsequently, different scientific societies and institutions recommended performing an ABMP to confirm the diagnosis of HT [
45] Therefore, the oscillometric sphygmomanometer was only used as a method for selecting study volunteers, and not as an analytical measure of the study.
The influence of the HS-LC extract on blood pressure was determined with an ABPM. The measurement of blood pressure was performed at baseline (prior to product consumption), at days 14, 56, and 84 from the beginning of the study. The ABPM method was done using a Holter Spacelabs Healthcare (holter) device [
46]. The cuff was placed on the non-dominant arm so that the subject could comfortably follow their daily life activities. The holter was engaged at the waist by a belt.
The volunteers were instructed to be quiet every time the cuff expanded. The holter was programmed to measure every hour for 24 h, and thus, after this time, the subjects returned the device. The values were displayed as the mean values of the measurements performed over 24 h, obtaining systolic and DBP determinations. Depending on the moment of the day, the daytime blood pressure and nighttime blood pressure were determined considering the morning hours and nighttime hours.
Other parameters related to blood pressure were also reported as the MBP that measures systemic irrigation providing information on the effectiveness with which blood reaches from the heart to the organs. The MBP was obtained from the values obtained along 24 h and as an hourly measurement, resulting from the following formula: MBP = DP + ((SBP-DBP)/3) [
47,
48].
Additionally, due to the importance of exercise on cardiovascular system, physical activity can be partly-determinant of the possible observed effects during the study. Therefore, in order to prove if some strategies are effective for the treatment of obesity, physical exercise must be measured and regulated. For this purpose, the metabolic equivalent of the task (MET) was determined throughout the whole length of the study. That fact led to the determination of metabolic equivalents that relates the intensity of physical activity with the kilocalories consumed by subjects, in order to standardize the exercise throughout the study. MET values were determined in accordance with the “Compendium of Physical Activities” [
49].
Statistic Determinations
Categorical variables were expressed as frequencies and percentages, and the continuous variables were expressed as the means and standard errors (SEs). The chi-square (χ2) test or the Fisher’s exact probability test was used for the comparison of categorical variables between the experimental and placebo groups. Quantitative variables were assessed using the analysis of variance (ANOVA) for repeated measures with two factors: time (baseline and final), as within-subject factors and intervention (experimental and placebo) as the between-subject factor with Bonferroni’s correction for pairwise comparisons. We considered 0.05 as the level of significance. Statistical analysis was carried out with the SPSS 21.0 software.