Melatonin supplementation reduces nighttime blood pressure but does not affect blood pressure reactivity in normotensive adults on a high-sodium diet

Abstract

High-sodium diets (HSDs) can cause exaggerated increases in blood pressure (BP) during physiological perturbations that cause sympathetic activation, which is related to cardiovascular risk. Melatonin supplementation has been shown to play a role in BP regulation. Our aim was to examine the effects of melatonin taken during an HSD on 24-h BP and BP reactivity during isometric handgrip (IHG) exercise, postexercise ischemia (PEI), and the cold pressor test (CPT). Twenty-two participants (11 men/11 women, 26.5 ± 3.1 yr, BMI: 24.1 ± 1.8 kg/m², BP: 111 ± 9/67 ± 7 mmHg) were randomized to a 10-day HSD (6,900 mg sodium/day) that was supplemented with either 10 mg/day of melatonin (HSD + MEL) or placebo (HSD + PL). Twenty-four-hour ambulatory BP monitoring was assessed starting on day 9. Mean arterial pressure (MAP) was quantified during the last 30 s of IHG at 40% of maximal voluntary contraction and CPT, and during 3 min of PEI. Melatonin did not change 24-h MAP (HSD + PL: 83 ± 6 mmHg; HSD + MEL: 82 ± 5 mmHg; P = 0.23) but decreased nighttime peripheral (HSD + PL: 105 ± 10 mmHg; HSD + MEL: 100 ± 10 mmHg; P = 0.01) and central systolic BP (HSD + PL: 97 ± 9 mmHg; HSD + MEL: 93 ± 8 mmHg; P = 0.04) on the HSD compared with the HSD + PL. The absolute and percent change in MAP during IHG was not different between conditions (all P > 0.05). In conclusion, melatonin supplementation did not alter BP reactivity to the perturbations tested on an HSD but may be beneficial in lowering BP in young healthy normotensive adults.

NEW & NOTEWORTHY BP reactivity was assessed during isometric handgrip (IHG) exercise, postexercise ischemia (PEI), and the cold pressor test (CPT) after 10 days of a high-sodium diet with and without melatonin supplementation. Melatonin did not alter BP reactivity in healthy normotensive men and women. However, melatonin did decrease nighttime peripheral and central systolic BP, suggesting it may be beneficial in lowering BP even in those with a normal BP.

INTRODUCTION

One in two United States adults ≥20 yr of age have hypertension (HTN) (1), and high-sodium diets (HSDs) contribute to the development of HTN as they are associated with increases in resting blood pressure (BP) (2). BP reactivity or the BP response to a physiological perturbation causing sympathetic activation is exaggerated during an HSD compared with a low-sodium diet, as shown in both preclinical and clinical studies (3, 4). Indeed, healthy adults who followed an HSD for 1 wk experienced an increase in both systolic and diastolic BP reactivity to the cold pressor test (CPT) (5). Furthermore, normotensive individuals who consumed salt pills for 1 wk experienced an increase in mean arterial pressure (MAP) during isometric handgrip (IHG) exercise (4). Because exaggerated BP reactivity has been linked to the development of HTN (6, 7), an increase in BP reactivity induced by an HSD has clinical relevance.

Exploring strategies to mitigate the effects of an HSD on BP and BP reactivity is critical as the average sodium consumption in the United States continues to surpass recommendations, and efforts to lower its consumption have been unsuccessful (8, 9). One novel strategy may be melatonin supplementation. Melatonin is a hormone released by the pineal gland primarily at night that can influence the diurnal variation in various physiological systems including the cardiovascular system (10). In particular, melatonin has been shown to influence BP and heart rate (11–14). Ingestion of melatonin has been demonstrated to reduce systolic, diastolic, and mean BPs; resting plasma noradrenaline during standing; and pulsatility indices, a measure of vascular resistance, in healthy adults (11–13, 15). Furthermore, melatonin ingestion attenuated muscle sympathetic nerve responses to lower body negative pressure, an orthostatic challenge, in healthy men and women (16). Chronic supplementation (3–4 wk) of melatonin has reduced nighttime BP in several clinical populations, including HTN, metabolic syndrome, and type II diabetes (13, 17–21). However, only one study has investigated the effect of chronic melatonin supplementation on BP in normotensive young adults and they found a reduction in nighttime BP (13). Therefore, it remains unknown whether melatonin can alter BP responses to sympathoexcitatory perturbations in normotensive adults during an HSD.

The role of melatonin on BP reactivity has been less studied, and the results have been mixed in young healthy adults. Ray (16) examined the effects of one-time administration of melatonin on BP reactivity to IHG and CPT and reported no changes in BP responses compared with a placebo. However, IHG was performed at 30% of maximal voluntary contraction (MVC) and it is not known whether a higher percentage of MVC would elicit a different response nor was postexercise ischemia (PEI) assessed. In contrast, Muller et al. (22) observed that a one-time administration of melatonin decreased MAP during mental stress compared with placebo. Although the results of acute melatonin supplementation are mixed, whether chronic supplementation may be more beneficial remains to be determined. Therefore, the purpose of this study was to examine the effects of melatonin supplementation on BP and BP reactivity after 10 days of HSD. We hypothesized that melatonin supplementation would decrease BP reactivity during IHG and PEI, and CPT on an HSD compared with an HSD alone. In addition, we assessed 24-h BP as we hypothesized that melatonin would lower BP on an HSD.

MATERIALS AND METHODS

Participants

The study protocol was approved by the University of Delaware Institutional Review Board (Protocol no. 1534734) and conformed to the provisions of the Declaration of Helsinki. The study was registered on clinicaltrials.gov (NCT04325191). Healthy young men and women (aged 18–45 yr) were recruited. Inclusion criteria included a BP of <130/80 mmHg; free of cardiovascular, liver, or kidney disease and malignant cancer; nonsmoker; and a nonobese body mass index (BMI: 18.5–29.9 kg/m²). Exclusion criteria included history of sleep disorders and/or night shift work; the use of attention deficit/hyperactivity disorder or mood disorder medications, melatonin, or other antioxidant supplementation in the past 3 mo; highly trained endurance athletes; and habitual consumption of less than 2,000 mg of sodium per day.

After providing written informed consent, participants completed a medical history questionnaire, a menstrual cycle form (women only), and a global physical activity questionnaire (GPAQ) (23). Anthropometrics and seated BP (dominant arm; Dash 2000, GE medical systems, Chicago, IL) were measured. A venous blood sample was collected to examine the metabolic panel and lipid profile. Lipids were assessed at screening and at the end of each condition as melatonin has been shown to have antilipidemic effect (19, 24). Participants recorded their food consumption over 2 weekdays and 1 weekend day to assess their habitual sodium intake. They were provided with handout of two-dimensional food models to help with estimating portion sizes. Diet records were reviewed by a trained researcher and diet records were analyzed using Nutrition Data System for Research software (NDSR, 2020, University of Minnesota, Minneapolis).

Experimental Design

This study used a randomized double-blind placebo-controlled crossover design consisting of two 10-day conditions separated by a washout period of at least 14 days. A washout of 2 wk was chosen as data suggest that balance is achieved within a few days although there is variation among participants (25, 26). During both 10-day periods, participants consumed salt pills and either supplemented with 10 mg of melatonin (HSD + MEL) or a placebo (HSD + PL) capsule containing lactose daily. Melatonin and placebo capsules were manufactured by SaveWay Compounding Pharmacy (Newark, DE) and matched in weight and appearance. Ten milligrams of oral melatonin are reported to have an absorption half-life of 6 min, to reach maximal concentration in 41 min, and have an elimination half-life of 54 min with a return to baseline plasma melatonin levels approximately 5 h after consumption (27). Furthermore, melatonin has also been shown to cross the blood-brain barrier (28). Participants took the melatonin/placebo capsule 30 min before they planned to go to sleep, including the night before the experimental visit. This dosage of melatonin has been used previously with no reported side effects in several populations (20, 29).

A total of 6,900 mg/day of sodium in both 10-day conditions was achieved by taking 12 enteric coated, slow-release salt pills, each containing ∼380 mg (∼4,600 mg total from pills), and by consuming 2,300 mg of sodium in the diet. This level of sodium is consistent with our prior studies (30, 31). Research staff provided participants with instructions and strategies to consume a 2,300 mg sodium diet during both conditions. Participants recorded their dietary intake during the last 3 days of the condition (days 7, 8, and 9) and were instructed to replicate their diet as much as possible during both conditions.

Measurements

Ambulatory BP and urine collection.

Participants wore a 24-h ambulatory BP monitor (OSCAR 2 with SphygmoCor, SunTech Medical, Morrisville, NC) on their upper arm starting on day 8. The OSCAR 2 has been validated according to the European Society of Hypertension International Protocol (32). The BP cuff was programmed to inflate every 20 min during the day and every 30 min during the night. BP readings were downloaded using AccuWin Pro 4. At least 75% of the readings was needed for the measurement to be considered as valid (33). Peripheral and central BP was collected using the OSCAR 2. Central pressures were determined from noninvasively recorded brachial artery waveform using a generalized transfer function. The 24-h urine collection started on day 9. Urine was assessed for electrolyte concentrations (Easy-Electrolyte Analyzer, Medica, Bedford, MA) and osmolality (Advanced 3D3 Osmometer, Advanced Instruments, ON, Canada) from a mixed aliquot from the 24-h collection container. Urine flow rate was calculated and used to determine 24-h sodium excretion. For urine collections to be considered valid, they had to be collected within 20–28 h, there could not be ≥2 missed collections and the urine volume could not be <500 mL. The first morning void was collected separate from the 24-h urine collection in the morning of day 10 to assess melatonin compliance by measuring urinary 6-sulfatoxymelatonin (ELISA kit, Alpco, Salem, NH). A 4-parameter logistics (4PL) curve fit was used to obtain the calibrator curve with SoftMax Pro Software (Molecular Devices). Values at or above the maximum detectable level (MDL) of the 6-sulfatoxymelatonin ELISA assay were arbitrarily assigned the MDL value for purposes of statistical analysis.

Physical activity monitoring and scoring.

Participants wore an accelerometer (ActiGraph wGT3X-BT, Pensacola, FL) on the hip for the entire duration of both conditions except to sleep and activities involving water. Participants recorded the times when the accelerometer was worn, and not worn, to improve compliance and data quality. Data were acceptable if there were at least 4 days with 10 h of wear time (34). Participants returned the device at the experimental visit on day 10. Accelerometer data were analyzed using ActiLife software. Wear time was validated using the Troiano algorithm (35) and activity variables were calculated using the Freedson Combination 1998 algorithm (36).

Sleep monitoring and scoring.

Participants wore a wrist accelerometer (Motionlogger Micro Watch, Ambulatory Monitory Inc., Tokyo, Japan) on the nondominant wrist for 9 days and 9 nights, except for activities involving water. Data were collected in the zero-crossing mode (ZCM) and saved in 1-min epochs. The University of California San Diego algorithm, which is validated to produce accurate and reliable sleep estimates relative to polysomnography (37), along with Action W-2 software (Ambulatory Monitoring Inc., Ardsley, NY), was used to score the data. Data were included if the accelerometer was worn for at least 7 of the 9 days (38). Variables of interest were sleep duration, sleep variability, and sleep efficiency. Sleep duration was defined as the total time spent asleep from sleep onset to wake onset. We present the mean (quantified for each night that the accelerometer was worn) of the sleep duration. Sleep variability was quantified as the standard deviation (SD) of sleep duration across the 9-day sleep monitoring period. Sleep efficiency was calculated as the total sleep time expressed as a percentage of the total time spent in bed. Participants were also asked to fill out the Pittsburgh Sleep Quality Index (PSQI) questionnaire at the two experimental visits.

Experimental Visit

Experimental visits were conducted in the morning (7 AM–10 AM) on day 10 and at the same time for both conditions. Participants were asked to fast for at least 9 h, avoid caffeine and alcohol for at least 12 h, and not exercise for 24 h. Upon arrival, body mass was measured. Next, BP was measured on a beat-by-beat basis via finger photoplethysmography on the middle finger of the participant’s nondominant hand (NOVA, Finapres Medical Systems, Enschede, The Netherlands). Brachial BP was measured as well using an automated oscillometric BP cuff placed on the right arm (Dash 2500, GE Healthcare, Milwaukee, WI) to confirm finger cuff values. A single-lead ECG was used to measure heart rate and a strain-gauge pneumograph (Pneumotrace, Stoelting, Wood Dale, IL) was placed around their abdomen to avoid Valsalva. Participants then underwent two different physiological perturbations.

Isometric handgrip exercise and postexercise ischemia.

Participants performed three maximal voluntary contractions (MVCs) with their dominant hand with 1 min of rest in between trials by squeezing a grip force transducer device (ADInstruments, Colorado Springs, CO) at maximal effort. The three attempts were averaged and used to calculate the 40% relative work rate. A 10-min baseline period occurred followed by IHG, which consisted of a static voluntary contraction for 2 min at 40% MVC. Participants were provided visual feedback during the duration of the isometric exercise and were asked to provide their effort level using the Rating of Perceived Exertion (RPE) scale at baseline and at the end of each minute (39). During the last 5 s of exercise, a cuff on the right upper arm was inflated to 240 mmHg to induce PEI and remained inflated for 3 min and 15 s. After the cuff release, measures were recorded for an additional 2 min during recovery. PEI was used to isolate activation of the metaboreflex.

Cold pressor test.

After the participant’s BP recovered to baseline values following the IHG trial, a 2-min baseline for CPT was started, and then their dominant hand was submerged in a slurry ice/water mixture for 2 min (1°C–5°C confirmed with a digital thermometer) (40, 41). Data were recorded for an additional 2 min after the CPT was terminated.

Data Analysis

Beat-to-beat BP and ECG signals were recorded continuously (LabChart Pro 8, ADInstruments, Colorado Springs, CO) at 1,000 Hz and stored for offline analysis. Baseline BP was calculated using the average of the 1-min baseline before the start of the exercise. Maximum BP was determined as the average BP during the last 30 s of IHG, and the last 3 min of PEI. BP recorded in the first 15 s of PEI was not included in the analysis as participants experience a robust and transient decrease in BP (42). Baseline BP before the CPT was calculated using the average of the 1-min baseline. Maximum BP was determined as the average BP of the last 30 s of the CPT. BP reactivity is reported as a change from baseline (Δ) and as the percent change (Δ expressed as a percentage of baseline BP) for IHG/PEI and CPT (42).

Statistical Analysis

Our primary outcome was the change in MAP for the melatonin and placebo conditions. Muller et al. (22) observed that a one-time administration of 3 mg of melatonin decreased MAP by 3 mmHg (standard deviation of 2 mmHg) in response to a mental stress challenge compared with placebo. Considering that our condition was not acute, and our dosage was higher (10 mg), we ran a more conservative a priori power analysis. With an α of 0.05, 95% power, effect size 1, and assuming a percent difference in MAP of 3% (standard deviation of 2 mmHg) at the end of the two conditions, the necessary sample size was 16 participants using a paired samples t test (G* Power). JMP Pro 16.0.0 (SAS, Cary, NC) was used for analysis. Data were assessed for normality, linearity, homoscedasticity, and multicollinearity. Paired t tests were used to compare changes in blood and urine variables as well as BP during IHG, PEI, and CPT. Data are expressed as means ± standard deviation (SD).

RESULTS

Twenty-two participants took part in this study. Participant screening characteristics and bloodwork are presented in Table 1. We had an equal distribution of men and women. Participants were young and normotensive with a BMI in the normal weight range. Fasting blood work was within normal limits. Furthermore, there was no difference in lipid levels between the two conditions.

Table 1.

Participant screening characteristics

	Value
n, men/women	11/11
Age, yr	26.5 ± 3.1
Race, Asian/Black/White	10/1/11
Body mass, kg	69.4 ± 11.2
Body height, cm	169.4 ± 10.2
BMI, kg/cm2	24.1 ± 1.8
Brachial systolic BP, mmHg	111 ± 9
Brachial diastolic BP, mmHg	67 ± 7
Heart rate, beats/min	66 ± 11
Hemoglobin, g/dLa	14.1 ± 1.4
Hematocrit, %a	42.3 ± 4.1
Glucose, mg/dLb	73 ± 16
BUN, mg/dL	12.4 ± 3.6
Creatinine, mg/dL	0.9 ± 0.1
eGFR, mL/min/1.73	108.2 ± 13.6
Sodium, mmol/L	140.8 ± 1.7
Chloride, mmol/L	101.7 ± 1.9
Total cholesterol, mg/dLa	192 ± 30
LDL cholesterol, mg/dLa	114 ± 30
VLDL cholesterol, mg/dLa	17 ± 7
HDL cholesterol, mg/dLa	61 ± 15
Triglycerides, mg/dLa	93 ± 42

Values are expressed as means ± SD; n = 22; ^an = 20; ^bn = 19. BMI, body mass index; BP, blood pressure; BUN, blood urea nitrogen; eGFR, estimated glomerular filtration rate; HDL, high-density lipoprotein; LDL, low-density lipoprotein; VLDL, very-low-density lipoprotein.

The effect of 10 days of HSD with and without melatonin supplementation on hemodynamic, anthropometric, blood, and urine measures is shown in Table 2. Urinary sodium excretion values were in the range expected for the level of sodium consumption in this study, indicating compliance with the salt pills. These data as well as the other variables were not different between the conditions. Urinary 6-sulfatoxymelatonin was greater in the melatonin supplementation condition, demonstrating that the participants were compliant with taking the supplements. Furthermore, there were no differences in physical activity as quantified by accelerometry between the two conditions. The time participants spent in sedentary, light, moderate, vigorous, and very vigorous physical activity was not different nor were the number of steps taken (HSD + PL: 6,202 ± 1,578 steps/day; HSD + MEL: 5,858 ± 3,107 steps/day; P = 0.64), and participants wore the devices for the same amount of time. Finally, there was no difference in dietary intake as recorded on days 7–9 of each condition. Participants consumed similar amounts of energy (HSD + PL: 2,032 ± 775 kcal/day; HSD + MEL: 2,020 ± 515 kcal/day; P = 0.93) and sodium (HSD + PL: 3,212 ± 1,331 mg/day; HSD + MEL: 3,303 ± 1,165 mg/day; P = 0.76) as well as the other nutrients (data not shown), demonstrating that they followed a similar diet.

Table 2.

Participant characteristics at the end of each condition

Variable	HSD + PL	HSD + MEL	P Value
Weight, kg	70.7 ± 10.7	70.6 ± 10.8	0.25
Brachial systolic BP, mmHg	113 ± 9	114 ± 3	0.75
Brachial diastolic BP, mmHg	69 ± 6	68 ± 7	0.80
Heart rate, beats/min	71 ± 11	71 ± 13	0.83
Serum sodium, mmol/La	140.1 ± 1.8	140.6 ± 3.0	0.36
Serum chloride, mmol/La	105.8 ± 1.5	106.0 ± 2.6	0.55
Plasma osmolality, mosmol/kgH2Ob	291.1 ± 8.5	288.9 ± 5.3	0.45
Hemoglobin, g/dLa	13.8 ± 1.8	13.6 ± 1.5	0.55
Hematocrit, %a	41.9 ± 4.5	41.3 ± 3.9	0.48
Total cholesterol, mg/dLc	174 ± 32	173 ± 34	0.78
LDL cholesterol, mg/dLc	102 ± 25	100 ± 30	0.58
HDL cholesterol, mg/dLc	55 ± 14	56 ± 16	0.56
VLDL cholesterol, mg/dLc	17 ± 7	17 ± 7	0.99
Triglycerides, mg/dLc	90 ± 40	91 ± 41	0.90
Urinary sodium excretion, mmol/24 hc	314.4 ± 94.7	285.8 ± 94.5	0.26
6-Sulfatoxymelatonin, ng/mL	50.9 ± 33.6	395.7 ± 27.3*	0.001

Values are expressed as means ± SD; n = 22; ^an = 21; ^bn = 18; ^cn = 20. BP, blood pressure; HDL, high-density lipoprotein; LDL, low-density lipoprotein; MAP, mean arterial pressure; HSD + MEL, high-sodium diet plus melatonin; HSD + PL, high-sodium diet plus placebo; VLDL, very-low-density lipoprotein. Paired t tests between conditions were run.

Melatonin supplementation did not alter sleep duration (HSD + PL: 467 ± 43 min; HSD + MEL: 456 ± 54 min; P = 0.47), sleep variability (HSD + PL: 73 ± 30 min; HSD + MEL: 63 ± 38 min; P = 0.53), or sleep efficiency (HSD + PL: 95 ± 2.5%; HSD + MEL: 95 ± 2.7 min; P = 0.32) as assessed by the sleep actigraphy. The PSQI (Global score: HSD + PL: 3.4 ± 1.8 min; HSD + MEL: 3.8 ± 1.9 min; P = 0.30) was also not different between conditions.

Twenty-four-hour ambulatory BP results for 18 participants are shown in Fig. 1. Four participants were excluded as they did not wear the BP cuff for the required time. There were no differences in 24-h systolic BP, 24-h diastolic BP, or 24-h MAP at the end of each condition. Melatonin supplementation lowered peripheral nighttime systolic BP (HSD + PL: 105 ± 10 mmHg, HSD + MEL: 100 ± 10 mmHg; P = 0.01), whereas nighttime MAP was lower and approaching significance (HSD + PL: 73 ± 7 mmHg, HSD + MEL: 70 ± 8 mmHg; P = 0.07) (Fig. 1). Consistent with this, central nighttime systolic BP was also lower with melatonin supplementation (HSD + PL: 97 ± 9 mmHg, HSD + MEL: 93 ± 8 mmHg; P = 0.04). No other differences were observed for the other peripheral (Fig. 1) or central BP variables (Supplemental Table S1). Melatonin supplementation resulted in greater nighttime systolic dipping compared with the placebo group that was approaching significance (HSD + PL: 13.2 ± 7.1%, HSD + MEL: 16.4 ± 6.9%; P = 0.05), whereas diastolic nighttime dipping (HSD + PL: 20.3 ± 8.6%, HSD + MEL: 22.2 ± 7.8%; P = 0.36) was not different between conditions.

Figure 1.

Twenty-four-hour ambulatory blood pressure at the end of the high-sodium diet plus placebo condition (HSD + PL) and high-sodium diet plus melatonin conditions (HSD + MEL). Twenty-four-hour systolic blood pressure (SBP) (A); 24-h diastolic blood pressure (DBP) (B); 24-h mean arterial pressure (MAP) (C); daytime SBP (D); daytime DBP (E); daytime MAP (F); nighttime SBP (G); nighttime DBP (H); and nighttime MAP (I). n = 18 (9 M/9 W). Data were analyzed using paired t tests and are presented as average and individual data points. Data points from a single participant are connected. *P < 0.05 between conditions.

During IHG, average calculated MVC was not different at the end of each condition (HSD + PL: 265.6 ± 123.2 N, HSD + MEL: 271.1 ± 120.9 N; P = 0.58). To determine whether participants exerted 40% of their MVC during the 2 min of IHG, we compared the force they exerted with the calculated 40% of MVC within the same condition. Our data show that participants did not reach their calculated 40% MVC in either condition. During the placebo and melatonin treatment, participants exerted an average force of 36.8% and 36% of the calculated MVC, respectively. Despite this, achieved force during the 2 min of IHG was not different between the two conditions (HSD + PL: 97.8 ± 44.0 N, HSD + MEL: 97.6 ± 47.0 N; P = 0.97).

MAP responses calculated as a delta change and percent change to IHG, PEI, and CPT are shown in Fig. 2. After both conditions, participants experienced a robust increase in MAP in response to IHG, but there were no differences in the systolic BP, diastolic BP, nor MAP responses to melatonin supplementation. Furthermore, RPE was not different at the end of minute 1 (HSD + PL: 12 ± 2 N, HSD + MEL: 13 ± 2 N; P = 0.15) or minute 2 (HSD + PL: 16 ± 2 N, HSD + MEL: 16 ± 2 N; P = 0.90) between the two conditions for IHG. Similarly, while evaluating the muscle metaboreflex using PEI, MAP increased, but there were no differences between conditions. Finally, BP responses to the CPT were not different between conditions despite increases in MAP.

Figure 2.

Effect of high-sodium diet plus placebo (HSD + PL) and high-sodium diet plus melatonin (HSD + MEL) on the blood pressure responses to isometric handgrip (IHG) exercise, postexercise ischemia (PEI), and the cold pressor test (CPT). Change in mean arterial pressure (MAP) (Δ) in response to IHG (A); percent change in MAP to IHG (B); change in MAP (Δ) in response to PEI (C); percent change in MAP in response to PEI (D); change in MAP (Δ) in response to CPT (E); percent change in MAP in response to CPT (F). n = 22 (11 M/11 W) for IHG and PEI; n = 19 (11 M/8 W) for CPT. Data were analyzed using paired t tests and are presented as average and individual data points. Data points from a single participant are connected.

DISCUSSION

Using a randomized, crossover, double-blind, controlled trial, we tested the novel hypothesis that melatonin supplementation would decrease BP reactivity to two different physiological perturbations causing sympathetic activation and would decrease in 24-h BP on an HSD. The primary finding of this study is that melatonin supplementation did not decrease the BP responses to IHG, PEI, and CPT during an HSD compared with HSD plus placebo. However, melatonin supplementation did decrease nighttime peripheral and central systolic BP assessed via a 24-h ambulatory BP monitor in this sample of young healthy normotensive adults. Notably, the melatonin-related improvements in nocturnal dipping occurred independent of sleep duration and quality. These findings suggest that melatonin supplementation may improve nighttime BP regulation, offering potential benefits for cardiovascular health in young normotensive adults.

Chronic consumption of melatonin is known to exert antihypertensive effects in clinical and healthy populations (13, 17–21). In agreement with these studies and in line with our hypothesis, we report decreases in peripheral and central nighttime systolic BP with melatonin supplementation compared with that of a placebo while consuming an HSD. These are clinically important findings as prospective studies have highlighted that increases in nighttime BP are a significant risk factor for cardiovascular morbidity and mortality even in the general population and after adjustments are made for daytime BP (43). We also observed a trend for greater peripheral systolic BP dipping induced by melatonin supplementation compared with that of the placebo. Although nighttime systolic BP fell in response to melatonin, overall and daytime BP values did not differ at the end of each condition. Furthermore, we did not observe any central BP differences except for nighttime central systolic BP. Although we did not evaluate the mechanism behind these BP differences, several mechanisms may be involved. Melatonin may indirectly regulate BP via modulation of the central nervous system and/or catecholamine secretion, through its antioxidant and anti-inflammatory effects, its ability to relax smooth muscle in blood vessels, and its influence on nitric oxide production and Ca²⁺ signaling (44–46).

Although 10 days of melatonin supplementation was beneficial in lowering nighttime systolic BP in our participants, it is possible this could be an acute response to the most recent dose. Two acute studies, one in young men (11) and the other in young women (12), demonstrated a reduction in systolic and diastolic BP 90 min following supplementation with 1 mg melatonin. In contrast, hypertensive men were not responsive to an acute dose of 2.5 mg of melatonin, however, after 3 wk of supplementation, they experienced declines in nocturnal systolic and diastolic BP (17). Therefore, it is not clear whether this is an acute response or whether some populations need chronic consumption to achieve a BP-lowering effect or whether this is a dose effect. Given that the hypertensive men were not responsive to an acute dose that was greater compared with the aforementioned acute studies in healthy young men and women suggests that dose may be important. This is clearly an area for future investigation.

Studies evaluating the effects of melatonin on BP reactivity in humans are scarce, and the different perturbations used in these studies make comparisons challenging. In healthy young participants, a one-time 3-mg dose of melatonin did not change beat-to-beat BP responses to IHG at 30% of MVC and CPT compared with the placebo condition (16). We hypothesized that using a higher force production would elicit different results. Therefore, our participants performed IHG at 40% of MVC; however, we did not see differences in our melatonin supplementation condition compared with placebo. We were also interested in evaluating PEI to isolate the metaboreflex that would allow for a deeper evaluation of the effects of melatonin on the ANS, but we did not observe any differences in BP reactivity between the placebo and melatonin conditions during PEI. Our results are in line with those of Cook and Ray (47), they found that 3 mg of melatonin did not change beat-to-beat BP responses to head-down rotation, another perturbation known to increase sympathetic responses, in the same population. Finally, in contrast to our results, a single 3-mg dose of oral melatonin attenuated beat-to-beat MAP responses induced by mental stress in young healthy participants (22). Therefore, BP reactivity to melatonin supplementation may be stimulus specific, and future studies should examine BP reactivity to additional perturbations, such as (sub)maximal exercise.

Melatonin has been shown to influence sympathetic output through several pathways. Some of these pathways overlap with the proposed mechanisms by which an HSD can increase BP. First, an HSD causes an increase in sympathetic nervous activity outflow through an increase in excitability of neurons in the rostral ventrolateral medulla (RVLM) (48), whereas melatonin epigenetically modifies the area postrema neurons, which inhibit RVLM activity (49). Second, an HSD may increase extracellular sodium and plasma osmolality levels, which are sensed by neurons in the circumventricular organs in the brain (50). These organs are connected to the paraventricular nucleus (PVN) of the hypothalamus, which projects to the RVLM. Melatonin is known to stimulate (GABA)-ergic signaling, inhibiting the PVN by the suprachiasmatic nucleus, and thereby, inhibiting the RVLM (51, 52). Further research involving clinical populations is warranted to investigate the potential of melatonin to attenuate the impact of sodium, as this may have therapeutic relevance.

Strengths and Limitations

This study has several important strengths including the use of ambulatory BP monitoring, which allowed us to evaluate BP values over 24 h. Furthermore, we assessed several variables that could have impacted our results. First, we evaluated the lipid profiles as melatonin may have antilipidemic effects by improving total cholesterol and triglycerides levels in different populations, and a relation with BP is known (19, 24). Second, we evaluated sleep and physical activity objectively with actigraphy as they may modulate BP responses (53). Third, participants were compliant with taking the salt pills in both conditions as we did not observe differences in 24-h urinary sodium excretion, urine osmolality, or sodium intake at the end of each condition. Finally, we confirmed compliance to the melatonin pills by quantifying 6-sulfatoxymelatonin, which increased in the melatonin supplementation group.

Our study does have some limitations. Although sodium regulation varies throughout the menstrual cycle and estrogen may modulate BP in premenopausal females (54, 55), we did not control for menstrual cycle phase. Although our purpose was to compare BP responses between conditions under an HSD, we could not corroborate whether an HSD caused increases in BP as neither was there a low-sodium control nor was baseline 24-h ambulatory BP measured. In addition, our sample consisted of young healthy normotensive adults and results are not generalizable to other populations.

In conclusion, in this randomized controlled crossover trial, we sought to determine whether melatonin supplementation would decrease 24-h BP and BP reactivity to two different physiological perturbations that cause sympathetic activation under high-sodium conditions. Our findings showed that melatonin supplementation did not alter BP reactivity to IHG, PEI, or CPT, but it decreased nighttime peripheral and central systolic BP in young healthy normotensive adults, suggesting it may be beneficial in reducing future cardiovascular risk.

Perspectives and Significance

Sodium intake continues to exceed recommended guidelines, and this has implications for cardiovascular health. Sodium has been to shown to increase BP reactivity in response to physiological perturbations, resulting in sympathetic activation even in those with a normotensive BP. Melatonin supplementation has been used in several clinical populations to reduce BP and improve other cardiovascular risk factors. Although melatonin was unsuccessful in altering BP reactivity in our study of young normotensive adults, it did lower nocturnal systolic BP, suggesting it may have some beneficial effect even for those with normal BP. Future studies should examine longer supplementation periods as well as middle-aged and older adults.

DATA AVAILABILITY

Data are available upon reasonable request to the principal investigator after institutional data transfer agreement approvals.

SUPPLEMENTAL DATA

Supplemental Table S1: https://doi.org/10.6084/m9.figshare.22732877.v1.

GRANTS

This research was supported by a National Heart, Lung, and Blood Institute Grant HL145055 (to S.L.L.).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

M.R.G., W.B.F., M.A.W., D.L.K., and S.L.L. conceived and designed research; M.R.G., M.R.A., K.E.K., and A.J.L. performed experiments; M.R.G. and S.L.L. analyzed data; M.R.G., A.J.L., and S.L.L. interpreted results of experiments; M.R.G. and S.L.L. prepared figures; M.R.G. drafted manuscript; M.R.G., M.R.A., K.E.K., A.J.L., W.B.F., M.A.W., D.L.K., and S.L.L. edited and revised manuscript; M.R.G., M.R.A., K.E.K., A.J.L., W.B.F., M.A.W., D.L.K., and S.L.L. approved final version of manuscript.