“We show that the autonomic system is susceptible to a hyperglycemic episode,” the kinesiologists from Louisiana State University and Columbus State University wrote. “Furthermore, using a rigorous study design, we show the efficacy of a naturally-rich source of amino acids, L-citrulline and L-arginine to preserve heart rate variability (HRV) during a hyperglycemic episode.”
Published in the journal Nutrients, the study was funded by the National Watermelon Promotional Board.
Watermelon juice and autonomic dysfunction
Autonomic dysfunction—defined as a dysfunction of the nerves that regulate nonvoluntary body functions, such as heart rate, blood pressure and sweating—is an emerging mechanism in the development of cardiometabolic disease. It is easily assessed through HRV i.e., the variation from two consecutive heartbeats, the study noted.
Previous research links autonomic dysfunction and low HRV to increased cardiovascular disease risk and mortality, specifically through visceral adipose tissue accumulation, hyperglycemia, endothelial dysfunction and increased inflammation. The researchers also hypothesized that watermelon juice would boost bioavailability of nitric oxide (NO)—which has been shown to improve vascular function and insulin synthesis—to mitigate the decline in HRV during the blood sugar spike.
“The loss of NO bioavailability and reduction in HRV during an OGC suggests that these phenomena may be causally linked,” the authors wrote. “To date, no studies have explored the protective effects of NO donor therapy on hyperglycemia-induced autonomic dysfunction and metabolic function.”
The study also evaluated the impact on metabolic parameters including energy expenditure and substrate oxidation.
The randomized, double-blind, placebo-controlled crossover trial recruited 18 healthy men and women between the ages of 18 and 40 years and assigned them to either 500 ml a day watermelon juice supplement or placebo group for two weeks.
After the supplementation period, participants completed a 75-g oral glucose challenge (OGC). HRV, resting energy expenditure and respiratory exchange ratio (RER) were measured for 30 min before glucose drink ingestion and 60 min and 120 min after. Participants then crossed over to the other intervention group after a two-week washout period between treatments.
“In the current study, we demonstrate that watermelon juice supplementation preserves aspects of autonomic function during acute hyperglycemia (OGC) but does not alter metabolic responses when compared to placebo treatment,” the researchers wrote. “This strengthens the argument that vascular and autonomic dysregulation precedes the development of whole-body metabolic impairments.”
In addition, the study did not observe improvements in fasted, baseline HRV between placebo and watermelon juice treatment, leading them to induce that “hyperglycemia and oxidative stress may be necessary mediators for reduced HRV.”
The researchers suggest that future work emphasize the importance of NO bioavailability in autonomic dysfunction in cardiometabolic disease and that studies on participants with obesity and existing metabolic dysfunction could provide more insight into the connection between autonomic and metabolic regulation.
Source: Nutrients 2023, 15(4), 810
“The Effect of Watermelon Juice Supplementation on Heart Rate Variability and Metabolic Response during an Oral Glucose Challenge: A Randomized, Double-Blind, Placebo-Controlled Crossover Trial”
Authors: Rachel Matthews et al.