A recent study has shown the significant effects of microencapsulated watermelon rind (MWR) on vascular function and tissue oxygen saturation in young adults. The study explores how a single dose of MWR containing 4 grams of L-citrulline improves flow-mediated dilation (FMD) while not significantly affecting tissue oxygen saturation (StO2) parameters. The research was conducted by Mônica Volino-Souza and her colleagues, aiming to assess the potential of a widely discarded food waste, watermelon rind, to promote better vascular health.
Background and Objectives
Watermelon, specifically its rind, contains a rich concentration of L-citrulline, an amino acid that plays a crucial role in the production of nitric oxide (NO) in the body. NO is vital for regulating vascular function, as it helps relax blood vessels, enhancing blood flow. Previous research has shown that consuming L-citrulline can improve vascular health more effectively than L-arginine supplementation, which is also involved in NO synthesis. This is because L-arginine is heavily metabolized by the intestines, making its supplementation less efficient.
Given the rich L-citrulline content of watermelon rind, which is typically discarded as food waste, the researchers aimed to investigate whether microencapsulated watermelon rind could be used as a functional food to boost vascular health. The specific objectives were to assess the impact of MWR on flow-mediated dilation, tissue oxygen saturation, and plasma levels of key amino acids such as L-citrulline, L-arginine, and L-ornithine.
Study Design and Methodology
The study followed a randomized, single-blind, placebo-controlled, and crossover design. A total of 11 young adults (six females and five males) participated, with an average age of 28.3 years. The participants visited the laboratory on two separate occasions, spaced one week apart, during which they ingested either 30 grams of MWR or a placebo (maltodextrin).
The participants’ vascular function was measured using flow-mediated dilation (FMD), a test that evaluates the function of the endothelium (the inner lining of blood vessels), and near-infrared spectroscopy to assess tissue oxygen saturation (StO2) in their forearm muscles. Blood samples were taken before and after ingestion to measure plasma amino acid concentrations.
The watermelon rind was microencapsulated, a process that improves the stability and delivery of bioactive compounds like L-citrulline. Each dose contained 4 grams of L-citrulline, a concentration known to support vascular function, compared to smaller doses used in other studies that yielded less significant results.
Key Findings: Flow-Mediated Dilation Improves
One of the most significant findings was that a single dose of microencapsulated watermelon rind increased the FMD response. FMD is a measure of macrovascular function, particularly the health of large blood vessels. In this study, FMD improved 60 minutes after ingestion of MWR, indicating enhanced vascular function in young adults.
This result aligns with previous studies showing that L-citrulline supplementation can improve vascular health. The researchers noted that the dose of L-citrulline (4 grams) used in this study was likely responsible for the observed benefits, as lower doses in prior research did not produce similar results. For instance, past studies on postmenopausal women and overweight or obese individuals using much smaller doses of L-citrulline failed to demonstrate significant improvements in FMD.
No Significant Changes in Tissue Oxygen Saturation
Despite the positive effects on FMD, MWR ingestion did not result in significant changes in tissue oxygen saturation (StO2) parameters. StO2 is a measure of microvascular function, assessing the responsiveness of small blood vessels in muscle tissue. While the study observed improvements in macrovascular health (FMD), the lack of changes in StO2 suggests that MWR’s effects may be more pronounced in larger blood vessels rather than in the microvasculature.
The researchers suggest that this difference could be due to varying contributions of NO to vasodilation in macrovascular versus microvascular systems. Larger blood vessels, like those measured in FMD, rely more heavily on NO for dilation, whereas microvessels may be less dependent on NO and more influenced by other factors.
Plasma Amino Acid Levels: A Positive Shift
Another critical finding was the significant increase in plasma L-citrulline and L-arginine levels after MWR ingestion. L-citrulline levels rose at both the 60 and 120-minute marks after ingestion, while L-arginine levels showed a notable increase at the 60-minute point. These findings confirm that MWR effectively elevates the availability of these key amino acids, which are crucial for NO synthesis and, by extension, vascular function.
While the study did not directly measure NO levels, previous research has demonstrated that L-citrulline supplementation can enhance NO bioavailability. The increase in plasma L-citrulline and L-arginine observed in this study supports the hypothesis that MWR ingestion boosts NO production, contributing to the improved FMD response.
Limitations and Future Research
Although the results are promising, the researchers caution that the findings should be interpreted with care due to the small sample size. With only 11 participants, the study may not be generalizable to the broader population. Moreover, the study did not measure NO metabolites (such as plasma nitrite and nitrate), which could have provided direct evidence of increased NO synthesis following MWR ingestion.
The researchers recommend future studies with larger sample sizes and more comprehensive measurements of NO production. Additionally, they suggest that populations at risk for cardiovascular disease, such as those with hypertension or diabetes, could benefit from similar studies. These individuals often suffer from vascular dysfunction, making them ideal candidates for interventions targeting vascular health.
Conclusion
This study adds to the growing body of evidence supporting the health benefits of watermelon, particularly its often-overlooked rind. By demonstrating that a single dose of microencapsulated watermelon rind can improve macrovascular function without affecting microvascular responsiveness, the researchers offer a potential new avenue for dietary interventions aimed at promoting cardiovascular health.
As food waste continues to be a global issue, the findings also highlight the potential of using discarded food parts, like watermelon rind, in health-promoting products. The study’s promising results pave the way for future research into the broader applications of microencapsulated watermelon rind, particularly for individuals at risk for cardiovascular disease.
In summary, while more research is needed to fully understand the potential of watermelon rind as a functional food, this study offers a compelling case for its use in promoting vascular health and reducing food waste.