Nitric Oxide, Blood Flow, and Performance: New Research Behind Healthmasters’ HGH Stimulate

Nitric oxide is one of the body’s most important signaling molecules for healthy circulation. Produced within the lining of blood vessels, nitric oxide tells the smooth muscle surrounding an artery to relax, allowing the vessel to widen and blood to move more freely. That seemingly simple reaction can influence blood pressure, oxygen delivery, nutrient transport, exercise capacity, and the health of the vascular endothelium itself [1-3].

This pathway is also the most compelling modern scientific story behind Healthmasters’ HGH Stimulate. Although the product’s name reflects longstanding interest in amino acids and growth-hormone physiology, its current formula is particularly well suited to supporting nitric oxide production. Each scoop combines 2 grams of L-arginine, 500 mg of L-citrulline, 500 mg of quercetin, 300 mg of vitamin C, and 240 mg of mixed tocopherols, along with folate. Instead of relying on a single ingredient, the formula approaches circulation through nitric-oxide precursors, flavonoid support, and antioxidant protection.

New research published in 2025 and 2026 is giving us a better understanding of why that combination may be useful.

Nitric Oxide Starts With the Endothelium

The endothelium is the thin layer of cells lining the inside of blood vessels. Far from being an inert coating, endothelial cells constantly respond to changes in blood flow, hormones, inflammation, oxygen demand, and oxidative stress. One of their most important jobs is producing nitric oxide through an enzyme called endothelial nitric oxide synthase, or eNOS [2].

L-arginine is the amino acid eNOS uses directly to manufacture nitric oxide. Once nitric oxide is released, it moves into the smooth muscle surrounding the blood vessel and triggers relaxation, allowing the vessel to expand. When this system functions properly, arteries can adjust their diameter to match changing demands for blood and oxygen [2].

L-citrulline enters the same pathway from another direction. The body can convert citrulline back into arginine, which then becomes available for nitric-oxide production. This is particularly interesting because oral arginine and citrulline are handled differently during digestion and metabolism. Citrulline largely bypasses the extensive intestinal and liver metabolism that affects arginine and can subsequently raise circulating arginine availability [2].

That makes the combination found in HGH Stimulate biologically interesting: arginine supplies the immediate substrate for nitric-oxide synthesis, while citrulline can help replenish the body's arginine pool.

New 2026 Trial Connects Citrulline With Endothelial Health

One of the newest human trials investigated L-citrulline in adults experiencing persistent problems after hospitalization for COVID-19, a population in which endothelial dysfunction can remain elevated long after the original infection [1].

Researchers randomized 43 adults to receive either 4 grams of L-citrulline per day or no citrulline for three months. They then examined several biomarkers related to endothelial function along with body composition, handgrip strength, and performance on a six-minute walking test [1].

The most interesting change occurred in ICAM-1, or intercellular adhesion molecule-1. ICAM-1 is expressed on endothelial and immune cells and tends to rise when the vascular lining is activated by inflammation. After three months, ICAM-1 fell by approximately 32.6 ng/mg of protein in the citrulline group, compared with essentially no meaningful change in the control group [1].

Physical capacity improved as well. Participants receiving citrulline increased their six-minute walking distance by approximately 141 meters, compared with about 68 meters in the control group [1]. Both groups improved, which is important because the participants were also recovering over time, but the improvement was significantly greater in those receiving citrulline. The study did not find significant between-group improvements in body composition or handgrip strength, showing that the benefit was not universal across every measurement [1].

This was a small, open-label trial in a specific post-COVID population, so it would be inappropriate to claim that citrulline will necessarily produce a 141-meter improvement in healthy people. It does, however, provide new human evidence that increasing citrulline availability can influence both a marker of endothelial activation and functional walking capacity.

Better Blood-Vessel Dilation in Randomized Trials

A broader 2025 systematic review and meta-analysis examined eight randomized controlled trials involving 176 middle-aged and older adults who received L-citrulline or related interventions [2]. Researchers focused on two measurements of vascular function: pulse-wave velocity, which provides information about arterial stiffness, and flow-mediated dilation, or FMD [2].

Flow-mediated dilation measures how much an artery expands in response to increased blood flow. Because that response depends heavily on endothelial nitric-oxide activity, FMD is widely used as an indicator of endothelial function.

Across the included trials, L-citrulline supplementation increased FMD by approximately 1.81 percentage points, a statistically significant improvement [2]. Overall pulse-wave velocity did not significantly improve, although a subgroup examining brachial-ankle pulse-wave velocity did show a significant reduction [2].

Those mixed results are useful. They suggest that citrulline should not simply be described as universally “making arteries less stiff.” The stronger evidence in this analysis involved the endothelium’s ability to dilate blood vessels, which fits directly with the nitric-oxide pathway.

Citrulline May Become Particularly Important Under Vascular Stress

Another 2026 meta-analysis looked at a more unusual situation: what happens to blood pressure when people are exposed to cold [3].

Cold causes blood vessels to constrict, which can rapidly increase blood pressure. Researchers pooled six randomized controlled trials involving 162 participants and found that L-citrulline significantly reduced the rise in blood pressure caused by cold exposure [3].

Across the studies, systolic pressure was approximately 9.3 mmHg lower, while diastolic pressure was about 5.3 mmHg lower with citrulline supplementation [3]. Brachial and aortic systolic pressures were both significantly improved, although the brachial diastolic result did not reach statistical significance [3].

These findings reinforce the idea that supporting nitric-oxide availability may become particularly relevant when blood vessels are being challenged to maintain normal dilation.

Why Combine Arginine and Citrulline?

While newer studies have focused heavily on citrulline, earlier randomized research provides an important reason for combining it with arginine.

In a randomized, double-blind crossover trial, male collegiate soccer players consumed 1.2 grams of L-arginine plus 1.2 grams of L-citrulline daily for seven days before completing a 10-minute maximal cycling test [4].

Compared with placebo, the combination significantly increased plasma arginine, citrulline, and nitric-oxide metabolites. Average cycling power was also higher, at approximately 242 watts compared with 231 watts during placebo, while participants reported less perceived leg soreness and greater ease of pedaling [4].

The study was small and limited to young male athletes, so it cannot tell us what the same combination will do in older adults or less active people. Nevertheless, it provides human evidence supporting the biochemical logic of supplying both amino acids rather than treating them as interchangeable ingredients.

This is where the formulation of Healthmasters’ HGH Stimulate begins to make more sense. Its 2 grams of arginine provide the direct nitric-oxide precursor while citrulline feeds the pathway by helping replenish circulating arginine. Importantly, HGH Stimulate contains less citrulline than the 4-gram dose used in the recent post-COVID trial, so the clinical outcomes from that study should not be attributed directly to one scoop of the product. The formula instead uses citrulline as part of a broader multi-ingredient nitric-oxide strategy.

Quercetin Adds Another Vascular Angle

HGH Stimulate also supplies 500 mg of quercetin, a plant flavonoid with antioxidant and vascular effects.

A systematic review and meta-analysis of seven randomized controlled trials involving 587 participants found that quercetin supplementation significantly reduced both systolic and diastolic blood pressure [5]. When researchers separated the trials by dose, significant reductions were seen in studies using 500 mg per day or more, with systolic pressure falling by approximately 4.45 mmHg and diastolic pressure by about 2.98 mmHg in that subgroup [5].

The researchers cautioned that indirect comparisons did not conclusively prove that the higher dose was superior, so 500 mg should not be viewed as a magic threshold [5]. Still, it is noteworthy that HGH Stimulate provides 500 mg of quercetin per scoop, placing the formula directly within the dose range used in much of the human cardiovascular research.

Quercetin also complements the amino-acid pathway in a different way. Rather than serving as raw material for nitric-oxide production, its antioxidant and cellular effects may help support the vascular environment in which nitric oxide operates.

Protecting the Endothelium From Oxidative Stress

Nitric oxide production is only half of the equation. The body also has to prevent nitric oxide from being rapidly destroyed.

Oxidative stress can reduce nitric-oxide availability by reacting with it before it has the opportunity to carry out normal vascular signaling. This is one reason antioxidant nutrients often appear alongside nitric-oxide precursors in cardiovascular formulas.

HGH Stimulate provides both vitamin C and mixed tocopherols, the vitamin E family. A large meta-analysis of 44 randomized trials previously found that vitamin C supplementation improved endothelial function overall, with stronger effects among people with cardiovascular or metabolic disorders [6]. Researchers proposed that one mechanism involves vitamin C helping preserve nitric-oxide availability under conditions of increased oxidative stress [6].

More recently, a 2026 systematic review and meta-analysis of 12 randomized trials examined vitamin E and endothelial adhesion molecules. Vitamin E significantly reduced VCAM-1 overall, while effects differed considerably depending on the vitamin E form, dose, population, and study duration [7]. Particularly interesting reductions in VCAM-1 appeared in studies using tocotrienols or mixed vitamin E isoforms [7].

Healthmasters uses mixed tocopherols in HGH Stimulate rather than relying exclusively on isolated alpha-tocopherol, adding another layer of antioxidant support to a formula already centered on nitric-oxide production.

Conclusion

HGH Stimulate is fundamentally a nitric-oxide and circulation formula.

The newer research gives Healthmasters’ HGH Stimulate a stronger and more relevant story than growth hormone alone. Its combination of L-arginine and L-citrulline targets the nitric-oxide pathway from two complementary directions, while quercetin, vitamin C, and mixed tocopherols help support the vascular environment in which nitric oxide has to function. That matters because healthy circulation is central to far more than cardiovascular health; it affects how efficiently oxygen and nutrients reach working muscles, how well blood vessels adapt to changing demand, and how effectively the body supports performance and recovery.

The recent studies on citrulline and endothelial function strengthen the scientific rationale behind the formula. Rather than relying on stimulants for a temporary boost, HGH Stimulate is built around supporting the circulation and nitric-oxide production that help the body perform more efficiently from the inside out.

References

[1] Salgado-Fernández, M. F., Orea-Tejeda, A., Sierra-Vargas, M. P., González-Islas, D., Debray-García, Y., Aztatzi-Aguilar, O. G., et al. (2026). Effect of L-citrulline supplementation on endothelial function and body composition in post-COVID-19 syndrome: A randomized clinical trial. Nutrients, 18(11), 1706. https://doi.org/10.3390/nu18111706.

[2] Luo, P., Li, Z., Liu, K., & Gao, W. (2025). Effects of L-citrulline supplementation and watermelon intake on arterial stiffness and endothelial function in middle-aged and older adults: A systematic review and meta-analysis of randomized controlled trials. Frontiers in Nutrition, 12, 1632952. https://doi.org/10.3389/fnut.2025.1632952.

[3] Luo, P., Chen, J., Liu, K., & Zhang, J. (2026). Effect of L-citrulline intake on blood pressure in cold environments: A systematic review and meta-analysis of randomized controlled trials. Food Science & Nutrition, 14(3), e71603. https://doi.org/10.1002/fsn3.71603.

[4] Suzuki, T., Morita, M., Hayashi, T., Kamimura, A., & Suzuki, Y. (2019). A combination of oral L-citrulline and L-arginine improved 10-min full-power cycling test performance in male collegiate soccer players: A randomized crossover trial. European Journal of Applied Physiology, 119(5), 1075-1084. https://doi.org/10.1007/s00421-019-04097-7.

[5] Serban, M. C., Sahebkar, A., Zanchetti, A., Mikhailidis, D. P., Howard, G., Antal, D., et al. (2016). Effects of quercetin on blood pressure: A systematic review and meta-analysis of randomized controlled trials. Journal of the American Heart Association, 5(7), e002713. https://doi.org/10.1161/JAHA.115.002713.

[6] Ashor, A. W., Lara, J., Mathers, J. C., & Siervo, M. (2014). Effect of vitamin C on endothelial function in health and disease: A systematic review and meta-analysis of randomised controlled trials. Atherosclerosis, 235(1), 9-20. https://doi.org/10.1016/j.atherosclerosis.2014.04.004.

[7] Karimi, M., Abdi, S., Hosseini, S., et al. (2026). Effect of vitamin E supplementation on biomarkers of endothelial function in adults: A GRADE-assessed systematic review and dose-response meta-analysis. Artery Research, 32, 2. https://doi.org/10.1007/s44200-025-00098-4.

*The matters discussed in this article are for informational purposes only and not medical advice. Please consult your healthcare practitioner on the matters discussed herein.

*These statements have not been evaluated by the Food and Drug Administration. Healthmasters' products are not intended to diagnose, treat, cure, or prevent any disease.