L-Citrulline and HIIT: Same Stack, Different Biology

The same supplement can hit different bottlenecks

L-citrulline is easy to oversell because the first part of the story is tidy. It raises circulating L-arginine, which can support nitric oxide production, which can improve blood flow. Neat pathway, nice diagram, solid supplement marketing. Real biology is less obedient. The best way to think about L-citrulline is not as a universal workout booster, but as a substrate-restoring tool whose effects depend on what problem the body has upstream.

That age-specific framing matters here. In a 2022 randomized trial by Marcangeli et al., researchers tested high-intensity interval training, or HIIT, with or without 10 g/day of L-citrulline for 12 weeks in obese older adults. In a 2025 randomized trial by Rodríguez-Carrillo et al., researchers tested HIIT plus L-citrulline, HIIT plus placebo, or L-citrulline alone for 12 weeks in obese adolescents with metabolic-dysfunction-associated steatotic liver disease, or MASLD. Same broad supplement idea. Same training format. Very different bodies. Very different results.

The older-adult trial points toward muscle and mitochondrial biology. The adolescent trial showed improvements in lipids, specifically triglycerides, VLDL-C, and non-HDL-C, but no measurable improvement in arterial stiffness. That split is the whole story. L-citrulline seems to help most where nitric-oxide-related substrate supply is colliding with the dominant dysfunction in that tissue, not where we wish it would help because the mechanism sounds elegant.

Why citrulline instead of arginine?

This part of the evidence is stronger than the performance hype. As a 2022 trial by Maharaj et al. explains, oral L-citrulline has a pharmacokinetic edge over oral L-arginine. Citrulline is converted to arginine in the kidney, bypasses substantial gut and liver extraction, and does not trigger the same arginase upregulation problem that can blunt arginine-based strategies. A 2024 meta-analysis by Smeets et al. supports that general framework in adults: L-citrulline and watermelon interventions produced modest but statistically significant improvements in vascular and cardiometabolic markers, though effect sizes were small and some vascular outcomes depended on duration.

So the basic biochemistry is credible. Supplementing citrulline can raise the arginine pool available to endothelial nitric oxide synthase. What does not follow automatically is better real-world performance, better arteries on every timescale, or better muscle function in every population. Biology does not grade on pathway diagrams.

Older adults: plausible muscle biology, incomplete outcome evidence

The older-adult study had a serious design. Marcangeli et al. randomized 81 obese older adults to HIIT plus L-citrulline or HIIT plus placebo for 12 weeks, with 45 participants in the citrulline arm and 36 in the placebo arm. Everyone trained three times per week. Researchers measured handgrip strength, quadriceps force, leg extensor power, a 6-minute walk test, and a chair stand test. They also collected muscle and adipose tissue biopsies to assess mitochondrial content, biogenesis, dynamics, and mitophagy. That combination of functional endpoints and tissue biology is what you want if you are testing a mechanism instead of just admiring a blood marker.

The rationale also makes sense. Aging muscle loses mitochondrial quality control, a process closely tied to sarcopenia, as reviewed in a 2023 review by Tian et al. HIIT can stimulate mitochondrial adaptation on its own. L-citrulline could, in theory, improve blood delivery during intervals and support nitric-oxide-linked signaling relevant to mitochondrial biogenesis. That is a coherent hypothesis for older adults whose main bottleneck may be impaired muscle energetics and endothelial substrate availability.

But coherent is not the same as confirmed. The published information available here describes the trial design and its mechanistic targets, yet does not provide extractable between-group numerical results for the functional tests or biopsy outcomes. That means we cannot say whether HIIT plus L-citrulline beat HIIT plus placebo on the prespecified primary outcomes, by how much, or with what statistical certainty. This is not a fussy technical complaint. It is the difference between "good hypothesis" and "proven additive effect."

There is another structural limit: the trial had no sedentary control and no citrulline-only arm. Since every participant did HIIT, the study cannot isolate the independent effect of citrulline from the effect of training itself. If you want to know whether citrulline adds something meaningful to HIIT in older adults, the design gets you part of the way there. If you want to know whether citrulline alone helps, it cannot answer that question.

Adolescents with MASLD: lipids improved, arteries did not

The adolescent trial is clearer on outcomes. Rodríguez-Carrillo et al. randomized obese adolescents with MASLD to three groups for 12 weeks: HIIT plus L-citrulline, HIIT plus placebo, and L-citrulline alone, with 14 participants in each exercise arm and 15 in the supplement-only arm. The HIIT-plus-citrulline group improved triglycerides, VLDL-C, and non-HDL-C. The prespecified vascular endpoints, pulse wave velocity and augmentation index corrected to 75 beats per minute, did not improve significantly.

That result is more interesting than a vague "mixed findings" label. It suggests that in these adolescents, the more responsive target may be lipid handling rather than structural arterial stiffness. The proposed mechanism fits the population: oxidative stress in MASLD can increase arginase activity and reduce nitric-oxide-relevant arginine availability. Because citrulline restores the arginine pool without driving further arginase upregulation, it could plausibly shift metabolism in a way that improves atherogenic lipid markers.

The null vascular finding still matters. Pulse wave velocity and augmentation index were prespecified outcomes, and they did not move. That limits any claim that 12 weeks of L-citrulline plus HIIT improves arterial health in metabolically stressed adolescents. A reasonable explanation is that adolescent arteries remain structurally compliant enough that short-term changes are hard to detect, especially when the expected effect is small. The Smeets et al. meta-analysis found only modest vascular effects in adults, with arterial stiffness outcomes that were small and duration-dependent. A 12-week trial in teenagers was never likely to produce dramatic pulse-wave fireworks. Arteries are not mood rings.

Even here, caution is warranted. The available published excerpt does not clearly confirm the L-citrulline dose used in the adolescent trial, so dose-specific conclusions are weaker than they look at first glance. And with only 14 participants per exercise arm, the study is too small for confident subgroup or mediation claims.

The big brake: nitric oxide support does not reliably improve performance

The strongest moderating evidence comes from a 2022 systematic review and meta-analysis by d'Unienville et al. This review found limited and inconsistent effects of L-citrulline and L-arginine on endurance exercise performance across randomized trials. That does not erase the mechanistic logic behind citrulline. It does force a more disciplined conclusion.

Raising plasma arginine is not the same as improving strength, power, or endurance in a way people can feel or measure consistently. Biomarkers and pathways can move while performance stays flat. That is exactly why the older-adult study needs readable outcome data, not just a plausible rationale, and why the lipid improvements in adolescents should not be stretched into claims about universal exercise enhancement.

Supportive side evidence from food-based trials using watermelon or watermelon juice, including a 2025 pilot trial by Singh et al. and a 2023 crossover trial by Matthews et al., adds context that citrulline-rich interventions can influence blood pressure, autonomic markers, or metabolic responses. But those studies used different populations, different interventions, and different endpoints. They support biological plausibility. They do not settle what isolated L-citrulline adds to HIIT in obese older adults or adolescents with MASLD.

What the evidence supports right now

If you are an older adult doing HIIT, L-citrulline has a credible mechanistic case and a trial specifically built to test muscle and mitochondrial outcomes. The catch is that the available evidence here does not let us verify the additive functional results. The honest position is cautious: the intervention is biologically plausible, and 10 g/day was the tested dose in the older-adult trial, but confirmed gains in strength, power, or walking performance remain unproven from the accessible outcome data.

If you are dealing with adolescent obesity plus MASLD, the evidence is more concrete but narrower. Over 12 weeks, HIIT plus L-citrulline improved triglycerides, VLDL-C, and non-HDL-C, while arterial stiffness measures did not improve. That makes L-citrulline look more like a targeted metabolic adjunct than a vascular reset button.

Across both ends of the age spectrum, the cleanest conclusion is also the least glamorous. L-citrulline is not a blanket performance enhancer. It is a substrate-restoring intervention whose benefits seem to depend on which physiological bottleneck matters most in the person taking it. In older muscle, that may involve mitochondrial quality control and endothelial substrate supply. In adolescents with fatty liver disease, it may involve lipid dysregulation linked to arginase competition. The mechanism is credible. The clinical payoff is narrower, smaller, and more endpoint-specific than the supplement aisle would like you to believe.

Verdict: treat L-citrulline as a targeted metabolic tool with modest, population-specific upside, not as a guaranteed way to turn HIIT into something magical.