Citrulline for Endurance: Does It Really Work?

meta-analysis PMID: 34965876

Quick Summary: A recent study looked at whether eating foods with citrulline, like watermelon, can boost your endurance during exercise. The research found that citrulline didn't improve performance, but foods with nitrates (like beets) and polyphenols (like pomegranate) did show a small benefit.

Does Citrulline Help with Exercise?

This research reviewed many studies to see if certain foods could improve how long you can exercise. They looked at foods containing citrulline, nitrate, and polyphenols. The surprising result? Citrulline-rich foods didn't seem to make a difference in endurance.

What The Research Found

Citrulline: Eating foods high in citrulline didn't improve endurance performance.

Nitrate: Foods like beets showed a small improvement in endurance.

Polyphenols: Foods like pomegranate also showed a small boost in endurance.

Gender Differences: The benefits of nitrate and polyphenol-rich foods were seen in men, but not in women.

Study Details

Who was studied: The researchers looked at the results of 118 different studies. These studies included people of different fitness levels.

How long: The studies varied in length, from single-dose tests to longer-term supplementation.

What they took: Participants consumed foods naturally high in citrulline, nitrates, or polyphenols.

What This Means For You

Citrulline: If you're looking to boost your endurance, eating foods high in citrulline (like watermelon) might not be the best strategy.

Nitrates: Consider adding beets to your diet. They might give you a small edge during exercise.

Polyphenols: Foods like pomegranate could also offer a slight benefit.

Men vs. Women: The study suggests that men might benefit more from nitrate and polyphenol-rich foods than women.

Study Limitations

Citrulline Studies: There weren't many studies specifically on citrulline, so more research is needed.

Dosage: The exact amount of citrulline consumed wasn't always clear, making it hard to know the best dose.

Other Factors: The study didn't fully explore why some foods worked better than others or why there were gender differences.

Key Findings

This meta-analysis found no significant effect of foods rich in L-citrulline on endurance exercise performance (standardized mean difference [SMD] = -0.03, p = 0.24). In contrast, nitrate-rich foods (SMD = 0.15, p < 0.001) and polyphenol-rich foods (SMD = 0.17, p < 0.001) demonstrated trivial but statistically significant improvements in performance metrics like time-trial, time-to-exhaustion, and intermittent tests. Benefits were more pronounced in well-trained males, while no effects were observed in females. Food-specific responses were noted: beetroot (nitrate) and certain polyphenol sources (e.g., pomegranate, Montmorency cherry) improved performance, but others (e.g., red spinach, green tea) did not.

Study Design

The study was a systematic review and random-effects meta-analysis of 118 randomized controlled trials (RCTs) up to 2021. It evaluated endurance performance outcomes after consuming foods containing nitrate, polyphenols, L-arginine, or L-citrulline. Subgroup analyses were conducted based on food type, sex, fitness level, test type, and supplementation duration.

Dosage & Administration

The three L-citrulline studies used food-based interventions, but specific doses were not reported in the summary. Administration was via dietary sources (e.g., watermelon, citrulline-enriched foods), with protocols including both acute and chronic supplementation. Other subgroups (nitrate/polyphenols) used standardized food sources (e.g., beetroot juice, pomegranate extract), though dosing variability limited detailed analysis for L-citrulline.

Results & Efficacy

L-citrulline-rich foods showed no ergogenic benefit (SMD = -0.03, 95% CI not specified, p = 0.24). Nitrate and polyphenol-rich foods had small but significant effects (SMD = 0.15 and 0.17, respectively, p < 0.001), though benefits were absent in females. Beetroot (nitrate) and specific polyphenol sources (grape, pomegranate) improved performance, while red spinach, Swiss chard, rhubarb (nitrate), and cocoa/green tea (polyphenols) did not. Heterogeneity was high in polyphenol studies (I² not quantified).

Limitations

Only three RCTs assessed L-citrulline, limiting statistical power. Dose-response relationships for L-citrulline were unclear due to unreported dosages. Subgroup analyses (e.g., sex, food types) may have been underpowered, particularly for females. Heterogeneity in polyphenol studies could reflect food-specific mechanisms or variations in study design, participant demographics, and supplementation protocols.

Clinical Relevance

For athletes, L-citrulline from food sources does not appear to enhance endurance. However, nitrate-rich beetroot and select polyphenol-containing foods (e.g., pomegranate, Montmorency cherry) may offer minor performance benefits, especially in well-trained males. These findings suggest that food composition and individual factors (e.g., sex, fitness level) influence efficacy. Practitioners should prioritize beetroot or polyphenol-rich fruits over L-citrulline for endurance support, though effects are trivial and may not translate to meaningful real-world improvements. Future research should clarify sex-based differences and optimize dosing for polyphenols.