Nitrate supplementation has consistently been shown to benefit exercise performance due to its capacity to increase nitric oxide (NO), a signaling molecule that plays an important role in skeletal muscle function during exercise. Once ingested, nitrate is converted to nitrite via anaerobic bacteria in the oral cavity and nitrite is converted to NO especially under conditions of low oxygen availability and low pH, such as those that may be found in skeletal muscle during exercise. Inorganic nitrate is considered by the International Olympic Committee as one of only five dietary supplements with sufficient evidence to improve exercise performance in specific scenarios. Despite this, improved exercise performance with nitrate intake is not a universal finding among studies. Importantly, while nitrate appears to be effective in men, most studies conducted in women have not shown performance improvements with its supplementation. Although a logical interpretation of this is that nitrate is not performance-enhancing for the female athlete, we believe it is important to recognize that the literature on nitrate supplementation and women is small and limited, and more research is needed before drawing definitive conclusions.
Female representation in research on nitrate supplementation and exercise is severely lacking, with only eleven studies to date conducted exclusively with women, while almost 100 investigations have been performed with men. Of those studies with women, most (8/11) of these did not find an improvement in exercise performance with nitrate, although there are some data to the contrary. A closer look at the results of a recent systematic review and meta-analysis may explain why many of the studies in women showed no benefit from nitrate supplementation. This meta-analysis suggested that the performance-enhancing benefit from nitrate ingestion may not occur when there is no control for antibacterial mouthwash use (since this can eliminate important bacteria that converts the nitrate to nitrite); with acute doses ≤4.9 mmol or ≥15 mmol and chronic doses ≤4.9 mmol or ≥10 mmol; when nitrate is ingested <150 min before exercise; for exercises with duration ≤120 s or >600 s. Accordingly, all studies to date that have demonstrated a lack of a performance-enhancing effect of nitrate in women possess at least one of these characteristics that may potentially compromise benefits from nitrate supplementation. Additionally, several of the nitrate studies in women did not control for the menstrual cycle phase and/or did not consider the use of oral contraceptives. This is important given that menstrual cycle phase and oral contraceptive use may influence exercise performance and the response to nitrate supplementation.
Our recently published article in the Translational Journal of the American College of Sports Medicine aimed to highlight this discrepancy in the literature, namely that there is currently insufficient evidence to draw strong conclusions about the efficacy of nitrate supplementation for female athletes. There is a clear need for well-designed studies to determine whether women can benefit or not from this dietary supplement. This is especially important at a time when the number of female athletes at the Olympic Games is equal to that of male athletes and given the fact that even the smallest performance gains can make the difference between winning gold and failing to achieve the podium. To help researchers interested in this field, we have provided suggestions for studies investigating the effectiveness of dietary nitrate in female performance:
Use a placebo-controlled, crossover, double-blind, and randomized study designs, due to the short duration of nitrate supplementation and its short washout period.
To increase performance-enhancing effects (as based upon studies using predominantly males), studies should employ a 5-9.9 mmol dose of nitrate, using beetroot juice as a vehicle, ingested ≥150 min prior exercise lasting 120-600 s (e.g.: 4-km cycling time-trial, 1500 m and 3000 m running time-trial and 2000 m rowing) and avoid antibacterial mouthwash.
Compare monophasic oral contraceptives users and a men-only group to determine if inherent physiological differences may lead to distinct performance responses to nitrate supplementation between women and men.
Consider whether menstrual cycle phase alters the response to nitrate supplementation through comparisons of the effects observed between two or more phases.
Women seeking to gain a competitive advantage from nitrate:
We suggest that female athletes consult with a registered dietitian to determine the need for nitrate supplementation. If the decision to trial nitrate is taken, we suggest that female athletes rigorously and systematically test nitrate supplementation in, preferably in repeated competition simulations and compare it to their performance without nitrate, to verify if this strategy can enhance exercise performance and does not lead to side effects (e.g., mild gastrointestinal problems). If dietary nitrate proves to be consistently effective in improving performance during simulations and side effects are not observed, the athlete could consider its use in competition.
For more detailed recommendations and explanations about the topic, please read the paper “Directions for Future Studies to Determine Dietary Nitrate Efficacy in Female Athletes."
Arthur Carvalho, B.Sc., is a master’s student at the Faculty of Medicine of the University of São Paulo and a member of the Applied Physiology and Nutrition Research Group at the University of São Paulo, where he researches sex differences in exercise performance response to inorganic nitrate supplementation.
Breno Duarte, MSc, is a Ph.D student at the Faculty of Medicine of the University of São Paulo and a member of the Applied Physiology and Nutrition Research Group at the University of São Paulo, where he researches physiological and performance responses to different sources of dietary nitrate supplementation.
Bryan Saunders, Ph.D., is a researcher and lecturer in sport and exercise science at the Faculty of Medicine of the University of São Paulo, Brazil and a senior member of the Applied Physiology and Nutrition Research Group at the University of São Paulo. His main areas of research include nutritional supplementation to improve exercise performance, determination of the mechanisms through which these supplements act and what factors might moderate these effects. Bryan has applied sport science experience having provided performance analysis and physiology support to several UK-based football teams, has worked with elite cycling in Brazil and was previously a member of the São Paulo Cycling Federation. He currently serves on the editorial board of Frontiers in Nutrition and the Translational Journal of the American College of Sports Medicine.