Athletes are always looking for methods to improve their performance, and trying a nutritional supplement is a natural step toward this improvement. Both male and female athletes may be willing to take sports nutrition supplements that promise increased time to fatigue, improvement in strength, or decreased body composition. Research on collegiate female athletes suggests that more than half (65.4%) are using either traditional (single and multivitamin/mineral supplements) or nontraditional (herbals, botanicals, and other biologic and nutrient) sports supplements at least one time per month. However, has the research on the safety and efficacy of sports supplements included adequate female representation as study participants? At least one recent article is saying, “not so much.”
Women are complex (on multiple levels), which certainly comes as no surprise to anyone reading this article! When focusing on physiology specifically, females differ from males in hormonal systems, structural characteristics, and many other processes that impact sports performance outcomes—including metabolism, microbiomes, hydration, thermoregulation, and biomechanical differences. Accounting for these areas requires additional time, energy, and expense associated with research methodology, leading fewer researchers to use dual-sex or women-only designs. As a result, prior research has found that women are study participants in just 4-13% of studies. Evidence-based practitioners recommendations rely on the idea that studies have the ability to be reproduced, transferred, and apply to the female client sitting in their office. However, if women haven’t been represented in these studies, it could be difficult, if not impossible, to develop current best-practice supplementation protocols that benefit the female athlete.
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The current study
To study how this lack of female representation in study participants plays out with supplements, an auditing protocol was developed to guide the evaluation of female study participants. Six performance supplements were looked at: β-alanine, caffeine, creatine, glycerol, nitrate/beetroot juice, and sodium bicarbonate. (These specific supplements were included within the audit due to their “strong scientific evidence for use in specific situations in sport using evidence-based protocols.”) An electronic audit was then conducted with specific search terms applying to original research papers among human participants, published in English, and current to Sept. 1, 2021.
When the final articles meeting the criteria for the six supplements identified were included, 1,826 studies with 34,889 participants revealed just 23% of participants were women. In this same vein, only 34% of the studies included at least one woman as a participant. Women-exclusive studies were almost non-existent. While these numbers may sound dismal for females looking to rely on the evidence on these six supplements and swim-bike-run with it, the good news was that female-only studies did have seen an upswing over the previous eight years. However, even with this upturn, male-only studies (59-77%) are approximately nine times higher than the female-only cohorts.
The influence of the menstrual cycle
Various mechanisms, including altered muscle activation, substrate metabolism, and thermoregulation, have been proposed to change throughout the menstrual cycle, all of which could alter the potential outcomes of performance when testing a sports supplement. Research has suggested that greater strength and power may be exhibited when progesterone is lower during the late follicular phase (while estrogen peaks). These alterations in sex hormone concentration could allow for changes in force production, impacting muscle strength and power—two important testing areas for sports supplements. Hormonal fluctuations throughout the month undoubtedly make studying women more difficult, as this requires both menstrual status classification (normal or abnormal cycle length) and methodological control.
While a total of 614 studies included female participants, 86% did not attempt to classify participants’ menstrual cycle—making it impossible to know where they were in their cycle. In addition, 99.5% involved inadequate methodological design addressing the category of phase and standardization of menstrual status. The lack of inclusion of menstrual status can lead to substantial variations in the outcomes of the supplements examined. In essence, this suggests that current research lacks the ability to account for differing menstrual characteristics, making it more challenging to provide meaningful and credible recommendations to female athletes on the supplements examined.
Health versus performance outcomes
No supplement is worth taking if it is not safe or does not improve or maintain overall health. Considering a female-only design, research regarding these supplements were overwhelmingly focused on their ability to improve performance. However, absolutely no studies examined health outcomes. These numbers are concerning, considering that an athlete should never consider a supplement without an eye toward preserving overall health.
The bottom line
A perfect study design accounting for every individual variation is impossible in research. It is challenging to obtain an equal representation of participation types for various reasons, some of which are out of the researcher’s control. However, considering supplements are currently recommended by expert groups as evidence-based safe and effective products, female participants must be represented in more significant numbers in high-quality research. Sports science specialists with knowledge of female physiology must take the lead to adapt experimental designs using gold-standard techniques to these female-specific considerations, including the menstrual cycle, to develop evidence-based guidelines for both sports dietitians, coaches, and female athletes. As triathletes taking these supplements, it is essential to consider that the current evidence has typically been extrapolated to females and continue to support the companies and research institutions making sure women are included in these designs.
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Kim Schwabenbauer, PhD, RD, CSSD, LDN, is a former professional triathlete turned registered dietitian, professor, consultant, speaker and triathlon coach with an emphasis in overall health, wellness and sports nutrition.