In a previous blog post, Dr. Kirsty Elliot-Sale nicely highlighted that differences in anatomy, physiology and psychology between males and females make it unclear whether females need different exercise and nutrition advice than their male counterparts. In this blog we will highlight the metabolic differences in fuel storage and utilization between males and females and what these differences may mean for female athletes.
Females store and use more fat in skeletal muscle
Looking at males and females it is clear that fat storage differs between the sexes with males storing more fat around their mid-section and females storing more fat around the hips and thighs. However, storage of fat within tissues also differs between the sexes with females storing more fat within skeletal muscle.
Within the muscle we find fat cells (adipocytes) in between muscle fibres. This fat is usually linked to lower quality muscle, inflammation and insulin resistance. This is called intermuscular fat. There is also fat inside the muscle cells. This fat, known as intramucular triglycerides (IMTG) or intramyocellular lipids (IMCL; the abbreviation we will use here), serves as an important fuel source to support energy needs during exercise. It is actually higher in athletes and the fat is stored immediately next to the mitochondria where it can be used for energy. Not only do women store more IMCL within their muscles, but their muscles are also primed to use fat as a fuel source during exercise.
In fact, research has consistently shown that during exercise females rely on fat stores (adipose triglycerides and/or IMCL) to support exercise energy needs more than males. This means that during exercise females are sparing carbohydrate (liver and/or muscle glycogen) stores. Importantly, depletion of glycogen stores is tied to fatigue. So from a performance perspective the slower depletion of glycogen stores by females during exercise translates to females being more fatigue resistant and better able to maintain performance during longer-duration events. (Also read this blog: Will women soon outperform men in ultra endurance events).
Female athletes have a greater reliance on fat as a fuel source during exercise compared to males, which may aid performance in longer-duration events due to sparing of muscle glycogen.
Differences in fuel use during exercise are mediated by estrogen
Research has shown that fuel storage and utilization during exercise is mediated by the sex hormone estrogen. Giving estrogen to males for 8 days so that estrogen levels were similar to that found in females during the luteal phase of the menstrual cycle, resulted in males relying more on fat stores during a bout of moderate-intensity cycling and an increase in the content of proteins related to fat metabolism within skeletal muscle. These findings have also been reported in animal studies where estrogen administration to male and oophorectomized female rats (having had the ovaries removed) increased IMCL content, reduced reliance on glycogen stores and improved performance during exhaustive exercise. Furthermore, fuel storage and use during exercise also fluctuates across the menstrual cycle with muscle glycogen stores being higher and reliance on fats being greater in the luteal phase of the menstrual cycle when estrogen levels are high.
What do these differences in fuel storage and metabolism mean for the female athlete?
At this point the answer is that we don’t really know. In a recent editorial we highlighted how little research has been conducted in females (4). While we have a clear understanding of sex- and menstrual-phase differences in fuel storage and metabolism, we do not yet understand how (or if) these differences may relate to performance and optimal dietary strategies for the female athlete. What has been reported thus far is that exercise performance may be slightly reduced during the early follicular phase of the menstrual cycle. However, as noted by the authors, these findings need to be interpreted with caution due to large between trial variability and the inclusion of poor-quality studies in the analyses.
Furthermore, while advancements have been made in the realm of sports nutrition with recent evidence-based recommendations being made for energy, carbohydrate and protein intake specific to the female athlete, more work in this area is required to determine the efficacy of sports supplements and optimize nutrition across the menstrual cycle. One further important consideration is that due to large differences in ovarian hormone concentrations between females and cycles, simply considering the female athlete based on the day of their menstrual cycle may lead to inappropriate exercise and nutrition recommendations and thus a more personalized approach must be considered. It is also possible that the differences between males and females, and between different phases of the menstrual cycle, are relatively small compared to the day to day differences that are required for periodisation of nutrition.
Summary
There are well established sex- and menstrual phase-based differences in fuel storage and utilization that are mediated by estrogen. However, due to a paucity of high-quality research involving female participants whether female (and menstrual phase)-specific exercise and nutrition recommendations are required remain unknown.
Maybe the most important takeaway is that there are differences in metabolism that reach statistical significance. Whether these differences are large enough to have a meaningful effect in the context of exercise performance remains to be seen. At this time there is insufficient evidence to suggest that nutrition recommendations should be different for men and women, or should change in different phases of the menstrual cycle.
References:
Devries MC, Sex-based differences in endurance exercise muscle metabolism: impact on exercise and nutritional strategies to optimize health and performance in women. Exp Physiol, 2016; 101(2):243-9.
Beaudry KM, Devries MC. Sex-based differences in hepatic and skeletal muscle triglyceride storage and metabolism. Appl Physiol Nutr Metab, 2019; 44(8):805-813
Moore DR, Sygo J, Morton JP. Fuelling the female athlete: Carbohydrate and protein recommendations. Eur J Sport Sci, 2021; 20:1-13 epub ahead of print.
Devries MC, Jakobi JM. Importance of considering sex and gender in exercise and nutrition research. Appl Physiol Nutr Metab, 2021; 46(6):iii-vii.
McNulty KL, Elliot-Sale KJ, Dolan E, Swinton PA, Ansdell P, Goodall S, Thomas K, Kicks KM. The effects of menstrual cycle phase on exercise performance in eumenorrheic women: A systematic review and meta-analysis. Sports Med, 2020; 50(10):1813-1827.
