The Female vs. Male Athlete: Differences in Fuel Storage and Utilisation 

In this article, we will discuss the metabolic variations between males and females in fuel storage and utilisation, as well as what these differences may entail for female athletes. 

Females retain and use more fat in their skeletal muscle than males. 

When males and females are compared, it is obvious that fat storage differs, with males storing more fat around their midsection and females storing more fat around their hips and thighs. However, fat storage inside tissues vary across sexes, with females storing more fat in skeletal muscle. 

Fat cells (adipocytes) are found between muscle fibres within the muscle. This fat is commonly associated with poor muscle quality, inflammation, and insulin resistance. This is known as intermuscular fat. Inside the muscle cells, there is also fat. This fat, also known as intramucular triglycerides (IMTG) or intramyocellular lipids (IMCL; the term we'll use here), serves as a vital fuel source to meet energy requirements during activity. It is higher in athletes, and the fat is stored just adjacent to the mitochondria, where it may be used for energy. Women not only have more IMCL stored in their muscles, but their muscles are also predisposed to utilise fat as a fuel source during exercise. 

Females rely on fat stores (adipose triglycerides and/or IMCL) to meet exercise energy needs more than males, according to research. This suggests that when women exercise, they save carbohydrate (liver and/or muscle glycogen) resources. Importantly, glycogen depletion is linked to weariness. So, from a performance standpoint, females' slower glycogen depletion during exercise corresponds to females being more fatigue resistant and better able to maintain performance over longer-duration activities. 

Estrogen regulates differences in fuel utilisation during exercise. 

According to research, the sex hormone oestrogen regulates fuel storage and usage during exercise. Giving oestrogen to guys for 8 days to mimic female oestrogen levels 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 amount of proteins related to fat metabolism inside skeletal muscle. These findings have also been reported in animal research, where oestrogen administration to male and oophorectomized female rats (the ovaries were removed) raised IMCL content, decreased reliance on glycogen stores, and improved performance during strenuous exercise. Furthermore, fuel storage and utilisation during exercise vary throughout the menstrual cycle, with muscle glycogen stores being higher and reliance on lipids being higher during the luteal phase of the menstrual cycle when oestrogen levels are high. 

What implications do these differences in fuel storage and metabolism have for female athletes? 

The answer at this point is that we don't fully understand. While we have a good understanding of sex and menstrual-phase differences in fuel storage and metabolism, we don't know how (or whether) these differences relate to female athlete performance and optimal nutritional strategies. So far, it has been documented that exercise performance may be modestly impaired during the menstrual cycle's early follicular phase. However, as the authors point out, these findings should be interpreted with caution due to high trial variability and the inclusion of low-quality research in the analysis. 

Furthermore, while recent evidence-based recommendations for energy, carbohydrate, and protein intake specific to the female athlete have been made, further work in this area is required to establish the efficacy of sports supplements and optimise nutrition across the menstrual cycle. Another important factor to consider 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 result in inappropriate exercise and nutrition recommendations, necessitating a more personalised approach. It is also feasible that the differences between males and females, as well as between different phases of the menstrual cycle, are minor in comparison to the day-to-day differences required for nutrition periodisation. 

Conclusion 

There are well-documented gender and menstrual phase differences in fuel storage and consumption that are regulated by oestrogen. However, due to a lack of high-quality studies involving female participants, it is unclear if female (and menstrual phase)-specific activity and nutrition guidelines are necessary. 

The most important message is that there are metabolic differences that are statistically significant. It needs to be determined whether these variations are significant enough to have an influence on exercise performance. There is currently little evidence to suggest that nutrition recommendations should differ for men and women, or that they should change during the menstrual cycle.