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