Carbohydrates and fat are the primary energy sources for ATP (energy) production and muscle contraction during endurance exercise. However, their relative contribution is not equal and varies depending on the intensity and duration of the exercise. Additional parameters may also influence carbohydrate and fat metabolism such as: genetic factors, VO2 max, training load, activity level, body weight, body fat percentage, diet, age and sex. Interestingly, sex related metabolic differences during exercise have been studied in the past but only recently gained increased research attention.
In general, women bear a higher percent of fat mass and display smaller muscle fiber areas compared to men . Women also tend to store fat around the hips and thighs, whereas men are prone to storing fat around the stomach. In-vitro adipose tissue biopsy data has shown that women display differences in lipolytic activity between upper body and lower body adipose tissue compared to men . In other words, women burn fat differently than men. On the same matter, men’s upper body burns fat more efficiently than women. .
In the case of endurance athletes, such general observations of sex differences in fat deposition, fat oxidation and muscle fiber composition may not apply directly to exercise. Nonetheless, twin studies have shown that male and female differences in body size, shape and composition are equally influenced by heritability, lifestyle and activity level [3, 4].
Moreover, genetic and environmental factors not only affect body composition but also coordinate a whole milieu of metabolic regulators such as pancreatic and liver functionality, digestion ability, the orchestration of the nervous system, and metabolic related hormonal activity. Hormones are considered master regulators of multiple organs that participate in metabolism. Therefore, fundamental sex hormonal differences between male and female play a pivotal role in the use of fat versus carbohydrate during given efforts and the storage of energy in the body. .
In men, testosterone has a major influence on body fat composition and muscle mass. For example, testosterone deficiency is associated with an increased fat mass, reduced insulin sensitivity, impaired glucose tolerance, elevated triglycerides and cholesterol and low HDL-cholesterol . On the other hand, estrogen, estradiol and well known hormonal fluctuations in pre-menopausal women influence the metabolic activity during the menstrual cycle. Evidence shows that the effects of estrogen and estradiol on metabolism may be regulated via the effect of estrogen on glucogenic hormones (those associated with using carbohydrates for energy) or lipolytic enzymes (those associated with breaking down fats) [7-9].
Specific research on sex differences in endurance athletes has been challenging in the past (in terms of normalizing the subjects on genetic, physiological and psychological parameters for meaningful comparisons) and therefore, the results have been somehow contradicting as evidenced by this recent review: ). However, present research has been more consistent and supports evidence that during low intensity efforts women oxidize more fat, less carbohydrates and less amino acids compared to men [8, 11-16]. Although the exact biochemical and regulatory pathways of these findings are yet to be clarified and further research has yet to be conducted, it is more often supported that during the same relative intensity of endurance exercise, women rely less on liver and muscle glycogen and display a greater capacity of utilizing fat compared to men [10, 12, 14-17]. Additionally, there have been reported sex differences of the nervous system responses in catecholamines during endurance exercise (such as epinephrine, norepinephrine and dopamine) [18-20]. These central nervous system differences affect stress levels, preferred use of fuel and mood during exercise and may be a key difference between sexes.
These recent results may initiate the importance and reasoning behind sex related different exercise regimes, different responses to altered nutritional habits (e.g. women may have inherited ketogenic responses to exercise and therefore, ketogenic diets could be less effective ), and perhaps different responses to supplementation  (e.g. effects of protein breakdown during creatine supplementation [12, 22, 23]). One might summarize that at a given weight and effort level women would need less carbohydrates than their male counterparts. Because women look to be more efficient at burning fat they may be better suited for ultra-long distance racing.
The above observations further initiate a more scientifically based reasoning behind the ability of women to perform and occasionally out-perform their male competitors in endurance and ultra-endurance events. Could the above findings shift the nutritional interest and supplementation towards more male and female specific products or simply a shift in how products are used? The future of research on the matter looks promising and we anticipate novel results in the not so far forthcoming.
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