Hormonal
During mild-to-moderate intensity exercise (40% VO2max), women oxidize a significantly higher proportion of fat and a lower proportion of carbohydrate compared to men, a difference driven by greater lipolytic sensitivity to catecholamines despite lower absolute catecholamine levels in women.
If you are a woman exercising at a moderate intensity (where you can hold a conversation), your body is likely prioritizing fat burning more efficiently than a man doing the same relative effort. This is due to hormonal differences, specifically how your body responds to stress hormones like epinephrine. This doesn't mean you should avoid carbs, but it does mean your fuel utilization strategy during endurance activities differs from men's.
During exercise, women derived proportionally more of the total energy expended from fat oxidation (50.9 +/- 1.8 and 43.7 +/- 2.1% for women and men, respectively, P < 0.02), whereas men derived proportionally more energy from carbohydrate oxidation (53.1 +/- 2.1 and 45.7 +/- 1.8% for men and women, respectively, P < 0.01)... As circulating glycerol levels were not different between the two groups, this suggests that women may be more sensitive to the lipolytic action of the catecholamines.
Why this rating
Randomized controlled trial with rigorous controls (control day, matched training status, controlled diet), though sample size is moderate (n=27 total).
Source
Fuel metabolism in men and women during and after long-duration exercise
Tracy J. Horton et al. · Journal of Applied Physiology · 1998
DOI 10.1152/jappl.1998.85.5.1823
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