Gender Differences in Carb/Fat use During Endurance Exercise...Females may have an Advantage..

I'm just going to quote my sports nutrition book here:

"Available data support the notion of gender differences in carbohydrate metabolism in exercise.  During submaximal (steady state) exercise at equivalent percentages of VO2max (or same relative workload), women derive a SMALLER proportion of total energy from carbohydrate metabolism than men.  This gender difference in substrate oxidation does not persist into recovery."  Interesting.  So, women rely more on fat than tap into their carb stores than men, which is why females may be able to run/bike/swim longer, in GENERAL, not everyone.  And, especially once you start getting into very long distances.

And looking into the physiology further..." With similar endurance-training protocols, both women and men show a decrease in glucose flux for a given submaximal power output.  But at the same relative workload after training, women display an exaggerated shift toward FAT catabolism (fat breakdown), whereas men do NOT.  This suggests that endurance training induces greater glycogen-sparing at a given relative submaximal exercise intensity for women than for men.  This gender difference in substrate metabolism's response to training may reflect differences in sympathetic nervous system adaptation to regular exercise (i.e. a more blunted cholamine response for women).  The sex hormones estrogen and progesterone may affect metabolic mixture indirectly via interactions with the catecholamines or directly by augmenting lipolysis (fat breakdown) or glycolysis (carb break down).  Five potential sites for endocrine regulation of a substrate (fat/carbs) use include:

1.  Substrate availability (via effects on nutrient storage)
2.  Substrate mobilization from body tissue stores
3.  Substrate uptake at tissue site of use
4.  Substrate uptake within tissue itself
5.  Substrate trafficking among storage, oxidation, recycling

Any glycogen-sparing metabolic adaptations to training could benefit a woman's performance during high-intensity endurance competition."

Wow, cool!  So, men are more powerful and stronger than women, but it seems women can last longer when it comes to endurance events, especially ultra's.  I have been searching around for research to back up that theory I have been reading about over the course of 2 years. 

From the book Sports and Exercise Nutrition by William D. McArdle, Frank L. Katch, and Victor L. Katch.