Researchers are only just beginning to understand how the sexes differ in utilising fat as an energy source during exercise. The body composition of females is almost identical to males until around puberty, at which point the pituitary hormones trigger an increase in production of estrogen in females and testosterone in males. Generally, this results in an increase in body fat for females and a larger body frame and greater muscle mass for males. Some people may see this as a disadvantage for women, but research is beginning to show that it has some advantages for female endurance athletes. In men and women, fat is stored all over the body as well as within the muscle in fat cells called adipocytes. Even though we have little or no control over which of these adipocytes decide to release their fat for energy during exercise, the latest research has found that women and men metabolise body fat differently during aerobic exercise. So why does fat seem to come off men so much easier than women? Originally, one study from Denmark found that even though men and women burn fat from the muscle at the same rate, it was the release of fat from fat cells that was less in women. However, a more recent study in the United States found that women metabolised more fat than men. They took 21 men and 21 women, all with similar fitness levels, and had them work out for 90 minutes a day at a moderate intensity level. Prior to this, they all ate the same controlled diet for eight days. The results revealed that the women on average had a 25 per cent decrease in muscle fat, while the men's intramuscular fat levels were relatively unaffected from the training. The researcher's explanation was the additional fat metabolised by women came from fat located in the muscle and not from fat deposited beneath the skin. Regardless of the differing results, both demonstrate that women and men metabolise fat differently. For those women who are not particularly active, this may not be the greatest of news, especially if weight and fat loss is their goal. But for women who exercise or train in endurance events such as long distance running, cycling or swimming, this ability to utilise intramuscular fat stores is a great asset since fat burning is vital to success in this area. Some examples of women demonstrating this natural ability to access fat stores in endurance events include Ann Trason, an ultra-marathoner who was placed second overall in 1995 in the Leadville 160km running race. Her time of 18 hours and 40 minutes was only six minutes behind the male leader. And the record for swimming the English Channel is held by a woman at seven hours and 40 minutes. The men's time is 32 minutes slower at eight hours and 12 minutes.