Janice,
In a nutshell, you could be eating enough to maintain scale weight, but have body comp be slowly shifting to a decreased muscle mass and increased fat weight. How? When the body gets glycogen depleted, as is the case after intense exercise or prolonged periods without food--we're talking hours here, not days--it will make glycogen from other sources, primarily protein. The protein can come from dietary sources or from lean muscle tissue. If it comes from the latter, over time, this leads to a loss of muscle and an increase in fat storage without any change in body weight.
For the person who isn't wanting to exercise at all and is on what is basically a starvation-type diet, she is losing muscle mass at a high rate. Her eating habits are so poor, she may be looking at more than a loss of skeletal muscle--remember, the heart is a muscle, too, and extreme dieting can lead to heart damage. She's on a yo-yo diet from hell and without exercise, it's even worse.
There is no particular magic number in terms of duration for cardio exercise and muscle catabolism. Highly fit people have much greater glycogen stores in the muscles than untrained people. People who eat regularly and don't exercise on an empty stomach will be able to exercise longer and/or more intensly before becoming glycogen depleted. The duration and intensity will be inversely proportional, though--training that is intense will go through the glycogen/glucose supply much faster than lower intensity exercise.
In the case of the triathletes, they have several things working in their favor--their greatly enhanced glycogen storage capacity, their higher VO2 max, and their tremendous increase in fat burning enzymes--as a result of their training. Still, excluding injury,the limiting factor in marathoners and triathletes overwhelmingly is glycogen depletion--aka "hitting the wall" or "bonking". Carb loading and replenishing during the race helps.
On the other hand, most ultra endurance athletes lose muscle mass both during their distance training and the event. But, their primary concern is winning their event--not necessarily preserving lean muscle mass--so if some muscle has to be sacrificed in order to make glucose, if it lets them finish the race when they otherwise would have bonked, it's worth it to them. This is why most of the endurance athletes appear defined, but very stringy. IMHO, these people overtrain on an ongoing basis--the amount of overuse injuries I treat in this population is very high--much more so than in most any other sport.
Does this help?
Maribeth
