If you train for a marathon or triathlon, surely your body fat will melt away. Correct?
Wishful thinking. If you are an endurance athlete who complains, “For all the exercise I do, I should be pencil-thin,” take a look at your 24-hour energy expenditure. Do you put most of your energy into exercising, but then tend to be quite sedentary the rest of the day as you recover from your tough workouts?
A study with of male endurance athletes who reported a seemingly low calorie intake indicates they did less spontaneous activity than their peers in the non-exercise parts of their day. Yes, it's really easy after a long run to lounge around and eat bon-bons because you "deserve" them...
Even when you are marathon training, you need to keep taking the stairs instead of the elevators, and keep moving in non-exercise parts of your day. Again, if weight is an issue, you should eat according to your whole day's activity level, not according to how hard you trained for an hour or two that day.
For more info:
Thompson, J., M. Manore, J. Skinner, E. Ravussin, M. Spraul.Daily energy expenditure in male endurance athletes with differing energy intakes. Med Sci Sports Exerc27::347-54, 1995.