obesity

Anyone who has studied basic human metabolism should be able to figure this out. Exercise alone simply is a bad way to lose weight.

The total energy expenditure (TEE) of the human body is determined by the following equation:

TEE = BMR + PA + TEF

BMR = Basal metabolic rate
This is proportional to the lean body mass, not the BMI (which is a really bad measure of obesity). This is typically 60 - 70% of your TEE

PA = Energy expended during physical activity
This consists of around 20% of your energy expenditure

TEF = Thermic effect of food
This is the energy expended to digest food, typically 10% of kcal's consumed. This really doesn't really come into play in weight gain since eating more food still gives you excess calories (albeit at 90%) and eating less is still fewer calories.

In other words, the majority of your energy expenditure is determined by your basal metabolic rate by a ratio of around 3.5 to 1. This is especially true in children whose BMR's are naturally higher than most adults'. This is not to say that exercise isn't useful. BMR is determined by lean body mass, which is determined by your muscle mass, which is determined by genetics and exercise. Exercise does help you lose weight, but it takes a lot longer than diet. Exercise also has independent benefits on cardiovascular health and a host of other health measures.

So all those people who tell you that losing weight is 80% diet and 20% exercise aren't lying. That's simply the science.