Reply #3: Since you're young and active, you may have thyroid or insulin resistance

[AllieB]

At your age, you should be effectively burning calories just by the activity you described. Make sure you have your thyroid checked to rule out metabolic issues. Allergies to grains, gluten, and/or dairy can also make people gain weight. You may want to try a low-carb, high-protein diet to rule out insulin resistance. Cutting calories and fat don't necessarily help people with insulin resistance.

Carbohydrates, aside from the indigestible ones, all break down in the body into straight sugar. Most people's body's run off "blood sugar" to function. Carbohydrates cause an insulin spike when ingested relative to the amount eaten and also relative to how fast they break down. For instance a honey bun will break down into glucose much faster than spinach. Insulin is the hormone responsible for driving this sugar into the body's cells for use. This is how the body begins storing fat. When you eliminate carbs, or at least are very restrictive of them, your body switches its system to burning fat for fuel instead of sugar. Carbohydrates are turned into mostly palmitic acid (a saturated fat) which is what is stored mostly in your belly. This is why saturated fat and cholesterol are being blamed for coronary heart disease, when really they are innocent.

Some books to look at:

Life Without Bread
Protein Power
Protein Power Lifespan Program
Dr. Atkins Diet Revolution
Atkins for Life
Dr. Atkins Diabetes Revolution
Dr. Bernstein Diabetes Solution
Dr. Bernstein Diabetes Diet
Dangerous Grains
Neanderthin
The Paleo Diet