Q: i am 14 and want to replace my fat with muscle how do i do it?

A: At your age it's actually quite easy to do as long as you have a little patience and discipline. Your body naturally 'wants' to be quite lean during teen years -- plenty of muscle and little fat. So as long as you work with your body and not against it, losing fat and gaining muscle is quite easy.

What it will take however is eating right (this is BY FAR the most important thing at your age), allowing a year or two for the transformation, and staying reasonably active in the meanwhile.

Eating right means eating as I describe in my Nutrition article. Please read that article carefully a couple of times to really get all of its points down. Do NOT worry about how much you eat -- at your age it's not crucial, just eat plenty in 3-4 meals daily -- but do pay attention to WHAT you eat. It's all described in the article I linked. Small changes in your diet make a huge difference over time. Eating a pop-tart and a lean pocket make for a horrible breakfast, while a bowl of old-fashioned oatmeal, strawberries, and cottage cheese make for a great one! A big bowl of green salad is awesome but if you dump half a cup of salad dressing on it it becomes next to junk food. So please make sure you learn what foods are good for you and eat only them!

Diet aside, simply stay active -- play sports, do pushups, pullups, sit-ups, jumping jacks, run, jog, ski, bike, swim, shovel snow, mow lawns, even clean around the house. If you like you can start on weight training with adult supervision but it's not absolutely necessary. That's it! Now you just need to stay patient and allow your body to do its thing. It won't be particularly fast but in 1-2 years you'll find yourself completely transformed. Just don't give up too early and be patient and disciplined. This 1-2 years of effort will pay huge dvidends the rest of your life. Good luck!