Question

Why might a person eat fewer servings than recommend by the Food Guide Pyramid and still gain an unhealthful amount of weight

Answer 1

They may have a slow metabolism because they less than they should.

To make up for the lack of calories, your body's metabolism slows, which means it holds on to the calories you do eat for energy.

Answer 2

Because they are eating the nonhealthy foods instead of the healthy foods and when you tend to not eat as much food as you should  your body is eating away well really it stores trans-fat that you dont really need.