Even though there does not seem to be a lot of research available on why some people should be underweight, or why they have difficulty gaining weight, the main reason seems to be the reverse of why people become overweight.

Underweight people tend to eat less calories and also tend to exercise more. Just like many overweight people eat more than they think, many underweight people eat less than they think.

Healthy Weight Gain Diet

The key to healthy weight gain is to eat calorie-dense, but healthy foods. A high-fat diet of junk foods or high in saturated fat is not recommended and will eventually lead to poor health and diet habits.

Sample Weight Gain Diet

The average normal calorie requirement for adult women is approximately 1900 calories per day.

This means the optimum amount to help most underweight women gain weight is about 2400 calories. For a healthy sample weight gain diet plan to help you put on weight, see Weight Gain Diet 1 or Weight Gain Diet 2.