People who have a high metabolism may have trouble gaining weight. While this condition is less common than being overweight, it can be just as harmful. Underweight individuals may be subject to vitamin and mineral deficiencies or even lose bone and muscle tissue. If you've been unable to maintain a minimum desired weight, it's wise to get a checkup from a doctor. Various medical conditions, such as an overly active thyroid, can cause weight loss or make weight gain difficult. Such problems should be ruled out before making dietary or lifestyle changes. Assuming your health is normal, you'll simply need to increase the number of calories you eat. For healthy weight gain, add nutritious foods from all five food groups, limiting items that are high in sugar and fat. When you do eat fat, choose monounsaturated oils like olive and canola oil which are better for your heart. Eat three good-sized meals a day and have healthy snacks in between. You might also try concentrated-calorie milkshakes that derive their most of their calories from carbohydrates. Don't give up exercise just to speed your weight gain; exercise has numerous health benefits. Instead, consider adding weight training workouts to build muscle and help fill out your body.