If you are one of the small percentage of people who actually need to gain weight, you don't want to do so by eating unhealthy food with lots of calories. Bulking up on junk food can be detrimental to your health, as these foods are also often high in saturated fat, trans fats and sugars. Instead, stick with healthy foods -- just eat them more often and in slightly larger amounts so you end up consuming more calories.
If you tend to feel full before you eat enough calories to gain weight, try eating foods that are more energy-dense but that are still healthy. Nuts, avocados, nut butter, seeds, olives and cheese are good examples, since although they contain both a lot of calories and a lot of nutrients, and, except for the cheese, contain mainly heart-healthy unsaturated fat instead of saturated fat. Add these foods to your snacks and meals to increase the calories without greatly increasing the volume of food. You can add both nutrients and calories by adding dried milk powder into cooked oatmeal, mashed potatoes, casseroles and soups.
Most of the foods you eat should be nutrient-dense foods, or foods that contain a lot of essential nutrients per a given amount of food. Fruits, vegetables, whole grains, eggs, low-fat dairy products, nuts and lean protein foods like seafood, poultry and beans are all nutrient-dense. These foods will help you make sure you get enough vitamins, minerals, protein and fiber in your diet and correct any nutritional deficiencies you might have from being underweight.
Instead of drinking water or non-caloric beverages, try drinking fruit smoothies. These provide you with a lot of nutrients and help you increase the amount of calories you are consuming in a healthy way. However, avoid drinking anything within half an hour of a meal, since beverages can fill you up so you don't eat enough at meals.
Being underweight makes it more likely you will eventually develop osteoporosis and can also cause anemia, a weakened immune system or difficulty becoming pregnant. Besides the foods you eat, other diet and lifestyle changes can make weight gain more likely, including eating more frequently and strength training to add muscle.
Based in Massachusetts, Jessica Bruso has been writing since 2008. She holds a master of science degree in food policy and applied nutrition and a bachelor of arts degree in international relations, both from Tufts University.