Approximately 2 percent of people in the United States are underweight, and some of these people have trouble gaining weight no matter what they try. You could be one of these people, or you could be underestimating how many calories you're burning during the day or overestimating how much you're really eating. The key to gaining weight is eating more calories than you burn through your daily activities. Add about 500 calories to your typical daily intake to gain weight at a healthy rate of 1 pound per week, which will make it more likely you gain muscle and not fat.
You May Be Overestimating the Calories You Eat
If you eat a lot of foods that are low in energy density, or calories per gram, it's possible to fill up without eating a lot of calories. Fruits, vegetables, broth-based soups and salads consisting mainly of vegetables would all fall into this category. You may want to increase the energy density of the food you're eating to help you consume more calories and eat higher-calorie foods toward the beginning of the meal instead of the end so you don't fill up on lower-calorie foods first. While it may be tempting to turn to high-fat sweets and junk food, this could lead to other health problems. It's better to increase calories by eating more healthy foods, such as whole grains, nuts, seeds, dairy products and healthy fats. Dried fruit will give you more calories in a concentrated package than fresh fruit.
Dietary Changes for Weight Gain
Keeping a food and exercise journal, either on paper or online, can help you figure out about how many calories you're actually eating and estimate how many calories you're burning during the day. You can use it to figure out what changes you can make to help increase your caloric intake and gain weight. Try eating more often and drinking between meals so liquids don't fill you up at mealtime.
Opt for nutrient-dense, high-calorie beverages, such as whole milk, smoothies or 100 percent fruit juice, instead of water or other noncaloric beverages, but skip the unhealthy options like soda. Protein-rich foods are also great choices, including lean meat, seafood, legumes and poultry, as you need adequate protein to build more muscle as you exercise. Add calories and healthy fats to foods by drizzling roasted veggies with olive oil, spreading nut butter on apples or bananas or adding avocado to sandwiches. Include calorie-dense whole grains -- whole grain bread, cereals or pasta -- and starchy vegetables, such as corn or sweet potatoes, at each meal and snack.
You May Be Underestimating Your Calorie Needs
A man needs about 14 or 15 calories per pound to maintain his weight if he's sedentary -- or about 18 calories per pound if he's very active. Women need fewer calories, with a sedentary woman needing just 12 or 13 calories per pound and about 16 if she's very active. A very active person exercises daily at a fairly vigorous level of exertion, which means you're working too hard to hold a conversation.
You also burn calories during your daily activities, so if you have a job where you walk around a lot, it could add significantly to your calorie needs. For someone who weighs 155 pounds, an hour of desk work burns about 130 calories, an hour of coaching sports burns about 298 calories and an hour of carrying boxes burns about 520 calories. Working as a police officer or bartender burns about 186 calories an hour, construction work burns about 410 calories an hour and a firefighter burns about 892 calories an hour. These "hidden" calorie-burning activities can add up and mean that you need to eat more to gain weight. Keep in mind that these are just estimates, and if you have trouble gaining weight due to a high metabolism, it may take some trial and error to determine just the right caloric intake for weight gain.
Exercise and Weight Gain
If you do a lot of cardio, you may be burning too many calories to gain weight. People trying to gain weight may want to concentrate more of their efforts on resistance training, which will help you put on weight in the form of muscle. An hour of running at a pace of 5 miles per hour burns about 606 calories for a person who weighs 160 pounds, which is way more than the 365 calories the same person would burn in an hour of resistance training. Try to fit in three or four resistance training workouts each week on alternating days and concentrate on doing fewer repetitions with higher weights to maximize increases in muscle mass.
A documentary on the BBC followed 10 naturally slim people as they tried to double their usual caloric intake and greatly limit their activity levels for four weeks and found that although some of the people gained significant amounts of weight, others did not. This weight was usually put on in the form of fat, but one person ended up putting on weight mainly as muscle. There's a lot of genetic variation in how people put on weight, with about 50 percent of weight being determined by genetic factors and about 50 percent by environment, according to Dr. Rudy Leibel of Columbia University. Researchers are still trying to determine just what makes some people gain more weight than others given the same conditions, although one theory is that some people have more growth hormone than others, causing them to use more energy through their daily activities.
Medical Issues Limiting Weight Gain
Check with your doctor if you're having difficulties gaining weight, as it could be due to certain health conditions or medications. An overactive thyroid, undiagnosed diabetes or chronic digestive issues could be at least partly responsible for the problem, although you could also just have a very high metabolism that makes it difficult to gain weight. If there's an underlying health condition, getting that condition treated could make it easier for you to gain weight again.