Losing, gaining or staying at the same weight all depend on how many calories you eat and how many calories your body uses over time. If you eat more calories than you use, you will gain weight; conversely, if you eat fewer calories than you use, you will lose weight. The Academy of Nutrition and Dietetics’ Healthy Weight Gain webpage provides some information and advice on how to gain weight and remain healthy.
For Tips on Gaining Weight, visit FNIC's General Nutrition and Health Information page. Because many Americans are overweight, there are many resources geared toward losing weight. Some of these resources explain the principles of weight balance and can provide guidance for you to gain weight in a healthy manner; you will just need to focus on portion sizes for weight gain, rather than weight loss. One such resource is Maintaining a Healthy Weight On The Go from the National Institute of Health’s National Heart Lung and Blood Institute.
If you would like personalized advice, or you want to know how many calories or what types of foods are best for you, Registered Dietitians (RD) are health professionals who can physically assess you and your needs. In the United States, the Academy of Nutrition and Dietetics has a referral service to registered dietitians. You can find a dietitian in your area by using the Find a Registered Dietitian referral service on their website.