For those of you who would like to gain weight and maybe some strength besides here's the best approach.
First of all, realize that a gain of 1/2 to 1 pound a week no more is about right. Next, count calories for three days or so to get your average daily intake. This will give you a rough idea of how many calories you require to maintain your current weight.
According to sports nutritionist Ellen Coleman, you need about 2,500 calories more per week to gain 1 pound of muscle mass. (This figures to around 350 more calories a day.)
To achieve this, Coleman suggests eating larger portions of foods you already eat, or adding a midmorning, midafternoon, or bedtime snack. As far as what to snack on, foods such as cereals and grains are good for you, but they're relatively bulky and low in calories. So try a cup of nonfat milk, a skinless chicken breast, or a high-protein drink instead.
Finally, to ensure that what you gain is muscle weight and not fat, Coleman suggests you begin a weightlifting program, too. If you take this advice, be sure to add to your total caloric intake about 100 calories per half-hour of weight training.
With added muscle weight, you'll be stronger, you'll look better, and you may run better, too.