The more protein your body stores—in a process called protein synthesis—the larger your muscles grow. But your body is constantly draining its protein reserves for other uses—making hormones, for instance. The result is less protein available for muscle building. To counteract that, you need to "build and store new proteins faster than your body breaks down old proteins.
Eat Meat
Shoot for about 1 gram of protein per pound of body weight, which is roughly the maximum amount your body can use in a day, according to a landmark study in the Journal of Applied Physiology. (For example, a 160-pound man should consume 160 grams of protein a day—the amount he'd get from an 8-ounce chicken breast, 1 cup of cottage cheese, a roast-beef sandwich, two eggs, a glass of milk, and 2 ounces of peanuts.) Split the rest of your daily calories equally between carbohydrates and fats.
Eat More
In addition to adequate protein, you need more calories. Use the following formula to calculate the number you need to take in daily to gain 1 pound a week. (Give yourself 2 weeks for results to show up on the bathroom scale. If you haven't gained by then, increase your calories by 500 a day.)
A. Your weight in pounds.
B. Multiply A by 12 to get your basic calorie needs.
C. Multiply B by 1.6 to estimate your resting metabolic rate (calorie burn without factoring in exercise).
D. Strength training: Multiply the number of minutes you lift weights per week by 5.
E. Aerobic training: Multiply the number of minutes per week that you run, cycle, and play sports by 8.
F. Add D and E, and divide by 7.
G. Add C and F to get your daily calorie needs.
H. Add 500 to G. This is your estimated daily calorie needs to gain 1 pound a week.