Unfortunately, it is safe to say that scientists know a lot less about the energy demands of muscle gain than they do about fat loss. We know that if you were to overeat 3,500 calories you should gain close to a pound (0.45 kg) of body fat, but most athletes are not looking to gain body fat. Furthermore, as mentioned earlier, there is considerable variation in the amount of weight that would be gained by consuming the extra 3,500 calories. Some would gain almost exactly one pound (0.45 kg), and others, because they can “waste” a higher percent of the excess energy they consume, would gain a bit less. 

Gaining muscle, however, is somewhat more complex. One pound of muscle contains quite a bit less energy than fat—about 2,500 calories compared to 3,500 calories—but additional energy costs are required to promote gains in lean tissue. These include the cost to run the body’s protein-making machinery, to arrange the amino acids consumed in the diet into the proper sequence, and to perform the resistance training that stimulates muscle gain. 

Thus, as much as I hate to say it, the caloric recommendations for weight gain are not based on a lot of hard science. Typically, to keep things simple and allow for a fudge factor of some sort, most practitioners use a figure of 3,500 calories to represent the cost of gaining one pound of muscle tissue.We do, however, know quite a bit about the potent stimuli for promoting muscle growth, often called anabolism, which includes feeding (as compared to fasting) and resistance training. Although feeding alone is not sufficient to induce muscle growth—otherwise most people in the Western world would look like professional bodybuilders—it supplies the amino acid building blocks and elevates many anabolic hormones including insulin. 

Insulin is important because it helps muscles take up and use amino acids for growth. Resistance exercise, as you know, provides the stimulus for muscle hypertrophy by promoting protein synthesis, which may be elevated for up to 48 hours following just a single bout of strenuous exercise. Eating in close association with resistance exercise is important because protein breakdown also increases after exercise, and food intake helps the body make more muscle than it degrades. Thus, the timing of nutrient delivery relative to the bout of exercise is also important. The general thinking is that feeding should occur about 20 to 30 minutes after exercise.