This is an excerpt from Physiology of Sport and Exercise 7th Edition With Web Study Guide-Loose-Leaf Edition by W. Larry Kenney,Jack Wilmore & David Costill.
Interaction Between Resistance Training and Diet
Muscle hypertrophy in response to resistance training can be either limited or enhanced by nutrition. As mentioned previously, a net positive protein balance (more synthesis than breakdown) is the necessary condition under which muscle hypertrophy occurs. Without adequate protein in the diet, protein synthesis is compromised and muscles cannot increase their protein content and hypertrophy. Ingesting protein within a few hours after a bout of resistance exercise increases the rate of protein synthesis and thus adds to the net positive protein balance. Increased protein intake over the subsequent 24 h period will continue to support muscle anabolism. Therefore, nutrition and exercise are powerful stimulators of skeletal muscle protein synthesis.5
Recommendations for Protein Intake
An international group of researchers recently performed a systematic analysis of 49 published studies (1,863 subjects) to determine if dietary protein supplementation enhances the gains in muscle mass and strength with resistance training.15They surveyedrandomized controlled trials in which subjects performed resistance training for at least 6 weeks and took various amounts of dietary protein supplementation.Their analysisof this large sample showed that dietary protein supplementation significantly enhanced changes in strength as measured by 1-repetition maximum tests and muscle size (fiber cross-sectional area and whole-muscle cross-sectional area). However, protein intakes greater than ~1.6 g/kg of body weight per day did not further contribute to these gains. So, although the current U.S. Dietary Reference Intake (DRI) for protein for people over 18 years of age, regardless of physical activity status, is 0.8 g/kg per day, athletes engaged in resistance training may require protein intakes in the diet as high as 1.7 g/kg per day. Although ingestion of relatively small amounts of protein (5-10 g) can stimulate muscle protein synthesis in young men and women, to make muscles larger, one should consume larger amounts of protein, on the order of 20 to 25 g, immediately after resistance exercise.1
What type of protein should be ingested and how much? The best forms of protein for muscle hypertrophy are easily and rapidly digested and rich in essential amino acids, especially leucine. Whey protein found in milk is one source that meets both of these goals. In practice, after resistance training, athletes should consume a small amount of high-quality protein along with adequate carbohydrate in order to stimulate muscle proteins and replenish muscle glycogen stores after exercise. This can be accomplished with either a recovery beverage or foods such as milk or yogurt, a small sandwich, or a protein-rich energy bar. Adding carbohydrate to postexercise protein ingestion does not markedly affect muscle protein balance but does have other benefits, including aiding in the resynthesis of muscle glycogen.
Is there an optimal timing of protein ingestion when an individual is trying to optimize the hypertrophic response to successive exercise sessions? A single bout of exercise stimulates muscle protein synthesis rates for several hours, and intake of protein further enhances postexercise muscle protein synthesis. The protein synthesis-stimulating effect of a single dose of amino acids is transient and lasts only 1 to 2 h. Ingesting repeated small doses of protein during recovery from resistance training may be more effective in increasing muscle hypertrophy than eating just one large meal. However, elevated muscle protein synthesis rates are not totally limited to the few hours of acute postexercise recovery. The so-called window of opportunity lasts from just before the start of resistance exercise to several hours postexercise. Providing protein before or during exercise can enhance muscle protein synthesis during exercise and is a good strategy for prolonged or repeated workouts.