Muscle mass in a non-exercising adult stays relatively constant because muscle growth, or skeletal muscle protein synthesis (SMPS), is balanced out by muscle loss, or skeletal muscle protein breakdown (SMPB). Muscle gain occurs when growth (SMPS) exceeds loss (SMPB). Conversely, muscle loss occurs during catabolic periods when breakdown exceeds synthesis.
Weight lifting exercise causes muscle synthesis to exceed muscle loss. For up to at least three hours following resistance exercise, muscle growth rates are two- to four-fold greater than muscle loss. Combined with proper nutrition, repeated resistance exercise can lead to muscle gain over time.
Amino acids, the building blocks of protein, are necessary for adding muscle mass following exercise. Blood flow to muscles is elevated in the hours after exercise, accelerating the delivery of amino acids. The three-hour period following a workout is referred to as the "anabolic window" — where amino acids stimulate additional muscle synthesis. Research shows weight training and protein intake during the anabolic window improve muscle gain. More importantly, failure to feed exercised muscles during this recovery window can actually lead to muscle loss.
Protein digestion rates, which vary among protein sources, also impact muscle growth. Dairy proteins are absorbed rapidly (whey) or slowly (casein), while protein absorption from soy tends to be intermediate. Whey protein quickly stimulates synthesis and opens the "anabolic window," and soy protein potentially extends the time the window is maximally open. Casein prolongs the delivery of amino acids to muscles and keeps the window from closing.
Consuming a combination of proteins with different digestion rates may enhance lean body mass gains by promoting longer periods of muscle growth after exercise. Scientific evidence is rapidly changing the way nutritionists and exercise scientists now think about protein ingestion and its impact on muscle growth. The concept of a "time released protein blend," which combines different sources of protein that release their amino acids at varying rates, is an emerging opportunity in sports nutrition.
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