Muscle mass declines gradually after age 40.  Muscle loss eventually leads to decreased strength and quality of life.  Muscle is in a constant state of synthesis and breakdown, and requires adequate protein intake to maintain size, even when overloaded through weight training.

Amino acids that make up proteins serve as chemical signalers and building blocks for muscle proteins.

Stuart Phillips from McMaster University summarized how weight training and protein intake cause muscle growth.  Leucine is the key amino acid for turning on muscle protein synthesis (mTOR pathway), but muscle hypertrophy grinds to a halt without the full complement of essential amino acids.  Following intense resistance exercise, muscle protein synthesis occurs in direct proportion to protein intake, peaking at 20-40 grams.  The mTOR pathway is most active in fast twitch muscle fibers, which respond best to high intensity weight training.  Peak muscle growth occurs when you concentrate on these fibers by emphasizing high stress training programs.

People over 40 are less responsive to key amino acids, so they should consume protein often during the day to maintain muscle mass and prevent atrophy.  Older adults should combine weight training and adequate protein intake to preserve muscle mass as they age.  (Applied Physiology Nutrition Metabolism, 34:403-410, 2009)