Muscle mass declines gradually in middle age, a condition called sarcopenia. 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.
Researchers 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 years of age are less responsive to key amino acids, so they should consume protein often during the day to maintain muscle mass and prevent atrophy (muscle breakdown). Older adults should combine weight training and adequate protein intake to preserve muscle mass as they age.