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Muscle Mass
**Muscle mass** (skeletal muscle mass) is the total weight of the body's striated, voluntarily controlled muscle tissue — the largest component of lean body mass and a primary site of glucose disposal and amino acid storage.
What drives muscle mass
Muscle mass reflects the long-run balance between two opposing processes: muscle protein synthesis (MPS) and muscle protein breakdown (MPB). Hypertrophy occurs only when MPS exceeds MPB over time (Phillips et al., Sports Medicine, 2014, PMID:24791918). The two principal non-genetic drivers are resistance training and adequate dietary protein; hormones such as insulin, which suppresses breakdown, and estrogen, which influences synthesis, also modulate the rate.
Supplemental protein combined with resistance training increases fat-free mass without significant changes in body fat (PMID:31565912). Source matters less than total intake and training: in an 84-day comparator-controlled trial of 50 sedentary adults, pea protein and whey protein produced comparable gains in muscle mass (2.3% vs 2.4%, P = 0.92). For a fuller treatment of intake and training variables, see the protein for athletes guide.
Why muscle mass matters for metabolism and aging
Skeletal muscle is metabolically active tissue, and its mass influences whole-body glucose handling and resting energy expenditure. The progressive, accelerated loss of muscle mass and function with age is termed sarcopenia (Age and Ageing, 2010, PMID:20392703). A contributing mechanism is anabolic resistance: aging is characterized by a blunted increase in muscle protein synthesis following protein intake (PMID:23558692), an effect that decreased physical activity can produce even in younger people.
Because of this, older adults often require more protein than the standard RDA to preserve muscle mass. The ESPEN Expert Group recommends 1.0–1.2 g/kg body weight per day for older adults to maintain muscle mass and function (Clinical Nutrition, 2014, PMID:24814383). Middle-aged and older women experience steeper declines in muscle mass and strength than men of the same age, linked in part to estrogen’s role in protein synthesis.
Protein quality and muscle mass
Not all protein supports muscle mass equally. Animal proteins generally carry higher protein quality scores than plant proteins, and leucine is the primary amino acid trigger for muscle protein synthesis. Plant sources can still drive a meaningful response: consuming 25 g of potato protein isolate twice daily stimulated muscle protein synthesis rates at rest and after resistance exercise in young women, identifying it as a high-quality plant-based protein (Nutrients, 2020, PMID:32349353).
Leucine content explains much of the difference between sources. When a plant-based blend was fortified to match whey’s leucine, its synthetic response became statistically indistinguishable from whey, indicating that closing the leucine gap can close the anabolic gap.
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