Research brief · 2026-09-09
Why Does Building Muscle Get Harder With Age?
Muscle loss is usually framed as something that happens abruptly in old age. The physiology says otherwise. A 2018 review in Ageing Research Reviews by Wilkinson, Piasecki and Atherton described muscle mass declining from middle age at roughly one percent per year, reaching as much as half of total muscle by the eighth or ninth decade in severe cases. Individual fibers shrink, and whole fibers are lost outright.
Underneath that decline sits a measurable phenomenon called anabolic resistance. Give an identical dose of protein, or an identical bout of exercise, to younger and older adults, and older muscle mounts a smaller protein-building response. A 2023 review in Nutrition Reviews by Aragon, Tipton and Schoenfeld framed this as central to how sarcopenia progresses even in people doing the right things.
For a sense of the ceiling being lost, Witard's 2014 study in the American Journal of Clinical Nutrition measured muscle protein synthesis in 48 resistance-trained young men after leg exercise. Twenty grams of whey raised synthesis 49 percent above the no-protein response, and 40 grams added no significant extra. Older muscle appears to need a larger or more targeted stimulus to approach a comparable response.
The candidate mechanisms, laid out in the 2013 review by Burd, Gorissen and van Loon, run in parallel: weaker anabolic signaling inside the cell, reduced blood flow delivering amino acids to muscle after a meal, and more of the eaten protein retained by the gut and liver. That review also noted that physical activity before protein intake increased how much of it was used for muscle building.
None of this is settled. Reviews describe multiple mechanisms without a confirmed hierarchy, and no study shows anabolic resistance can be fully reversed rather than partly offset. A defined protein threshold for older adults, comparable to the 20-gram figure in young men, has not been established. Anyone concerned about muscle loss should raise it with a physician.
Sources
- Wilkinson DJ, Piasecki M, Atherton PJ. The age-related loss of skeletal muscle mass and function
- Ageing Research Reviews 2018
- Aragon AA, Tipton KD, Schoenfeld BJ. Age-related muscle anabolic resistance: inevitable or preventable?
- Nutrition Reviews 2023
- Witard OC et al. Myofibrillar muscle protein synthesis rates in response to increasing doses of whey protein
- American Journal of Clinical Nutrition 2014
- Burd NA, Gorissen SH, van Loon LJC. Anabolic resistance of muscle protein synthesis with aging
- Exercise and Sport Sciences Reviews 2013
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