Resistance Training vs Leucine

Leucine is being marketed as the muscle-building supplement for aging women. A September 2025 double-blind RCT just found that resistance training reversed frailty and increased muscle protein synthesis — and leucine added nothing to those results.

First, what leucine actually is. It is one of the nine essential amino acids, found in high concentrations in animal proteins like chicken, eggs, beef, dairy, and fish. It is the primary amino acid responsible for activating mTORC1, the molecular switch that turns on muscle protein synthesis. This is why the supplement industry has built an entire category around it.

Jacob et al., published in GeroScience in September 2025, randomized 19 pre-frail and frail older women into a 12-week resistance training program. All women ate an optimized protein diet of 1.2g per kilogram of bodyweight per day. Half received 7.5g of leucine daily. Half received a placebo.

Resistance training increased basal muscle protein synthesis significantly in both groups. Frailty reversed in both groups. Strength, lean body mass, and myofiber profiles all improved in both groups. The leucine group showed no additional benefit over placebo on any outcome.

7.5g per day is a substantial dose, well above the threshold typically associated with maximal mTORC1 activation in aging muscle. The women were eating adequate protein and training consistently. The anabolic machinery was already saturated. The supplement had nowhere to add value.

Honest caveat: this study was in women with a mean age of 77.5 years, not perimenopausal women specifically. But the principle translates. When protein intake is adequate and training is consistent, the evidence for leucine producing additional benefit is thin