Optimising nutritional and exercise strategies is essential to preserve muscle health and physical function in frail older adults. This study aims to investigate the effects of a protein intervention during progressive resistance training (PRT) with varying training intensities on muscle strength, muscle mass, and physical performance in frail community-dwelling older adults. We were particularly interested in whether these effects differed according to variations in habitual protein intake and resistance training intensity.

Secondary analysis of a randomized controlled trial.

This 12-week RCT randomized 295 frail community-dwelling older adults into PRT-only or PRT with a protein intervention (PRT-Pro). Frailty was defined by receipt of in-home care services or a Tilburg Frailty Indicator score ≥5.

All participants performed under one-to-one supervision, twice-weekly full-body resistance training performed until muscle failure with varying training intensities (20–80% of one-repetition maximum (1RM)). Participants in the protein intervention group received dietary counselling and tailored daily provision of whey protein supplements (20 g) to support achieving a protein intake of 1.2–1.5 g/kg/day. To prevent over-feeding, for participants with a BMI > 30 kg/m ² , bodyweight was adjusted using a BMI of 27.5 kg/m ² .

Primary outcome was leg press muscle strength (1RM). Secondary outcomes included appendicular lean mass and physical performance.

Participants had a mean age of 73.9 ± 6.0, with 69% being female. Protein intake increased in PRT-Pro compared to PRT-only (0.4 g/kg, 95%CI: 0.3−0.5, p < 0.001). Overall, the protein intervention did not result in significantly greater gains in leg press muscle strength, appendicular lean mass, or physical performance compared to PRT-only (p > 0.05). In exploratory subgroup analyses, participants with baseline habitual protein intake <1.2 g/kg showed greater improvements in leg press strength with protein intervention (6.4 kg, 95% CI: 1.3–11.4, p = 0.013), with the largest effect observed in those <0.8 g/kg (10.9 kg, 95% CI: 1.5–20.4, p = 0.025) compared with PRT-only. Training intensity did not modify the effects of the protein intervention.

A protein intervention during resistance training did not enhance overall muscle strength, muscle mass, or physical performance in frail older adults. The protein intervention was associated with greater strength gains in exploratory analyses among participants with lower baseline protein intake (<1.2 g/kg/day), with the most pronounced associations observed in those consuming <0.8 g/kg/day. These findings highlight the potential importance of baseline protein status, although they require confirmation in larger trials involving frail older populations.

NL-OMON54919 (at ICTRP Search Portal), date of registration: 09-02-2021.