Effects of a self-selected vegetarian vs. omnivorous diet on protein intake and muscle mass in older adults during resistance exercise: the IMPACT randomized controlled trial.

BACKGROUND: A predominantly plant-based diet may reduce the risk of non-communicable diseases and benefit sustainability, but may also result in lower protein intake. OBJECTIVE: This study investigated the effects of a lacto-ovo vegetarian vs an omnivorous diet, both combined with resistance exercise training, on protein intake and appendicular muscle mass in adults aged 55 years and older. METHODS: In this randomized non-inferiority trial, 108 adults (66 [62-69] years; 58% female; BMI: 25.7 kg/m2) received dietary counseling for either a lacto-ovo vegetarian or an omnivorous diet. Participants in both groups completed a 12-week resistance training program (3 sessions/week) while self-selecting foods to achieve a protein intake of ≥1.2 g/kg/d, targeted at either ≥60% plant-based protein (vegetarian diet) or ≥60% animal-based protein (omnivorous diet). Primary outcomes were protein intake (3-day food record) and appendicular muscle mass (DXA), with non-inferiority margins of 0.12 g/kg/d and 0.25 kg, respectively. Secondary outcomes included leg press strength, chair stand test, and handgrip strength. RESULTS: Non-inferiority of the vegetarian diet was not established for protein intake (-0.33 g/kg/d; 95% CI -0.49, -0.17) and appendicular muscle mass (-0.33 kg; 95% CI -0.69, 0.03). Both groups met their protein intake and plant:animal protein ratio targets: 1.24 (1.14, 1.35) g/kg/d and 76% plant-based protein in the vegetarian group, and 1.58 (1.48, 1.69) g/kg/d and 67% animal-based protein in the omnivorous group. Appendicular muscle mass significantly increased in both the vegetarian [+0.38 (0.12, 0.64) kg, p=0.004] and the omnivorous group [+0.66 (0.40, 0.93) kg, p<0.001]. Leg press strength and chair stand test significantly improved in both groups, with no between-group differences. CONCLUSIONS: Protein intake was lower in the vegetarian group and non-inferiority for appendicular muscle mass was not demonstrated. Yet, both groups achieved a protein intake of ≥1.2 g/kg/d and improved muscle mass, strength, and function during resistance training. Trial registration ClinicalTrials.gov: NCT06172725 (https://clinicaltrials.gov/study/NCT06172725).