Background: Key areas of research on the preservation of lean tissue mass (LTM) during aging are determinations of the protein requirement and optimal protein intake at meals.Objective: The aim of this study was to determine the effect of protein supplementation at breakfast and lunch for 24 wk beyond habitual intakes on whole-body LTM in healthy adults aged 50-70 y.Methods: In a single-blinded, randomized, controlled design, 60 healthy older men and women (aged 61 5 y) with a body mass index (in kg/m(2)) of 25.8 3.6 consumed either 0.165 g/kg body mass of a milk-based protein matrix (PRO) or an isoenergetic, nonnitrogenous maltodextrin control (CON) at breakfast and midday meals, the lower protein-containing meals of the day, for 24 wk. Dual-energy X-ray absorptiometry was used to measure the change in LTM.Results: After the intervention, protein intake in the PRO group increased from 0.23 0.1 to 0.40 0.1 g/kg for breakfast and from 0.31 0.2 to 0.47 2 g/kg for the midday meal. In response, LTM increased by 0.45 (95% Cl: 0.06, 0.83) kg in the PRO group compared with a decrease of 0.16(95% CI: -0.49, 0.17) kg in the CON group (P= 0.006). Appendicular LTM accounted for the majority of the difference in LTM, increasing by 0.27 (95% Cl; 0.05, 0.48) kg in the PRO group compared with no change in the CON group (P = 0.002).Conclusions: Protein supplementation at breakfast and lunch for 24 wk in healthy older adults resulted in a positive (+0.6 kg) difference in LTM compared with an isoenergetic, nonnitrogenous maltodextrin control. These observations suggest that an optimized and balanced distribution of meal protein intakes could be beneficial in the preservation of lean tissue mass in the elderly. This trial was registered at clinicaltrials.gov as NCT02529124.