Introduction <p>There is interest in the potential impact of increased protein intake on muscle function in older adults. However, there is paucity of information on effects of different protein types.</p> Objectives <p>To determine the impact of protein sources (plant vs dairy) on the plasma metabolomic profile, and to explore associations between metabolites and handgrip strength.</p> Methods <p>In this secondary analysis of a 12-week double-blinded RCT, 171 healthy adults (≥50 years) were assigned to high plant protein (23 g/d), high dairy protein (22 g/d) or low protein (2 g/d) supplement. Plasma samples were collected pre- and post-intervention and analysed using a targeted LC-MS platform. Main and interaction effects were estimated using ANOVA-simultaneous component analysis (ASCA). Spearman correlation analysis evaluated associations between metabolites and handgrip strength, with false discovery rate correction (q &lt; 0.05).</p> Results <p>ASCA revealed no significant main effect of group (p = 0.23) or group × time interaction (p = 1.00), while time had a significant effect on the metabolomic profile (p &lt; 0.05). After FDR correction, 26 metabolites were significantly correlated with baseline handgrip strength. The strongest positive correlations were observed for creatinine (r = 0.53), leucine (r = 0.38), proline (r = 0.35) and isoleucine (r = 0.33). Baseline levels of leucine, isoleucine, PCaeC322 and SMOHC222 were associated with positive changes in handgrip strength (p &lt; 0.05).</p> Conclusions <p>Metabolomic changes were not related to protein type. However, metabolite levels were correlated with handgrip strength outcomes, suggesting the necessity to evaluate the metabolic state of individuals prior to interventions.</p> Trial registration <p>This study is registered on isrctn.com as ISRCTN16651067.</p>