In a randomized controlled trial, 82 older adults (>65y) with (or at risk of) undernutrition (n=82) were randomly allocated to 12 weeks of supplementation with a novel supplement (586 kcal, 22 g protein of which 50% whey and 50% casein, 206 mg ursolic acid, 7 g free BCAAs, 11 µg vitamin D) or standard care (600 kcal, 24g protein of which 100% casein, 4 µg vitamin D). Body weight increased significantly in the 12 weeks, both in the intervention group (+1.6 ± 0.2 kg, p<.0001) and in the standard care group (+1.8 ± 0.2 kg, p<.0001). Gait speed during 4m and 400m tests improved over time in the intervention group, whereas the standard care showed no improvements (time*treatment effects 400m: p=0.038 and 4m: p=0.048). Gene sets related to mitochondrial functioning were strongly upregulated in the participants receiving the intervention product. We showed that a novel oral nutritional supplement improves gait speed in older adults via improvements in mitochondrial functioning.