Technical Abstract: Objectives: The interactions between nutritional supplementation and physical activity on changes in physical function among older adults remain unclear. The primary objective of this study was to examine the impact of nutritional supplementation plus structured physical activity on 400M walk capacity in mobility-limited older adults across two sites (Boston, USA and Stockholm, Sweden). Design: All subjects participated in a physical activity program (3x/week for 24 weeks), involving walking, strength, balance, and flexibility exercises. Subjects were randomized to a daily nutritional supplement (150kcal, 20g whey protein, 800 IU vitamin D) or placebo (30kcal, non-nutritive). Setting: Participants were recruited from urban communities at 2 field centers in Boston MA USA and Stockholm SWE. Participants: Mobility-limited (Short Physical Performance Battery (SPPB) </=9) and vitamin D insufficient (serum 25(OH)D 9-24 ng/ml) older adults were recruited for this study. Measurements: Primary outcome was gait speed assessed by the 400M walk. Results: 149 subjects were randomized into the study (mean age= 77.5 +/- 5.4; female= 46.3%; mean SPPB= 7.9 +/-1.2; mean 25(OH)D= 18.7 +/- 6.4 ng/ml). Adherence across supplement and placebo groups was similar (86% and 88%, respectively), and was also similar across groups for the physical activity intervention (75% and 72%, respectively). Both groups demonstrated an improvement in gait speed with no significant difference between those who received the nutritional supplement compared to the placebo (0.071 and 0.108 m/s, respectively (p=0.06)). Similar effects in physical function were observed using the SPPB. Serum 25(OH)D increased in supplemented group compared to placebo 7.4 ng/ml versus 1.3 ng/ml respectively. Conclusion: Results suggest improved gait speed following physical activity program with no further improvement with added nutritional supplementation.