Background. Obesity exacerbates age-related decline in glucometabolic control. Undercarboxylated osteocalcin (UcOC) regulates pancreatic insulin secretion. The long-term effect of lifestyle interventions on UcOC and insulin secretion has not been investigated. Methods. One hundred seven frail, obese older adults were randomized into the control (N=27), diet (N=26), exercise (N=26), and diet-exercise (N=28) groups for 1 year. Main outcomes included changes in UcOC and disposition index (DI). Results. UcOC increased in the diet group (36 ± 11.6%) but not in the other groups (P<0.05 between groups). Although similar increases in DI occurred in the diet-exercise and diet groups at 6 months, DI increased more in the diet-exercise group (92.4 ± 11.4%) than in the diet group (61.9 ± 15.3%) at 12 months (P<0.05). UcOC and body composition changes predicted DI variation in the diet group only (R2=0.712), while adipocytokines and physical function changes contributed to DI variation in both the diet (∆R2=0.140 and 0.107) and diet-exercise (∆R2=0.427 and 0.243) groups (P<0.05 for all). Conclusions. Diet, but not exercise or both, increases UcOC, whereas both diet and diet-exercise increase DI. UcOC accounts for DI variation only during active weight loss, while adipocytokines and physical function contribute to diet-exercise-induced DI variation, highlighting different mechanisms for lifestyle-induced improvements in insulin secretion. This trial was registered with ClinicalTrials.gov number NCT00146107.