Quantifying the effect size of acute exercise on insulin sensitivity (SI exercise) and simultaneous measurement of glucose disappearance (R d), endogenous glucose production (EGP), and meal glucose appearance in the postprandial state has not been developed in humans. To do so, we studied 12 healthy subjects [5 men, age 37.1 Ϯ 3.1 yr, body mass index 24.1 Ϯ 1.1 kg/m 2 , fat-free mass (FFM) 50.9 Ϯ 3.9 kg] during moderate exercise at 50% V O2max for 75 min, 120 -195 min after a triple-tracer mixed meal consumed at time 0. Tracer infusion rates were adjusted to achieve constant tracer-to-tracee ratio and minimize non-steadystate errors. Glucose turnover was estimated by accounting for the nonstationary kinetics introduced by exercise. Insulin sensitivity index was calculated in each subject both in the absence [time (t) ϭ 0 -120 min, SI rest] and presence (t ϭ 0 -360 min, SIexercise) of physical activity. EGP at t ϭ 0 min (13.4 Ϯ 1.1 M·kg FFM Ϫ1 ·min Ϫ1 ) fell at t ϭ 120 min (2.4 Ϯ 0.4 M·kg FFM Ϫ1 ·min Ϫ1 ) and then rapidly rose almost eightfold at t ϭ 180 min (18.2 Ϯ 2.6 M·kg FFM Ϫ1 ·min Ϫ1 ) before gradually falling at t ϭ 360 min (10.6 Ϯ 0.9 M·kg FFM Ϫ1 ·min Ϫ1 ). Rd rapidly peaked at t ϭ 120 min at the start of exercise (89.5 Ϯ 11.6 M·kg FFM Ϫ1 ·min Ϫ1 ) and then gradually declined at t ϭ 195 min (26.4 Ϯ 3.3 M·kg FFM Ϫ1 ·min Ϫ1 ) before returning to baseline at t ϭ 360 min. SI exercise was significantly higher than SI rest (21.6 Ϯ 3.7 vs. 12.5 Ϯ 2.0 10 Ϫ4 dl·kg Ϫ1 ·min Ϫ1 per U/ml, P Ͻ 0.0005). Glucose turnover was estimated for the first time during exercise with the triple-tracer technique. Our results, applying stateof-the-art techniques, show that moderate exercise almost doubles postprandial insulin sensitivity index in healthy subjects.oral minimal model; exercise; insulin sensitivity IT IS WELL ESTABLISHED THAT exercise increases rates of glucose uptake (R d ) and that rates of endogenous glucose production (EGP) must increase to meet the increased metabolic demands of the exercising muscle to prevent hypoglycemia (13,16,38,40). These changes in glucose fluxes are facilitated by falling insulin and rising glucagon and catecholamine levels during exercise in healthy individuals (43). Although numerous studies have demonstrated increased R d and EGP during physical activity in individuals with and without diabetes (19,25) in the postabsorptive state, very few have examined the effects of exercise in the postprandial state in individuals with and without type 2 diabetes (8, 9, 23, 26 -28) and none in individuals with type 1 diabetes. Furthermore, very few studies (12) have used methods that minimize fluctuations in tracer-totracee specific activity to enable accurate continuous (every 10 min) measurement of glucose turnover and during the transition from rest to exercise in nondiabetic subjects. This latter point is important when developing a mathematical model for the next-generation artificial pancreas for type 1 diabetes.Models of insulin action and secretion in response to physiological perturbations...