To determine whether exercise training improves insulin actions through concomitant body weight loss (BWL).

Subjects (aged 55±8 years) with metabolic syndrome (MetS), prediabetes (fasting blood glucose: 111±2mg·dL⁻¹, HbA1c: 5.85±0.05%) and abdominal obesity (waist circumference: 104±7.9cm) were randomly allocated to either a group performing aerobic interval training (EXER; n=76) or a sedentary group receiving lifestyle counselling (CONT; n=20) for 16 weeks.

At baseline, insulin sensitivity (according to HOMA2 and intravenous glucose tolerance test; CS_I), body composition and VO_2max were similar between the groups. After the intervention, both groups had similar BWL (1–2%), but only the EXER group showed decreased [mean (95% CI)] trunk fat mass [from 18.2 (17.4–18.9) to 17.3kg (16.6–17.9); P<0.001] and HOMA2 scores [from 1.6 (1.5–1.7) to 1.4 (1.3–1.5); P=0.001], and increased VO_2max [from 2.07 (1.92–2.21) to 2.28 (2.11–2.45) LO₂·min⁻¹; P<0.001]. However, CS_I did not improve in any group. Within-group subdivision by BWL (≤0%, 0–3%, ≥3%) revealed higher CS_I in those with BWL≥3% in both groups. Trunk fat mass reductions were closely associated with CS_I and HOMA-IR improvement (r=−0.452–0.349; P<0.001).

In obese MetS subjects with prediabetes, 3% BWL is required for consistent improvement in insulin sensitivity. Thus, exercise-training programmes should be combined with calorie restriction to achieve BWL levels that prevent the development of diabetes.