The Effects of the OPTIFAST Diet and Aerobic Exercise on Insulin Sensitivity

BACKGROUND: Clinical weight loss (CWL) (5-10%) combined with aerobic exercise improves insulin sensitivity. The OPTIFAST program is a medically supervised diet for weight loss, however, there is minimal data available on the OPTIFAST diet for improvement in insulin sensitivity. METHODS: Twenty-nine sedentary, overweight, and obese adults (Age: 44.8 ± 9.7 yrs\; Weight: 95.6 ± 12.8 kg\; BMI: 34.2 ± 3.3 kg/m2) completed a 10-week intervention of supervised aerobic exercise training with an OPTIFAST program to achieve bodyweight loss of =7%. The aerobic exercise training consisted of 300 metabolic minutes (MET min) per week and increased by 50 MET min each week until 700 MET min per week was reached. The OPTIFAST diet included meal-replacement products (~800 kcals per day) and weekly classes about nutrition and behavior modification guided by a registered dietician. A 2-hour oral glucose tolerance test (OGTT) was performed, which involved ingestion of 75g of dextrose\; serum blood samples were obtained at 0, 30, 60, 90, and 120 minutes and analyzed for glucose and insulin levels. Insulin sensitivity was calculated using the Matsuda Index. A normal score for the Matsuda index is =3. RESULTS: After the intervention, Matsuda Index (3.1, p<0.001) and relative VO2peak (2.5 mL/kg/min, p<0.001) increased, while weight (-9.1 kg, 9.4%, p<0.001), waist circumference (-8.5 cm, p<0.001), BMI (-3.2 kg/m2, p<0.001), percent fat mass (-2.1%, p<0.001), VAT mass (-0.11 kg, p<0.001), VAT volume (-120.9 cm3, p<0.001), VAT area (-23.2 cm2, p<0.001), and lean mass (-3.5 kg, p<0.001) decreased. Changes in insulin sensitivity via the Matsuda Index were associated with changes in percent weight loss (r=-0.59, p<0.001), body weight (r=-0.46, p=0.012), waist circumference (r=-0.54, p=0.033), BMI (r=-0.49, p=0.007), VAT mass (r=-0.38, p=0.041), VAT volume (r=-0.38, p=0.040), and VAT area (r=-0.39, p=0.039). No other significant relationships were observed between changes in Matsuda Index with fitness or other body composition measurements (e.g., lean mass and % fat mass). CONCLUSIONS: An OPTIFAST weight loss program combined with aerobic exercise improved insulin sensitivity in overweight and obese adults, with changes in weight and body composition representing important predictors of insulin sensitivity. Future research should investigate the effect of the OPTIFAST diet and aerobic exercise on insulin sensitivity during a weight maintenance period