Insulin sensitivity and secretion in young, healthy subjects are not changed by Zone and Mediterranean diets

The Zone diet (ZD) is centreed primarily on protein intake. Previous findings have assessed the role of ZD on glucose metabolism, but the outcomes have been conflicting. So far, there is no clear evidence of the effects of ZD on insulin response. Furthermore, the effect of ZD on insulin sensitivity has never been clearly determined yet. Our aim was to evaluate in healthy subjects how ZD acts on body composition, lipid profile and overall on insulin sensitivity and secretion in dynamic conditions. We also compared ZD effects with those of the Mediterranean diet (MD). Twelve volunteers were studied. Subjects were randomized either to MD or to ZD for 8 weeks. A wash-out period followed. Afterwards, subjects were crossed over to the other diet for other 8 weeks. At any end-point subjects underwent standard 75 g oral glucose tolerance test for measurements of glucose, insulin, C-peptide, proinsulin and glucagon. Lipids and free fatty acids were determined at fasting. Insulin sensitivity was assessed by the oral glucose insulin sensitivity index. Insulin secretion was determined by mathematical modelling of insulin and C-peptide data. No significant differences were observed following both diets in body weight, fat mass and plasma glucose, insulin, C-peptide, proinsulin and glucagon levels. Similarly, insulin sensitivity and secretion were not different in both absolute values and changes from baseline. Decrease in triglyceride and increase in HDL–cholesterol levels were observed after ZD compared with MD. Thus, it was determined that intake of high-protein diet for 2 months does not modify insulin action and secretion, but improves lipid profile.