In recent years, several alternative dietary approaches, including high-protein and low-glycaemic-load diets, have produced faster rates of weight loss than traditional low-fat, high-carbohydrate diets. These diets share an under-recognised unifying mechanism: the reduction of postprandial glycaemia and insulinaemia. Similarly, some food patterns and specific foods (potatoes, white bread, soft drinks) characterised by hyperglycaemia are associated with higher risk of adiposity and type 2 diabetes. Profound compensatory hyperinsulinaemia, exacerbated by overweight, occurs during critical periods of physiological insulin resistance such as pregnancy and puberty. The dramatic rise in gestational diabetes and type 2 diabetes in the young may therefore be traced to food patterns that exaggerate postprandial glycaemia and insulinaemia. The dietary strategy with the strongest evidence of being able to prevent type 2 diabetes is not the accepted low-fat, high-carbohydrate diet, but alternative dietary approaches that reduce postprandial glycaemia and insulinaemia without adversely affecting other risk factors.