The zone diet is based on the theory that excess insulin, a hormone that the body secretes to control our blood sugars, makes us fat and keeps us fat. As our diet plan review explains, the aim of the zone diet is to regulate our blood sugar levels closely and therefore keep our insulin in a tight "zone" so the body burns fat more efficiently and consequently loses weight.

To achieve this aim we need to control the insulin levels that affect the blood sugar levels, by getting the perfect balance of carbohydrates, proteins and fats in every meal. In essence this is a low carbohydrate, high protein diet with moderate amounts of fat. Many fans of the zone diet advocate a 40:30:30 ratio. That is each diet meal plan should contain 40% carbohydrate, 30% protein and 30% fat.

Barry Sears, who is credited with discovering the zone diet, came up with the idea of 'food blocks' to help followers keep up with the complicated diet meal plans and demands of the diet. A food block contains a standardized amount of carbohydrate, protein or fat. A certain number of blocks are allocated for each meal and snack, and recipes are devised specifically around these food blocks, to enable the programme's followers to lose weight.

This programme was primarily conceived as a weight loss plan not based on calorific thinking but more in line with how the body actually evolved. There are many critics who claim that it is irresponsible to advocate an eating plan that is low in essential nutrients and vitamins whilst being unnaturally high in protein and fats. Also the long-term consequences of such an eating regime are unknown, and for this reason, it is not recommended by the British or American Heart Association.