soozmac, it sort of categorises food into 3 categories, high, medium and low. Its all to do with how much sugar is in the foods. Its the diet a diabetic would follow and it is the only way I can lose weight at all. You might have a problem with your insulin receptors and your bidy might be overloaded with sugar anyway. For example, you might pick eating a fruit salad which on the surface is brilliant but ifits gor fruit that high on the glycaemic index it might be a cause of weight gain. Its very easy to follow, in Australian supermarkets they label foods up with there GI Index and Tesco is following suit too. It works in strange ways too though, depends how well you want to follow it, cooking foods in different ways will give them a different index count, because of the way the food breaks down.