In the past, it was believed that protein, fat and carbohydrate foods, taken in equal quantities, satisfy our appetite equally. We now know from recent research that the satiating (making us feel full) capacity of these three nutrients is not equal.

Fatty foods, in particular, have only a weak effect on satisfying appetite relative to the number of kilojoules they provide. This has been demonstrated clearly in experimental situations where people are asked to eat until their appetite is satisfied. They over-consume kilojoules if the foods they are offered are high in fat. When high carbohydrate and low-fat foods are offered, they consume fewer kilojoules, eating to appetite. So, carbohydrate foods are the best for satisfying our appetite without over satisfying our kilojoule requirement.

When we eat more carbohydrate, the body responds by increasing its production of glycogen. Glycogen is stored glucose, the critical fuel for our brain and muscles. The size of these stores is limited, and they must be continuously refilled by carbohydrate from the diet. Good glycogen stores ensure a well-fuelled body and make it easier to exercise. Even when we are not exercising, the body will use a mixture of carbohydrate and fat, attempting to match the carbohydrate : fat ratio of the fuel mixture to that which has been eaten in the diet.

Because fat is less satisfying to our appetite, it is easy to over-consume fatty kilojoules. That is why reducing the dietary fat intake is a far more effective means of achieving weight control while satisfying the appetite than restricting carbohydrate intake. By eating a high carbohydrate diet it will be easier to lower your fat intake, and by choosing that carbohydrate from low G.I. foods, you make it even more satisfying.

What’s more, even when the kilojoule intake is the same, people eating low G.I. foods may lose more weight than those eating high G.I. foods. In a South African study, the investigators divided overweight volunteers into two groups: one group ate a low kilojoule, high G.I. diet and the other, a low kilojoule, low G.I. diet. The amount of kilojoules, fat, protein, carbohydrate and fibre in the diet was the same for both groups. Only the G.I. factor of the diets was different. The low G.I. group included foods like lentils, pasta, porridge and corn in their diet and excluded high G.I. foods like potato and white bread. After 12 weeks, the volunteers in the group eating low G.I. foods had lost, on average, 9 kilograms—2 kilograms more than people in the group eating the diet of high G.I. foods.

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