High protein diets are useful for weight reduction

True, but indirectly because a high-protein diet is usually a low-fat diet. To achieve loss of body fat, either energy intake (kilojoules) needs to be reduced or energy expenditure increased. In the short term, a reduction in energy intake can be worthwhile, but in the longer

term it is preferable to increase energy expenditure whilst attending to the quality of the food eaten (see WE SHOULD CHOOSE FOOD ACCORDING TO FOOD GROUPS). In the long-term the 'prudent diet' low in animal fat, is to be encouraged (see WHAT WE KNOW ABOUT FATS AND OILS IN FOOD).

The question is whether, in the short term, there is favourable effect on body composition (i.e. relative quantities of fat, muscle and other lean body mass, body water and bone) when on a low-energy diet. The diet may contain one macronutrient (protein, fat, carbohydrate, alcohol) more than another. The advantage of relatively high protein content in such diets is that it might reduce the loss of high-protein tissues like muscle. At very low energy intakes (less than 3360 kilojoules or 800 kilocalories per day) this appears to be the case. The increased protein in these diets ought to be accompanied by a reduction in fat content, since art from providing essential polyunsaturated fatty acids and enabling absorption of fat-soluble vitamins, has no recognised nutritional value. Obviously there should be no alcohol, which, like fat, is high in kilojoules! There is not much opportunity within a 3360 kilojoule diet to include carbohydrate, since 100 grams of protein already contributes 1680 kilojoules of energy.

It should be emphasised that this reduction from the usual dietary intake of carbohydrate will be associated with a loss of body carbohydrate stores (as glycogen) liver and muscle and the loss of water associated with the glycogen. The initial loss of body weight as water is almost inevitable with reduction of energy intake; it lasts a few days, contributes nothing to loss of body fat, and the weight returns when usual eating habits are resumed.

In an every-day diet for weight maintenance, it is healthy to include low-fat versions of high-protein foods, especially those derived from meat (trimmed, lowfat cuts) and reduced and low-fat dairy products rather than their high-fat counterparts. A high-protein diet itself is of no particular value.