THE ROLE OF FAT IN ENERGY BALANCE
In the traditional teaching of energy balance, it has been assumed that, in humans, there is a relatively free conversion of non-fat calories to fat calories for storage. This is no longer so; now fat nutrients are thought to be more fattening than nutrients gained from either carbohydrate or protein.
The reasons that fat was thought to be a major contributor to obesity were twofold: its high energy density of 9 kcal/g, and its relatively inexpensive conversion to fat storage (about 3-5 per
cent of its energy) compared to carbohydrate at 4 kcal/g and about 25 per cent of its energy needed to convert it into fat for storage. A better understanding of the physiology of macronutrient (carbohydrate, protein, fat, alcohol) balance has now added a more powerful reason: macronutrients are largely handled separately by the body and, under modem living conditions, dietary fat is essentially the only macronutrient which ends up in fat stores. Indeed, obesity can be seen as the ‘price’ paid for eating a high-fat diet and burning little fat through exercise since it is the gain in fat mass and fat-free mass which brings the body back into energy and fat balance.
Physiologically, therefore, all calories are not equal and fulfil quite different roles in energy balance. The role of fat is to provide energy reserves and fat in the diet has little impact on the active maintenance of energy balance through appetite mechanisms or stimulating fat burning. But does this mean that fat calories are more important in the cause or treatment of obesity? Changing diet composition while maintaining the same total calories does not result in weight changes over the long term, and increasing the calorie intake, irrespective of its composition, does cause some weight gain in the long term. However, we don’t live in a calorie controlled environment. Under conditions of ad libitum (free) eating, where foods are selected from a wide variety of choices and eating is at liberty in generally unrestricted amounts, the composition of the diet does have a substantial and important effect.
Increasing the dietary fat content results in a slow fat gain in all but the very active and decreasing the dietary fat content results in slow fat loss in people who are overfat. Therefore, the quality of the diet (i.e. fat: carbohydrate ratio) affects the quantity of the diet (i.e. the total calories). In the development of obesity, high fat content and high total calories are both important because they occur together, but the critical insight is that fat is, to a large extent, responsible for total calories anyway.
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