Weight loss - background

A number of studies have shown that similar weight loss can be achieved on either a low fat (<25% fat, 40g) or modified fat diet (35%-40% fat, 60g fat mainly from monounsaturated olive oils), when total energy intake is restricted to about 1500kcal/day. The modified fat diet also resulted in higher HDL cholesterol levels, lower fasting triglycerides and insulin levels.

Short term human metabolic studies suggest that overfeeding with fat causes more fat deposition than overfeeding with carbohydrate. On the other hand, studies have shown that overfeeding carbohydrate results in less mobilisation and oxidation of fats. The findings of metabolic studies are inconclusive.

Epidemiological studies have not shown conclusively that fat intake is correlated with body fatness. Weight loss studies are also not definitive. No sustained weight loss has been documented using either low fat diets or restricted energy diets. There is limited data on long term programs and insufficient evidence in favour of selecting one method over another. Trials in which dietary fat has been reduced over 12-24 months have shown little change in body weight, particularly in the longer term. An 18 month study at Harvard University comparing a low fat diet with a high monounsaturated fat diet reported considerable and similar weight loss on both diets, but this levelled off after 6 months.

Flatt (Int J Obesity 1996; 20: S1-11) hypothesises that there is a limited range of carbohydrate stores in the body (equivalent to about 2000kcal per day). This is generally used up in a day and replaced by carbohydrate in food. It is only when excess fat is eaten that this is saved in the fat stores of the body and body fatness increases. With a higher rate of overall food intake, extra carbohydrate causes a greater ‘sparing’ of fat in the energy cycle and induces food fats to be deposited more readily in fat stores. Flatt suggests that this might be why Americans continue to get fatter despite the wide range of low-fat products on the market; they over-compensate by eating too much. He suggests that if a diet is very low in fat 15-20% (e.g. traditional Asian diets) it may be possible to eat very large amounts of carbohydrate. Because most diets are not this low, it may be necessary to restrict somewhat overall food and energy intake.

Overall, it is agreed that it is negative energy balance, not nutrient profile, which will determine weight loss. However, a varied approach to dietary counselling is clearly possible to accommodate higher fat styles of eating which may convey some advantages in improving cardiovascular risk factors. Whether higher or lower fat diets for weight loss have any unique advantages over one another in terms of satiety, compliance or maintenance of weight loss is unknown (Golay et al. AJCN 1996; 63:174-8.; Low et al Diabetes 1996; 445: 569-75).

The consensus about weight loss diets at the Boston Workshop coordinated by Harvard University nutrition experts is as follows (Sacks, Int Congress on Vascular Disease Prevention 1998.)“There is increasing scientific evidence that there are positive health effects from diets that are high in fruits, vegetables, legumes and wholegrains which include fish, nuts and low fat dairy products. Such diets need not be restricted in total fat as long as there is not an excess of calories and the diet is low in saturated fats and partially hydrogenated oils.”

The total amount of fat in the diet is determined by the culture and dietary habits of people around the world, which can range from 15% in Asia to 40% in the Mediterranean. Total fat intake is not critical, but somewhere in the range of 30-35% energy is realistic in western societies.

Why diets don’t work

Restricting food sends a signal to the body that there is a food shortage. The body thinks its own survival is threatened. It responds by slowing down its metabolic rate (i.e. the rate at which the body uses energy). This slows body fat loss so that it becomes harder to lose weight. When you diet, the body responds by conserving body fat. The weight that is lost is mainly water and muscle. Reduced muscle further slows down the bodys metabolism.

Rapid weight loss usually means 50% of fluid loss (rapidly regained), 25% muscle tissue and 25% fat. Research has shown that people find it more and more difficult to lose weight after repeated dieting, and they more readily gain weight. 95% of dieters regain the weight after the diet is over, and often regain more. Weight fluctuations associated with repeated dieting increase the risk of CHD. It is healthier to stabilise at a heavier weight than to repeatedly lose and gain weight.

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Last Updated: October 2003.