The role of dietary fat in obesity

Epidemiological evidence suggests that a high-fat diet promotes the development of obesity and that there is a direct relationship between the amount of dietary fat and the degree of obesity. The importance of this relationship has been shown in black prepubescent females, who consumed more calories as fat than white females. Moreover, black adult females are heavier and have significant higher cardiovascular disease mortality rates than white females.

An overview of animal studies had indicated that high-fat diets induce greater food intake and weight gain than high-carbohydrate diets. Several factors such as caloric density, satiety properties and post-absorbtive processing can contribute to this different response to high-fat diets. Accordingly, the satiating effects after meals with a high fat:carbohydrate ratio is less than for meals with a lower ratio. Some authors have reported that the most important variable influencing meal size is not the level of hunger but the nutrient content of the range of foods consumed. Thus dietary fat has a weak effect on satiety and we suggest that periodic exposure to a high-fat meal, particularly when hunger is high, may be sufficient to lead to overconsumption of energy as fat in obese patients.

Energy balance is well correlated with fat balance in lean controls, whereas there is no correlation with either carbohydrate or protein balances. Several authors have shown that carbohydrate and protein storage is closely regulated by adjusting oxidation to intake, whereas fat is almost exclusively used or stored in response to day-to-day fluctuations in energy balance. The positive relationship between fat intake and lipid oxidation seen in lean controls appears not to be present in obese patients. On high-fat diets, post-obese women failed to increase the ratio of fat to carbohydrate oxidation appropriately. Increasing dietary fat results in preferential fat storage in post-obese women, impaired suppression of carbohydrate and reduction of 24h energy expenditure.

Dietary fat induces overconsumption and weight gain through its low satiety properties and high caloric density. Obese and post-obese subjects do not appear to adapt to dietary fat, and therefore fat storage is increased.