The role of dietary fat in the prevention and treatment of obesity. Efficacy and safety of low-fat diets

BACKGROUND: Does dietary fat play a central role in weight gain and develop ment of obesity? Do low-fat diets have adverse effects on blood lipids? OBJECTIVE AND DESIGN: To answer these questions we have reviewed the eviden ce linking the dietary fat content to energy balance and obesity, and exami ned the efficacy and safety of ad libitum low fat, high carbohydrate/protei n diets in the prevention and management of obesity. RESULTS: Physiological studies have provided insight into the mechanisms by which the macronutrients differ in their effect on energy balance: (1) ene rgy from fat is less satiating than energy from carbohydrate, and a high fa t/carbohydrate ratio in the diet promotes passive overconsumption, a positi ve energy balance and weight gain in susceptible individuals; (2) fat is mo re readily absorbed from the intestine and fecal energy toss is much lower with a high dietary fat/carbohydrate ratio; (3) carbohydrate is more thermo genic than fat and energy expenditure is lower during positive energy balan ce produced by a diet with a high fat/carbohydrate ratio than during positi ve energy balance produced by a diet with a low fat/carbohydrate ratio. Ran domized intervention studies comparing low fat diets to normal fat diets sh ow that low fat diets prevent weight gain in normal weight subjects and pro duce weight loss in overweight individuals. In our meta-analysis of ad libi tum low fat interventions we included 16 trials involving 1728 individuals. The difference in weight loss between intervention and control groups was 2.5 kg (95% CI, 1.5 - 3.5; P < 0.0001). Weight loss was positively related to pre-treatment body weight (r = 0.52, P < 0.05) and to reduction in perce ntage energy as fat (0.37 kg/%, P < 0.005). Extrapolated to a body mass ind ex (BMI) similar to 30 kg/mz, and assuming a 10% reduction in dietary fat, the predicted weight loss would be 4.4 kg (95% CI, 2.06.8 kg), which has be en confirmed in subsequent studies. Newer studies have shown that replacing some carbohydrate with protein may enhance weight loss. CONCLUSION: The American Paradox, the observation that obesity prevalence i s increasing despite a slight decrease in population dietary fat consumptio n, is easily explained by the concomitantly decreasing physical activity, w hich reduces fat requirements and counteracts the beneficial effect of a sl ight reduction in dietary fat. Low fat diets with a high content of complex carbohydrates and protein do not produce any adverse effect on cardiovascu lar risk factors when weight loss is allowed to occur, and they have been s hown to decrease mortality among high risk subjects.