BACKGROUND: Does dietary fat play a central role in weight gain and development of obesity? Do low-fat diets have adverse effects on blood lipids?
OBJECTIVE AND DESIGN: To answer these questions we have reviewed the evidence linking the dietary fat content to energy balance and obesity, and examined the efficacy and safety of ad libitum low fat, high carbohydrate/protein 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) energy from fat is less satiating than energy from carbohydrate, and a high fat/carbohydrate ratio in the diet promotes passive overconsumption, a positive energy balance and weight gain in susceptible individuals; (2) fat is more readily absorbed from the intestine and fecal energy loss is much lower with a high dietary fat/carbohydrate ratio; (3) carbohydrate is more thermogenic than fat and energy expenditure is lower during positive energy balance produced by a diet with a high fat/carbohydrate ratio than during positive energy balance produced by a diet with a low fat/carbohydrate ratio.
Randomized intervention studies comparing low fat diets to normal fat diets show that low fat diets prevent weight gain in normal weight subjects and produce weight loss in overweight individuals. In our meta-analysis of ad libitum 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 percentage energy as fat (0.37 kg/%, P<0.005). Extrapolated to a body mass index (BMI)∼30 kg/m2, and assuming a 10% reduction in dietary fat, the predicted weight loss would be 4.4 kg (95% CI, 2.0–6.8 kg), which has been 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 is increasing despite a slight decrease in population dietary fat consumption, is easily explained by the concomitantly decreasing physical activity, which reduces fat requirements and counteracts the beneficial effect of a slight reduction in dietary fat.
Low fat diets with a high content of complex carbohydrates and protein do not produce any adverse effect on cardiovascular risk factors when weight loss is allowed to occur, and they have been shown to decrease mortality among high risk subjects.