Dietary fat and obesity: still an important issue

Abstract

The role of a reduction in dietary fat for weight loss and maintenance should be assessed by evidence-based principles giving most weight to randomized clinical trials. Four meta-analyses examined weight changes on ad libitum fat-reduced diets in intervention trials lasting for up to 1 year, and they all demonstrated a 3_/4 kg larger weight loss on the fat-reduced than on the normal-fat diet in normal-weight and overweight subjects. The analyses also show a dose_/response relationship, i.e. the reduction in percentage energy as fat is positively associated with weight loss. Weight loss is also positively related to initial weight: a 10% reduction in dietary fat is predicted to produce a 4_/5 kg weight loss in an individual with a body mass index of 30 kg m_2. The outcome of the meta-analysis of trials with long-term follow-up included only six studies, and none of the trials had an active intervention throughout the period. Short-term trials clearly demonstrate th at the non-fat diet components are at least as important for body-weight regulation as the fat content. Sugar in beverages is less satiating and more obesity promoting than sugar in solid foods, and replacement of energy from fat by sugar derived from sweetened beverages is not likely to produce weight loss. Protein is more satiating and thermogenic than carbohydrates, and a fat-reduced diet with a high protein content (20-25% of energy) may increase the efficacy of fat-reduced diets markedly. Whereas the glycaemic index of the carbohydrate may play a role for cardiovascular risk factors, there is very little evidence to support that low glycaemic index foods facilitate weight control. The evidence linking particular fatty acids to body fatness is weak. If anything, monounsaturated fatty acids (MUFA) may be more fattening than polyunsaturated and saturated fats, and no ad libitum dietary intervention study has shown that a normal-fat, high-MUFA diet is equivalent or superior to a low-fat diet in the prevention of weight gain and obesity. The current evidence strongly supports a diet with reduced content of fat and sugar-rich beverages, and more carbohydrates, rich in fibre and grain (whole-grain foods, fruit and vegetables) and protein (lean meat and dairy products) as the best choice for the prevention of weight gain, obesity, type 2 diabetes and cardiovascular disease. The use of a normal-fat, high-MUFA diet needs more evidence from randomized ad libitum dietary intervention trials before it can be recommended to the public. Keywords: fat-reduced diets; food pyramid; glycaemic index; low-fat diets; protein; weight loss