The diet-heart hypothesis, obesity and diabetes

Abstract

Human feeding studies show that dietary fat quality, but not total fat intake, influences levels of low-density lipoprotein (LDL) cholesterol. Metaanalyses indicate an association with reduced coronary heart disease (CHD) risk when saturated fatty acid is replaced with polyunsaturated fatty acid or with low-glycaemic index carbohydrates. A meta-analysis of eight small trials supports this benefit. Secular trends in populations that modified fat intake and quality show a consistent reduction in LDL  cholesterol levels and CHD risk. The increase in obesity and diabetes in many developed countries does not track consistently with the  implementation of dietary guidelines aimed at lowering fat intake. Obesity is more likely to be due to increases in total energy intake, coupled with an increasingly sedentary lifestyle. However, cohort studies indicate that poor dietary quality is associated with future weight gain. Both cohort studies and secular trends implicate the increased consumption of sugar-sweetened beverages as being associated with obesity, diabetes and cardiovascular disease. Weight reduction can be achieved with a range of energy-restricted diets, including low-fat, high-carbohydrate diets and low-carbohydrate, high-fat diets. Metabolic benefits are proportional to the degree of weight reduction, irrespective of the dietary approach used. The prevention of CHD requires an emphasis on fat quality, rather than fat quantity, while the prevention of obesity and diabetes requires a focus on energy balance and carbohydrate quality.