One-year effects of increasingly fat-restricted, carbohydrate-enriched diets on lipoprotein levels in free-living subjects

Restriction of all dietary fat is a popular strategy for restricting satura ted fat intake to lower LDL cholesterol. Some authorities advise the restri ction of fat intake to the extreme of less than 10% of daily energy on the assumption that more fat restriction is better. The two studies described h erein address questions relating to whether increasing fat restriction prod uces proportionally increasing benefit on cardiovascular risk factors in hy perlipidemic subjects. The first study is the Dietary Alternatives Study (D AS). The DAS was conducted in 531 male Boeing employees over a 2-year perio d. Subjects were defined as hypercholesterolemic (HC) or combined hyperlipi demic (CHL) based on age-specific 75th percentiles for plasma LDL-C and tri glyceride levels. Hypothesis test analyses were performed at 1 year. HC sub jects were randomized to diets taught to attain fat intakes of 30, 26, 22, and 18% (Diets levels 1-4, respectively). CHL subjects (slightly fewer in n umber) were randomized to Diets IS. After 1 year, subjects' total fat intak es were 27, 26, 25, and 22% of energy (en%), resulting in saturated fat int akes of 8, 7, 7, and 6%, respectively. In HC subjects the greatest LDL-C de crease was with Diet 2 (mean of 13.4%) and in CHL subjects with Diet 1 (7.0 %). Surprisingly, plasma triglyceride concentrations rose in HC subjects 20 % and 40% above baseline on Diets 3 and 4, respectively, with reciprocal re ductions in HDL cholesterol of 2.5% and 3%, respectively. Furthermore, apo B reductions were attenuated below Diet 2 in HC subjects and Diet 1 in CHL subjects, end no further reductions were seen in plasma glucose and insulin concentrations, blood pressure, or body weight. Measurements of plasma tot al fatty acid composition showed a slight increase in plasma palmitate, whe reas stearate decreased slightly, supporting the idea that de novo synthesi s of palmitic acid was increased in the chronic high-carbohydrate feeding c ondition. The second study asked if the most effective diet in HC subjects, Diet 2, has an equivalent effect in women and men. To answer this question , men and women Boeing employees were taught the closely similar National C holesterol Education Program (NCEP) Step II diet. After 6 and 12 months, eq uivalent reductions in LDL cholesterol were observed in women compared with men. HDL cholesterol levels in men were unchanged from baseline at 6 and 1 2 months, but were reduced 8% in HC women, with accompanying decreases of 1 8% in HDL2-cholesterol end 5% in apoprotein A-I (all P < 0.01). These data indicate that intakes of fat below about 25 en% and carbohydrate intake above <approximate to> 60 en% yield no further LDL-C lowering in HC and CHL male subjects and can be counterproductive to triglyceride, HDL-C, and apo B levels. This lack of benefit appears to be explained by an enhan ced endogenous synthesis of palmitic acid, which negates the benefit of fur ther saturated fat restriction. The HDL-C decrease in HC women may have a s imilar cause and points to an underlying male-female difference. Alternativ e dietary approaches to limit saturated fat intake deserve intensive study.