Dietary Fat and Lipid Reduction
March 1, 1998
Dietary Fat and Lipid Reduction
ABSTRACT & COMMENTARY
Synopsis: Restricting total fat intake to less than 26% of total calories may be counter-productive.
Source: Knopp RH, et al. JAMA 1997;278:1509-1515.
Knopp and colleagues undertook a study to evaluate dietary fat restriction and its effect on lipids. A total of 444 men had low-density lipoprotein cholesterol (LDL-C) levels above the 75th age-specific percentile. Subjects with triglyceride (TG) levels less than the 75th age-specific percentile were defined as hypercholesterolemic (HC), and those with TG levels at or above the 75th age-specific percentile were defined as combined hyperlipidemic (CHL).
Hypercholesterolemic subjects were randomized to diets 1, 2, 3, and 4, which contained 30%, 26%, 22%, and 18% fat, respectively, and the CHL subjects were randomized to diets 1, 2, and 3. All four diets were taught to subjects and spouses or partners in eight weekly two-hour classes.
Fat intake after one year declined from a mean of 34-36% of energy to 27%, 26%, 25%, and 22% in the four HC diet groups and 28%, 26%, and 25% in the three CHL diet groups. Mean LDL-C reductions were 5.3%, 13.4%, 8.4%, and 13.0% in the HC diet groups and 7.0%, 2.8%, and 4.6% in the CHL diet groups (P < 0.01 in all but 1 instance). Apoprotein B levels decreased 8.6%, 10.7%, 4.3%, and 5.3% in the HC groups and 14.6%, 11.4%, and 9.9% in the CHL groups (P < 0.05-0.01 in each instance). Triglyceride levels increased significantly in subjects following HC diets 3 and 4 (21.7% and 38.7%, respectively) but not in any CHL subjects. HDL-C decreased 2.8% and 3.2% in subjects on HC diets 3 and 4, respectively (P < 0.05 in both cases).
After one year, moderate restriction of dietary fat intake attains meaningful and sustained LDL-C reductions in HC subjects and apoprotein B reductions in both HC and CHL subjects. More extreme restriction of fat intake offers little further advantage in HC or CHL subjects and potentially undesirable effects in HC subjects.
COMMENT BY RALPH R. HALL, MD
As the authors point out in their discussion, "the intuitive appeal of extreme fat restriction traps us into trying to lower the dietary fat as much as possible." The four diets contained 30%, 26%, 25%, and 18% of energy from fat and 300, 200, 100, and 100 mg/d of cholesterol. The ratio of polyunsaturated fat was 1.0.
The study addressed two main questions: 1) Does greater restriction of fat intake attain greater LDL lowering after one year? and 2) Does lipoprotein response (HC and CHL subjects) differ between dietary regimens?
After 12 months, each group had significant reductions in their body weight of 2-3 kg, and greater fat restriction was not associated with greater weight reduction. A maximum and equal amount of LDL-C lowering was observed on diets of 26% and 18% of energy from fat and an achieved reduction of 26% and 22%. In CHL subjects, maximum LDL-C lowering was achieved with a target fat restriction of 30% and an achieved restriction of 28% fat. Further, no significant benefit was obtained in weight reduction, glucose insulin, or blood pressure levels with greater restriction of fat intake.
There were increases in triglyceride levels in HC subjects taking the lower fat diets and decreases in their HDL-C. This suggests a conversion to a more unfavorable lipid profile containing the more damaging small, dense LDL-C that has greater atherogenic properties.
Several important points are worth noting. Despite intensive instruction of educated couples and a highly motivated group of subjects, the groups with goals of less than 26% of total calories were not able to reach these goals. Hypercholesterolemic subjects consuming 25% and 22% of their calories in fat had increases in their triglycerides and significant decreases in their HDL-C levels. Knopp et al also observed that their subjects were free living, as compared with the subjects of Ornish,1 who were receiving only 10% of their calories as fat, but were not free living, were in exercise programs, and were taking fat-lowering agents. Thus, the results of the lipid and weight changes in the two groups cannot be compared.
This is one of the most useful and important studies for practicing physicians on dietary fat intake and compliance issues to be published. It deserves to be read in its entirety by any physician who is caring for hyperlipidemic patients. (Dr. Hall is Emeritus Professor of Medicine, University of Missouri-Kansas City School of Medicine.)
Reference
1. Ornish D, et al. Lancet 1990;336:129-133.
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