Since dietary modification is an initial step in the management of hypercholesterolemia, it seems logical to assume that greater fat restriction may lead to larger decreases in low-density lipoprotein (LDL) cholesterol levels. Knopp and colleagues conducted a randomized comparison trial of four different low-fat diets to determine the effects of varying degrees of fat restriction on lipid levels.

A total of 444 men were recruited from an industrial firm for the one-year study. Each participant had two LDL determinations greater than the 75th percentile (for age). Subjects with triglyceride levels below the 75th age-specific percentile were classified as having hypercholesterolemia and those with triglyceride levels above the 75th age-specific percentile were classified as having combined hyperlipidemia. Men were excluded from the study if they had medical conditions that might cause hyperlipidemia, such as diabetes mellitus, nephrotic syndrome or hypothyroidism, or if they were receiving medications that might affect lipid levels, such as beta-adrenergic blockers, thiazide diuretics and lipid-lowering drugs.

Four fat-restricted diets were studied: 30 percent fat and 300 mg per day of cholesterol; 26 percent fat and 200 mg per day of cholesterol; 22 percent fat and 100 mg per day of cholesterol; and 18 percent fat and 100 mg per day of cholesterol. Men with combined hyperlipidemia were not assigned to the diet containing 18 percent fat. Study subjects received training on following the fat-restricted diets. A four-day food diary was collected at baseline and at five other times during the study. The average baseline fat intake was 34 to 36 percent. Lipoprotein analysis was performed at one, three, six, nine and 12 months.

The men on the lowest-fat diets did not lose significantly more weight than those on the less-restrictive diets. Men in the 30 percent fat intake group actually ingested a diet containing only 27 to 28 percent fat. The group adhering to the 26 percent fat diet met their goal intake. The men assigned to diets containing 22 and 18 percent fat did not meet their goals, ingesting 25 and 22 percent fat, respectively.

LDL levels were reduced significantly in all of the study groups. In the hypercholesterolemic men, LDL levels fell by 5.3, 13.4, 8.4 and 13.0 percent on the 30, 26, 22 and 18 percent fat diets, respectively. However, high-density lipoprotein (HDL) cholesterol levels decreased 2.8 percent and 3.2 percent in the hypercholesterolemic men consuming the two most restrictive diets. Plasma triglyceride levels increased by 22 percent and 39 percent in the hypercholesterolemic men on the two most fat-restrictive diets.

In the combined hyperlipidemia group, the decrease in LDL levels was not as great as that in the hypercholesterolemic group. LDL levels dropped 7.0, 2.8 and 4.6 percent with the three different diets these men followed. Triglyceride levels did not increase in the men with combined hyperlipidemia who were assigned to the most restrictive diet.

The authors conclude that long-term moderate restriction of fat intake (both total and saturated fat) is effective in reducing LDL levels. More aggressive fat restriction offers little further advantage and may have undesirable effects in patients with hypercholesterolemia.