Nutritional regulation of cholesterol synthesis and apolipoprotein B kinetics: studies in patients with familial hypercholesterolemia and normal subjects treated with a high carbohydrate, low fat diet.

High carbohydrate, low fat diets decrease plasma low-density lipoprotein cholesterol (LDL-C) and apolipoprotein B (apoB) mass in normal subjects and in patients with familial hypercholesterolemia (FH). To investigate the mechanisms for these effects, four normal, four FH heterozygous, and one FH homozygous subjects were studied on a basal (45% carbohydrate, 40% fat) diet and during continuous nasogastric infusion of Vivonex (90% carbohydrate, 1% fat). For the entire group, the mean changes in total cholesterol, LDL-C, high-density lipoprotein cholesterol (HDL-C) and triglycerides were -90, -95, -14 (all P less than 0.01) and +114 (P less than 0.02) mg/dl, respectively. Fecal sterol balance measurements demonstrated a 24% decrease in whole body cholesterol synthesis in normals, from 8.4 +/- 4.4 (mean +/- SD) to 6.4 +/- 1.3 mg/kg per day and in FH subjects, a 58% decrease, from 11.4 +/- 5.6 to 4.8 +/- 1.7 mg/kg per day (both P less than 0.05). ApoB kinetic studies were performed using a [3H]leucine tracer in two normals and three FH heterozygotes on both basal and Vivonex regimens, and the results were analyzed by compartmental modeling using the SAAM program. Total apoB production was not altered in a consistent manner by carbohydrate feeding. ApoB secretion, however, was shifted from the production of small VLDL/IDL-like particles to large VLDL by Vivonex, with an accompanying increase in intrahepatic assemblage time before secretion. In the two normal subjects, Vivonex induced an increase in apoB loss as VLDL/IDL; however, in the FH patients no such loss occurred. A decrease (P less than 0.05) in the residence time of LDL-apoB occurred for all subjects and was the primary determinant of the fall in plasma LDL concentration, since LDL-apoB transport did not change consistently. Thus, in FH patients, a high carbohydrate, low fat diet results in suppression of cholesterol synthesis and a fall in plasma LDL concentration due to an increased plasma clearance rate for LDL.