The aim of the present study was to investigate cholesterol absorption and cholesterol and bile acid synthesis and relate these values of kinetics of low-density lipoprotein (LDL) apoprotein (apo) B in 50- and 75-year-old men to find out why and by which mechanism serum cholesterol level decreases with advancing age under normal home-living conditions. The daily calorie, fat, and cholesterol intakes were lower in the 75-year-old men because the physiological requirements of daily energy are reduced in old age. However, absolute body weight was identical in the two groups, indicating isocaloric energy intake. Serum levels of total and LDL cholesterol were insignificant lower but those of LDL apo B significantly lower, so that the LDL cholesterol/apo B ratio was higher in the elderly men. The mean reduction of LDL apo B by 26% (P < .05) in the old men was associated with a 30% (P < .05) decrease in transport rate (TR) and a 3% (P = NS) decrease in removal (FCR) for LDL apo B. However, at the comparable apo B levels, both TR and FCR for apo B were significantly lower in the old than in the younger men. Cholesterol absorption efficiency, bile acid synthesis, fecal neutral and total sterol excretion, and cholesterol turnover but not synthesis were reduced in the elderly men.(ABSTRACT TRUNCATED AT 250 WORDS)