Kinetic mechanisms determining variability in low density lipoprotein levels and rise with age.

Levels of plasma low density lipoproteins (LDL) vary among individuals at any given age and frequently rise with increasing age. Both production rates and fractional clearance rates (FCRs) of LDL theoretically could affect the plasma levels of LDL. To evaluate the relative importance of these two factors, turnover rates of LDL apoprotein (apoLDL) were determined in two groups: 19 young adult men aged 23 to 29 years and 15 middle-aged men aged 40 to 60 years. Results were compared to a group of six healthy young adults (aged 22 to 28 years) who we previously studied and who were on a cholesterol-lowering diet. In both groups in the current study, a diet resembling the average American diet was consumed, and LDL levels ranged from low-to-high normal. On average, the 19 young adult men had lower levels of total cholesterol and LDL cholesterol than did the middle-aged men. The younger men also had significantly higher FCRs and lower production rates of apoLDL. When data from all subjects were pooled, apoLDL levels were negatively and significantly correlated with FCRs and positively and significantly correlated with production rates. Similar relations were found with LDL cholesterol levels. These results show that both FCRs and production rates of apoLDL are important regulators of plasma LDL levels; the correlation suggests that the FCR is more influential at lower LDL concentrations, and that production rates are more influential at higher LDL concentrations.