Estimation of turnover of low density lipoproteins by simplified methods

This study was carried out to evaluate simplified methods for estimating parameters of kinetics of low density lipoproteins (LDL). Initially, LDL turnover studies were carried out in 78 patients, and fractional catabolic rates (FCRs) for LDL were determined from die-away curves of plasma radioactivity and from urine-to-plasma (U/P) ratios of radioactivity. When U/P ratios were calculated on a daily basis and throughout the study and averaged by a weighting procedure, the weighted U/P ratios were highly correlated with plasma decay FCRs (r = 0.92, P less than 0.001). One simplified method involved calculation of FCR for LDL from the U/P ratio on the seventh day after injection of radioactivity. Seventh-day U/P ratios also were correlated significantly with plasma decay FCRs (P less than 0.001), but the strength of correlation between the two was relatively weak (r = 0.52). Finally, another correlation was found to be significant. This was the plasma decay FCR vs. the fraction of injected dose disappearing from plasma during the first 24 h (r = 0.86). This comparison was extended to 140 turnover studies, and the correlation remained high (r = 0.90, P less than 0.0001). Thus, estimation of FCR for LDL from the fraction of dose disappearing at 24 h appears superior to that determined from the 7th-day U/P ratio.