Improved estimation of body masses and turnover of cholesterol by computerized input--output analysis

In 23 patients, the decay curves of serum cholesterol specific activity after a single intravenous dose of radioactive cholesterol were measured for 16-66 wk and were subjected to computerized input-output analysis. Of 17 patients with decay curves followed for longer than 50 wk, a three-exponential curve fit was better in 12, and a two-exponential curve fit in 5, according to computerized F tests. Of six patients with decay curves followed for less than 50 wk, a two-exponential curve fit was better in five and a three-exponential curve fit in one. In the 13 patients who exhibited three-exponential curve fits, the third exponential appeared after 13-43 wk of observation (average, 25 wk). In 12 patients of this group who were followed for 50 wk or more, turnover rates and exchangeable masses of cholesterol were measured at maximum lengths of the curves (50-66 wk), and these parameters were then compared with measurements made with curves successively shortened down to 10-12 wk. The average differences between analyses of the minimum vs. the maximum lengths of the curves were: It (input rate: absorbed dietary plus biosynthesized cholesterol), 14% larger (1.24 vs. 1.09 g/day); M(a) (rapidly exchangeable mass of cholesterol), no change (34 vs. 33 g); M (total exchangeable mass), 26% smaller (67 vs. 91 g); M - M(a) (remaining exchangeable mass), 39% smaller (40 vs. 65 g). Significant differences in It, M, and M - M(a) (minimum vs. maximum curve lengths) were found in both normolipidemic and hypercholesterolemic patients, and the differences were of similar magnitude in the two groups. Since only 12 of 17 patients followed for 50 wk or longer demonstrated three-exponential curve fits, various means were sought by which it might be predicted at the outset whether a given patient must be studied for so long a time; none was found. However, in the group with two exponentials the value of M(a) was significantly larger than those with three-exponential curve fits, and this difference was apparent at as early as 10-12 wk.