Tamoxifen and toremifene lower serum cholesterol by inhibition of delta 8-cholesterol conversion to lathosterol in women with breast cancer

PURPOSE

Long-term effects of tamoxifen and toremifene, a new antiestrogen that closely resembles tamoxifen, were investigated on serum lipids and cholesterol metabolism.

PATIENTS AND METHODS

The study group consisted of 24 postmenopausal Finnish women with advanced breast cancer from an international multicenter study of 415 patients. Cholesterol metabolism was evaluated by measuring the cholesterol precursor (delta 8-cholestenol, desmosterol, and lathosterol, reflecting cholesterol synthesis) and plant sterol (markers of cholesterol absorption) and cholestanol levels by gas-liquid chromatography.

RESULTS

Tamoxifen and toremifene lowered significantly serum low-density lipoprotein (LDL) cholesterol levels after 12 months of treatment by 16% and 15%, with no change in high-density lipoprotein (HDL) cholesterol or serum triglyceride levels. Serum delta 8-cholestenol was increased 40- and 55-fold during toremifene and tamoxifen treatment, respectively, while the increase of desmosterol less than doubled and was lacking for lathosterol by toremifene. Plant sterols and cholestanol were only inconsistently increased in serum.

CONCLUSION

Tamoxifen and toremifene inhibit the conversion of delta 8-cholestenol to lathosterol so that serum total and LDL cholesterol levels are lowered by downregulation of cholesterol synthesis. Thus, inhibition of the delta 8-isomerase may be the major hypolipidemic effect of these agents. Reduced risk of coronary artery disease will probably occur also during long-term toremifene treatment, because the drug reduces cholesterol and its synthesis, similarly to tamoxifen.

Reduction of serum cholesterol with sitostanol-ester margarine in a mildly hypercholesterolemic population

BACKGROUND

Dietary plant sterols, especially sitostanol, reduce serum cholesterol by inhibiting cholesterol absorption. Soluble sitostanol may be more effective than a less soluble preparation. We tested the tolerability and cholesterol-lowering effect of margarine containing sitostanol ester in a population with mild hypercholesterolemia.

METHODS

We conducted a one-year, randomized, double-blind study in 153 randomly selected subjects with mild hypercholesterolemia. Fifty-one consumed margarine without sitostanol ester (the control group), and 102 consumed margarine containing sitostanol ester (1.8 or 2.6 g of sitostanol per day).

RESULTS

The margarine containing sitostanol ester was well tolerated. The mean one-year reduction in serum cholesterol was 10.2 percent in the sitostanol group, as compared with an increase of 0.1 percent in the control group. The difference in the change in serum cholesterol concentration between the two groups was -24 mg per deciliter (95 percent confidence interval, -17 to -32; P < 0.001). The respective reductions in low-density lipoprotein (LDL) cholesterol were 14.1 percent in the sitostanol group and 1.1 percent in the control group. The difference in the change in LDL cholesterol concentration between the two groups was -21 mg per deciliter (95 percent confidence interval, -14 to -29; P < 0.001). Neither serum triglyceride nor high-density lipoprotein cholesterol concentrations were affected by sitostanol. Serum campesterol, a dietary plant sterol whose levels reflect cholesterol absorption, was decreased by 36 percent in the sitostanol group, and the reduction was directly correlated with the reduction in total cholesterol (r = 0.57, P < 0.001).

CONCLUSIONS

Substituting sitostanol-ester margarine for part of the daily fat intake in subjects with mild hypercholesterolemia was effective in lowering serum total cholesterol and LDL cholesterol.

The effect of cholesterol absorption inhibition on low density lipoprotein cholesterol level

The degree of serum cholesterol lowering by up to almost maximal inhibition of cholesterol absorption was tested during neomycin and neomycin + sitostanol treatment in six hypercholesterolemic men. Neomycin decreased cholesterol absorption efficiency by 49% and the combination by 79%, and serum cholesterol level by 27% and 36%, respectively. The correlation between the absorption percentage and low density lipoprotein (LDL) cholesterol was significant (r = 0.510), and the regression equation (y = 0.04x + 2.59) suggested that the mean LDL cholesterol content would be about 2.5 mmol/l at zero cholesterol absorption. In conclusion, in hypercholesterolemic subjects, the lowering of LDL cholesterol appears to be limited to a low normal range only by almost totally inhibiting cholesterol absorption.

Sitostanol ester margarine in dietary treatment of children with familial hypercholesterolemia

In familial hypercholesterolemia (FH) the lowering of serum cholesterol levels should be started in childhood in order to prevent coronary artery disease later in life. However, treatment of children is problematic. We studied the effects of sitostanol (3 g/day) ester dissolved in rapeseed oil margarine as a hypocholesterolemic agent in one homozygous and 14 heterozygous children with FH maintained on a low cholesterol diet for 6 weeks, using a double-blind crossover design. Absorption and synthesis of cholesterol were evaluated by measuring serum plant sterol and cholesterol precursor proportions to cholesterol by gas-liquid chromatography. The compliance was good, and the children could not distinguish by taste the two margarines without and with sitostanol ester. Sitostanol margarine significantly reduced serum total, intermediate density (IDL), and low density lipoprotein (LDL) cholesterol by 11, 26, and 15%, respectively, and increased HDL/LDL cholesterol ratio by 27%. The proportions of serum delta 8-cholestenol, lathosterol, and desmosterol were significantly increased by 36, 19, and 18%, and those of serum cholestanol, campesterol, and sitosterol were significantly decreased by 9, 42 and 29%, respectively, suggesting that cholesterol absorption was decreased and synthesis was compensatorily increased. High basal precursor sterol proportions predicted a high decrease in LDL cholesterol levels. In conclusion, partial replacement of normal dietary fat consumption by sitostanol ester margarine appears to be an effective and safe hypocholesterolemic treatment in children with FH.

Postprandial vitamin A and squalene clearances and cholesterol synthesis off and on lovastatin treatment in type III hyperlipoproteinemia

Postprandial fat clearance and absorption, fecal elimination and synthesis of cholesterol, bile acid synthesis, and cholesterol precursors and plant sterols in serum were studied in five patients with type III dyslipoproteinemia off and on lovastatin. The basal values were related to those in nontreated normolipidemic control subjects with apolipoprotein E3/3 phenotype (apo E3 controls, n = 16). On regular home diets, cholesterol precursor concentrations and cholesterol precursor/cholesterol ratios were high in the type III group. However, cholesterol absorption efficiency, bile acid and cholesterol synthesis measured with sterol balance technique and the precursor sterol/plant sterol ratios in serum were similar to the control values, suggesting that cholesterol absorption and metabolism was normal in these subjects. Lovastatin normalized the increased lipoprotein concentrations and reduced biliary cholesterol secretion, absolute absorption of cholesterol, precursor sterol/cholesterol and precursor sterol/plant sterol ratios in serum, fecal neutral and total sterol outputs and cholesterol synthesis. Lovastatin had no effect on cholesterol absorption efficiency or bile acid synthesis. Despite normalization of the triglyceride-rich lipoprotein levels by lovastatin, the postprandial vitamin A and squalene peak concentrations and the areas under the curves remained above the control ranges. The findings show that in type III hyperlipidemia, the precursor sterol/cholesterol ratios do not predict cholesterol synthesis. The latter, bile acid synthesis, precursor sterol/plant sterol ratios in serum, and cholesterol absorption are normal under basal conditions. The normalization of increased lipids by lovastatin is mainly due to reduced synthesis and absolute absorption of cholesterol, while the retarded postprandial fat clearance was not normalized by the drug.

Two different allelic mutations in a Finnish family with lecithin:cholesterol acyltransferase deficiency

Lecithin:cholesterol acyltransferase (LCAT) deficiency is a genetic disorder associated with low levels of serum HDL cholesterol. The proband of the Finnish LCAT-deficient family had corneal opacities, proteinuria, anemia with stomatocytosis, low serum HDL cholesterol (0.27 mmol/L), and low LCAT activity. Sequence analysis of his LCAT gene revealed compound heterozygosity for two different mutations: a C insertion in exon 1 between nucleotides 932 and 937 and a C-to-T point mutation in exon 6 at position 4976. The C insertion in exon 1 is predicted to result in premature termination and a truncated polypeptide containing only 16 amino acids. The C-to-T point mutation in exon 6 substitutes cysteine for arginine at residue 399. The functional significance of the Arg399-->Cys mutation was examined by expressing the mutated and wild-type LCAT cDNAs in COS cells. COS cells transfected with mutated and wild-type cDNAs showed comparable levels of mature LCAT mRNA. However, LCAT activity in the cell media of COS cells transfected with the mutant LCAT cDNA was significantly lower than that of COS cells transfected with the wild-type cDNA (1.4% versus 12.0% cholesterol esterified, respectively). A polymerase chain reaction-based duplex assay, in which both mutations can be detected simultaneously, was used for preliminary screening of Finnish subjects with serum HDL levels below 0.9 mmol/L; two additional individuals heterozygous for the Arg399-->Cys mutation were identified.(ABSTRACT TRUNCATED AT 250 WORDS)

Accumulation of 99mTc-low-density lipoprotein in human malignant glioma

Low-density lipoprotein (LDL) uptake in gliomas was studied to find out if LDL has potential as a drug carrier of boron, especially for boron neutron capture therapy. Single photon emission tomography (SPET) was performed 2 h and 20 h after intravenous injection of autologous 99mTc-labelled LDL in four patients with untreated and five patients with recurrent glioma. 99mTc-LDL uptake was compared with the uptake of 99mTc-labelled human serum albumin (HSA), an established blood pool marker. The intra- and peritumoral distributions of radioactivity in the SPET images were not identical for radiolabelled LDL and HSA. The mean LDL tumour to brain ratio, determined from transversal SPET slices at 20 h post injection, was 1.5 in untreated and 2.2 in recurrent gliomas; the corresponding ratios for HSA were 1.6 and 3.4. The brain to blood ratio remained constant at 2 h and 20 h in both types of tumours. These data are not consistent with highly selective, homogeneous uptake of LDL in gliomas. However, the different tumoral distribution and rate of uptake of 99mTc-LDL, as compared with 99mTc-HSA, indicate that the uptake of LDL is different from that of HSA and that further studies on the mechanism of LDL uptake in glioma are warranted.

Cholesterol absorption and metabolism and LDL kinetics in healthy men with different apoprotein E phenotypes and apoprotein B Xba I and LDL receptor Pvu II genotypes

Apoprotein (apo) E, apoB Xba I, and LDL receptor gene Pvu II polymorphisms are associated with LDL cholesterol level, but little is known about cholesterol and LDL metabolism in subjects with the latter two genetic polymorphisms alone or in combination with different apoE phenotypes. We studied cholesterol absorption efficiency, cholesterol and bile acid synthesis, and LDL apoB kinetics in 52 healthy men and related the metabolic results to the apoB Xba I and LDL receptor Pvu II restriction fragment length polymorphism (RFLP) and apoE phenotypes. New findings were as follows. ApoB Xba I polymorphism was not associated with the metabolic variables of cholesterol, but LDL receptor Pvu II RFLP was associated with fractional catabolic rate for LDL apoB, cholesterol absorption, and cholesterol and bile acid synthesis. ApoE polymorphism exerted the most powerful effect on the LDL cholesterol concentration, so that the apoE2 subjects had the lowest LDL cholesterol and apoB levels and cholesterol absorption, and the highest fractional catabolic rate and bile acid and cholesterol synthesis compared with the apoE3 or especially apoE4 phenotypes in different genetic combinations. In multiple stepwise regression analysis with LDL cholesterol as the dependent and the genetic and metabolic parameters as the independent variables, 47.0% (n = 35, P < .001) of the variability of LDL cholesterol was explained by the apoE polymorphism, 7.1% (P < .05) by the LDL receptor Pvu II RFLP, and 11.3% (P < .01) by bile acid synthesis, while the contribution of the apoB Xba I RFLP was nonsignificant.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of cholesterol and low- and high-density lipoproteins in primary biliary cirrhosis: cholesterol absorption and synthesis related to lipoprotein levels and their kinetics

Cholesterol absorption, elimination, and synthesis, and low-density lipoprotein (LDL) and high density lipoprotein (HDL) kinetics were studied in patients with mild to severe primary biliary cirrhosis (PBC) (n = 16) to show how this cholestatic disease modified cholesterol and lipoprotein metabolism as compared with healthy controls (n = 50). Serum total and lipoprotein cholesterol and triglyceride levels were similar in the two groups, but in PBC, especially in severe forms, very low density lipoprotein (VLDL) was rich in apoprotein (apo) B and cholesterol and low in triglycerides, whereas LDL was rich in triglycerides and low in esterified cholesterol, and HDL was enriched by surface lipids, phospholipids, and free cholesterol. In severe PBC, the fractional catabolic rate (FCR) for LDL apo B was reduced. The transport rate (TR) for LDL apo B was unaffected and it tended to correlate with the LDL apo B and LDL cholesterol levels in PBC, whereas in the controls the LDL apo B concentration was regulated by both the FCR and TR, and LDL cholesterol was regulated only by FCR. FCR for apo A-I in HDL was unaltered in PBC, but TR for apo A-I was reduced in the severe cases. Cholesterol absorption efficiency was significantly reduced in PBC (14.5 +/- 3.0% in severe PBC and 34.0 +/- 2.5% in mild PBC vs. 47.4 +/- 1.4% in the controls, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol absorption and cholesterol and bile acid synthesis in two brothers with IDDM and diarrhea

OBJECTIVE

To determine the best possible treatment for two brothers who had IDDM and bile acid malabsorption-associated disabling diarrhea that was resistant to cholestyramine, antibiotics, clonidine, loperamide, or anticholinergic drugs.

RESEARCH DESIGN AND METHODS

Our study paired these two brothers with control subjects. Serum noncholesterol sterols and fecal elimination of cholesterol and plant sterols were quantitated by gas-liquid chromatography.

RESULTS

Fecal losses of bile acids exceeded four to six times the control values, and cholesterol synthesis was increased two to three times. Cholesterol absorption efficiency and serum cholesterol concentrations were, respectively, within the high and low control range.

CONCLUSIONS

Bile acid malabsorption might have resulted from deranged intestinal motility, possibly contributed by gastrointestinal neuropathy, and some genetic factor.

Serum cholesterol and cholesterol and lipoprotein metabolism in hypercholesterolaemic NIDDM patients before and during sitostanol ester-margarine treatment

Cholesterol absorption and metabolism and LDL and HDL kinetics were investigated in 11 hypercholesterolaemic non-insulin-dependent diabetic men off and on a hypolipidaemic treatment with sitostanol ester, (3 g sitostanol daily) dissolved in rapeseed oil margarine, by a double-blind crossover study design. Serum total, VLDL and LDL cholesterol and apoprotein B fell significantly by 6 +/- 2, 12 +/- 6, 9 +/- 3 and 6 +/- 2%, mean +/- SEM, and HDL cholesterol was increased by 11 +/- 4% (p < 0.05) by sitostanol ester. LDL cholesterol and apoprotein B were significantly decreased in the dense (1.037-1.055 g/ml), but not light, LDL subfraction due to a significantly diminished transport rate for LDL apoprotein B, while the fractional catabolic rate was unchanged. HDL kinetics, measured with autologous apoprotein A I, was unaffected by sitostanol ester. Cholesterol absorption efficiency was markedly reduced from 25 +/- 2 to 9 +/- 2% (p < 0.001) during sitostanol ester followed by proportionately decreased serum plant sterol proportions. Cholesterol precursor sterol proportions in serum, fecal neutral sterol excretion, and cholesterol synthesis, cholesterol transport, and biliary secretion were all significantly increased by sitostanol ester. We conclude that the sitostanol ester-induced decrease in cholesterol absorption compensatorily stimulated cholesterol synthesis, had no effect on fractional catabolic rate, but decreased transport rate for LDL apoprotein B so that serum total, VLDL and LDL cholesterol levels were decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of serum cholesterol level in middle-aged and elderly men. Relation of cholesterol absorption and synthesis to lipoprotein metabolism

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)

Postabsorptive metabolism of dietary squalene

Serum squalene, a non-steroid intermediate of cholesterol biosynthesis, originates mainly from endogenous cholesterol synthesis and partly from diet, especially in populations consuming a lot of olive oil rich in squalene. Its postabsorptive metabolism has not been studied in detail in humans. Its presence in chylomicrons and VLDL suggests that the removal of dietary squalene may reflect the metabolism of intestinal lipoproteins. Accordingly, we studied the postabsorptive metabolism of 1 g dietary squalene in 16 healthy subjects with apolipoprotein (apo) E 3/3 phenotype and in five type III hyperlipidemic apo E 2/2 homozygotes known to have a retarded chylomicron remnant removal, and compared the results with vitamin A fat load test. About 40% of the basal and 90% of the postabsorptive squalene was in lipoproteins < 1.019 g/ml. The peak concentrations of chylomicron squalene were at 6 h, and of triglyceride-rich nonchylo-fraction at 9-12 h in the controls. The peak values occurred later than those of vitamin A. At 24 h the levels still exceeded the basal ones. In type III dyslipoproteinemia, most of the basal and postabsorptive squalene was in lipoproteins of density less than 1.019 g/ml, the peak postabsorptive values occurred later than in the controls and the serum values remained above the control levels for up to 24 h. The squalene and vitamin A areas under the incremental response curves (AUC) were higher than in the control group. The AUCs of the two markers in chylomicron were correlated negatively and those in LDL+HDL were correlated positively with fasting HDL cholesterol levels, the respective correlations being opposite with fasting VLDL triglycerides. The postabsorptive profile of squalene levels resembled that of vitamin A in both groups, except that the squalene curves were shifted to a later time period. Thus, a delayed clearance of chylomicron remnants could be detected by analyzing serum squalene 6-24 h after the squalene-supplemented fat meal.

Deletion of exon 15 of the LDL receptor gene is associated with a mild form of familial hypercholesterolemia. FH-Espoo

We describe a mutation of the low-density lipoprotein (LDL) receptor gene, designated familial hypercholesterolemia (FH)-Espoo, which deletes exon 15 of the LDL receptor gene. The mutant receptor is predicted to lack 57 amino acids, including 18 serine and threonine residues, which are the sites of the clustered O-linked sugars of the receptor. Studies on 10 carriers of this gene revealed that FH-Espoo is associated with an exceptionally mild form of FH. Thus, in conditions in which cell proliferation was rendered dependent on the function of LDL receptors, lymphocytes from the patients with the FH-Espoo allele had a growth rate intermediate between those from healthy subjects and patients with the FH-Helsinki gene, a mutation known to abolish LDL receptor function. The in vivo fractional catabolic rate of LDL apolipoprotein B was lower than normal in the two FH-Espoo heterozygotes studied. Although higher than those in healthy controls, the serum LDL cholesterol concentrations in patients with the FH-Espoo gene were significantly lower than those in patients with the FH-Helsinki mutation. The thickness of the Achilles tendons was within the normal limits in subjects with the FH-Espoo gene. Our study suggests that moderate varieties of hypercholesterolemia, ie, those not considered to represent FH, may occasionally be due to subtle LDL receptor gene mutations.

Metabolism of cholesterol and apolipoprotein B in celiac disease

To test the association of cholesterol malabsorption with cholesterol and lipoprotein metabolism, we determined low-density lipoprotein (LDL) apolipoprotein (apo) B kinetics simultaneously with measurements of cholesterol absorption and synthesis in six patients with celiac disease (CD) before and during the gluten-free diet (GFD). The basal condition was characterized by low cholesterol absorption, enhanced cholesterol synthesis, and high removal and transport rate of LDL apo B. The GFD markedly improved cholesterol absorption and decreased intestinal influx of cholesterol, fecal neutral steroids, and cholesterol synthesis. Of plasma total and lipoprotein cholesterol levels, only plasma high-density lipoprotein (HDL) was enhanced by the GFD proportionately to cholesterol absorption. The plasma LDL apo B level remained unchanged because of simultaneous decreases in the fractional catabolic rate (FCR) and transport rate of LDL apo B. In fact, the more cholesterol absorption was improved by the GFD, the more the FCR and transport rate for LDL apo B were decreased, and their reductions were closely related to the decrease in cholesterol synthesis. The present results show that cholesterol absorption, cholesterol synthesis, hepatic B/E receptor activity, and LDL apo B transport rate are closely associated with each other and that their levels can change markedly with no detectable change in serum levels of LDL cholesterol or apo B.

Postabsorptive retinyl palmitate removal is retarded in lecithin-cholesterol acyltransferase deficiency

Postabsorption fat clearance of intestinal lipoproteins indicated by retinyl palmitate was studied in two siblings with the classical lecithin-cholesterol acyltransferase (LCAT) deficiency and in a control group of 21 healthy subjects with similar apoprotein E 3/3 phenotype. Relatively high pre- and postabsorptive cholesterol esterification percentage of chylomicrons suggested that acyl coenzyme A:cholesterol acyltransferase activity was not inhibited. Postabsorptive levels of plasma lipids increased and decreased roughly similarly in the cases and controls with the postabsorptive peak values at about 4 h. Plasma total and chylomicron levels of retinyl palmitate were not affected by LCAT deficiency, while the removal of the vitamin from very low and intermediate density lipoproteins was clearly reduced so that the peak concentrations, values under the response curves and the time of the peak concentrations were markedly higher than in the controls. The long-lasting circulation of chylomicron remnants of density < 1.006-1.019 g ml-1 may have some clinical significance because postabsorptive lipoproteins are suspected to have atherogenic potentiality.

Effects of ketoconazole on cholesterol precursors and low density lipoprotein kinetics in hypercholesterolemia

Ketoconazole, an inhibitor of cholesterol synthesis at 14 alpha-demethylation of lanosterol, effectively reduces serum total and low density lipoprotein (LDL) cholesterol levels. We studied the effects of ketoconazole (400 mg/day for 5 weeks) on serum lipids, free and esterified noncholesterol sterols, and kinetics of LDL apolipoprotein B (apoB) in seven patients with heterozygous familial hypercholesterolemia (FH) and in three patients with primary hypercholesterolemia (nonFH). The total, intermediate density, and LDL cholesterol levels were significantly reduced by 24, 27, and 29%, respectively, and LDL apoB by 23%. Serum total and lipoprotein triglycerides were unchanged. The LDL cholesterol/apoB ratio decreased significantly. Serum ratios of lanosterols to cholesterol were increased over 50 times, almost the same in all lipoproteins and mainly as the unesterified form; free delta 8-precursor sterols, 2-5 times; cholestanol, slightly; while ratios of lanosterol of desmosterol, lathosterol, and plant sterols were virtually unchanged. Inconsistent esterification of methyl sterols might indicate unaltered acyl CoA:cholesterol acyltransferase activity. LDL apoB transport was decreased in all nonFH subjects but inconsistently in FH. The fractional catabolism rate (FCR) for LDL apoB was increased significantly in FH by 13% and inconsistently by 4% in nonFH. In a subgroup of three FH patients, more dense LDL (d 1.037-1.055 g/ml) was transported and catabolized faster on than off ketoconazole so that the serum level of this more dense LDL subfraction was unchanged, the decrease of LDL being due to a reduction of the less dense LDL subfraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiologic mechanisms for reduced apolipoprotein A-I concentrations associated with low levels of high density lipoprotein cholesterol in patients with normal plasma lipids

Low plasma concentrations of high density lipoprotein (HDL) cholesterol and apolipoprotein A-I (apoA-I) are major risk factors for coronary heart disease (CHD). Low HDL levels are common in patients with hypertriglyceridemia, but they also occur in those with normal plasma lipids; the latter include obese patients and cigarette smokers, though other patients with low HDL levels are neither obese nor smokers. The present study was designed to define metabolic causes of low apoA-I levels in normal-weight, normolipidemic patients. ApoA-I tracer studies were carried out in two groups of normolipidemic patients having low HDL levels to determine input rates and residence times for ApoA-I; these patients included 11 nonobese nonsmokers and 11 nonobese cigarette smokers. Their results were compared to those of 20 normal-weight, normolipidemic controls with normal HDL levels and 12 obese nonsmokers also having low HDL. In all three groups manifesting low HDL-cholesterol and low apoA-I levels, residence times for plasma apoA-I were reduced by approximately 30%, compared to control subjects with normal HDL levels. In contrast, average input rates for apoA-I were similar among the three low-HDL patients and control subjects. No differences in apoA-I kinetics were observed among any of the three groups with low apoA-I concentrations. Within each of the four groups of the study, however, input rates for apoA-I were highly correlated with plasma concentrations of apoA-I. Thus, for individuals with normal levels of plasma lipids, both residence times and input rates for apoA-I appeared to be important determinants of apoA-I levels. Residence times for apoA-I were reduced in almost all patients with low apoA-I levels, regardless of concomitant factors, whereas input rates were highly variable among individuals.