Physiological Study of Human Cholesterol Metabolism and Excretion

Reverse Cholesterol Transport in Humans

The underlying hypothesis is that whole body cholesterol - including cholesterol present in tissues that cannot be measured by standard blood tests - is related to heart disease risk. Endogenous cholesterol will be labeled with an intravenous infusion of one type of cholesterol tracer and dietary cholesterol will be labeled with another. These tracers will be used to measure how fast cholesterol is synthesized and excreted using mass spectrometry to distinguish the tracers. Data will be related to circulating biomarkers (blood tests) and to the thickness of the lining of the carotid artery. The effect of the drug ezetimibe on these processes will also be determined. Successful completion of this study will give us more knowledge about cholesterol metabolism that may be useful in designing new drugs and treatments for patients with heart disease, especially those that are already receiving maximum amounts of current medications.

The central hypothesis of this proposal is that reverse cholesterol transport is related to coronary heart disease (CHD) risk. It is complementary to the concept that reduction of cholesterol biosynthesis with statin drugs prevents CHD, but it focuses on whole body cholesterol metabolism and kinetic cholesterol transport rather than on static levels of circulating lipoproteins. Although this is an old idea, it has not been adequately tested in humans because of lack of suitable methods. In this proposal we will apply innovative stable isotope and mass spectroscopic technology to study reverse cholesterol transport in human subjects. The first specific aim is to improve the preparation of intravenous deuterated cholesterol tracer, a critical limiting element in the study of whole body cholesterol metabolism. The second aim is to use that intravenous tracer, along with a different oral tracer, to partition fecal cholesterol into excreted endogenous cholesterol, unabsorbed dietary cholesterol and newly-synthesized cholesterol derived from the liver and intestine. Measurements will be made during consumption of a controlled diet provided by the metabolic kitchen. The pool size of the rapidly-mixing body cholesterol pool will be measured along with the fractional rate of cholesterol catabolism. These direct measures of reverse cholesterol transport will be correlated with plasma biomarkers and with metabolic covariates. The relation of reverse cholesterol transport to carotid intima-media thickness will be determined. The third specific aim will use similar methods to study the mechanism of action for the widely-used drug ezetimibe. The fractional rate of endogenous cholesterol excretion and the rate of plasma cholesterol turnover will be determined in two periods, one with drug and one with placebo treatment. This work represents a new direction for cholesterol research with the potential to develop new and complementary methods of reducing CHD risk that can be added to diet and statin drug treatment.

Inclusion Criteria:

• Aim II. 100 subjects aged 30-80 with stable medical and/or surgical illnesses.
• Aim III. 30 subjects age 18-80 with LDL cholesterol <190, fasting triglycerides<250 and stable medical or surgical illnesses.

Exclusion Criteria:

• Aim II. Subjects taking ezetimibe, bile acid sequestrants or with gastrointestinal or liver disease will be excluded since these may affect whole body cholesterol metabolism.
• Subjects with coronary heart disease or other medical illnesses will not be excluded if medically stable.
• Adults under age 30 and children will be excluded because in our current database there is no relation between carotid intima-media thickness and cardiovascular risk factors in this younger group.
• Aim III. Individuals have risk factors for coronary heart disease that mandate drug treatment according to the National Cholesterol Education Program guidelines will be excluded.