PREVENTIVE MEDICINE 8, 679-714 (1979)

Conference on the Health Effects of Blood Lipids: Optimal Distributions for Populations’

Workshop Report: Clinical-Pathological Section

AMERICAN HEALTH FOUNDATION, APRIL 11 AND 12, 1979

320 East 43rd Street New York, New York 10017

Participants:

BARRY LEWIS, Chairman

Department of Chemical Pathology and Metabolic Disorders,

St. Thomas’s Hospital Medical School

DAVID BLANKENHORN

Department of Medicine, University of Southern California

JOE C. CHRISTIAN

Department of Medical Genetics, Indiana University Medical School

ANTHONY M. GOTTO, JR.

Department of Medicine, Methodist Hospital-Baylor College of Medicine

RUTH M. KAY

Department of Surgery, Toronto Western Hospital

HAQVIN MALMROS

Research Department, University Hospital, Lund, Sweden

MARIO MANCINI

Universita di Napoli, Naples, Italy

NORMAN E. MILLER

Department of Chemical Pathology and Metabolic Disorders,

St. Thomas’s Hospital Medical School

ESKO A. NIKKILA

Third Department of Medicine, University of Helsinki Central

Hospital

GOTTHARD SCHETTLER

Medizinische Klinikum der Universitat Heidelberg, Heidelberg,

Federal Republic of Germany

ABRAHAM SILVERS

Department of Medicine, Methodist Hospital -Baylor College of

Medicine

JEREMIAH STAMLER

Department of Community Health and Preventive Medicine Northwestern University

PETER D. WOOD

Stamford Heart Disease Prevention Program, Palo Alto, California

1 Address for reprints: Barry Lewis, M.D., Department of Chemical Pathology and Metabolic Disorders, St. Thomas’ Hospital Medical School, London SE1 7EH, England.

679 0091-7435/79/060679-36$02.00/0 Copyright @ 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

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INTRODUCTION

Disparities exist between statistically normal and optimal values for many risk- related variables, including blood pressure and body weight. In none is the present position more confusing, and the need for clearer understanding more urgent, than the definition of optimal plasma concentrations of lipids and lipoproteins.

Epidemiological evidence discussed in a parallel workshop has provided pro- tiles for the relationship between ischemic heart disease (IHD) risk within popula- tions, and serum cholesterol level, triglyceride level (at least in univariate analysis), and cholesterol level in high-density lipoprotein (HDL) and low-density lipoprotein (LDL). These indicate that a risk gradient exists even between the lowest two quartiles of serum cholesterol and LDL-cholesterol in high risk popu- lations in the United States and Europe; a risk gradient is also evident throughout the distribution of HDL-cholesterol concentration, which is inversely related to risk.

Although no threshold value is apparent from prospective studies in high-risk countries, the decrement in risk when cholesterol levels are less then 180 mg/dl may prove to be small compared with the gradient between 200 and 250 mg/dl. In this workshop we have sought information from other sources which is pertinent to the identification of optimal lipid and lipoprotein levels. These data are considered under the following headings:

1 Common hyperlipoproteinemic states II Clinical studies of the genetic hypolipoproteinemias III Health statistics in populations with low lipid levels IV Cellular lipid requirements V Kinetics of human intimal lipoprotein transport VI Regression of atheroma in man VII Lipoproteins and the familial aggregation of coronary heart disease VIII Hypertriglyceridemia

A parallel consideration reviewed by us is the feasibility in westernized popula- tions of attaining lipoprotein concentrations within or approaching an optimal range. In a field impinging on both science and politics it is necessary to address the art of the possible as well as the art of the soluble. Hence we have considered also:

IX Diet: Lipoprotein and lipid responses to unifactorial and multifactorial dietary change X Genetic variations of plasma lipids XI Physical activity and plasma lipoproteins XII Plasma lipids and lipoproteins in a low-risk community: Naples, Italy XIII Summary and conclusions

I. COMMON HYPERLIPOPROTEINEMIC STATES

A parallel workshop has summarized the epidemiological evidence that the prevalence and incidence of clinical endpoints of coronary disease (myocardial infarction, angina pectoris, sudden death) are more closely related to the concen- trations of the different plasma lipoproteins than to the plasma total cholesterol and triglyceride concentrations. Support for this concept has been provided by: (i) clinical studies of the familial dyslipoproteinemias; (ii) angiographic and elec- trocardiographic assessment of coronary artery disease in relation to plasma lipo-

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proteins; (iii) measurements of body cholesterol pool size and tissue cholesterol content; and (iv) histopathological examination of human postmortem specimens.

The majority of plasma cholesterol and triglyceride in fasting plasma resides in the low-density (1.006- 1.063 g/ml: LDL) and very low-density (< 1.006 g/ml: VLDL) lipoproteins, respectively.

Accordingly these lipoproteins have received most attention in relation to coro- nary disease. More recently the relationship of high-density lipoprotein (1.063 - 1.21 g/ml; HDL) (containing lo-30% of plasma cholesterol) to atheroscle- rosis has also been explored. The Familial Dyslipoproteinemias

Fredrickson et al. (5) first summarized the evidence that certain familial hyper- lipoproteinemias are associated with an increased susceptibility to atherosclero- sis. More recent experience has left no doubt that familial hypercholesterolemia, familial combined hyperlipidemia, and familial type III hyperlipoproteinemia (broad-beta disease) are accompanied by accelerated atherogenesis (10). In con- trast, familial hypertriglyceridemia does not appear to have this effect (4), while familial hypobetalipoproteinemia and hyperalphalipoproteinemia are charac- terized by a rarity of coronary heart disease and increased longevity (6). Most of these conditions have been shown to be due to a single inherited defect in lipo- protein synthesis or catabolism, thus providing powerful evidence for a close relationship between lipoprotein metabolism and atherogenesis. In the present context it is pertinent that the occurrence of atherosclerosis is enhanced when hypercholesterolemia reflects a familial increase in LDL concentration, but is reduced when it reflects a familial increase in HDL concentration. Angiographic Studies

Heinle et al. (8) first reported that the majority (54%) of patients with angio- graphic evidence of coronary atherosclerosis had either the Type II (increased LDL concentration) or Type IV (increased VLDL concentration) plasma lipo- protein phenotype of the Fredrickson classification. Attempts to correlate the concentrations of different plasma lipoproteins with the severity and distribution of coronary atherosclerosis have since been made by several groups. Jenkins et al. (9) examined coronary angiograms from 41 middle-aged subjects being investi- gated for chest pain. On multivariate analysis, an “atherosclerosis score,” based on the number and sizes of the lesions, was independently correlated with age (positively), and with the plasma total cholesterol concentration (positively), HDL-cholesterol concentration (negatively), but not with the VLDL-triglyceride concentration. An essentially identical result was obtained by Moore et al. (13) in a study of 101 males aged 30-59 years who had already suffered a first myocardial infarction 6-36 months previously. The studies of Pearson et al. (17) have indi- cated that these relationships probably apply to both sexes, and have demon- strated a particularly high prevalence of low HDL-cholesterol levels in patients with left main coronary artery disease. Preliminary observations by Hammett et al. (7) indicate that patients with high atherosclerosis scores have decreased levels of the HDL, subclass of HDL, while the HDL, subclass-which is relatively cholesterol poor-is reciprocally increased.

Patients with angiographically proven peripheral vascular disease tend also to have elevated VLDL and LDL concentrations and reduced HDL concentrations (3).

682 CLINICAL-PATHOLOGICAL SECTION

Cerebrovascular disease appears to be associated with low HDL-cholesterol, but normal plasma total cholesterol, levels (18, 19).

Exercise Electrocardiography Olsson et al. (16) examined the correlations between plasma lipoprotein con-

centrations and electrocardiographic evidence of myocardial ischemia during exercise-testing in hyperlipidemic, but apparently otherwise healthy, subjects. On multivariate analysis, degree of ST-segment depression was positively correlated with age, VLDL-triglyceride, and LDL-cholesterol, and negatively correlated with HDL-cholesterol. The most consistent relationships were with age and LDL-cholesterol.

Autopsy Studies As part of the International Atherosclerosis Project, the Oslo Heart Study group

have recently completed a preliminary analysis of the relationship of plasma lipids and lipoproteins to the extent of coronary atherosclerosis, quantified directly in postmortem specimens from about 200 subjects. The extent of atherosclerosis was positively correlated with the plasma total cholesterol concentration, indepen- dently negatively correlated with HDL-cholesterol, and unrelated to the plasma triglyceride concentration (nonfasting) and cigarette consumption (personal com- munication to N. E. Miller). Earlier comparisons of population samples from different countries had already demonstrated a correlation between the severity of coronary atherosclerosis in 10 countries and mean plasma cholesterol levels in population samples in these countries (21).

Tissue Cholesterol Content Familial increases in the VLDL and/or LDL concentrations are commonly

associated with cholesterol deposition in connective tissue and skin (xanthomas). In contrast, familial hyperalphalipoproteinemia does not induce xanthoma forma- tion (15). Using multicompartmental analysis of plasma cholesterol specific activ- ity: time curves following intravenous infusion of radiolabled cholesterol, Miller et al. (11) found a strong negative correlation between body cholesterol pool size and HDL-cholesterol concentration in hyperlipidemic subjects of similar age and LDL concentration. Subjects with hyperalphalipoproteinemia (HDL-cholesterol, 85 mg/dl) have recently been shown by the same method to have a much smaller slowly exchanging pool of cholesterol (by about 60%) than controls with normal HDL-cholesterol levels (15). Similar measurements in type II hyperlipo- proteinemic subjects have indicated that this cholesterol pool is also independently positively correlated with LDL concentration (20).

Bondjers et al. (2) found that the cholesterol content of human mesenteric arterial wall, measured directly by fluorimetry was positively correlated with LDL-cholesterol and negatively correlated with HDL-cholesterol. Nestel and Poyser (15) examined the cholesterol content of arterial biopsy specimens taken during coronary artery surgery. Patients whose tissue cholesterol values were in the upper quartile had, on average, higher VLDL and LDL concentrations and lower HDL concentrations than did subjects with lower tissue cholesterol values.

CONFERENCE: HEALTH EFFECTS OF BLOOD LIPIDS 683

Conclusions Present information from the clinicopathological studies outlined is clearly con-

sistent with the epidemiological finding that atherosclerosis shows independent and opposing relationships with the plasma LDL- and HDL-cholesterol concen- trations, and that knowledge of both permits more accurate prediction of coronary risk than does the plasma total cholesterol level. Whether or not plasma VLDL concentration is directly related to atherogenesis remains uncertain. Assessment of the contribution of VLDL to coronary risk is complicated by the paucity of data from fasting subjects on VLDL-cholesterol, as opposed to VLDL-triglyceride concentration. Statistical problems are also created by the considerable biological variability of VLDL concentration. In a recent longitudinal study in healthy young adults conducted over a 60-week period, fasting plasma triglyceride concentration was by far the most variable (mean coefficient of variation, 27%) of the lipid/ lipoprotein risk factors for coronary disease (12). Until the uncertainty concerning VLDL is resolved it would seem wise to regard this lipoprotein as atherogenic, and to consider the optimal distribution of lipoproteins at any given plasma total cholesterol concentration as a high ratio of HDL to VLDL and LDL.

The limited information at present available from clinicopathological studies suggests that the relationship of coronary artery stenosis and tissue cholesterol content to the HDL cholesterol level is continuous over a wide range of concen- trations (25- 100 mg/dl), without any clear threshold of concentration below which cholesterol deposition becomes particularly marked. Data on LDL cholesterol mostly concern the “normal-to-high” range (150-400 mg/dl), within which the relationships again appear to be continuous. Until more data are collected over the “low-to-normal” range, however, it will not be possible to confirm or refute the suggestion, based on tissue culture studies (see below), that there is an upper physiological limit of LDL cholesterol concentration of the order of 50 mg/dl (at which concentration LDL receptors are saturated in vitro).

REFERENCES 1. Avogaro, P., Cazzolato, G., Kostner, G., and Holasek, D. R. A. Familial hyperalphalipo-

proteinaemia. Further studies on serum lipoproteins and some serum enzymes. Clin. Chim. Acta 77, 139- 145 (1977)

2. Bonjers, G., Gustafson, A., Kral, J., Schersten, T., and Sjostrom, L. Cholesterol content in arterial tissue in relation to serum lipoproteins in man. Artery 2, 200-207 (1976).

3. Bradby, G. V. H., Valente, A. J., and Walton, K. W. Serum high density lipoproteins in peripheral vascular diseases. Lancer 2, 127 1- 127 (1978).

4. Brunzell, J. D., Schrott, H. G., Motulsky, A. G., and Bierman, E. L. Myocardial infarction in the familial forms of hypertryglyceridemia. Metabolism 25, 313-320 (1976).

5. Fredrickson, D. S., Levy, R. I., and Lees, R. S. Fat transport in lipoproteins-An integrated approach to mechanisms and disorders. New Engl. J. Med. 276, 34-44 (1967).

6. Glueck, C. J., Gartside, P., Fallat, R. W., Sielski, J., and Steiner, P. M. Longevity syndromes- familial hypobeta-and familial hyperalphalipoproteinemia. J. Lab. Cbn. Med. 88, 941-957 (1976).

7. Hammett, F., Saltissi, S., Miller, N., Rao, S., Van Zeller, M., Coltart, J., and Lewis, B. In preparation (1979).

8. Heinle, R. A., Levy, R. I., Fredrickson, D. S., and Corlin, R. Lipid and carbohydrate abnor- malities in patients with angiographically documented coronary artery disease. Amer. J. Car- &o/. 24, 178- 186 (1969).

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9. Jenkins, P. J., Harper, R. W., and Nestel, P. J. Severity of coronary atherosclerosis related to lipoprotein concentration. Bril. Med. J. 2, 388-391 (1978).

10. Lewis, B. Hypothesis into theory-the development of aetiological concepts of ischaemic heart disease: a review. J. Roy. Sot. Med. 71, 809-818 (1978).

11. Miller, N. E., Nestel, P. J., and Clifton-Bligh, P. Relationships between plasma lipoprotein cholesterol concentrations and the pool size and metabolism of cholesterol in man. Afheroscle- rosis 23, 535-547 (1976).

12. Mjqs, 0. D., Rao, S. N., and Miller, N. E. Unpublished observations (1979). 13. Moore, R. B., Long, J. M., Matts, J. P., Amplatz, K., Varco, R. L., Buchwald, H., and the

POSCH group. Plasma lipoproteins and coronary arteriography in subjects in the Programme on the Surgical Control of the Hyperlipidemias. Atherosclerosis 32, IOl- 119 (1979).

14. Nestel, P. J., Miller, N. E., and Magill, P. Unpubtished observations (1979). 15. Nestel, P. J., and Poyser, A. Cholesterol content of the human atrium is related to plasma lipo-

protein levels. Afherosclerosis 30, 177-183 (1978). 16. Olsson, A. G., Ekelund, L.-G., and Carlson, L. A. Studies in asymptomatic primary hyper-

lipidaemia IV ECG at rest and during exercise and its relation tc various lipoprotein classes. Acta Med. Stand. 198, 55-73 (1975).

17. Pearson, T. A., Bulkley, B. H., Achuff, S. C., Kwiterowich, P. O., and Gordis, L. Association of low plasma HDL cholesterol with left main coronary artery disease. Circulation 55, Ill- 194 (1977).

18. Rossner, S., Kjellin, K. G., Mettinger, K. L., Siden, A., and Soderstrom, C. Normal serum-cholesterol but low HDL-cholesterol concentration in young patients with ischaemic cerebrovascular disease. Lancer 1, 577-579 (1978).

19. Sirtori, C. R., Gianfranceschi, G., Gritta, I., Nappi, G., Brambilla, G., and Paoletti, P. Decreased high density lipoprotein-cholesterol levels in male patients with transient ischaemic attacks. Atherosclerosis 32, 205-211 (1979).

20. Smith, F. R., Dell, R. B., Noble, R. P., and Goodman, Dew. S. Parameters of the three-pool model of the turnover of plasma cholesterol in normal and hyperlipidaemic subjects. J. C/in. Invesf. 57, 137- 148 (1976).

21. Strong, J. P., and Eggan, D. A. Risk factors and atherosclerotic lesions, in “Atherosclerosis” (R. J. Jones, Ed.), pp. 355-364. Springer-Verlag, Berlin, 1970.

II. CLINICAL STUDIES OF THE GENETIC HYPOLIPOPROTEINEMIAS Tangier disease, abetalipoproteinemia, and hypobetalipoproteinemia represent

an opportunity to examine the role of high-density lipoproteins (HDL) and low- density lipoproteins (LDL) in triglyceride and cholesterol transport. These ab- normalities help identify the possible lower levels of serum cholesterol concentra- tions necessary for normal growth and health.

Patients with Tangier disease present with a number of clinical findings: these are large orange-colored tonsils, splenomegaly, and often peripheral neuropathy (9). All of these clinical manifestations are due to an abnormal storage of choles- teryl esters in the form of foam cells in many tissues of the body (12). The intracellular lipid deposit consists mostly of histiocytes which, when lipid laden, are called foam cells. These are present in the tonsils, bone marrow, spleen, skin, and submucosa of the jejunum, in Schwann cells of peripheral nerves, and in nonvascular smooth muscle cells (6, 9).

There is a complete absence of “normal” HDL, since levels of apoA-I and apoA-II are very low (1, 2, 6, 9). The mean concentration of apoA-I in normal males is 120 mg/dl and in normal females 135 mg/dl.