a Department of Chronic Diseases Epidemiology, National Institute of Public Health and the Environment, Bilthoven, The Netherlands.
b Department of Human Nutrition and Epidemiology, Wageningen University, Wageningen, The Netherlands.
c Lipid Reference Laboratory, Academic Hospital Rotterdam, Rotterdam, The Netherlands.
d Division of Public Health Research, National Institute of Public Health and the Environment, Bilthoven, The Netherlands.
Saskia Houterman MSc, Department of Chronic Diseases Epidemiology, National Institute of Public Health and the Environment, PO Box 1, 3720 BA Bilthoven, The Netherlands. E-mail: saskia.houterman{at}rivm.nl
Abstract
Background The aim of this study is to describe trends in plasma total and high density lipoprotein (HDL) cholesterol in The Netherlands between 1993 and 1997 and to examine whether these trends in cholesterol could be explained by changes in body mass index, smoking, alcohol intake, use of cholesterol lowering medication, intake of saturated fat, trans fatty acids and dietary cholesterol.
Methods Each year a random sample of men and women aged 2059 years living in three towns in The Netherlands was invited to participate in the study. In total more than 21 000 people were examined.
Results Between 1993 and 1997 plasma total cholesterol decreased significantly by 0.19 mmol/l in men and by 0.27 mmol/l in women. During this period HDL cholesterol remained stable in both men and women. Small decreases were observed in the intake of saturated fat, trans fatty acids and dietary cholesterol in both men and women. The use of cholesterol lowering medication and for women oral contraceptives and prescribed oestrogens increased significantly. After adjustment for these determinants in multivariate analyses the trend in total cholesterol remained highly significant.
Conclusions Between 1993 and 1997 the mean total cholesterol level decreased significantly while the mean HDL cholesterol remained stable in both men and women in The Netherlands. The observed trend in total cholesterol could only for a small part be explained by changes in the determinants studied.
Keywords Total cholesterol, HDL cholesterol, trends, determinants
Accepted 20 December 2000
Since 1972, age-adjusted coronary heart disease mortality has declined in The Netherlands by about 40%, but it is still a major public health problem.1 A further decline in coronary heart disease mortality is therefore desirable. A decline in plasma total cholesterol could have a significant impact on mortality. It is known from longitudinal studies that a plasma total cholesterol reduction of 1% results in a decrease of coronary heart disease mortality of 23%.2 Earlier results of monitoring projects in The Netherlands between 1974 and 1992 showed that plasma total and high density lipoprotein (HDL) cholesterol levels were relatively stable, except for a decline in total cholesterol of 0.2 mmol/l in men aged 3337 years in the early 1980s and a decline of 0.1 mmol/l in 19911992 in men aged 2059 years.3,4
Nutritional and lifestyle factors affect plasma total and HDL cholesterol levels. Obesity, smoking and a high intake of saturated and trans fatty acids and dietary cholesterol increase plasma total cholesterol. Obesity, smoking and trans fatty acids decrease HDL cholesterol while physical activity, saturated fat and alcohol consumption increase HDL cholesterol.511
The first aim of the present study is to describe trends in plasma total and HDL cholesterol in The Netherlands between 1993 and 1997. The second aim is to examine if the observed trend in total cholesterol could be explained by changes in body mass index, smoking, use of cholesterol lowering medication, intake of saturated fat, cholesterol and trans fatty acids.
Methods
Study population
The Monitoring Project on Risk Factors for Chronic Diseases (MORGEN-project) was carried out in The Netherlands from 1993 to 1997. The general purpose of this project was to determine both the prevalence of risk factors for chronic diseases (e.g. plasma cholesterol, blood pressure and smoking habits) and the prevalence of some specific chronic conditions in a sample of the general population. The project was carried out at the municipal health services in three towns in The Netherlands: Amsterdam, Doetinchem, and Maastricht. Each year a new random sample of men and women aged 2059 years was selected from the municipal registry of Amsterdam and Maastricht, stratified by age and sex. In Doetinchem, the study population consisted of individuals aged 2659 years who had participated in the previous Monitoring Project on Cardiovascular Disease Risk Factors (19871991) and a new random sample of men and women aged 2025 years from the municipal registry of Doetinchem to cover the whole age range of 2059 years.4 The overall response rate for the three towns was 48% in 1993, 51% in 1994, 44% in 1995 and 1996, and 40% in 1997. Each year the number of respondents from each town was about equal. In the analyses, respondents were excluded if they were pregnant (n = 137) or if they had no measurements for the potential confounders (n = 452). In all, data from 9773 men and 11 678 women were used for the analysis.
Measurements
The subjects who agreed to participate received a general questionnaire and a semi-quantitative food frequency questionnaire, to fill in at home.
The general questionnaire asked for demographic variables, lifestyle factors, psycho-social factors, (family) history of diseases, medication use and (for women) reproductive history. Alcohol intake was measured using a standardized questionnaire as number of drinks per day and subsequently converted to grams of alcohol by multiplying each glass of beer, wine, or spirits by its alcohol content. Educational level, measured using a standardized questionnaire, was classified into three categories: low (intermediate secondary education or less), medium (intermediate vocational or higher secondary education) and high (higher vocational or university education). Information on cigarette smoking (current, former or never) was obtained from a standardized questionnaire. Physical activity was measured between 1994 and 1997 using an extended version of a validated physical activity questionnaire.12 For the analyses, we classified physical activity in three categories: (1) <3.5 h per week physical activity (2) >3.5 h per week physical activity, of which <2 h strenuous and (3) >3.5 h per week physical activity, of which >2 h strenuous.
The semi-quantitative food frequency questionnaire was designed to assess the habitual consumption of 178 food items during the previous year. In addition, this questionnaire provided information on the use of a prescribed diet. In 1991 and 1992, the reproducibility and relative validity of the nutrients were assessed in a validation study.13 Nutrient and energy intake were quantified for each individual using an extended version of the 1996 computerized Dutch food composition table.14 Because of the recent interest in trans fatty acids a food table was constructed with data on the amount of trans fatty acids in the products.15 Chemical analyses were available for edible fats in 199016, 199517,18 and 1996.17 In addition, cookies and pastries as well as dairy products and meats were analysed, contributing to 95% of the total mean fat intake of the Dutch national survey of 1992.18 The trans fatty acid contents of fruit, vegetables, legumes, nuts, and game and offal were derived from the UK food table19 or based on other analyses.20,21 The trans fatty acids content of the remaining foods was derived from recipes or deduced from other foods. Trans fatty acid concentrations were corrected for measurement error due to the use of different methods of chemical analysis. To estimate the trans fatty acid concentration in 1993 and 1994, interpolation was done, assuming that the decline in trans fatty acid concentration was linear between 1990 and 1995. For 1997 the same food table was used as for 1996, because the assumption was made that there was no further decline in trans fatty acid concentration in this period.
During a physical examination weight and height were measured. Body mass index was calculated by dividing weight (kg) by the square of height (m2).
Total and HDL cholesterol determination
Non-fasting blood samples were obtained in all three towns using a standardized protocol. Venous blood samples were collected in vacutainers containing 7.5% tri potassium (K3) ethylenediaminetetraacetic acid (Safety-Monovette® tubes, Sarstedt, Tilburg, The Netherlands). More than 30 minutes after the blood collection, the samples were centrifuged at room temperature for 10 minutes at 3000xg. After centrifugation the plasma was separated from blood cells, and stored at 20°C. Once a week the blood samples were transported from the three towns to the laboratory in Rotterdam.
Total and HDL cholesterol determinations were performed in the Lipid Reference Laboratory (LRL) of the University Hospital Dijkzigt in Rotterdam using standardized enzymatic methods. Total cholesterol was measured using a CHOD-PAP method (Boehringer);22 HDL cholesterol was determined in the supernatant after precipitation of apoB-containing lipoproteins with phosphotungstic acid/MgCl2 (Boehringer).23 Although the specimen type was EDTA-plasma, results are standardized to a serum matrix. For lipid standardization, there is international consensus that the Centres for Disease Control (CDC) Reference Methods are solely used in combination with a serum matrix. Field methods may use an alternative matrix but standardization should be performed, by means of a split sample comparison, against serum reference values. Accordingly, lipid levels in EDTA-plasma, as measured in the present study, are traceable to serum values.
The LRL Rotterdam is a permanent member of the international Cholesterol Reference Method Laboratory Network,24 performing the Abell-Kendall cholesterol reference method exactly as it is performed at the Centres for Disease Control (CDC), Atlanta, Georgia, USA and being standardized to CDC through participation in the CDC-National Heart Lung and Blood Institute (NHLBI) Lipid Standardization Program. For HDL cholesterol the accuracy base at the LRL Rotterdam is the HDL cholesterol Designated Comparison Method.24,25 Performance for enzymatic total and HDL cholesterol measurements fulfilled National Cholesterol Education Program (NCEP) recommendations throughout the entire study period.24,26,27 Furthermore, the internal quality control data of the LRL were analysed to detect whether, within the recommendations made by NCEP, there had been any laboratory drift over time that would influence the observed trend in plasma total cholesterol. For HDL cholesterol, adjustment was necessary for the period January 1993 to November 1994. During this period HDL cholesterol was determined manually and from November 1994 to December 1997 automatically. In total 40 samples were analysed with both methods. This crossover experiment was performed according to the NCCLS guideline EP9-A. The HDL cholesterol levels ranged between 0.65 and 3.1 mmol/l (i.e. encompassing the NCEP cutpoints of 0.91 and 1.55 mmol/l) and the distribution was Gaussian. Consequently, slope and intercept of the calculated regression line of the method comparison are reliable and accurate. The following orthogonal regression equation was found: Y (automatic method, mmol/l) = 1.055 (manual method, mmol/l) + 0.029. Due to this change, HDL cholesterol was adjusted by +6.5% in the period January 1993 to November 1994.
Hypercholesterolaemia was defined as a plasma total cholesterol concentration of 6.5 mmol/l or higher. For low HDL cholesterol the cut-off point of 0.9 mmol/l was used.28
Statistical analyses
All data were age-standardized to the age distribution of the Dutch population aged 2059 years in 1995. Univariate regression analysis was used to analyse trends over time with time (day) as independent variable and plasma total or HDL cholesterol as dependent variable. The multivariate regression analysis concerning total cholesterol was adjusted for age, town, body mass index, intake of saturated fat, intake of trans fatty acids, dietary cholesterol, education, smoking, cholesterol lowering medication, cholesterol or fat lowering diet, and (for women) use of oral contraceptives and oestrogens. Intake of saturated fat and trans fatty acids was expressed as a percentage of total energy. Whether trends in total and HDL cholesterol differed between the three towns, the 10-year age categories, and the different educational levels was studied by including interaction terms in the regression models between town and day, age group and day, and educational level and day. All analyses were carried out for men and women separately. The SAS computer package (version 6.12) was used for all statistical analyses (SAS Institute Inc., Cary, North Carolina, USA, 1989).
Results
Age-standardized yearly means and prevalence of high-risk levels of plasma total and HDL cholesterol for both men and women, are given in Table 1. The changes over time in plasma total and HDL cholesterol in both men and women are shown in Figure 1
. Between 1993 and 1997 there was a decrease in total cholesterol in both men and women, with the largest decline between 1994 and 1995. Consequently, the prevalence of hypercholesterolaemia also decreased in men and women during this period. The concentration of HDL cholesterol remained almost stable from 1993 to 1997 in men and women and the prevalence of a low HDL cholesterol level (<0.9 mmol/l) declined somewhat.
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In Table 2 the changes in determinants of total and HDL cholesterol are shown. Between 1993 and 1997 the intake of trans fatty acids decreased significantly by 0.4% of energy in both men and women. The intake of saturated fat decreased significantly by 0.4% of energy in men and 0.5% of energy in women. The intake of cholesterol decreased significantly by 0.6 mg/MJ in men and 1.9 mg/MJ in women. The use of cholesterol lowering medication increased significantly by 1.5% in men and 0.4% in women. The use of oral contraceptives increased significantly by 3.0% and prescribed oestrogens increased significantly by 1.3%.
Multivariate regression models were used to examine if the changes in the determinants mentioned above could explain the trend in plasma total cholesterol (Table 3). In men, the univariate and multivariate regression coefficient for the change in total cholesterol was comparable. The multivariate regression coefficient in women was somewhat lower than the univariate coefficient for the change in total cholesterol, but still significant. This means that the observed trend in total cholesterol could only partly be explained by changes in the determinants studied. The multivariate regression coefficients for the most important determinants of total cholesterol are given in Table 4
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The plasma total cholesterol level and the prevalence of hypercholesterolaemia decreased significantly between 1993 and 1997 in men and women. The HDL cholesterol level remained stable during this period. The observed trend in total cholesterol in men and women could only be explained in part by changes in the determinants studied, i.e. intake of saturated fat, trans fatty acids, dietary cholesterol, use of cholesterol lowering medication and for women use of oral contraceptives and prescribed oestrogens.
The response rate ranged from 48% in 1993 to 40% in 1997. Each year, a small non-response survey was carried out. Plasma cholesterol levels could not be measured in non-responders, so educational level, an important determinant of non-response and associated with biological determinants like cholesterol level and blood pressure, was used to assess possible selection bias.29,30 From the non-response surveys it can be concluded that people with a lower level of education are possibly under-represented in the MORGEN-project. Furthermore, the number of respondents with a high level of education increased while the number of respondents with a low level of education decreased during the study period in both men and women. This change in level of education could be due to the declining response rate between 1993 and 1997 but could also be the result of the known increase in the level of education of the general Dutch population. However, we included level of education in our multivariate model so educational level cannot explain the trend in total cholesterol.
Previous monitoring projects carried out in The Netherlands between 1974 and 1992 showed almost no changes in plasma total and HDL cholesterol, except for a decline in total cholesterol of 0.2 mmol/l in men aged 3337 years in the early 1980s and a decline of 0.1 mmol/l in 19911992 in men aged 2059 years.3,4 In the present study however, plasma total cholesterol decreased significantly by 0.19 mmol/l in men and by 0.27 mmol/l in women in the relatively short period between 1993 and 1997. The HDL cholesterol level remained stable in both men and women. Results from the Zutphen Elderly Study showed a decline in serum total cholesterol of similar magnitude in Dutch men aged 70 years and older in the early 1990s as observed in our study.31 Therefore, a decline in total cholesterol levels seems to be generalizable to the total Dutch population.
Saturated fat is one of the most important dietary factors that influence plasma total cholesterol.8,9 Between 1993 and 1997 a small, but significant, decline was observed in the intake of saturated fat in both men and women. The intake of polyunsaturated fat remained stable in men and increased in women whereas dietary cholesterol declined in both men and women. Using the equation of Keys et al.,32 a decline in mean plasma total cholesterol of 0.03 mmol/l in men and 0.06 mmol/l in women from 1993 to 1997 was predicted based on the change in the intake of saturated fat, polyunsaturated fat and dietary cholesterol during this period.
Trans fatty acids from the diet also increase plasma total cholesterol.9,11 Since 1985, the trans fatty acid content of margarines and spreads has substantially declined in The Netherlands.15,33 In the present study a significant decline was observed in the intake of trans fatty acids in men and women between 1993 and 1997. Mensink and Katan11 estimated that each additional per cent of dietary energy as trans fatty acids results in an increase in total cholesterol of 0.026 mmol/l. Extrapolated to this study, the observed change in trans fatty acid intake would lead to a decline in mean total cholesterol of 0.01 mmol/l in both men and women between 1993 and 1997. This means that the change in the intake of the dietary determinants would result in a decline in total cholesterol of 0.04 mmol/l (20%) in men and 0.07 mmol/l (29%) in women.
The reproducibility and relative validity of the nutrients, like total fat intake, was assessed in a validation study and seemed to be adequate.13 This was not investigated for saturated fat and trans fatty acids. Random misclassification of dietary exposure, due to error in the quantification of saturated fat and trans fatty acids, cannot be excluded. However, using the equation of Keys, it can be calculated that a decline in the intake of saturated fat of almost 3% of energy in men and 4% of energy in women would be needed for a reduction of total cholesterol by 0.19 mmol/l in men and 0.27 mmol/l in women. This calculated decline in saturated fat is about ten times higher than the real decline observed in this study. It is therefore unlikely that misclassification of saturated fat explains the findings of this study.
In our study the use of cholesterol lowering medication, mostly statins, increased significantly by 1.5% in men and 0.4% in women. In a meta-analysis of randomized clinical trials it was estimated that people who used statins had a 20% lower total cholesterol level compared with people not using such cholesterol lowering medication after a mean follow-up period of 5 years.34 This observed change in the use of cholesterol lowering medication would, in our population, lead to a decline in total cholesterol of 0.02 mmol/l in men and 0.004 mmol/l in women.
The observed trend in plasma total cholesterol could only be explained in part by changes in the determinants studied, i.e. trans fatty acids, saturated fat, dietary cholesterol, use of cholesterol lowering medication and for women use of oral contraceptives and oestrogens. Including these determinants and the variables age, town, body mass index, education, smoking and diet in the regression models only slightly changed the coefficients for change in total cholesterol over time. It is unclear which (other) factors are responsible for the trend.
In many other countries cholesterol levels have also declined during the last decades. In the WHO MONICA Project, carried out in 38 populations in 21 countries, the mean total cholesterol level declined by 0.008 mmol/l per year in men and 0.015 mmol/l per year in women between 1985 and 1995.35 In the WHO MONICA Project dietary data were only collected in subsamples of some of the populations studied. The most important determinants of population changes in total cholesterol could therefore not be studied. In Finland a declining trend in serum total cholesterol has been observed since 1972. Serum total cholesterol decreased by 0.9 mmol/l in men and by 1.2 mmol/l in women aged 3059 years between 1972 and 1992,36 which seemed to be explained by dietary changes. The intake of polyunsaturated fatty acids increased by 2% of energy, saturated fat decreased by 5% of energy and dietary cholesterol decreased by about 25 mg/1000 kcal in both men and women. The expected decline in total cholesterol based on these dietary changes would be 0.6 mmol/l. A change from boiled to filtered coffee could have reduced the cholesterol levels by another 0.3 mmol/l. This means that dietary changes fully explained the decline in total cholesterol in men. In women the decrease in weight was also an important determinant in addition to the dietary changes.37
In the US the mean serum total cholesterol level declined by 0.2 mmol/l and HDL cholesterol increased slightly by 0.03 mmol/l between 1976 and 1991 in men and women aged 2074 years.38 This change was fully explained by changes in the intake of polyunsaturated fatty acids, saturated fat and dietary cholesterol.39
The decline in plasma total cholesterol of 4% in men and 5% in women between 1993 and 1997 in The Netherlands could only be explained in part by changes in trans fatty acids, saturated fat, dietary cholesterol and use of cholesterol lowering medication. A decline in total cholesterol of this magnitude is of great public health importance because it could lead to a decline in mortality from coronary heart disease of about 815% in the future.2
KEY MESSAGES
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This study was financially supported by the Ministry of Public Health, Welfare and Sports of The Netherlands and the National Institute of Public Health and the Environment. The development of the food frequency questionnaire was supported by the Europe Against Cancer Programme of the European Union. The authors wish to thank the epidemiologists and field workers of the Municipal Health Services in Amsterdam, Doetinchem and Maastricht for their important contribution to the data collection for this study. Furthermore we like to thank the investigators of the Department of Nutritional Epidemiology of the TNO Nutrition and Food Research Institute, Zeist, The Netherlands, and the investigators of the Division of Human Nutrition and Epidemiology of the Wageningen University, Wageningen, The Netherlands for the data on the amount of trans fatty acids in different foods.
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