Background
Since the landmark Framingham Heart study in 1948 [
1], there have been several hundred prospective cohort studies on cardiovascular disease and associated risk factors.
In Gothenburg the first cohort of 50-year-old men (the Study of Men Born in 1913) was examined in 1963 [
2]. Younger cohorts of 50-year-old men (i.e. men born in 1923, 1933, and 1943) have later been examined every 10
th year [
3‐
5].
The present study adds new data by including the 5th cohort of 50-year-old men (men born in 1953). We have also examined 50-year-old women (born in 1953) in addition to a follow-up examination of the 4th cohort of 50-year-old men (born in 1943), now aged 60 years.
With an increasing prevalence of excess body weight and obesity in the population, the metabolic syndrome (MetSyn) has attracted considerable attention during the past decade as an important risk factor in cardiovascular disease. There are at least five definitions of Metsyn [
6], which create considerable confusion regarding which definition to use. Furthermore, with so many definitions, it is difficult to obtain consistent research results.
The aim of the present study was twofold: (1) to acquire current information about risk factors in cardiovascular disease in a middle-aged Swedish population and (2) to analyse the prevalence of the MetSyn using three popular definitions.
Methods
Participants
The study population consists of three cohorts: one third of all men (n = 993) and women (n = 994) born in 1953 and living in Gothenburg in 2003 were randomly sampled from the population register and invited to the examination. Gothenburg, which is a maritime and industrial city on the West coast of Sweden, is the second largest city in Sweden with approximately 450,000 inhabitants. The third cohort, men born in 1943, was a random sample in 1993 and now consists of all persons who were examined in 1993 (n = 798, 55% of those invited), except for those individuals that had died (n = 34) or moved abroad (n = 15). This leaves 749 men, now aged 60 years that were invited to participate in the present study. Based on those individuals examined, the participation rate was 60% (595 of 993) among men born in 1953, 67% (667 of 994) among women born in 1953 and 87% (655 of 749) among men born in 1943.
Examination procedures
The examinations took place between August 2003 and December 2004. All participants were mailed a questionnaire on smoking habits and physical activity during leisure time. Each item was rated on a scale from 1 to 4, where 1 = no physical activity, 2 = moderate activity (e.g. walking, riding a bicycle and light gardening) for a minimum of 4 hours per week, 3 = regular, strenuous activity for a minimum of 3 hours per week and 4 = athletic training (competitive sports regularly). Regular smoking was defined as smoking at least one cigarette per day. Ex-smokers were defined as having quit smoking at least one month before they mailed the questionnaire. A snuff taker (snuffer) is a person who uses snuff (wet tobacco) daily.
The participants also answered questions about chest pain, psychological stress, family history of cardiovascular disease and cancer (parents and siblings), previous and current medical history and ongoing medication. Psychological stress was rated on a six-point scale with 0 = no stress, (1–3 = various grades of intermediate stress), 4 = continuous stress during the past year and 5 = continuous stress during the past 5 years. Diabetes was defined as having a physician's diagnosis of diabetes. Hypertension was defined as a physician's diagnosis and/or systolic blood pressure ≥ 140 and/or diastolic blood pressure (phase 5) ≥ 90 (physician measurement) and/or treatment for hypertension. Individuals who returned the questionnaire were invited to the examination, which was performed in the morning after an overnight fast. One reminder was sent out to the participants who did not return the first questionnaire but after that no further action was taken. The results are based solely on those persons that were examined.
The study was done in the morning. The participants were asked to fast overnight. A question relating to when they had last eaten revealed that close to 90% had complied with the request to fast. A study nurse measured height (cm) and weight (kg) with indoor clothing and without shoes. Waist circumference was measured at the level of the umbilicus (cm) and hip circumference at the level of the anterior iliac crest (cm) with the participant standing and breathing normally. After five minutes of rest, blood pressure was measured automatically in the right arm in the seated position with the OMRON 711 monitor. A 12-lead electrocardiogram was recorded with the participant relaxed and supine. Blood samples (fasting state) were taken for analysis of plasma glucose, serum total cholesterol, high-density lipoprotein (HDL) cholesterol and serum triglycerides (standard methods at the accredited university hospital laboratory in Gothenburg). During 2002, the analysis equipment at the laboratory was upgraded from Hitachi 917 Roche to Modular Roche, which resulted in an 11% increase of the mean HDL cholesterol levels [Flenner E, personal communication]. Low-density lipoprotein (LDL) cholesterol was calculated using the Friedewald formula [
7]. Blood samples were frozen (-70°C) until further analysis.
After the first part of the study was completed, the participants were served a light breakfast. During breakfast, they completed another questionnaire on social and psychosocial factors, social network [
8], education, working times, various complaints, sleeping habits and self-ratings on a seven-point scale regarding their health, economy, family situation, memory, energy, sleep, ability to handle stress, and simultaneous capacity [
9].
A physician administered a structured interview after breakfast. The same physician also checked the questionnaires. The physician measured blood pressure using exactly the same method as in 1963 [
2], i.e. with a mercury sphygmomanometer (cuff size 12 × 23 cm) in the right arm after five minutes of rest with the participant in the seated position. If potential medical problems were identified, the participants were referred for further work-up (severe hypertension, chest pain or other alarming symptoms). All participants received a letter with the results from the examination and, if needed, advice about lifestyle changes.
The review board of the Ethics Committee at the University of Gothenburg approved the study. All participants signed a written informed consent form.
Statistical methods
The analyses were conducted using the SAS statistical software package [
10]. Descriptive statistics were used. The prevalence of the MetSyn was calculated based on three definitions recently reported in the literature (Table
1).
Table 1
Three definitions of the metabolic syndrome.
1. Fasting P-glucose ≥ 6.1 mmol/l (≥ 110 mg/dl) | 1. Fasting P-glucose ≥ 5.6 mmol/l (≥ 100 mg/dl) or drug treatment for elevated glucose. | 1. Fasting p-glucose ≥ 5.6 mmol/l (≥ 100 mg/dl) or known type 2 diabetes |
2. Blood pressure ≥ 130/≥ 85 | 2. Systolic BP ≥ 130 or diastolic BP ≥ 85 or treatment for hypertension | 2. Systolic BP ≥ 130 and/or diastolic BP ≥ 85 or treatment for hypertension |
3. Triglycerides ≥ 1.7 mmol/l (≥ 150 mg/dl) | 3. Triglycerides ≥ 1.7 mmol/l (≥ 150 mg/dl) or drug treatment for elevated triglycerides | 3. Triglycerides ≥ 1.7 mmol/l (≥ 150 mg/dl) or specific treatment |
4. HDL-cholesterol <1.03 mmol/l (<40 mg/dl, men) or <1.29 mmol/l (<50 mg/dl, women) | 4. HDL-cholesterol <1.03 mmol/l (<40 mg/dl, men) or <1.29 mmol/l (<50 mg/dl, women) or drug treatment for low HDL-cholesterol | 4. HDL-cholesterol <1.03 mmol/l (<40 mg/dl, men) or <1.29 mmol/l (<50 mg/dl, women) or specific treatment |
5. Waist circumference >102 cm (men), >88 cm (women) | 5. Waist circumference ≥ 102 cm (men), ≥ 88 cm (women) | |
Discussion
The present study investigated the prevalence of cardiovascular risk factors in random samples of middle-aged Swedish men and women examined in the beginning of the 21st century. Using recent definitions of desirable levels, very few (2–9%) of the participants presented no risk factors at all. Moreover, we found that the prevalence of the MetSyn varies substantially depending on which definition is used. More precisely Metsyn varied from 11 to 16% among 50-year-old women, from 16 to 26% among 50-year-old men and from 20 to 35% among 60-year-old men.
There are two limitations that need to be acknowledged and addressed regarding the present study. The first limitation concerns that our study used a cross-sectional design and thus the predictive value of the MetSyn for the development of cardiovascular disease and diabetes cannot be determined because we do not yet have follow-up data. Another limitation is that our populations were not representative of the general population because only two thirds of those invited actually participated. In the first study of 50-year-old men in Gothenburg in 1963 the participation rate was 88% [
2]. In later population studies in Gothenburg the participation rate dropped to 76% in 1983 [
5] and to 65–69% in the GOT-MONICA project in 1995 [
14]. It is known that mortality is higher among non-participants than among participants in the Gothenburg population studies [
15,
16]. Analyses of non-participants in other population studies are almost non-existent.
Smoking among 50-year-old men has decreased from 56% in 1963 to 22% in the present study. Twenty-two percent is among the lowest rates reported in developed countries [
17]. Among the 50-year-old women, 26% were smokers. This percent value could be compared with the final survey of the international MONICA study [
17] where smoking rates among women were lowest in Lithuania (5%) and highest in Denmark (42%).
The mean serum cholesterol level of 5.4–5.5 mmol/l in our study ranked in the middle between the lowest (4.5 mmol/l in China) and the highest level (6.3–6.6 mmol/l in Switzerland) in the MONICA study [
17]. The higher HDL cholesterol level in our study versus the previous Gothenburg studies [
14] may be explained by methodological differences (see Methods) as well as by a higher level of physical activity of the cohorts in our study. In our cohorts 24% had regular or intense physical exercise during leisure time, whereas in the previous Gothenburg cohorts (using the same methodology as in our study) from 1985–1995 [
14] the figures were 8–11% for women aged 45–54 years, 13–23% for men aged 45–54 years, and 12–19% for men aged 55–64 years. In comparison with the previous Gothenburg cohorts, the prevalence of obesity, as measured by BMI, has increased slightly among the participants in our study [
14].
Using cut points for cardiovascular risk factors as described in the IDF definition of the MetSyn [
13], together with smoking and cholesterol ≥ 5.2 mmol/l (≥ 200 mg/dl) very few (2–9%) of the participants had no risk factors at all. In the recent Swedish INTERGENE study [
18] 10% of the men and 13% of the women had "optimal" risk factor status. The Norwegian HUNT study [
19] reported similar findings. The researchers of the HUNT study concluded that if the 2003 European guidelines on prevention of cardiovascular diseases are implemented, most Norwegians (which they stated to be one of the healthiest populations in the world) would be classified as at high risk for fatal cardiovascular disease [
20].
Using different definitions of the MetSyn the prevalence of the MetSyn was found to vary (as expected) in our study as well as in two German studies [
21,
22] and one Greek study [
23], although the figures were higher in the German studies than in our study when comparing similar age cohorts. The German GEMCAS study [
22], however, was not a strict population study but participants were those who visited general practitioners. Even Norwegians have a slightly higher prevalence of the MetSyn [
20] using the AHA [
6] and the IDF [
13] definitions than the men and women of similar ages in our study. In Western societies the MetSyn is more common in men than in women, but in a Chinese [
24] and an Arab population [
25] it was more common in women than in men.
Especially after the introduction of the IDF definition [
13] of the MetSyn, there has been an ongoing debate about its usefulness in clinical practice and as a predictor of cardiovascular disease and diabetes [
26,
27]. It has also been suggested that the syndrome should be dumped entirely [
28]. Recently, it was reported that the MetSyn (NCEP criterion) was negligibly linked to incident vascular disease in the elderly [
29]. Fasting blood glucose alone was a better predictor of incident diabetes than the MetSyn [
29]. In an accompanying editorial [
30], the clinical usefulness of the syndrome was questioned. However, another Swedish study [
31] concluded that the MetSyn predicts cardiovascular mortality in 50-year-old men, even when taking established risk factors (e.g., smoking and elevated cholesterol) into account.
Conclusion
This study provides up-to-date information about the prevalence of cardiovascular risk factors and the MetSyn in middle-aged Swedish men and women. Our study reveals that risk factor status has improved, especially regarding smoking. The prevalence of smoking, at least in men, is among the lowest in the world. Based on our findings and those from other studies, the usefulness of the MetSyn is suspect, primarily because it creates uncertainty about which definition to use.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
All authors participated in the design of the study and the examination of the participants. LWe drafted the manuscript and performed the statistical analyses. All authors read and approved the final manuscript.