a Research Centre for Chronic Degenerative Diseases and Department of Internal Medicine and Prevention and Health Biotechnology, University of MilanBicocca, Milan, Italy.
b Istituto di Ricerche Farmacologiche Mario Negri, Milan, Italy.
c Department of Laboratory Medicine at Hospital of Desio, University of MilanBicocca, Milan, Italy.
Correspondence to: Marco Ferrario, Centro Ricerche Patologia Cronico-Degenerativa; Università degli Studi di MilanoBicocca; Ospedale San Gerardo, Villa SerenaVI piano; Via Donizetti, 106; 20052 Monza (MI), Italy. E-mail: marco.ferrario{at}unimib.it
Abstract
Background The goals are to estimate time trends (19861994) of major coronary risk factors in an industrialized low CHD incidence population and to assess education class (EC) differences in risk factor prevalence and in time trends.
Methods Three population surveys were conducted in 19861987, 19891990 and 1993 1994 on independent and two-stage age- and gender-stratified random samples (1906 men and 1941 women) of 3564 year old residents of Brianza, an affluent region of northern Italy. The protocol for data collection, clinical measurements and biochemical determinations adhered to the WHO MONICA manual and underwent repeated quality control assessments. EC were identified according to gender- and 5-year birth-cohort specific tertiles.
Results In the initial, middle and final surveys 1258, 1259 and 1330 subjects were enrolled, corresponding to participation rates of 70.1%, 70.3% and 74.3%, respectively. Over the 8-year period, in men systolic blood pressure and smoking habits declined, body mass index and serum total cholesterol increased. In women systolic blood pressure showed a constant reduction, total cholesterol and BMI increased and the prevalence of smokers remained stable. Overall inverse associations with EC were found for body mass index, for prevalence of cigarette smokers in men and for systolic blood pressure in women. Decreases in blood pressure were more evident in the lowest EC. Cigarette smoking was on the decline in the higher EC in men. BMI and total cholesterol increased in all EC with the notable exception of the low EC in women.
Conclusions Favourable changes of the risk factor profile in the low socio-economic classes may have contributed to reduce CHD rates in this population. Specific policies oriented to lowest socio-economic classes are needed to continue to combat the smoking epidemic.
Keywords Coronary risk factors, trends, socio-economic status, Italy
Accepted 12 December 2000
In Italy consistent reductions in cardiovascular and coronary mortality rates have been reported in both gender groups1,2 since the first half of the 1980s. These trends have not occurred uniformly in all regions of the country:3 in northern Italy, a region with recognised low coronary attack rates,4 a sharp and stable decline in cardiovascular mortality was found in the 1970s and in the first half of the 1980s.5 The trends are of the same magnitude as the declines observed in other industrialized countries,6 and persisted in the second half of the 1980s.7 Among populations included in the WHO MONICA (MONItoring of CArdiovascular diseases) Project, downward trends of coronary event attack rates in Brianza, an industrialized northern Italian population, have been recently reported.8
Education class (EC) is a largely used index of socio-economic status (SES), and in previous studies it was repeatedly reported to be associated with risk factor prevalence.9 Furthermore, information on education is relatively easy to obtain, reliable and available for all population strata.10,11 Some studies in the US,1216 Australia,17 and Finland18 have focused on the associations between EC and time trends of coronary risk factor prevalence. Whether differences in CHD risk factors across education class have increased in the last decades remains an issue of considerable public health concern.
The primary goal of the present analysis is to estimate time trends (19861994) of major coronary risk factors, in representative samples of middle-aged residents in Brianza, and to investigate differences in prevalence and time trends of these risk factors among EC. Although reports on trends in coronary risk in Italy are available up to the mid-eighties19,20 no results have been published on differences in time trends among socio-economic classes.
Methods
Population characteristics and survey samples
Brianza is located in the region of Lombardia, northern Italy, between Milan and the Swiss border. It is characterized by a high level of industrialization and urbanization, with one of the highest average incomes in the country. Some economic problems occurred in the first half of the 1990s, due to a political crisis and a subsequent industrial recession.
Three population surveys were conducted in Brianza in 19861987, 19891990 and 19931994. All surveys started at the beginning of autumn and continued through mid summer, with the exception of one month in the winter season (middle of December to middle of January). No seasonal shift in participants' distributions was detected among surveys.
Ten-year age and gender stratified random samples were selected from municipality rolls among 2564-year-old residents of five towns, identified to represent the level of urbanization of the target population. People selected in previous surveys were not included in the subsequent ones, therefore surveys are independent. In the present analysis only 3564-year-old participants were included, due to the relatively low participation rate found in the youngest age stratum (2534-year-old) in the middle survey. Telephone interviews in all age strata were conducted on random samples of non-respondents to determine the reason for lack of response as well as their education level, health conditions and health knowledge.
Measurements of coronary risk factors
In all three surveys the adopted procedures for data collection and measurement assessment strictly adhered to the WHO MONICA manual, reported in detail elsewhere.21,22 Briefly, trained technicians took blood pressure measurements on the sitting subject, at rest for at least 10 minutes, using a standard mercury sphygmomanometer. Systolic and diastolic blood pressure were taken at the first and fifth phase of the Korotkoff sounds, respectively. Two measurements, taken 5 minutes apart, were obtained for each subject. In the initial survey sphygmomanometers were equipped with only one cuff bladder (13 cm) while in the following two surveys a larger cuff (17 cm) was also made available, although its use was limited to less than 2% of the participants in both surveys.
Smoking habits were assessed by means of a standardized interview administered by trained interviewers. Smoking status was classified as a dummy variable which identified current smokers (regular and occasional smokers) and non-smokers (never and past smokers).
Following the blood pressure measurements, venous blood specimens were taken from the ante-cubital vein on sitting subjects who had fasted for 12 hours. A tourniquet was used, only if necessary, for no longer than 3 minutes. Specimens were refrigerated at 4°C and shipped to the Clinical Laboratory of Desio Hospital where they were analysed within 4 hours. Total cholesterol determinations were made on sera by an automatic enzymatic method. In all surveys results of the external quality controls were within acceptable limits, but small and negative overall average biases were detected both in the initial (2.9%) and middle (2.5%) surveys.23 For this reason total cholesterol data from the two first surveys were corrected according to the survey-specific detected biases.
Height and weight were measured on subjects without shoes and wearing light clothing. Body mass index (BMI) was computed as weight in kilograms divided by height in square metres.
Classification of education classes (EC)
In our study information on education achievement and the number of years of education was included in a standardized interview. The number of years of full-time education was ascertained by asking the single question How many years have you spent at school or in full time study? This index was preferred to education achievement for classifying EC because it allows for a wider range of values and is therefore more suitable for identifying cutoff points. Three EC were identified using gender- and 5-year birth-cohort specific tertiles.18 This allowed for the consideration of relevant differences in education levels among gender and age groups, and helped to avoid the introduction of differential misclassification of EC among surveys taking place at different time periods.
Statistical analysis
Age-standardized means and proportions of risk factors were computed by survey, by EC, and by survey and EC groupings. The direct method was applied using the world standard population weights: 12/31, 11/31 and 8/31 for the three 10-year age strata in the age range 3564.
Differences in age-standardized mean levels of coronary risk factors among EC classes were assessed for continuous variables by using weighted analysis of variance, and for smoking, a dichotomous variable, the Mantel-Haenszel chi-squared test. This approach was adopted to detect non-monotonic changes.
Trends over time in levels of risk factors were estimated, assuming the linearity of trends, by fitting simple linear regression models24 on the entire sample and on education strata, separately for each gender group. For smoking weighted logistic regression models were used. All models included as main independent variables surveys and education classes, as dummy variables, and age as a continuous covariate. Differences in trends of risk factors among levels of EC have been tested including in the models the appropriate interaction terms.
The models are in the form: y = a + b*(t-1990) + e, where t is the calendar year of the measurement, a is the estimated mean or prevalence of the risk factors at the middle of the study period (1990), b is the average annual change in mean or prevalence over the 8-year study period, and e is the error term in the regression model. The 10-year average changes and relative standard errors were extrapolated from the data as being ten times the corresponding annual change or 10*b. The 95% confidence intervals (two-sided) of the regression coefficients were calculated assuming the normal distribution of errors.
Results
The numbers of participants in the initial, middle and final surveys were 1258, 1259 and 1330, corresponding to participation rates of 70.1%, 70.3% and 74.3%, respectively, without noticeable variations among age- and gender-strata. Participants did not show any statistically significant differences in main socio-demographic and health conditions in comparison to survey-specific random samples of non respondents, with the exception of weight and height. In non-participant women these were reported on average of 0.7 kg less and 2 cm greater, respectively, in comparison to measurements recorded on study participants.
Table 1 reports age-standardized mean values of coronary risk factors for each survey by gender group and 10-year trends. Systolic blood pressure shows statistically significant decreasing trends of 6.6 and 8.1 mm Hg in 10 years in men and women, Total cholesterol levels increased in both gender groups mainly in the final survey. These non-monotonic trends in total cholesterol would have been greater without correction according to the detected biases in the initial and middle surveys (uncorrected values of total cholesterol for the initial and middle surveys were 5.64 and 5.57 mmol/l for men and 5.50 and 5.55 mmol/l for women). A continuous reduction in the proportion of current smokers was detected in men but not in women. BMI shows a statistically significant increase (over 10 years BMI increased by 1 kg and 0.8 kg per square metre in men and in women, respectively).
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Statistically significant reductions in the proportion of current smokers were found in men in the intermediate EC. In the high EC the value of the trend coefficient is of the same magnitude, but not non-statistically significant due to the smaller sample size Non-statistically significant trends in the proportion of current smokers were found in women in all EC and in the lower EC in men.
The interaction terms between the time trends and EC were statistically significant for systolic blood pressure in both groups, and for total cholesterol in women only. BMI showed statistically significant increases over the 10 years period both in the high and the intermediate EC. In women, even if none of the EC-specific trends in BMI were statistically significant, the large initial difference in BMI among EC (Figure 1) was somewhat reduced in the third survey due to an increasing trend in the high and intermediate class and a decreasing trend in the low one.
Discussion
The present study, conducted on three consecutive and independent random samples from a northern Italian population, provides evidence that from the mid-1980s to the early 1990s the cardiovascular risk factor profile changed in a complex fashion. In men, systolic blood pressure and smoking habits showed a marked progressive reduction; BMI and serum total cholesterol showed increases. In women systolic blood pressure showed a marked and progressive reduction, while total cholesterol and BMI increased and the prevalence of smokers remained stable.
The reported changes in coronary risk factors cover a more recent time period (19861994) than the ones reported in previous Italian studies, conducted on representative samples of the entire country.19,20 Systolic blood pressure in both sexes and the prevalence of current smokers in men have continued to decrease. The previously reported increasing proportion of smokers among women cannot be confirmed but these may be due to differences in smoking habits in northern Italy in comparison to the rest of the country. In our study self-reported smoking habits in the first two surveys were validated through serum thiocyanate determinations.25 Previously reported increasing trends in BMI in women were also confirmed, and now the same trends are observed in men too, as reported in most wealthy societies.26 Serum total cholesterol mean levels increased in both gender groups, mainly in more recent years. A likely explanation for such a trend is that in Italy the traditional Mediterranean diet is being abandoned in favour of hypercaloric, faster and easier-to-cook foods.27 The standardized methods of data collection and the stable participation rates give sufficient protection against possible biases in detecting trends. The increase in some risk factors (e.g. serum cholesterol and body weight) is a matter of public health concern and requires continued monitoring to determine whether this is a transient or progressive phenomenon.
Education level is a widely used proxy, and occupation and income are alternative measures of socio-economic status.9,10 Information on income is affected by confidentiality problems in most cultural settings. Occupational classes are widely used in European studies and have been reported to better describe the social gradient in mortality risk.28 In addition, unemployment and job insecurity has demonstrated relevant health effects,29 but they are subjected to fast changes in industrialized societies. In Italy, the use of occupation classes is problematic because of the high rate of retirement at an early age and the considerable proportion of housewives among middle-aged women.11 EC has been preferred in the present study because information on years of schooling was available in both gender groups and when EC are defined according to gender and birth-cohort specific tertiles it is an accurate index of socio-economic status. This approach in fact reduces misclassification biases attributable to preferential inclusion of younger people in the high and older people in the low EC, which may play an important role when changes over time are monitored.
Our population was not targeted by extensive preventive programmes in the community, thus our results are better compared with the results of studies carried out under the same conditions and using the same SES index.1214,18
In men, decreasing trends in prevalence of current smokers are reported in all these studies, but larger decreases have been found in the highest EC where lower prevalence was present at baseline, increasing the socio-economic gap in smoking habits. In women upward trends of current smokers in the lowest EC and concurrent downward trends in the highest EC are found. This result increases the SES gap as well. Taking into account such results, it is apparent that the descriptive model proposed by Lopez et al.30 on the dynamics of the cigarette epidemic in developed countries holds for the entire sample as well as for the higher EC but not for the lowest one. Therefore, to further reduce smoking-related diseases, including CHD, public health efforts need to be oriented to combat the smoking epidemic in the lowest socio-economic classes. In this perspective, the reinforcement of restriction policies as well as the involvement in prevention activities of non-profit organizations, already implicated in assistance of individuals in the low social classes, may be helpful.
With the exception of Eastern Finland, downward trends in blood pressure have usually been found to be greater in women and in the lower education classes, narrowing the SES gap. In our population it is likely that such trends are attributable to increased awareness and better treatment of hypertensive subjects.31
All studies reported increasing trends of BMI in all gender groups and in all EC, again with the exception of the low EC in women in Brianza. In affluent societies the upward trends in BMI is a major concern.26 It has been postulated that SES differences in BMI are greater in populations with wider differences in years of schooling between the highest and the lowest EC than in populations where education is more uniform.32 This suggests that the puzzling reduction of BMI in the low EC in northern Italian women may be in part explained by the rapid increase in education level achieved in recent years.
In conclusion, the reported downward trends of some CHD risk factors in the low EC, i.e. blood pressure in both gender groups and total cholesterol and BMI in women reduce socio-economic inequalities in this northern Italian population. An attractive implication, which requires further investigations, is that the reported risk factor trends in the low SES strata may in part be responsible of the unexpected reductions of coronary attack rates8 in this low CHD-incidence population. On the other hand, due to the lagged time required for some risk factor changes33 to produce effects on disease occurrence in the communities, the increase in total cholesterol occurred in recent years may still not have influenced CHD rates. In this perspective, continuation of the epidemiological surveillance of coronary events and risk factors may give more insights.
KEY MESSAGES
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Acknowledgments
The WHO MONICA Project in Brianza had been mainly funded by the Administration of Regione Lombardia with grants No. 9783/86 and 41795/93.
The MONICABrianza Research Group: Giancarlo Cesana and Marco Ferrario (Principal Investigators); Roberto Sega, Franco Valagussa, Giovanni De Vito, Callisto Bravi, Maria Teresa Gussoni, Felice Achilli, Paolo Brambilla, Carla Crespi, Paolo Mocarelli, Renzo Zanettini and Antonio Grieco.
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