Explaining inequalities in women’s heart disease risk
Research published in BMC Medicine, based on the Million Women Study, reports women with lower levels of education and living in more deprived areas of the UK are at higher risk of coronary heart disease due to differences in behaviour. Here, study co-author Dr Sarah Floud discusses what these findings mean in the context of addressing social and health inequalities.
Heart disease is a leading cause of death worldwide for men and women. Many observational studies show that individuals with lower socio-economic status have a higher risk of heart disease than those with higher socio-economic status.
However, relatively few studies have looked at social inequalities in heart disease risk for women in the UK. The Million Women Study gave us a good opportunity to examine social inequalities in heart disease risk and examine how much could be attributed to differences in health behaviours.
The Million Women Study is a large observational study of women’s health and involves more than one million UK women aged 50 and over. The MRC and Cancer Research UK are the study’s main funders. Approximately 1 in 4 UK women born in the 1930s and 1940s are in the study. These women are from the first generation in which a substantial proportion smoked for their entire adult lives.
During 12 years follow-up of 1.2 million women without prior heart disease, 72,000 developed the condition. These large numbers made it possible for us to look in detail at the risks of heart disease for women with varying levels of educational achievement, as well as for women living in areas with different levels of deprivation.
Using information on health behaviours (smoking, physical activity, alcohol consumption and body mass index), that women reported when they joined the study, we were able to look carefully at whether the differences in risk of heart disease were due to differences in health behaviours. Although body mass index is not a behaviour, we refer to it here as one because it tells us about behaviours such as dietary intake and physical inactivity.
Inequalities in heart disease risk
In analyses which did not take account of health behaviours, women with lower levels of education were about twice as likely to develop heart disease or die from it than women with college or university degrees. We found a similar disparity (again in analyses which did not take account of health behaviours) between women living in the most deprived areas compared to women in the least deprived.
Smoking, lack of exercise and obesity are major risk factors for heart disease and we know them to be more common in people of lower socio-economic status. We also found this in our study. Overall alcohol consumption was low in this cohort; it was slightly higher in the least deprived but on average the cohort consumed about one unit of alcohol per day.
Half of inequality due to smoking
Our main aim was to assess the extent to which these four health behaviours could explain the social inequalities in heart disease risk. This was particularly interesting because previous studies have produced differing estimates of the contribution of health behaviours.
We found that most of the social inequalities in heart disease risk were attributable to differences in health behaviours. Smoking alone accounted for about half of the associations of heart disease with education and deprivation. And all four factors together accounted for some 70-80% of the associations.
The true contributions of these four health behaviours could be even larger, because women only reported on them once – when they joined the study. We couldn’t account for differences in behaviour over time, such as quitting smoking. We know that women with a higher socio-economic status more commonly quit smoking. This would mean we may well be underestimating how much smoking and the other factors have contributed to social inequalities in heart disease.
Overall, it’s reasonable to conclude that most, if not all, of the social inequalities in heart disease incidence and mortality in these UK women could be explained by health behaviours. This is consistent with a growing body of evidence suggesting that health behaviours could account for much of the social inequalities in disease risk.
It is, however, important that we recognise these health behaviours are influenced by education and deprivation. It’s harder to change them if you don’t have the resources to do so.
Our results underline the importance of existing public health policies to reduce smoking and to promote healthy eating and exercise. The more disadvantaged members of society are often the hardest to reach. If we could reach them, these findings emphasise the potential gains we could make in reducing rates of heart disease.
Dr Sarah Floud is an epidemiologist working in the Cancer Epidemiology Unit, Nuffield Department of Population Health, University of Oxford. She has a PhD in epidemiology and a background in social policy, social psychology and social research methods. Sarah’s focuses her research on how social factors might affect health outcomes in the Million Women Study cohort.