Heart disease is a leading cause of death worldwide for men and women. Many observational studies have shown that there are inequalities in heart disease risk by people’s socio-economic status—that is, 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 provided a good opportunity to examine social inequalities in heart disease risk as well as to examine how much could be attributed to differences in health behaviours.
The Million Women Study is a large observational study of women’s health, involving more than one million UK women aged 50 and over. It is funded mainly by the UK Medical Research Council and Cancer Research UK. It includes approximately 1 in 4 UK women born in the 1930s and 1940s, who were 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 heart disease. 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 is largely a marker of 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. A similar disparity was found (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 are known to be more common in people of lower socio-economic status, which we also found in this study. Overall alcohol consumption was low in this cohort; it was slightly higher in the least deprived but an average consumption of alcohol was about one unit per day.
Half of inequality due to smoking
The main aim of our work was to assess the extent to which these four health behaviours could explain the social inequalities in risk of heart disease, particularly since 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.
It is plausible that the true contributions of these four health behaviours are even larger, because they were reported only once when women joined the study. We could not account for lifestyle changes over time, such as quitting smoking which has been found to occur more commonly in women with a higher socio-economic status. 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 is 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 to recognise that these health behaviours are themselves influenced by education and deprivation, and that it is 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, but the findings from this study emphasise the potential gains that could be made in reducing rates of heart disease if they are reached.