post by Paul Kelleher
In an article purporting to be on the adverse effects of income inequality, but which also spends a great deal of time on status inequality, Sir Michael Marmot is quoted as saying:
When Marmot and his colleagues controlled for lifestyle factors like smoking and lack of exercise in the lower socioeconomic groups, the gradient remained. "Those things accounted for about a third of the gradient," he says, noting that you have to look for the "causes of the causes" — the reasons that the lower classes might be driven to smoke, drink, take drugs or indulge in sweet, fatty foods. "It's now been described the world over," Marmot says.
Many who have read Marmot's work in the past know that it is often reported that the Whitehall Study of British civil servants "controlled for health behaviors," and that the health gradient remained. But follow-up research on Whitehall employees that was published last year seems to conflict with Marmot's original findings. And the conflict appears to trace to different (improved?) ways of assessing the impact of health behaviors on the gradient:
Design, Setting, and Participants Established in 1985, the British Whitehall II longitudinal cohort study includes 10 308 civil servants, aged 35 to 55 years, living in London, England. Analyses are based on 9590 men and women followed up for mortality until April 30, 2009. Socioeconomic position was derived from civil service employment grade (high, intermediate, and low) at baseline. Smoking, alcohol consumption, diet, and physical activity were assessed 4 times during the follow-up period.
Main Outcome Measures All-cause and cause-specific mortality.
Results A total of 654 participants died during the follow-up period. In the analyses adjusted for sex and year of birth, those with the lowest socioeconomic position had 1.60 times higher risk of death from all causes than those with the highest socioeconomic position (a rate difference of 1.94/1000 person-years). This association was attenuated by 42% (95% confidence interval [CI], 21%-94%) when health behaviors assessed at baseline were entered into the model and by 72% (95% CI, 42%-154%) when they were entered as time-dependent covariates. The corresponding attenuations were 29% (95% CI, 11%-54%) and 45% (95% CI, 24%-79%) for cardiovascular mortality and 61% (95% CI, 16%-425%) and 94% (95% CI, 35%-595%) for noncancer and noncardiovascular mortality. The difference between the baseline only and repeated assessments of health behaviors was mostly due to an increased explanatory power of diet (from 7% to 17% for all-cause mortality, respectively), physical activity (from 5% to 21% for all-cause mortality), and alcohol consumption (from 3% to 12% for all-cause mortality). The role of smoking, the strongest mediator in these analyses, did not change when using baseline or repeat assessments (from 32% to 35% for all-cause mortality).
Conclusion In a civil service population in London, England, there was an association between socioeconomic position and mortality that was substantially accounted for by adjustment for health behaviors, particularly when the behaviors were assessed repeatedly.
As Marmot notes, he always thought we need to ask why health behaviors themselves may be distributed in a graded fashion. But this new research seems to support the claim that this is the question. If that's true, it means we should focus much more on health behaviors than the summaries of the original Whitehall studies would have us do.
I would be grateful for pointers to additional research and/or commentary on these points.