RE: "SENSE OF COHERENCE AND MORTALITY IN MEN AND WOMEN IN THE EPIC-NORFOLK UNITED KINGDOM PROSPECTIVE COHORT STUDY"

John Macleod1, George Davey Smith2 and John Lynch3

1 Department of Primary Care and General Practice, University of Birmingham, Birmingham B15 2TT, United Kingdom
2 Department of Social Medicine, University of Bristol, Bristol BS8 2PR, United Kingdom
3 Department of Epidemiology, School of Public Health, University of Michigan, Ann Arbor, MI 48104-2548

Observational evidence of associations between psychosocial factors and mortality is widespread (1), but what these associations mean is unclear (2, 3). In a recent article, Surtees et al. (4) reported an association between a higher "sense of coherence" (SOC) and lower mortality in the large European Prospective Investigation into Cancer and Nutrition–Norfolk (EPIC-Norfolk) Study. Does their report suggest that intervening to promote SOC should lead to improvements in life expectancy?

Participants in the study who felt that their lives were more coherent were considerably more likely to be nonsmokers and to occupy nonmanual occupations. They were also considerably less likely to report feelings of hostility or neuroticism. The former associations signal a possible source of residual confounding of the association between SOC and mortality; the latter attest to the covariance, at times bordering on interchangeability, of many psychosocial constructs (5).

Surtees et al. dismissed the possibility of noncausal explanations for their findings, because most of their effect estimates remained conventionally significant following adjustment for age, diagnosed prevalent disease, adult occupational class, and smoking history. However, undiagnosed yet still experienced ill health may do more to erode a sense of well-being than diagnosed disease (3). Furthermore, the limitations of statistical adjustment in situations where correlated covariates are measured imprecisely are well known (6). Adult occupational class is a crude index of social position across the life course and the exposures associated with this. A growing number of studies show that socioeconomic disadvantage at various life-course stages independently predicts cardiovascular disease mortality (7, 8) and is associated with adult psychosocial characteristics such as hopelessness and hostility (9). A categorization of smoking status into current, former, and never smokers similarly provides a limited index of lifetime tobacco exposure.

Even with these imprecise measures, confounding appeared to account for approximately half of the apparent protective effect of higher SOC on cardiovascular disease mortality in men (a rate ratio of 0.75 associated with higher SOC was attenuated to 0.87). Therefore, the probability of residual confounding in relation to most estimates appears strong. The nonspecific character of the association between SOC and both cancer and cardiovascular disease mortality is another indicator of its potentially artifactual basis. The importance of specificity as a criterion for establishing causality has perhaps been underestimated (10, 11). Interestingly, investigators in the EPIC-Norfolk Study previously reported a strong, apparently protective but nonspecific association of vitamin C levels with cardiovascular and other causes of death (12). Evidence from a randomized trial now strongly suggests that this was noncausal (13), and empirical data demonstrate the degree to which confounding by life-course socioeconomic circumstances could have generated such an artifact (14). All of these considerations suggest that some degree of caution is warranted when attempting to draw causal conclusions based on observational evidence of a general association between greater SOC and better health.

Finally, adopting a "positive psychology" toward one’s life’s circumstances is an admirable and attractive idea, but achieving this goal may be difficult for persons facing manifest disadvantage. Indeed, as clinicians and public health advocates, we need to ask ourselves whether it is even appropriate that they be counseled to do so.

REFERENCES

  1. Marmot M, Wilkinson RG. Psychosocial and material pathways in the relation between income and health: a response to Lynch et al. BMJ 2001;322:1233–6.[Free Full Text]
  2. Macleod J, Davey Smith G. Psychosocial factors and public health: a suitable case for treatment? J Epidemiol Community Health 2003;57:565–70.[Abstract/Free Full Text]
  3. Macleod J, Carroll D. What should we make of associations between vital exhaustion and heart disease? Int J Epidemiol 2003;32:997–9.[Free Full Text]
  4. Surtees P, Wainwright N, Luben R, et al. Sense of coherence and mortality in men and women in the EPIC-Norfolk United Kingdom prospective cohort study. Am J Epidemiol 2003;158:1202–9.[Abstract/Free Full Text]
  5. Stamatakis KA, Lynch JW, Everson SA, et al. Self-esteem and mortality: prospective evidence from a population-based study. Ann Epidemiol 2003;14:58–65.[CrossRef][ISI]
  6. Phillips AN, Davey Smith G. How independent are "independent" effects? Relative risk estimation when correlated exposures are measured imprecisely. J Clin Epidemiol 1991;44:1223–31.[ISI][Medline]
  7. Davey Smith G, Hart C, Blane D, et al. Adverse socio-economic conditions in childhood and cause specific adult mortality: prospective observational study. BMJ 1998;316:1631–5.[Abstract/Free Full Text]
  8. Beebe-Dimmer J, Lynch JW, Turrell G, et al. Childhood and adult socioeconomic conditions and 31-year mortality risk in women. Am J Epidemiol 2004;159:481–90.[Abstract/Free Full Text]
  9. Harper S, Lynch J, Hsu WL, et al. Life course socioeconomic conditions and adult psychosocial functioning. Int J Epidemiol 2002;31:395–403.[Abstract/Free Full Text]
  10. Weiss NS. Can the "specificity" of an association be rehabilitated as a basis for supporting a causal hypothesis? Epidemiology 2002;13:6–8.[CrossRef][ISI][Medline]
  11. Davey Smith G, Ebrahim S. Data dredging, bias or confounding. BMJ 2002;325:1437–8.[Free Full Text]
  12. Khaw KT, Bingham S, Welch A, et al. Relation between plasma ascorbic acid and mortality in men and women in EPIC-Norfolk prospective study: a prospective population study. European Prospective Investigation into Cancer and Nutrition. Lancet 2001;357:657–63.[CrossRef][ISI][Medline]
  13. Heart Protection Study Collaborative Group. MRC/BHF Heart Protection Study of antioxidant vitamin supplementation in 20,536 high-risk individuals: a randomised placebo-controlled trial. Lancet 2002;360:23–33.[CrossRef][ISI][Medline]
  14. Lawlor DA, Davey Smith G, Bruckdorfer KR, et al. Those confounded vitamins: what can we learn from the differences between observational versus randomised trial evidence? Lancet 2004 (in press).