Letters

Association between birth weight and death from heart disease

BMJ 1999; 318 doi: https://doi.org/10.1136/bmj.318.7178.259a (Published 23 January 1999) Cite this as: BMJ 1999;318:259

Data do not support association

  1. R J Jarrett, Emeritus professor of clinical epidemiology
  1. 45 Bishopsthorpe Road, London SE26 4PA
  2. Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT
  3. Institute of Geriatrics, University of Uppsala, S-751 25 Uppsala, Sweden
  4. Department of Sociology, Stockholm University, S-106 91 Stockholm, Sweden

    EDITOR—Leon et al claim that their study is “the most persuasive evidence of a real association between size at birth and mortality from ischaemic heart disease.”1 However, the study is open to different interpretations and, in my view, inflicts a serious wound on the birth measurement and adult disease hypothesis.

    Their table 3shows that there was no significant association between birth weight and all cause mortality in either sex. Both sexes showed a positive association between birth weight and death from neoplasms and respiratory disease, though none of these were significant. The negative association between birth weight and deaths from circulatory disease was significant only in men. In women there were no significant associations between birth weight and any cause of death. Subsequently, most of the analysis concentrates on the association of death from ischaemic disease and birth measurements in men.

    This study is claimed (probably correctly) to have unique features. It certainly has large numbers of deaths to analyse. Yet no significant association was found in women, and that in men, with ischaemic heart disease, was presumably compensated for by other causes of death, which made the association with all cause mortality non-significant. Thus, of all the possible associations with birth weight, and despite the large number of deaths, only the association between birth weight and death from ischaemic heart disease in men remains significant. I do not see how these data justify the key message “adult mortality from ischaemic disease increases as size at birth declines.”

    There is also the question of socioeconomic confounding. Leon et al state that adjustment for socioeconomic circumstances produced only a small reduction in the strength of the association between birth weight and mortality from ischaemic heart disease. The problem of adjusting for socioeconomic factors in this context has been much debated. In this paper the adjustment brought the upper limit of the confidence intervals perilously near unity. If the precision of this measurement matched that of the others, even the single significant result might be in peril.

    References

    Authors' reply

    1. David A Leon, Reader in epidemiology,
    2. Ilona Koupilová, Lecturer,
    3. Paul McKeigue, Reader in metabolic and genetic epidemiology,
    4. Hans O Lithell, Professor of geriatrics,
    5. Rawya Mohsen, Programmer,
    6. Lars Berglund, Statistician,
    7. Ulla-Britt Lithell, Assistant professor,
    8. Denny Vµgerå, Professor of medical sociology
    1. 45 Bishopsthorpe Road, London SE26 4PA
    2. Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT
    3. Institute of Geriatrics, University of Uppsala, S-751 25 Uppsala, Sweden
    4. Department of Sociology, Stockholm University, S-106 91 Stockholm, Sweden

      EDITOR—Jarrett draws attention to the intriguing positive association between birth weight and neoplasms that we observed. Interest is growing in whether risk of certain cancers is increased in individuals who are large at birth.1 If true, this may indeed partly compensate for the negative effect of reduced size at birth on later mortality from circulatory disease. However, it is precisely the specificity2 of the negative association of size at birth with mortality from ischaemic heart disease that strengthens the case for this association having a causal element. Although not reaching significance, a similar association was observed in women, with risk of death from ischaemic heart disease falling with increased size at birth. We therefore stand by our assertion that risk of death from ischaemic heart disease increases as size at birth falls.

      The question of socioeconomic confounding is an important one that we addressed. Jarrett is wrong to focus on confidence intervals. The main criterion for judging the extent to which there may be inadequate adjustment for a confounder is the magnitude of the change in the estimates of effect, rather than changes in the P values or width of confidence intervals. In our case the rate ratio for ischaemic heart disease associated with a 1kg increase in birth weight was 0.77in the crude data and 0.82when adjusted for socioeconomic characteristics at the birth of the subject and at two points in adult life. We believe that this shows that although a small fraction of the crude association between size at birth and ischaemic heart disease is attributable to socioeconomic confounding, there remains a consistent and robust effect that is not.

      The evidence for impaired fetal growth being associated with ischaemic heart disease in men is steadily accumulating. Unlike other studies, ours has largely been able to exclude the role of bias and confounding. Further epidemiological studies in other populations are desirable, particularly concerning the association in women and the potential modifying effects of factors such as obesity in adult life. However, the understandable scepticism about the reality of these associations that prevailed when the fetal origins hypothesis was first put forward some years ago should now give way to critical thinking and investigation of plausible underlying mechanisms. Among many other questions that need to be addressed is the extent to which maternal nutritional status drives this association.

      References

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