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Re: The larger the better?

Dear Editor,

It is interesting to note that now birth size is shown to be related to income in later life! However, I wonder whether a correlation of 0.04 or 0.1 can be considered substantial. Although the P values are impressive (<0.0001), it is due to the relative large sample size in their study.

Another interesting point is that in this study Professor Barker and his co-authors used birth height rather than their favourite birth weight. I guess it might be because they want to compare their results to a previous study, but, however, it would be interesting to know if they can find similar correlations between birth weight/BMI/Ponderal index and income in later life.

Another point I would like to make is that in their conclusions they said "Protection of infant growth will not only reduce infant mortality and childhood morbidity, but will improve cognitive function, and reduce morbidity from coronary heart disease and type 2 diabetes in adult life". Notwithstanding the questionable statistical analyses in most studies on fetal origins of adult diseases hypothesis, [1] two studies found high birth weight increases the risk of breast cancer in women [2,3]. So does this mean that we should only protect the growth of male babies?

References

1. Tu Y-K, West R, Ellison GTH, Gilthorpe MS. Why evidence of fetal origins of adult diseases can be statistical artifact: the reversal paradox examined for hypertension. American Journal of Epidemiology 2005;161:27-32.

2. Ahlgren M, Melbye M, Wohlfahrt J, Sorensen TA. Growth patterns and the risk of breast cancer in women. New England Journal of Medicine 2004;351:1619-1626.

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Re: Childhood growth influences cognition in the Thousand Families study, but not income at age 50.

Dear Editor,

Barker et al. recently reported a relationship between short length at age 1 year and lower incomes in adulthood [1]. Citing results from an early report from the Newcastle Thousand Families Study showing a relationship between height at age 3 years and intelligence at age 11 [2]. Barker et al. interpreted this as evidence that slow infant growth may lead to a lifelong impairment of cognitive function, which may explain the relationship between infant growth and income 50 years later, assuming that childhood IQ is related to income 50 years later.

We tested this assumption in the Thousand Families Study using data on growth and IQ collected during childhood and total household income collected at age 50 [3]. Information on both IQ at age 11 and income at age 50 was available for 430 participants.

A recent multivariable analysis of the Thousand Families data suggests that there is a continuing relation between post-natal growth and childhood cognition beyond the age of 9 years [4]. There was a highly significant correlation between childhood IQ and household income at age 50 (p<0.0001, predicting an increase of around £300 per annum for every point increase in IQ at age 11, after adjusting for social class at birth). Results were similar for boys and girls. While our previous analysis showed that height at age 9 was significantly associated with IQ at age 11 (p<0.0001 after adjusting for social class at birth) we found that there was no such relationship between height at age 9 and later income (p=0.28). Our results suggest that while childhood height does predict childhood IQ, this is mainly due to postnatal growth and is an effect which may continue throughout childhood. While promotion of growth in infancy is important, promotion of growth throughout childhood may be more beneficial in maximizing cognitive function.

References

1. Barker DJP, Eriksson JG, Forsén T, et al. Infant growth and income 50 years later. Arch Dis Child 2005;90:272-273.

2. Miller FJW, Court SDM, Knox EG, et al. The school years in Newcastle upon tyne, 1952-62. London: Oxford University Press, 1974.

3. Lamont DW, Parker L, Cohen MA, et al. Early life and later determinants of adult disease: a 50 year follow-up study of the Newcastle Thousand Families cohort. Public Health 1998;112:85-93.