Growth, current size and the role of the 'reversal paradox' in the foetal origins of adult disease: an illustration using vector geometry

BACKGROUND Numerous studies have reported inverse associations between birth weight and a range of diseases in later life. These have led to the development of the 'foetal origins of adult disease hypothesis'. However, many such studies have only been able to demonstrate a statistically significant association between birth weight and disease in later life by adjusting for current size. This has been interpreted as evidence that the impact of low birth weight on subsequent disease is somehow dependent on subsequent weight gain, and has led to a broadening of the hypothesis into the 'developmental origins of health and disease'. Unfortunately, much of the epidemiological evidence used for both of these interpretations is prone to a statistical artefact known as the 'reversal paradox'. The aim of this paper is to illustrate why, using vector geometry. MATERIALS AND METHODS This paper introduces the key concepts of vector geometry as applied to multiple regression analysis. This approach is then used to illustrate the similar statistical problems encountered when adjusting for current size or growth when exploring the association between birth weight and disease in later life. RESULTS Geometrically, the three covariates – birth size, growth, and current size – span only 2-dimensional space. Regressing disease in later life (i.e. the outcome variable) on any two of these covariates equates to projecting the disease variable onto the plane spanned by the three covariate vectors. The three possible regression models – where any two covariates are considered – are therefore equivalent and yield exactly the same model fit (R2). CONCLUSION Vector geometry illustrates why it is impossible to differentiate between the effects of growth from the effects of current size in studies exploring the relationship between size at birth and subsequent disease. For similar reasons, it is impossible to differentiate between the effects of growth and the effects of birth weight. Assessing the 'independent' impact of growth on later disease by adjusting for either birth weight or current size is therefore illusory

Johannesburg's 'poor housing, good health' paradox: the role of health status assessment, statistical modelling, residential context and migrant status

The 'poor housing, good health' paradox observed by De Wet et al. (2011) across eight of Johannesburg's poorest Wards (neighbourhoods), was re-examined using: a more generic measure of self-reported health; better-specified adjustment for measured confounders; household data from a range of Wards and income strata across Johannesburg and Gauteng; and adjustment for migrant status. The present study examined the relationship between (formal vs. informal) housing and self-reported health (-limited work and/or social activities) across four subsamples of respondents to the 2013 Quality of Life survey undertaken by the Gauteng City Regional Observatory: n = 1494 from households in the eight Wards examined by De Wet et al. (2011); n = 3059 from households with the lowest income tertile in Johannesburg; n = 8263 from households throughout Johannesburg; and n = 24,727 from households throughout Gauteng Province, irrespective of Ward or income. The relationship between housing and self-reported health in each of these subsamples was examined before and after adjustment for measured confounders identified using a temporally determined causal path diagram in the form of a directed acyclic graph. Following adjustment for measured confounders, 'informal housing' was only associated with 'good' self-reported health in Johannesburg's poorest Wards (odds ratio [OR]: 1.39; 95% confidence interval [CI]: 1.07, 1.79) and Johannesburg as a whole (OR: 1.15; 95%CI: 1.00, 1.31). These associations were substantially attenuated following adjustment for migrant status (OR: 1.26; 95%CI: 0.97, 1.65; OR: 1.07; 95%CI: 0.93, 1.22, respectively). While the present study found that Johannesburg's 'poor housing, good health' paradox was still evident when using a more generic/subjective (self-reported) measure of health, the apparent paradox appears to reflect, at least in part: the differential aggregation of migrant-headed households in Johannesburg neighbourhoods exhibiting a high concentration of informal housing; and the likely impact of the health-related selection on the health of migrant-headed households. [Abstract copyright: Copyright © 2020 The Royal Society for Public Health. Published by Elsevier Ltd. All rights reserved.

A note on the small mammal fauna of Vaalkop Dam Nature Reserve

Opinio

The use of 'racial' categories in contemporary South African health research

A survey of articles published in the South African Medical Journal between 1992 and 199