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Chennai 1

The association between cause-specific mortality and body-mass index (BMI) has been studied mainly in high-income countries. Research published today in the Lancet Global Health investigated the relations between BMI, systolic blood pressure, and mortality in India.

In this study conducted by Dr Gajalakshmi, half a million men and women aged 35 years or older were recruited from the general population in Chennai, India, between 1998 and 2001. Participants were interviewed, measured (height, weight, and blood pressure) and followed up until 2015. Deaths were identified by linkage to state mortality records and through household visits from trained fieldworkers. There were 30,000 premature deaths (ie, before age 70) of which half were vascular.

In this South Asian population, the investigators found little association between BMI and either cardiac or stroke mortality, even though BMI was strong determinant of blood pressure and diabetes, both of which were strong determinants of vascular mortality. The limited vascular risks associated with overweight and obesity differed strikingly from the substantial risks seen in Europe and North America, where studies show cardiac mortality to be minimal in people with BMI in the range 20–22·5 kg/m², above which mortality doubles per 10 kg/m² increase in BMI. This contrast is surprising, especially because, at a given BMI, South Asians tend to have more body fat than Europeans.

These findings point to the existence of novel and important causes of vascular mortality in India that are closely correlated with below average BMI and act to counterbalance the  important adverse effects of BMI on blood pressure and diabetes. If understood, these correlates could well be of some relevance in all populations. If, however, these correlates are mainly poverty-related developmental factors that act before adulthood to affect BMI and vascular risk, then the adverse effects of becoming overweight in adult life could be about as great in South Asia as elsewhere.

The most immediately actionable finding from this study for individual or population health is confirmation here, as elsewhere, of the importance of blood pressure to vascular mortality. Effective antihypertensive drugs are available as generics, and trials have shown that they safely reduce stroke and myocardial infarction.