That obesity is a causal factor for many lifestyle diseases such as cardiovascular disease, and diabetes, which in turn increases the risk of death, has been well established in the developed world. Similarly, being underweight increases the risk of death. A study published online in The New England Journal of Medicine (NEJM) (“Association between body-mass index and risk of death in more than 1 million Asians” by Wei Zheng et al.,) finds a strong correlation between underweight and increased risk of death in all Asians. The study is an analysis of published data involving 1.1 million people living in Asian countries, including 287,000 Indians and Bangladeshis. The risk of death in the case of East Asians was high for those with a high body mass index (BMI), but not in the case of Indians and Bangladeshis. Indians and Bangladeshis were more prone to death when they were underweight. It is a fact that severely underweight people are malnourished, and hence very unhealthy. Low immunity levels seen in such people make them highly prone to several infections. Large-scale studies done in India, which has a huge malnourished population, are more likely to show a large percentage of deaths associated with lower BMI than higher BMI. Several small studies done in India have shown the risk of mortality increasing with higher BMIs.
The study reported in NEJM has several limitations. Being underweight increases the risk of infections but a person can become underweight as a result of an underlying infection. The researchers were not able to exclude, at the time of enrolment, underweight people already suffering from some infection, and hence at a higher risk. That many people were reported to be dying soon after enrolment suggests they may have had some underlying infection or disease at the time of enrolment. Waist-to-hip ratio or waist circumference is a better marker than BMI to know fat distribution in the body. Indians, even those who are thin, tend to accumulate fat in their waist. Abdominal adiposity is a causal factor for hypertension and diseases such as diabetes and cardiovascular disease. Those with a BMI between 22.6 and 25 with less central adiposity have the greatest chances of living free of any infections and lifestyle-related diseases. With more urban children becoming obese, public health messages should address the increasing chances of health complications when the BMI is lower or higher than the normal range. They should also stress the need to reduce abdominal adiposity even when the BMI is within the normal range.