Does ‘overweight’ need to be redefined?

A study shows that the ideal body mass index associated with lower mortality has been rising through the years and that overweight individuals have lower mortality than so-called normal weight individuals


Obesity and overweight are classified using BMI, which is weight in kilograms divided by height in metres squared.
Obesity and overweight are classified using BMI, which is weight in kilograms divided by height in metres squared.

New Delhi: Being overweight is often associated with bad health and diseases such as diabetes. But a new study from Denmark shows that the ideal body mass index (BMI) associated with lower mortality has been rising through the years and that overweight individuals have lower mortality than so-called normal weight individuals.

“The increased risk of all-cause mortality associated with obesity compared to normal weight decreased from 30% in 1976-78 to 0% in 2003-13,” says principal investigator Shoaib Afzal, Herlev Hospital, Copenhagen University Hospital, Denmark.

This research, published in the Journal of the American Medical Association (JAMA), looked at changes in the BMI (a measure of body fat based on height and weight) associated with the lowest all-cause mortality in three cohorts from Copenhagen which included more than 100,000 people and were observed over nearly 40 years. These individuals were examined in 1976-78, 1991-1994, and in 2003-2013 and were followed until 2014.

“The optimal BMI for the lowest mortality increased from 23.7 in 1976-78, through 24.6 in 1991-94, to 27 in 2003-13, while individuals with a BMI below or above the optimal value had higher mortality,” added Afzal in a statement.

Obesity and overweight are classified using BMI, which is weight in kilograms divided by height in metres squared. A BMI of 25-29.9 represents overweight, a BMI of 30 or greater represents obesity, while a BMI of 18.5-24.9 is considered normal weight.

What is the ideal BMI?

“Compared to the 1970s, today’s overweight individuals have lower mortality than so-called normal weight individuals. The reason for this change is unknown. However, these results would indicate a need to revise the categories presently used to define overweight, which are based on data from before the 1990s,” said senior author Borge G. Nordestgaard, University of Copenhagen and Copenhagen University Hospital.

This is not the first time that a study has suggested that overweight is not as associated with mortality as is widely believed. More and more research is coming up with similar findings in the last few years. A meta-analysis of 97 studies including 2.88 million people was published in January 2013 in JAMA.

The team of researchers led by an epidemiologist at the National Center for Health Statistics in Hyattsville, Maryland, reported that people considered overweight by international standards were 6% less likely to die than were those of normal weight over the same time period.

But this study caused much uproar as well, including in Harvard School of Public Health where the chair of the nutrition department said that the results of the study were flawed and extremely misleading. “The new statistics are completely misleading for anyone interested in knowing about their optimal weight. The fundamental reason is that the authors did not adequately separate people who are lean because they are ill from those who are lean because they are active and healthy. This will inevitably lead to wrong conclusions about the effects of body weight on risk of premature death,” nutrition department chair Walter Willett said on the Harvard School of Public Health website.

Willett also added that there was no reason to rethink the definition of overweight, but that in addition to looking at the weight chart one should keep track of change in weight and waistline over time.

However, Nordestgaard who is a senior author of the latest study, cautions the public that it is important that their results should not be interpreted as suggesting that now people can eat as much as they like, or that so-called normal weight individuals should eat more to become overweight.

“That said, maybe overweight people need not be quite as worried about their weight as before,” Nordestgaard added in a statement.

The JAMA research from Denmark has shown a U-shaped pattern in the association of BMI with mortality. It shows that although average BMI has increased over time in most countries, the prevalence of cardiovascular risk factors may also be decreasing among obese individuals over time.

Chennai-based diabetologist V. Mohan has recently returned from a public heath conference in Rome where data for BMI and mortality from nearly 25 countries was discussed. “There is more and more evidence coming in to show that all physiological variables and so-called risk factors for heart diseases are not as straight-lined as we thought. It is more a U-shaped curve,” explained Mohan, chief of diabetology at Dr Mohan’s Diabetes Specialities Centre, one of India’s largest diabetes clinic chains.

“Of course, too much weight or too little weight is bad for you, but there is a wide range of weights which comes under normal weight or a little overweight which is also fine, as long as one maintains physical fitness and a good diet,” Mohan said.

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