Being overweight as defined by the body mass index rating scale is not linked to an increase in death when considered separately from other health issues, a new study found.
Also known as BMI, the calculation measures a person's body fat based on height and weight. As currently used, the BMI scale divides adult populations into various degrees of body fat.
An adult is considered "overweight" if their BMI falls between 25 and 29.9, while a "healthy" or "normal" weight is defined as a BMI between 18.5 and 24.9, according to the US Centers for Disease Control and Prevention. A BMI of 30 or higher is considered obese.
"The real message of this study is that overweight as defined by BMI is a poor indicator of mortality risk, and that BMI in general is a poor indicator of health risk and should be supplemented with information such as waist circumference, other measures of adiposity (fat), and weight trajectory," said study first author Dr Aayush Visaria, an internal medicine resident physician at the Rutgers Robert Wood Johnson Medical School in New Brunswick, New Jersey.
However, limitations of the study make it hard to determine if the findings are due to BMI or other factors, according to experts not involved with the new research.
"The use of the word 'overweight' is misleading here, as it excludes anyone with a BMI above 30. In lay language 'overweight' would usually be interpreted as anyone with a weight above 'normal' and would include obese patients," said Dr Baptiste Leurent, a lecturer in medical statistics at University College London, in a statement.
"This paper found an unequivocal association between BMI and mortality, before and after adjustment for risk factors," said Leurent, who was not involved in the study.
In addition, observational studies can only show an association, not causation, said Dr Robert H. Shmerling, senior faculty editor for Harvard Health Publishing, a subsidiary of Harvard Medical School in Boston.
"They looked at death rates, but there are other outcomes that are also important that they didn't look at, such as quality of life or the development of new comorbidities such as new cases of diabetes or heart disease," said Shmerling, who was not involved in the study.
Death vs. disease
In the new study, published Wednesday in the journal PLOS ONE, researchers analysed data gathered on over 554,000 non-pregnant Americans older than age 20 from the 1999-2018 National Health Interview Survey and the 2019 US National Death Index.
Visaria and his coauthor, Dr Soko Setoguchi, a professor of medicine and epidemiology at the Rutgers Robert Wood Johnson Medical School and Rutgers School of Public Health, then compared BMI levels with deaths that occurred over the next 20 years.
The risk of death did rise by 18 percent to 108 percent for most people with BMI levels higher than 27.5, Visaria said, with risk rising as weight increased in a U-shaped curve.
There was one exception: adults older than age 65. There was no significant increase in mortality for any older adult with a BMI of between 22.5 and 34.9 - a range that included those with normal weight, overweight and obesity.
"This paper does not add anything new," said Naveed Sattar, a professor of cardiovascular and metabolic health at the University of Glasgow in Scotland, who was not involved in the study.
"We know that BMI often displays a U shaped curve with mortality, but this is due to many people (particularly older) at the lower end of the BMI range having unintentional weight loss due to illness," Sattar said in a statement.
Weight loss often goes hand in hand with development of dementia, cancer and accompanying loss of appetite in an older population. Prior research has found losing as little as 5 percent of body mass increased the risk of premature death among adults 65 and older, especially for men.
The most significant finding, Visaria said, was for people between ages 20 and 65 whose BMI was between 24.5 and 27.5 - the lower end of the overweight scale. There was no significant increase in the risk for death.
However, the risk of future disease is "probably a more important measure of health than all cause mortality," said Tom Sanders, professor emeritus of nutrition and dietetics at King's College London, who was not involved in the study.
"The main hazard of overweight (BMI 25 - 29.9) and moderate obesity (BMI 30-35) is a three times greater risk of developing diabetes which contributes to cardiovascular disease, renal failure and blindness," he said in a statement.
While the study did control for smoking and a variety of other diseases linked to early death, that information was only gathered once for each person in the survey. Therefore, the study could not follow that person to see if he or she later developed conditions such as hypertension or diabetes that might contribute to death - a limitation of the study, Visaria told CNN in an email.
"They also didn't look at the cause of death - it might have been from a car accident or something not health-related," said Shmerling, who is the former clinical chief of the division of rheumatology at Beth Israel Deaconess Medical Center in Boston.
"In addition, if you look at the big picture findings, they did find an increase of mortality with obesity - so it's not as if they've refuted the usefulness of BMI for all purposes," he said.
It's also possible, said Shmerling, that people in the overweight category visit the doctor more often, make lifestyle changes - such as increasing exercise or adopting a healthier diet - and get medical care so they don't develop diabetes, heart disease or other comorbidities.
"Death due to obesity may be due to development of morbidities like heart disease, but obesity is also its own independent disease that we know can independently increase risk of death," Visaria said. "The issue is how we diagnose obesity which may not be representative of the risks."
Waist circumference is a key measure
In addition to BMI, Visaria said the study also looked at data that measured waist circumference, or the thickest part of the stomach. Results showed using waist circumference "significantly modifies the association between BMI and all-cause mortality," he said.
"People with elevated waist circumference had higher risk of mortality compared to normal waist circumference within the same BMI groups," Visaria said in an email. "In the overweight BMI range (25-29.9), the risk of mortality was 17-27 percent higher among people with elevated waist circumference compared to lower waist circumference."
That type of deep fat that surrounds the body's organs, often called belly or visceral fat, has been linked to a 39 percent higher risk for dementia in older women and heart disease, frailty and early death in both sexes.
Measuring waist circumference should be paired with stepping on a scale as part of any health assessment, according to guidelines published in April 2021 by the American Heart Association. Abdominal obesity, as it is known, is defined as a waist circumference of 40 inches (102 centimetres) or above for men, and 35 inches (88 centimetres) or above for women.
The American Medical Association also recently adopted new guidelines calling for physicians to use more than BMI when evaluating an individual's health.
"BMI is based primarily on data collected from previous generations of non-Hispanic white populations," the AMA wrote. While it's "significantly correlated with the amount of fat mass in the general population, (it) loses predictability when applied on the individual level."
The use of BMI as a measure of potential health risk may not disappear from clinical practice as it does have a place in a thorough workup - but it should not be the only measure, Visaria said.
However, any discussion over how fat mass is measured does not change what science knows about the impact of excess weight upon the body, experts say.
"We know from other evidence that the higher our weight, the greater the risks of developing multiple conditions," said the University of Glasgow's Sattar.
"These conditions in turn adversely influence people's quality of life and their happiness," Sattar said in a statement. "It is these 'quality of life' metrics we need to focus more on, and, if possible, try to improve with relevant interventions at multiple stages of life."