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Body mass index (BMI) alone may not be an appropriate indicator of all-cause mortality risk, according to a new study. Henrik Sorensen/Getty Images
  • A new study casts further doubt on the value of body mass index (BMI) alone as a sole predictor of all-cause mortality.
  • According to the study authors, most other studies use older data that is insufficiently diverse, and they hope that the new study can correct that.
  • While a BMI indicating overweight or obesity can be a risk factor for various chronic, life threatening diseases, it may not be a good indicator of overall mortality risk.

A new study finds that a person’s body mass index, or BMI, is best considered in conjunction with other risk factors when predicting all-cause mortality. BMI as an independent factor may not be as clear an indicator of premature death as previously believed.

Researchers found that across both healthy and overweight BMI categories — from a BMI of 22.5 to 27.4 — there was little difference for adults in the risk of all-cause death.

The study, however, observed that the risk of all-cause death increased by 21% to 108% in adults whose BMI exceeded 30.

For older adults, no significant increase in mortality was seen between BMIs of 22.5 to 34.9, the upper range of which indicated obesity.

The study is published in PLOS ONE.

Most research regarding BMI and mortality is based on data from the 1970s that focused on non-Hispanic white adults.

Mindful of the changes in lifestyles since that period — including the rise in overweight and obesity — and seeking a more diverse study population sample, the new study analyzed fresher, more expansive data.

The analysis involved self-reported BMI data from 554,332 adults in the United States who participated in the 1999-2018 National Health Interview Survey and data from the 2019 US National Death Index.

Individuals averaged 46 years of age, with an equal number of men and women, and 69% were non-Hispanic white, with 12% being non-Hispanic Black.

Of the participants, 35% had a BMI between 25 and 30, which is typically considered overweight, and 27.2% had a BMI of 30 or higher, classified as obesity.

They were followed for an average of 9 years and a maximum of 20 years, during which 75,807 deaths were recorded.

Dr. Pedro J. Caraballo, medical director of the Clinical Decision Support Program at Mayo Clinic, not involved in this study, told Medical News Today:

“The use of BMI alone to define obesity or health is highly controversial. There are better definitions of obesity and different types of obesity that could influence health. However, BMI is easy to calculate and readily available in all medical records.”

The formula for one’s BMI is relatively simple: It is a person’s weight in kilograms divided by the square of their height in meters.

However, BMI fails to consider other body characteristics, such as fat versus muscle, the manner in which fat is distributed throughout the body, and one’s metabolic health. Fat around the waist, for example, particularly increases the risk of disease.

“[BMI] does not distinguish between muscle mass and fat mass, and some individuals like bodybuilders may have a high BMI because of more muscle mass,” explained Dr. Dagfinn Aune, a research associate in the Faculty of Medicine, the School of Public Health at Imperial College London in the United Kingdom, not involved in this study.

“In spite of these limitations,” said Dr. Aune, “BMI does a pretty decent job in capturing the increased chronic disease and mortality risk at the population level associated with adiposity.”

The study’s corresponding author, Dr. Aayush Visaria, a postdoctoral research fellow at Rutgers Center for Pharmacoepidemiology and Treatment Science, NJ, noted:

“I think the real message of this study is that overweight as defined by BMI is not an appropriate measurement tool, at least for all-cause mortality.

Overweight as a medical condition is still important, but will likely need to be diagnosed taking into account body composition and body fat distribution.”

Dr. Aune offered a long list of chronic conditions associated with an overweight BMI. These included coronary heart disease, stroke, heart failure, sudden cardiac death, atrial fibrillation, hypertension, type 2 diabetes, kidney stones, gallstones, diverticular disease, and a dozen different types of cancer.

“In addition,” Dr. Aune pointed out, “overweight/ obesity during pregnancy is associated also with a wide range of pregnancy complications including preeclampsia, gestational diabetes, gestational hypertension, stillbirth and infant death.”

To Dr. Caraballo, the findings of this research are old news. He cited his own study “with risk stratification based on comorbidities, and we showed that BMI is an independent risk factor only in extreme values, very low (<20) or very high (>40).”

Dr. Caraballo said multiple studies have suggested mild and moderate obesity “may help survival when considering a specific subpopulation that is under stress.”

He speculated that having energy reserves may be helpful for people, noting several publications on this topic for “heart disease, kidney disease, cancer, stroke, and rheumatoid arthritis, etc.”

“The U.S. has changed dramatically since the 20th century in terms of racial/ ethnic makeup, age distribution, healthcare access and treatments, and sociocultural behaviors,” suggested Dr. Visaria.

“These all can contribute to the relationship between BMI and all-cause mortality, so it is important to understand the relationship in a more contemporary population,” he noted.

Dr. Visaria also noted the value of using the most rigorous methods to reduce bias and be as nationally representative as possible with observational data.

With older people showing no increase risk of mortality up to a BMI of 35, Dr. Visaria speculated why this may be the case.

“We think this is in part due to loss of muscle mass in older age — termed sarcopenia — as well as bone-mineral density. Loss of these two types of weight can lead you to have inappropriately normal BMIs despite having excessive amounts of fat,” he told MNT.

“People with higher BMIs may be paradoxically healthier because of sustained muscle mass and bone density,” said Dr. Visaria.

“The relationship between obesity and mortality is very complicated,” said Dr. Caraballo.

“Obesity by itself, in the range of mild to moderate, may not be an independent risk factor,” he explained. “However, obesity is an important risk factor to develop many metabolic conditions which will increase the mortality risk after a few years (diabetes, heart disease, etc.). Also, people may gain weight when they are chronically ill due to less physical activity and poor diet.”

Dr. Visaria suggested that “physicians should consider supplementing BMI with other measures such as waist circumference, waist-to-height ratio, and waist-to-hip ratio.”

In the study, said Dr. Visaria, “We show that waist circumference significantly modifies the association between BMI and all-cause mortality.”

Bioimpedance scales,” Dr. Visaria continued, “are also an option to estimate total body fat [percentage], but they still need to be validated and are known to have some margin of error. Additionally, physicians should interpret adiposity measures in the context of their cardio-metabolic health parameters such as blood pressure, blood sugar, and cholesterol levels.”