The Obesity Paradox: Does BMI Increase Mortality?

A new study suggests that most overweight people may not be at a higher risk of all-cause mortality. Such findings provide new insights into the obesity paradox, which researchers have long tried to explain.

Obesity affects more than two in five adults (42.4%) and nearly one in five children (19.3%) in the United States. The condition has been associated with an increased risk of multiple serious illnesses, including type 2 diabetes and coronary heart disease.

However, researchers examining the link between obesity and all-cause mortality have faced the so-called "obesity paradox": previous studies found that individuals with body mass index (BMI) ranging from 25 to 35 had little or no increased mortality risk compared to people with healthy weight.

BMI is a measure that uses height and weight to determine if a person's weight is healthy. However, the tool often lacks accuracy, as it does not distinguish between excess fat, muscle, or bone mass. For example, muscular athletes may have high BMIs but be healthy.

In a new study published in the journal PLOS One, the researchers from Rutgers University analyzed data on 554,332 US adults from the 1999 to 2018 National Health Interview Survey and the 2019 US National Death Index.

Thirty-five percent of the study participants had a BMI between 25 and 30, categorizing them as overweight, and 27.2% had a BMI above or equal to 30, which typically indicates obesity.

Over a median follow-up of 9 years, the researchers observed 75,807 deaths. The risk of all-cause mortality was similar across a wide range of BMI categories. For older adults, there was no significant increase in mortality for any BMI between 22.5 and 34.9. For younger adults, BMI between 22.5 and 27.4 was also not associated with significantly higher mortality.

Overall, adults with a BMI of 30 or over had a 21% to 108% increased mortality risk attributed to their weight.

"Our study highlights the increasing reservations of using BMI alone to drive clinical decisions. There is no clear increase in all-cause mortality across a range of traditionally normal and overweight BMI ranges; however, that is not to say that morbidity is similar across these BMI ranges. Future studies will need to assess incidence of cardio-metabolic morbidities," the authors wrote.

Authors of another 2023 study say that because BMI does not account for body composition or how long a person has been overweight, a lack of such data may explain "the obesity paradox."

After examining data from 17,784 people collected over 27 years, the researchers from the University of Colorado Boulder discovered that 20% of the participants characterized as having "healthy" weight had been in the overweight or obese category in the decade prior. This group had a substantially worse health profile compared to those with stable weight.

Living with overweight and obesity can cause illnesses over time, which may result in rapid weight loss. Therefore, obtaining BMI data during this time can skew research results, the researchers say. The study also showed that individuals who gained weight only in the last decade were in better health, while those with low BMI (18.5 to 22.5) had the lowest mortality risk.

A 2016 systematic review published in the British Medical Journal included 230 studies with more than 3.74 million deaths from 30.3 million participants. Those with a BMI of around 25 were found to have the lowest risk of death. However, when the researchers examined subgroups, the lowest mortality was observed in the BMI range of 23 to 24 among never-smokers, 22 to 23 among healthy never-smokers, and 20 to 22 among studies of never-smokers with longer follow-up durations.

The study concluded that obesity, as measured by BMI, increases the risk of premature death. Additionally, higher mortality risk in underweight people could be partly explained by prediagnostic diseases distorting research results.

Research also suggests that comorbidities — having more than one disease or condition at the same time — may be better predictors of mortality risk except at extreme BMIs. Therefore, managing comorbidities more aggressively may provide better survival outcomes for patients with body mass between normal and moderate obesity.

While evidence on the relationship between BMI and mortality remains inconsistent, maintaining a healthy body weight may be essential for preventing diseases and maintaining a quality of life.