Study findings contradict prevailing wisdom on obesity and mortality risk

Being overweight or obese increases the risk of death by 22% to 91% — much higher than previously thought — and the risk of death from being mildly underweight may be overestimated, according to the new CU Boulder study.

The findings were published in the February 9 issue of the journal population studiescountering the widely held belief that being overweight only increases the risk of death in extreme cases.

The statistical analysis of nearly 18,000 people also revealed flaws in using body mass index (BMI) to study health outcomes, providing evidence that the preferred metric may be biasing findings. After accounting for these biases, it estimates that about one in six U.S. deaths is related to being overweight or obese.

Existing research may underestimate the mortality consequences of living in a country where cheap, unhealthy food is increasingly available and where a sedentary lifestyle is the norm. This study and others are beginning to reveal the true toll of this public health crisis. “

Ryan Masters, Associate Professor of Sociology, University of Colorado Boulder

Challenging the Obesity Paradox

While numerous studies have shown that heart disease, high blood pressure, and diabetes (often associated with being overweight) increase the risk of death, few studies have shown that people with higher BMIs have higher mortality rates.

Instead, most studies show a U-shaped curve in what some have called the “obesity paradox”: People in the “overweight” category (BMI 25-30) have the lowest mortality risk. There was little or no increased risk in the “obese” category (30-35) compared with the so-called “healthy” category (18.5-25). The risk of death increased for both being “underweight” (less than 18.5) and extremely obese (35 and over).

“The conventional wisdom is that elevated BMI generally does not increase the risk of death until it reaches very high levels, and that there is actually some survival benefit to being overweight,” said Masters, a social demographer who has spent his career studying death. rate trend. “I've always been skeptical of those claims.”

He points out that doctors and scientists often use BMI as a measure of health, which is based solely on weight and height and doesn't take into account differences in body composition or how long a person has been overweight.

“It's a reflection of a body at a point in time. That's it,” Masters said, noting that Tom Cruise (5-foot-7, at one point 201 pounds muscular) had a BMI of 31.5, which made him He made a name for himself in the “fat” category. “It doesn't quite capture all the nuances and different sizes and shapes of the body.”

To see what happens when these nuances are accounted for, Masters dug into the National Health and Nutrition Examination Survey (NHANES) from 1988 to 2015, looking at data on 17,784 people, including 4,468 deaths.

He found that a full 20 percent of the samples described as “healthy” weight fell into the overweight or obese category over the past decade. When separated, this group was much less healthy than those in the weight-stabilized category.

Masters points out that being overweight for life leads to diseases that, paradoxically, lead to rapid weight loss. If BMI data were captured during this period, it could distort the study results.

“I think we've been artificially inflating the risk of death in the low BMI category, including those with a high BMI who have recently lost weight,” he said.

Meanwhile, 37 percent of overweight people and 60 percent of obese people with a BMI had a lower BMI ten years earlier. Notably, those who had recently gained weight had better health outcomes.

“The health and mortality consequences of a high BMI are not like a light switch,” Masters said. “A growing body of research shows that consequences depend on duration.”

By including people who have been at a low BMI weight for most of their lives in the high BMI category, previous research has inadvertently made high BMI seem less risky than it really is, he said.

When he looked at differences in fat distribution across BMI categories, he also found that these differences made a huge difference in reported health outcomes.

exposed public health problems

Overall, the findings confirm that research has been “significantly influenced” by BMI-related biases.

When the numbers were recalculated without these biases, he found not a U-shape but a straight-up line, with those with a lower BMI (18.5–22.5) having the lowest risk of death.

Contrary to previous research, the study found no significant increase in mortality risk in the “underweight” category.

While previous research has estimated that 2 to 3 percent of U.S. adult deaths are due to high BMI, his study puts the number of deaths at eight times that figure.

Masters said he hopes the study warns scientists to be “extremely cautious” when drawing conclusions based on BMI.But he also hopes the work draws attention to what he sees not as a problem that individuals can solve alone, but as a public health crisis fueled by an unhealthy or “obesogenic” environment in America

“For people who were born in the 1970s or 1980s and lived their entire lives in this obese environment, the outlook for healthy aging into old age doesn't look good right now,” he said. “I hope this work influences higher-level discussions about what we as a society can do about it.”


University of Colorado Boulder

Journal reference:

Masters, RK (2023) Sources and severity of bias in estimates of BMI-mortality association. population studies. doi.org/10.1080/00324728.2023.2168035.

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