Based on clues she picked up from popular culture and public health advice, statistician Maya Mathur, Ph.D. from Stanford Medicine, always assumed that being overweight shortens life expectancy.
She was then surprised to come across research that suggested that the life expectancy of overweight people — people with a body mass index between 25 and 29.9 — is generally no shorter than that of people with a normal BMI range , when factors such as age and whether they have smoked are taken into account.
In fact, a 2013 study that analyzed nearly 100 studies involving more than 2.8 million people found that being overweight slightly reduced the risk of mortality. (This was not the case for people with a BMI of 30 or more.)
An analysis of around 240 studies from 2016 found a connection between obesity and higher mortality, but the effect was small.
Mathur believed that both studies had methodological problems, such as that factors such as diet and physical activity were not well controlled.
“My own exposure to public health messaging suggested that overweight BMI was a risk factor for mortality,” she said.
But after reviewing the research, she concluded, “This is simply not an evidence-based perception when you look at all of the literature.”
Mathur, an assistant professor at Stanford Medicine’s Quantitative Sciences Unit and of Pediatrics, wondered whether doctors harbored the same misconceptions.
Her mother, Vandana Mathur, is a practicing physician and biomedical researcher in the Bay Area who learned in her medical training that being overweight is harmful.
Since about one in three American adults is overweight but not obese, the pair decided to survey nearly 200 primary care physicians in the U.S. about their assessment of the mortality risks for this group.
A gap between belief and reality
For their study, published in Epidemiology, the mother-daughter duo found that 90% of doctors surveyed believed that being overweight shortened patients’ life expectancy, despite clinical guidelines from the American College of Cardiology and the American Heart Association saying being overweight is not associated with a higher risk of death.
“It seems like there is a really big gap between the empirical evidence and doctors’ perceptions,” Mathur said.
The researchers also gave doctors descriptions of two imaginary patients, 60-year-old women who were identical in every way except that one was overweight (but not obese) and one was of normal weight.
When they asked doctors to predict how likely each woman was to die from any cause in the next two decades, the doctors estimated that the obese woman had a 25% higher risk of dying.
When they asked doctors to generally estimate how obesity affects mortality risk, participants found an increase of nearly 60%.
“The estimates they presented found much stronger associations than even the studies suggesting increased mortality risk,” Mathur said.
Mathur suspects the discrepancy could be due to misleading messages doctors are receiving from the medical establishment.
For example, the clinical guidelines that state that being overweight does not increase the risk of death also ask physicians to “advise overweight and obese adults that the higher the BMI, the greater the risk of all-cause mortality.”
The Centers for Disease Control and Prevention website also lumps overweight and obese people together, claiming that both are at increased risk of dying earlier from all causes.
“Maybe the gap is actually between the evidence and the communication,” Mathur said. Social stigma could also play a role. “It seems very plausible that our culture gives us a lot of messages about BMI that are not based on facts.”
According to the American College of Cardiology and the American Heart Association, research shows that higher BMI in overweight and obese people is associated with a higher risk of coronary heart disease, cardiovascular disease, stroke and type 2 diabetes.
But, says Mathur, “Much of the literature does not differentiate between overweight and obesity.” Being either obese or underweight is associated with an increased risk of mortality.
Mathur says she’s concerned that doctors’ distorted views of weight will seep into interactions with patients. “Exaggerations about certain health risks could potentially lead to undue stress in patients with an overweight BMI,” Mathur said.
“And if the gap between communication and evidence comes to light, it could understandably also reduce patients’ trust in their doctors.”
Improving doctor-patient communication
Mathur urges doctors to follow research closely when discussing weight with patients. Such conversations are becoming more common as new weight loss drugs like Ozempic become more popular.
“It’s really important to be respectful and non-stigmatizing and evidence-based,” she said. “An excessive focus on obesity as a risk factor for mortality in its own right, independent of biomarkers or metabolic health, does not appear justified.”
Mathur believes future research could look at the factors that shape doctors’ beliefs and how prejudice affects patient encounters, as well as examine the views of doctors in other countries.
In the meantime, Mathur advises overweight patients who are bombarded with social and public health messages about weight to ask questions.
“When it is claimed that an overweight BMI is the primary cause of a particular health problem or risk, ask, ‘What is the evidence for this?'” she said. “Maybe there isn’t as much as you might think.”
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Written by Katia Savchuk/Stanford University.