A new finding could undermine many diet resolutions, a new government study shows that people who are overweight are less likely to die in any given period than people of normal weight. Even those who are moderately obese don’t have a higher-than-normal risk of dying.
Being substantially obese, based on measure called body mass index, or BMI, of 35 and higher, does raise the risk of death by 29%, researchers found.
But people with a BMI of 25 to 30—who are considered overweight and make up more than 30% of the U.S. population—have a 6% lower risk of death than people whose BMI is in the normal range of 18.5 to 25, according to the study, being published Wednesday in the Journal of the American Medical Association.
People who had a BMI of 30 to 35—considered the first stage of obesity—had a 5% lower risk of dying, but those figures weren’t considered statistically significant.
Still, health experts said that Americans shouldn’t treat the new study as a license to eat more.
“That would be a mistake—and this study did show an increase in mortality for people who are obese,” said Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, which conducted the study. “I don’t think anyone would disagree with the basic fact that being more physically active and eating a healthier diet is very important for your health.”
The new report is the latest, and largest, to document what scientists call the “obesity paradox.” Other studies have shown that people with heart disease, diabetes and other chronic health conditions tend to live longer if they carry excess pounds even though excess weight is associated with heightened risk of heart disease, Type 2 diabetes and several cancers which in turn raise the risk of premature death.
CDC researchers analyzed 97 studies involving nearly three million people and 270,000 deaths around the world. “The findings are very consistent across all different ages and continents,” said lead author Katherine Flegal, a senior scientist at the CDC. She stressed that the study looks at all causes of mortality, not overall health risks. “This is not meant to suggest that the conventional wisdom is wrong,” she said.
BMI is calculated by dividing weight in kilograms by height in meters squared. Someone who is 6 feet tall and 180 pounds would have a BMI of 24.4, considered normal; at 200 pounds, his BMI would be 27.1, or slightly overweight, and at 230, his BMI of 30 would be considered obese.
Other experts said the study underscores that BMI is an inexact measure of health—largely because it doesn’t differentiate between fat and muscle mass. Many athletes are technically obese, based on BMI, even though they are extremely fit.
Nor does BMI assess how fat is distributed, which experts say may be more important than total fat overall.
Excess belly fat seems to be particularly toxic, while extra fat in the buttocks and legs may have a protective effect, research suggests.
Doctors at the Mayo Clinic in Rochester, Minn., recently found that among 14,000 people, those with a normal BMI, but a high waist-to-hip ratio, were the most likely to die of cardiovascular disease during the 14-year follow up, even more than those in obese ranges.
“BMI gives the same value to good fat, bad fat and muscle, and it doesn’t make a lot of sense to mix all those things together,” said Francisco Lopez-Jimenez, a preventive cardiologist at Mayo, who wasn’t involved in the CDC study.
Some experts noted that studies of all causes of mortality include deaths from traumatic injuries, and that small amounts of excess fat may provide extra padding.
It isn’t clear why people with heart disease, diabetes, hypertension and kidney disease live longer if they are overweight or obese.
One theory is that extra pounds give such patients some metabolic reserve if they are unable to consume adequate nourishment.
The tendency for people to lose weight and waste away as they become severely ill could artificially inflate the death rate for those in normal BMI ranges, some experts said.
The new report didn’t assess mortality rates for people with below-normal BMI, Dr. Flegal said, because there weren’t enough underweight subjects to provide sufficient data. But other research associates BMIs below 18.5 with a slightly higher risk of death. In such comparisons, researchers measure only deaths that occur specific time period. The studies in the JAMA analysis ran for five to 15 years. Experts stressed that physicians should rely less on BMI and more on specific health measures such as blood pressure, blood sugar, cholesterol, triglycerides, waist circumference and strength and endurance tests.
Patients should focus more on staying fit and eating healthy than hitting a particular number on the scale.
“You’d hate to have the message get out there that it’s good to be overweight,” said Mercedes Carnethon, an epidemiologist at Northwestern University’s Feinberg School of Medicine. “The reality is that people who are overweight very often become obese and that’s clearly not good.”