According to a new study they’re wrong. And what these researchers found might affect you even if your body mass index, or BMI, is only slightly above normal. Here’s what you need to know.
The new study found that you don’t have to be morbidly obese for your waistline to steal years from your life.
How is BMI related to mortality?
Researchers at Colorado University at Boulder found that people who are simply overweight—not obese-- are nine times more likely to die than those who are not.This is alarming news when you consider that about 31 percent of the U.S. population is considered overweight.
For those who are in the obese category, the risk of death is eight times higher than previously believed—that affects 42 percent of the U.S. population.
How could the numbers be so much higher than once believed?
Well, Americans are getting heavier due to our poor processed diets and sedentary lifestyles, but that doesn’t explain it. The researchers say that doctors have underestimated the link between your waistline and your mortality because of conventional medicine’s reliance on their go-to health measure – the body mass index.
BMI doesn’t correlate with life expectancy
It’s not surprising that chronic health problems often linked to obesity such as cardiovascular disease, high blood pressure and diabetes, elevate mortality risk.However, very few studies demonstrate that higher BMIs alone induce higher mortality rates. In fact, most studies show a U-shaped mortality curve.
At the left upright of the U are those with a BMI below 18.5. This is labeled as “underweight” and puts people at a greater risk of death. At the right upright of the U are the extremely obese, with a BMI of 35 or more. These are also at greater risk of death.
Surprisingly – which is why it’s called the obesity paradox – the lowest mortality risk is found in adults in the overweight category with a BMI between 25 - 29.9. Those in the “obese” category (30 – 34.9) have little or no increased risk over the so-called “healthy” category (18.5 – 24.9).
Ryan Masters, associate professor of sociology at Colorado University at Boulder has spent his career studying mortality trends and was “suspicious” of these findings. He doubted whether the obesity paradox really existed.
“The conventional wisdom,” he said, “is that elevated BMI generally does not raise mortality risk until you get to very high levels, and that there are actually some survival benefits to being overweight.”
He didn’t buy this, so he took a deeper look into the individuals who are put into BMI categories and made some surprising discoveries.
BMI captures only one moment in time
BMI is calculated by dividing an adult's weight in pounds by their height in inches squared. This is captured at a single point in time and doesn’t account for how long a person has been overweight or for differences in body composition.An example of this, which Professor Masters points to, is the actor Tom Cruise, who at 5 feet 7 inches and very fit and muscular at one point with a weight of 201 pounds, found himself labeled “obese” with a BMI of 31.5.
“It isn’t fully capturing all of the nuances and different sizes and shapes the body comes in,” Professor Masters said. To see what happens when these nuances are considered, he mined data from the National Health and Nutrition Examination Survey. It included 17,784 adults over a period of up to 27 years.
BMI underestimates mortality: Blame it on once skinny people
Professor Masters discovered that 37 percent of those characterized as overweight and 60 percent as obese from their BMI had lower BMIs in the previous decade. By putting people who had spent most of their life with a low BMI in high BMI categories, it makes a high BMI seem less risky than it is, underestimating the mortality risk.Conversely, 20 percent characterized as having a “healthy” weight had been in the overweight or obese category in the previous decade. This group had a substantially worse health profile than those in the same category whose weight had been stable. This may be because excess weight over many years can lead to illnesses that, paradoxically, lead to rapid weight loss.
Professor Masters also looked at differences in fat distribution within BMI categories and found variations made a huge difference in reported health outcomes. When he reworked the numbers to take all these factors into account, he came up with quite different findings.
What BMI is associated with lowest mortality?
Instead of a U shape, the reworked data produced an upward sloping line. Adults with a low BMI (18.5–22.5) had the lowest mortality risk. And there’s no significant mortality risk increase for those in the “underweight” category. As weight increases so does the risk, with excess weight and obesity boosting the risk of death by 22 percent to 91 percent.Current estimates put two to three percent of U.S. adult deaths as due to high BMI. However, Professor Master’s study pegs the toll at eight times that.
Commenting on the findings he said: “Existing studies have likely underestimated the mortality consequences of living in a country where cheap, unhealthy food has grown increasingly accessible, and sedentary lifestyles have become the norm.
“For groups born in the 1970s or 1980s who have lived their whole lives in this “obesogenic” environment, the prospects of healthy aging into older adulthood does not look good right now.”
https://www.colorado.edu/today/2023/02/23/excess-weight-obesity-more-deadly-previously-believed
