Why ‘Waist Loss’ Is More Important Than ‘Weight Loss’
A new study helps clarify why BMI is not a perfect measurement of health
With summer and summer apparel coming, many people will take to their bathroom scales to see what havoc winter hath wrought. Of course, people like to look trim for cosmetic reasons, but NEWS FLASH: being overweight is far more importantly associated with increased morbidity (illness, disease) and mortality (death).
To put an individual’s weight into better perspective, we doctors often measure a body mass index (BMI), which is body weight in kilograms divided by the square of a person’s height in meters. A normal BMI is considered to be 18.5 to 24.9; 25.0 to 29.9 is overweight; 30.0 to 34.9 is obese, and anything higher than that is really obese.
BMI is not a perfect measurement because it can’t tell the difference between lean body mass (muscle) and fat, i.e., a kilogram is a kilogram is a kilogram. But if you graph body mass index and mortality, the result looks like a half-pipe in a snowboarding competition: mortality rises sharply as weight rises, until we get flung into the afterlife attempting a tricky half-cab quadruple backflip.
Are Overweight People Healthier?
In 2013, a study came out that seemed to shred this half-pipe BMI/mortality graph. It appeared that people with BMIs in the overweight range were actually healthier than those in the normal range, and that those in the mildest obese category did just as well as those with a normal BMI.
For public health officials, this was the equivalent of a study showing that light smokers were healthier than non-smokers. What!?! The results didn’t make sense, and it didn’t take long for experts to point out potential reasons why: smoking bucks the trend by lowering BMI but increasing mortality; and unintentional weight loss is typically a sign of ill health. Indeed, mortality rises for those with low BMIs.
A new study that came out earlier this month in Annals of Internal Medicine tried to clarify whether the BMI/mortality curve is indeed a clean half-pipe, or as the 2013 study suggested, one with a few moguls in it.
Researchers looked at data from three studies that followed over 200,000 nurses and doctors for 16 years. They used the participants’ maximum BMI over the entire study period, and for most of them, that was their most recent BMI. (Yes, we get bigger as we go.) But for some people, this was not the case — they had once been obese, a BMI of 32 perhaps, but had now fallen into the normal range with a BMI of 24.
Mystery Solved
That should be good news, but since most weight loss is unintentional and an indicator of looming or active disease, this was actually ominous: disease was pushing these people into the normal BMI category and skewing the mortality data. When taking this into account, the BMI mortality graph looked like the predictable half pipe: being overweight was modestly unhealthy, and being obese was very unhealthy — a dangerous Backside Triple Cork 1620.
An accompanying editorial by Jean-Pierre Després, a Canadian expert on cardiometabolic risk factors, put this latest research into a sharper clinical perspective.
Després reemphasizes that body mass index is imperfect because it cannot tell the difference between lean body mass and fat, or where the fat is. And that’s important because to humans, fat is like real estate: location matters.
Abdomen Fat ‘Far More Unhealthy’
So-called “visceral fat” is fat deposited in normally lean tissues such as the liver, heart or skeletal muscle, and perhaps most prominently in the drapery of tissue called the omentum that surrounds the intestines. You don’t have to drink beer to get this “beer belly,” also called “central obesity,” and this kind of fat seems to be far more unhealthy than the fat deposited just beneath the skin.
Although being in the overweight category (a BMI of 25 to 29.9) only modestly increases mortality risk, the editorial points out that the overweight category also has the largest individual variation in both body composition and body fat distribution. Overweight people with central obesity, perhaps best measured by a waist-to-hip ratio, had more than modest risk. Conversely, those with excess weight, but no “beer belly,” were at less than modest risk.
In a 2015 paper in The Canadian Journal of Cardiology, Després argues that weight loss should not be the sole target. He pointed out that weight loss studies using calorie deprivation alone have shown few cardiovascular benefits, while studies that emphasized good nutrition (not just limiting calories, but offering healthier calories) showed improved outcomes.
He also noted the “fat and fit phenomenon,” where people who are abdominally obese but still very active have a markedly lower risk of heart disease than inactive individuals.
A More Nuanced Approach
Després suggests six simple public health messages that might be more nuanced and helpful than the “drop the weight,” BMI-centric mantra.
One suggests placing less emphasis on calorie restriction and more on nutritional quality, and another suggests an improved focus on “waist loss,” not just weight loss.
Three of the six emphasize physical activity and fitness, and you can get some of that simply by trying to figure out where in the world you put your summer clothes.
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