Obesity is a major risk factor for a long list of chronic diseases, but counterintuitively, extra weight can sometimes correlate to a longer lifespan, some recent high-profile studies have shown.
These seemingly at-odds observations are sometimes dubbed the "obesity-mortality" paradox, and in an essay published today, two University of Pennsylvania researchers pinpoint the underlying problem: It’s how we’re making the measurements.
Anyone who has been to the doctor or to high school health class is familiar with Body Mass Index (weight divided by height squared in kg and meters). BMI is the standard measure for whether you are "normal" weight (18.5-24.9), overweight (25-29.9), or obese (above 30). Doctors use BMI all the time as a shorthand for predicting risk of diseases, as well as recommending actions like dieting and exercise.
But emerging science says that BMI many times doesn’t cut it when it comes to predicting health outcomes, the researchers write in the journal Science. That’s because it’s not just whether you’re fat that matters—but where the excess fat is stored They write:
Although it is widely used, the BMI does not accurately measure fat content, reﬂect the proportions of muscle and fat, or account for sex and racial differences in fat content and distribution of intra-abdominal (visceral) and subcutaneous fat.
Increasingly, scientists recognize that some kinds of fat is worse than others. Visceral fat, which often wraps deep around the belly, plays a role in the metabolic syndromes that increase the risk of diabetes and cardiovascular disease. A kind of flabby tissue called subcutaneous fat that lies just under the skin may not be as bad and can may even sometimes protect health. An estimated 10% of U.S. adults have an "obese BMI" but are metabolically healthy, the authors, Rexford S. Ahima and Mitchell A. Lazar, note.
The opposite is also true: you can be thin or normal weight and have the same metabolic syndrome and health risks as an obese person. For reasons not yet fully understood, some thin people can develop "severe fatty liver" and insulin resistance. These normal weight individuals who are "metabolically unhealthy" may comprise about 8% of U.S. adults, the researchers write.
BMI is the best measure most doctors have for assessing health risks related to weight. But the Science essay points out the urgent need for more accurate, practical, and affordable tools. People are putting thought into this—one new index dubbed the Body Shape Index is thought to already be a better predictor of mortality hazard than BMI, they note. But given the rising rates of chronic diseases in the U.S., there is a major public health imperative to figure this out and implement a change, if a better measure is found.
Pressing their case, the researchers get philosophical. To quote Galileo: "Measure what can be measured, and make measurable what cannot be measured."