Obesity. The word weighs heavily on a national scale. According to the most recent statistics from the Centers for Disease Control and Prevention, a whopping 78 million adults—more than a third of the U.S. population—are obese. And Eric Braverman, M.D., founder of the New York neurology practice PATH Medical and a clinical assistant professor of integrative medicine at Weill Cornell Medical School, says that a more accurate number could be up to twice that amount.
In a recent study he co-authored with current New York State health commissioner Nirav Shah, M.D., Dr. Braverman found that the body mass index (BMI)—the gold standard for diagnosing obesity—misidentified individuals who were actually obese nearly 40 percent of the time.
“BMI wasn’t designed to measure adiposity”—that is, fatness—says Braverman. “Fat, not weight, is the best predictor of disease.”
The BMI measurement, first devised in the 1830s, is a basic ratio of weight to height used throughout the healthcare industry to assess whether an individual’s weight is healthy or puts her in the danger zone for increased risk of cardiovascular disease, high blood pressure, type 2 diabetes and some cancers, including breast cancer. Though different organizations adhere to slightly varying cut-offs, the widely accepted rule of thumb is that an adult with a BMI of 30 or higher is considered obese.
But this measurement, Braverman asserts, dangerously underestimates the true prevalence of obesity. BMI, he contends, is a crude calculation that doesn’t accurately reflect a person’s body fat percentage. His research compared the BMIs of nearly 1,400 men and women to the results of a DXA scan, a type of full-body X-ray that can precisely discriminate and quantify fat, muscle and bone. While BMI characterized 26 percent of patients as obese, the DXA data put 64 percent in that category, based on percent body fat.
What’s more, the disparity was significantly larger among women than men. While BMI and DXA were in accordance three-quarters of the time for men, BMI misclassified half of the women as non-obese when they were actually obese. The paper recommends lowering the BMI threshold for obesity to 28 for men and 24 for women. But even that, Braverman contends, isn’t sufficient.
“BMI is letting women down,” he says. Braverman explains that at 20 years of age, the difference in body fat percentage between men and women is essentially negligible. We’re talking about two or three percentage points on average. But by age 50, that spread is likely to increase to 10 percent or more.
Beginning in their thirties, women experience a shift in hormones, including a drop in dopamine, a neurotransmitter that plays a role in pleasure and reward, and is involved in the hunger feedback loop. When you eat, fat cells produce the hormone leptin. That signals the hypothalamus to release dopamine, which shuts off appetite and (bonus!) stimulates the body to burn fat. When that pathway is obstructed, metabolism slows. It becomes harder to feel full and all the while, fat is accumulating.
Leptin can be a problem in overabundance as well, in the case of individuals who develop leptin resistance. People with too much fat produce an excess of leptin, which becomes bound to stress hormones, including cortisol, and other molecules in the bloodstream. When this happens, the leptin is not able to cross the blood-brain barrier to do its work on the hypothalamus. The brain is no longer receptive to leptin, no matter how much the body makes, throwing off metabolism, satiety and fat-burning—and basically perpetuating a cycle of eating more and more and getting fatter and fatter. It’s similar to the insulin resistance that leads to type 2 diabetes.
But this also gives doctors a powerful biological marker for predicting obesity. A blood-leptin level between zero and five is normal (since, when you release leptin, your body is supposed to make use of what’s there). Elevated leptin measurements, however, may indicate resistance, which is a strong risk factor for obesity. Braverman believes that everyone should have regular leptin blood tests done, the same way people get their cholesterol checked.
He used blood leptin to create an adjusted BMI scale that adds points to a person’s BMI score if leptin levels are elevated. Braverman says the leptin-adjusted BMI is as accurate as a DXA scan for predicting body fat content.
Ideally, Braverman wants everyone to get a DXA scan rather than relying on roundabout estimates of body fat. But most doctors don’t have the equipment, and the scans can be prohibitively expensive. That said, if you break a bone or go in for a bone density scan, which uses the same DXA scan, you can ask the technicians to measure body fat while they’re at it.
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