It’s been 20 years since the federal government, on the advice of an expert panel convened by the National Heart, Lung, and Blood Institute (NHLBI), lowered the body mass index (BMI) cutoff point for overweight, a move skeptics worried would spur Americans struggling to slim down to throw in the towel.
Soon after the guidelines were published, though, researchers began suggesting that the cutoff—the lowest BMI at which people are classified as overweight—still wasn’t low enough for at least 1 segment of the population: postmenopausal women. Misclassifying overweight postmenopausal women as normal weight means they could miss out on treatments that could reduce their risk of obesity-related health problems, particularly cardiovascular disease.
Body mass index is an estimate of body fat based on weight relative to height. Before the NHLBI expert panel recommended lowering the cutoff point for overweight to 25 for all adults, it had been 27.8 in men and 27.3 in women. The panel also recommended 30 as the cutoff point for obesity. Those moves brought the US definition of overweight and obesity in line with that of the World Health Organization (WHO).
According to the NHLBI expert panel’s final report, epidemiological studies showed that a BMI of 25 up to 30 was associated with a modest increase in mortality, while a BMI of 30 or more was associated with death rates 50% to 100% higher than among people with a BMI of 20 up to 25. When the federal government lowered the BMI cutoff for overweight, an estimated 29 million Americans who had been considered normal weight no longer were.
But BMI is only a proxy for body fat, and a recent study suggested that even the lowered cutoffs fail to capture most postmenopausal women whose actual body fat percentage would classify them as obese, let alone those whose body fat percentage would classify them as overweight. To improve the sensitivity of BMI in identifying postmenopausal women at risk of obesity-related diseases, the authors of the recent study concluded, the obesity cutoff might need to be set as low as 24.9, which is currently the top of the normal BMI range for the general adult population.
“BMI was put forth by the World Health Organization as this kind of one-size-fits-all solution to quantifying someone’s body composition,” said first author Hailey Banack, PhD, a postdoctoral fellow in epidemiology at the University at Buffalo, the State University of New York. “I think our research is showing that’s really not the case.”
Banack and her coauthors used whole body dual x-ray absorptiometry scans, which they called a gold standard for adiposity measurement, to assess the body composition of 1329 postmenopausal Buffalo, New York, women ranging in age from 53 to 85 years. The researchers defined obesity as a BMI of 30 or higher or a body fat percentage greater than 35%, 38%, or 40%. (They tested 3 different cutoff points because no consensus exists on what percentage of body fat should be used to define obesity, according to the authors.)
The standard BMI cutoff of 25 for overweight and 30 for obesity might be too high for postmenopausal women because their body composition changes over time. As they age, women tend to lose bone and muscle mass, which are heavier than fat. So even if a 65-year-old woman weighs the same as she did at 25 years of age, fat accounts for a larger share of her weight. And that fat isn’t distributed in her body the way it was at age 25 years. More of it is visceral fat stored in the abdomen, as opposed to subcutaneous fat, and the former is riskier than the latter. Visceral fat has been linked to metabolic dysfunction, including higher total cholesterol and lower-density lipoprotein (“bad cholesterol”) as well as insulin resistance. Banack and her coauthors found that most of the women whose body fat percentage was 35% or more—which meant they were obese and at a greater risk of obesity-related health problems—had a BMI below 30. Broken down by body fat percentage, only 32.4% of women with 35% body fat, 44.6% of women with 38% body fat, and 55.2% of those with 40% body fat had a BMI of 30 or greater in Banack’s study.
“I’m not surprised by the results,” said Gary Hunter, PhD, a professor in the human studies department at the University of Alabama, Birmingham, who has studied the age-related shift in visceral fat but was not involved with Banack’s study.
Men also accumulate more visceral fat as they age, but “it’s much more dramatic in women,” given that they start out with less than men, Hunter said. Between the ages of 25 and 65, the average woman will lose approximately 13 pounds of bone and muscle mass, while her visceral fat will nearly quadruple in size, Hunter said. By comparison, the average man’s visceral fat will double in size between the ages of 25 and 65, he said.
Hunter said he and others have developed cutoffs for the amount of visceral fat that’s probably safe and the amount at which it could increase health risks. Assessment of visceral fat is usually done with computed tomography or magnetic resonance imaging, but those technologies are too expensive for routine use, he said.
That’s one reason BMI is so widely used to screen people for overweight or obesity. Unlike measuring body fat percentage or visceral fat, calculating BMI does not require expensive equipment run by experienced technicians, Banack said. “It’s very useful for comparison purposes,” she said of BMI, but, she added, “I think there are subgroups in the population where it might not be as valid.”
Even the WHO acknowledges that is probably the case. “BMI provides the most useful population-level measure of overweight and obesity as it is the same for both sexes and for all ages of adults,” according to the WHO’s fact sheet on obesity and overweight. “However, it should be considered a rough guide because it may not correspond to the same degree of fatness in different individuals.”
Barbara Howard, PhD, a senior scientist and former president of the MedStar Health Research Institute in Hyattsville, Maryland, sat on the expert NHLBI committee that recommended lowering the BMI overweight cut point for all adults to 25. The panel recognized that there would be variations in body fat percentage at the same BMI among different groups, particularly racial/ethnic populations, said Howard, a professor of medicine at the Georgetown University School of Medicine. “We decided that to start making different cutoffs for different people would just confuse the field,” she said, noting that BMI is only 1 factor physicians should consider when deciding whether a postmenopausal patient needs to be treated for obesity. “That’s where medical judgment comes in.”
JoAnn Manson, MD, DrPH, chief of the Division of Preventive Medicine at Brigham and Women’s Hospital in Boston, echoed Howard. “You need to look at what their cardiovascular risk factors are in the context of their BMI,” Manson said, noting that there is some evidence that overweight or even obese individuals with normal blood lipids, insulin resistance, and blood pressure might not have an increased risk of cardiovascular disease. However, she noted, “Over time, some of these risk factors may increase. We don’t want to encourage complacency just because the risk factors are in a good range.”
Manson, who holds an endowed chair in women’s health at Harvard Medical School, said she finds that measuring waist circumference is “extremely valuable” in determining whether postmenopausal patients have too much visceral fat. In the 1998 report, the NHLBI’s expert panel was the first to recommend checking waist circumference in people whose BMI was between 25 and 34.9, because waistlines greater than 35 inches in women and 40 inches in men were associated with a greater chance of obesity-related risk factors. “If the waist circumference is high, no matter what the BMI is, there will be concerns about cardiovascular risk, diabetes risk, and cardiometabolic outcomes,” Manson said. “We do learn a lot from this simple measure.” Although the updated 2013 NHLBI guidelines referred to these cutoffs as “somewhat arbitrary,” it also noted that waist circumference measurements could provide some useful information.
Numbers don’t tell the whole story. “Regardless of what the cut point is, there will be a lot of individual variation in terms of what the BMI means,” she explained. Manson said she prefers to focus on lifestyle behaviors rather than weight, per se. If a postmenopausal woman is physically active and her waistline hasn’t changed much over the years, Manson said, she probably doesn’t have to worry that her BMI of 25 might be too high. “I always think about physical activity as the magic bullet of good health.”
Moving the BMI goal posts might end up discouraging women instead of motivating them, said Joann Pinkerton, MD, executive director of the North American Menopause Society and a professor of obstetrics and gynecology at the University of Virginia. “Telling someone they need to lose 20 or 30 pounds is off-putting,” Pinkerton said. “And we know that losing just 10 pounds can make a huge difference in health.”
Intuitively, it might make sense to lower the BMI cutoff for obesity in postmenopausal women, but research to determine whether it would benefit them is lacking. Samar El Khoudary, PhD, MPH, an associate professor of epidemiology at the University of Pittsburgh Graduate School of Public Health, called Banack’s conclusion that 30 is too high “an interesting suggestion.” However, said El Khoudary, “We still need to assess the impact of changing this cut point on health outcomes and risk in those women.”
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