Later this week, the Council of Fashion Designers of America will issue guidelines for runway shows that encourage a healthier lifestyle for models. In New York, catwalkers won't be tested for a minimum body mass index, as they are in Madrid and Milan. Is it possible to have a very low BMI and still be healthy?
Only if it's completely natural—and that's rare. Many people are predisposed to be thin, but if a model has a BMI of below 18—the threshold used in Madrid—she is almost certainly practicing unhealthy habits, such as restricting food intake or overexercising. Someone who's 5-foot-11, like most female models, would have to weigh less than 130 pounds to fall short of the threshold. To get to that point, even a natural waif would likely have to skip meals, exercise too much, or use diet aids.
People who engage in these activities can face a myriad of health problems, often because of their low body-fat levels. BMI doesn't measure body fat directly; it looks only at the ratio of a person's height and weight. Doctors say direct fat measurements—via skinfold calipers, for example—tell you more about someone's health. (BMI is popular because it's so easy to calculate.) Women with less than 12 percent body fat can be at risk for serious health problems or starvation.
Human bodies have evolved to be able to withstand starvation on occasion. In the absence of sufficient caloric intake, the body's metabolism slows down, and the liver, heart, and other organs may reduce their tissue reserves. For instance, the heart keeps backup capacity so that we can run very fast when we need to get away from a bad situation. The heart of someone who is starving, however, may not be able to hold extra muscle mass stockpiled for that sort of occasion.
Because of these adapting body changes, people who are too thin can function normally much of the time. But a major blow to the system, like a bad bout of influenza or gastroenteritis, could throw off the tenuous balance and lead to cardiovascular problems and fainting. Other long-term potential complications of being too thin could include a diminished ability for the bowels to absorb nutrients, lost menstrual cycle, and decreased thyroid function.
The standards set by Milan fashion-industry heavyweights require models to obtain a license issued by a panel of experts before being allowed to walk the runway. This panel is supposed to take models' ethnic and geographical background into consideration before issuing or denying someone a permit. Some research suggests that Asians, as well as members of some Australian Aboriginal tribes, may have a naturally lower BMI than Caucasians. However, the details on this are still unclear.
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Explainer thanks Ovidio Bermudez of the National Eating Disorder Association, Bill Dietz of the Centers for Disease Control and Prevention, Boyd Swinburn of Deakin University, and Terri Wheeldon of the Mayo Clinic.