Old Rule: Keep your BMI between 18.5 and 24.9.
New Rule: Eat healthy, exercise, and let your weight settle naturally.
Physicians use BMI (body mass index)—a ratio of your weight to your height—as a tool to diagnose obesity. But critics say BMI ignores muscle mass, and a 2011 Obesity study notes that it also ignores a person's hip circumference. "People come in different sizes and shapes," says Joanne Ikeda, nutritionist emeritus at the University of California, Berkeley. "The idea that everyone should fall under 25 is ludicrous." A person can have a high BMI and still be healthy, Ikeda argues. Research supports the theory: A Journal of the American Medical Association study found that fit women—even if they were overweight according to their BMI—were less likely to suffer a heart attack than those who were out of shape. Ikeda advises her patients to stop obsessing over their BMI, eat a nutritious diet, and log 150 minutes of exercise per week. "A healthy lifestyle results in a healthy weight."