Despite what the headlines declare, .
鈥攁 meta-analysis of 51 other studies鈥攆ound that those with a Body Mass Index (BMI) of 18.5 or below had a 1.8 times higher risk of dying than people with a BMI in the normal range. Meanwhile, people classified as obese, with a BMI of 30-34.9, were 1.2 times as likely to die, while the severely obese鈥攚ith a BMI of 35 or higher鈥攚ere 1.3 times as likely to die.
What does that mean, exactly? Not what everyone seems to think it does. BMI is a simple formula, a calculation of height vs. weight that can be a useful indicator of health. . It can鈥檛 differentiate between muscle and fat, so two people might be the same height and weight鈥攁nd thus the same BMI鈥攂ut have a very different muscle to fat ratio, and different overall health. 鈥淩ather, a 鈥榬obust body size鈥 is one where there is a good amount of muscle mass and a limited amount of abdominal fat,鈥 Dr. Joel Ray of St. Michael鈥檚 Hospital in Toronto, who led the study, says. 鈥淏MI is not the best indicator of obesity-related mortality risk, but it is in underweight people.鈥
You also have to remember that being over- or underweight is not necessarily the thing that鈥檚 going to kill you. Low weight can be caused by factors like drug use, alcohol use, and poor self-care; a higher BMI can be accompanied by heart problems and diabetes.
But that doesn鈥檛 mean you should ignore BMI either. According to Dr. Steven Heymsfield, a professor at Pennington Biomedical Research Center at Louisiana State University, 鈥淏MI is only a first step in evaluating an individual鈥檚 health.鈥 A high BMI is a good indicator that a patient needs other tests鈥攆or blood pressure, blood lipids and other risk factors.
If you want to be really careful, check both your BMI and and waist circumference. 鈥淭hat鈥檚 not a bad approach if you have the resources; the combined measure is a better index than either one alone in most studies,鈥 Heymsfield says.
But no matter your BMI it鈥檚 a good idea to talk to your doctor about what that number actually means for you and to focus on what really counts: staying active.