Among all the numbers your doctor takes at your appointment — blood pressure, cholesterol — your BMI (Body Mass Index) is likely the one to stick in your memory. That’s because the measure of your body mass index can tell you if you’re in the normal weight, overweight, or obese category.
It’s likely you’ve heard about the obesity epidemic. In fact, more than two in three adults are overweight or obese, according to the National Health and Nutrition Examination Survey. Public health experts are sounding the alarm, too, recognizing it as one of the driving forces behind many diseases. The financial toll is great: Obesity costs $190 billion a year in weight-related medical bills, the American Heart Association points out.
So why is knowing your BMI important? “Your BMI is a measurement of your risk for health problems,” says William Yancy, MD, associate professor of medicine at Duke University School of Medicine and program director of the Duke Diet and Fitness Center in Durham, North Carolina.
Obesity can put you at a greater risk for health conditions like type 2 diabetes, high blood pressure, joint problems, and gallstones, according to the Centers for Disease Control and Prevention (CDC).
To find your BMI, you calculate your weight in kilograms divided by your height in meters squared. You can avoid doing the calculation yourself by plugging your stats into an online calculator, like the one here: https://www.nhs.uk/Tools/Pages/Healthyweightcalculator.aspx
After finding your number, here’s where you fall:
- BMI 18.5 to 24.9 = Normal weight
- BMI 25 to 29.9 = Overweight
- BMI 30 and higher = Obesity
Why BMI Isn’t the Best Measure of Body Weight
Unfortunately, BMI isn’t perfect. As a way to measure a population as a whole, it’s rather advantageous, especially because it’s more widely used than other weight measurement, says Dr. Yancy. But BMI tends to “fall flat,” he says, when you’re looking at an individual person.
Why? For one, BMI can’t tell you how much lean mass (muscle, bone) you have compared with fat. That means that someone who is larger and more muscular can be inaccurately placed into the overweight or obese category because muscle weighs more than fat in a given space. Plus, not only is BMI not a direct measurement of body fat but it doesn’t indicate where someone is storing that fat, which may matter more.
“If BMI is the only thing your doctor uses, you should also use other measurements,” says Yancy, who advocates for knowing your waist circumference. “That’s actually a better predictor of your health, as carrying fat around your midsection is riskier than other parts of your body,” he says. This fat is called visceral fat, which hugs your body’s organs, causing damaging inflammation that can drive type 2 diabetes, increased cholesterol, and high blood pressure. The more fat you have around your middle, the more your health may take a hit, regardless of BMI, adds Patrick M. O’Neil, PhD, director of the weight management center and a professor in the department of psychiatry and behavioral sciences at the Medical University of South Carolina in Charleston.
For reference, a woman should have a waist measurement of less than 35 inches (89 cm); a man, less than 40 inches (101 cm). (Measure around your belly button.)
Research shows that waist circumference is just as accurate at measuring someone’s cardiorespiratory fitness as BMI. And more research has found that both measurements could estimate the risk of heart disease, as well.
Another glaring problem: Just because you’re overweight doesn’t mean you’re necessarily unhealthy — your healthy eating and exercising habits mean more, despite what the scale (or BMI calculator) says. While having a normal weight and being fit delivers the greatest protection against chronic disease as other weight categories, “most studies show that being overweight and fit is similar to being normal weight and unfit,” says Yancy.
The Possible Health Risks of Having a High BMI
A breadth of research shows that BMI correlates with numerous health problems, including:
High Blood Pressure: Both men and women who had higher BMIs were more likely to have hypertension compared with normal weight folks, per research.
Heart Disease: Having a BMI in the morbidly obese category (a BMI of 35 or greater) can make you nearly twice as likely to die from cardiovascular disease, one study shows. (Risk also went up for those who were underweight.)
Type 2 Diabetes: Being overweight may boost your odds for type 2 diabetes by 50 percent, research suggests. The risk gets greater as BMI increases. While that sounds like bad news, the more optimistic flip side is that if you’re obese, getting into the overweight category can help protect you from developing the full-blown disease.
Mental Health: Having a higher BMI has been shown to lead to symptoms of depression and generally a lower sense of well-being. One review of nine studies found that those who were obese were 32 percent more likely to have depression compared with healthy weight folks.
Life Expectancy: Other research has looked at the mortality rates depending on BMI. The authors of one study found that BMIs in the 30 to 35 range decrease survival by up to four years. At a BMI of 40 to 45, people lived up to 10 years less — about the same as smoking.
It’s important to put this all in perspective. While BMI may be associated with these health conditions, there are several other lifestyle considerations — history of smoking, alcohol use, rapid weight gain — and genetic components that factor into the equation, according to a review on the limitations of BMI.
Health Risks of Having a Low BMI
With all the fear surrounding being overweight or obese, you’d almost think that thinner was better — but that’s not necessarily the case. If you’re underweight (meaning you have a BMI less than 18.5), the risks may be harder to quantify. “It’s often difficult to tease out if someone is underweight because of an underlying health problem,” says Yancy.
But being underweight leaves you with less energy reserves in the event you do become ill. “People who are acutely ill have better survival or outcomes if they had a bit of fat storage to begin with. Those who are overweight seem to do best in these situations,” he says.
What’s more, being underweight is also associated with poorer mental health compared with having a normal weight, suggests research. If you’re concerned about your weight, talk to your doctor who can point you toward additional resources to getting the help you need.
Health Benefits of Having a Normal BMI
Losing weight to achieve a healthy BMI comes with numerous perks, including the reduction in risk from the diseases outlined above. “After losing weight, people feel better. They’re happier to be more fit, even if they don’t get to their goal,” says Dr. O’Neil. People tend to assume they’ll only feel happy at their “ideal” weight, when in reality they reach that same level of happiness even while weighing more, he explains. It’s all about making progress, which is most important.
What Factors Can Influence Your BMI?
The truth is that weight gain can be attributed to a complex mix of factors. People want to boil it down to eating too many potato chips, but that’s rarely all that’s going on.
That said, there are three main drivers for BMI, says O’Neil: “I tell people it’s who you choose for your parents and what you choose for dinner and exercise,” he says. “There’s a substantial genetic component to being obese and where you carry your fat,” he adds. There’s also your environment, including your access to healthy foods or the walkability of your town. Healthy habits like sleep and exercise matter. The guidelines for staying active — the government recommendations of 150 minutes of moderate exercise every week — still stand.
There are also drugs to consider, some of which are associated with weight gain and which many people take to treat chronic health problems in the first place. The best person to talk to about these side effects is an obesity medicine specialist or your primary care physician, says Yancy. Other conditions, like hypothyroidism (low thyroid) can contribute to weight gain. Injuries and illnesses can also factor in, since they can interfere with your ability to exercise or stay active in general, he adds.
Of course, there is the healthy eating part, too. But that can be equally as complex, particularly if you eat to cope with your emotions. In that case, a registered dietitian, especially one who specializes in emotional or intuitive eating, can help.
One final thought: Despite the perks of being a normal weight, you’ll still reap substantial benefits from a modest weight loss — no matter where your BMI stands now. The CDC recommends losing 5 to 10 percent of your current body weight, which can decrease your blood pressure, blood sugar, and cholesterol, they point out.