Body mass index, or BMI, is a common tool for estimating a person’s “fatness.” A ratio of weight to height, BMI has long been used to categorize people as either underweight, normal weight, overweight or obese. People who are classified in the obese range are then divided further into three levels of severity.
That said, BMI’s usefulness is often called into question by experts who say it misidentifies many people. Some people, for instance, may be unfairly labeled as overweight or obese when they’re in fact healthy, while others — much more consequentially — may be left unaware of potential health risks because they believe that a normal-weight BMI means they have no reason to worry.
So, what’s the point of learning your body mass index — if there’s a point at all? Let’s take a look at the measure’s promises and pitfalls to find out:
The Good
BMI does not measure body fat. Instead, it approximates body fat more accurately than looking at weight alone. It also provides a simple baseline measure against which you can measure your progress if you’re trying to lose weight or get more fit. Moving from the “overweight” to “normal weight” category is undoubtedly uplifting, even though some experts say it’s a bit of an arbitrary dividing line.
BMI also provides useful information to doctors and health and fitness professionals about potential health risks. For example, if you’re overweight or obese according to BMI, your provider may check for other risks related to excess body fat, such as hypertension, high cholesterol or Type 2 diabetes. At the very least, BMI provides a catalyst for potentially tough conversations between you and your healthcare providers.
The Bad
Because BMI does not distinguish between fat weight and muscle weight, some athletes and fitness enthusiasts have high BMIs because they’re more muscular. Consider J.J. Watt, an All-Pro defensive end for the Houston Texans. At 6 feet, 5 inches, his extremely muscular frame weighs 289 pounds. That means Watt’s BMI is 34.3 — a number considered on the very high end of the first level of obesity. Clearly, something is wrong here: Watt’s high BMI doesn’t reflect his completely normal metabolic functioning.
The more problematic scenario arises when the math works in the opposite direction, placing someone who is metabolically unhealthy into the normal-weight category. Low muscle strength and poor aerobic fitness are associated with an increased risk of Type 2 diabetes — regardless of one’s weight or BMI. The danger lies in the false sense of security that a normal BMI may provide.
There is also a segment of the population — although it’s unclear how large or small — that’s obese according to BMI, but metabolically healthy. These people aren’t athletes with a lot of muscle like Watt, and do in fact have a lot of excess fat, but don’t seem to have any of the health risks associated with carrying that extra weight. Some evidence suggests, however, that their disease-free state won’t last, since it appears that there’s only so long the human body can stay both fat and healthy.
The Bottom Line
The true value of BMI lies in its simplicity — not its precision.
You can find out your BMI by dividing your weight in kilograms by your height in meters squared or by simply using an online BMI calculator like the one the American Council on Exercise provides. The resulting value can be useful to keep tabs on your health — regardless of whether researchers find the dividing line between categories somewhat dubious or whether the number actually reflects your health. Remember: Your goal should be to improve, and you can only do that by comparing your current self to your former self. For some people, a decreasing BMI means they’re likely on a path to better health.
One last tip: No matter your BMI, it’s always a good idea to measure your waistline. Abdominal fat is riskier to your health than fat stored elsewhere in the body, largely due to its proximity to major organs and its strong correlation to cardiometabolic disease. Guidelines state that men are at an increased risk of health problems if their waist circumference is greater than 40 inches, while the threshold for women is 35 inches.
One of the keys to long-term success is measurable progress. BMI — along with waist circumference, steps taken and calories burned — can be used to track that progress. Keep those numbers moving in the right direction and you are sure to stay on track.
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Do You Need to Know Your Body Mass Index? originally appeared on usnews.com
