If you’ve ever walked into a doctor’s office, you’ve probably seen a body mass index (BMI) chart on the wall or listened to your physician explain which BMI category you fit into. BMI was even used as eligibility criteria for receiving a COVID-19 vaccine when the shots were first being given. But, aside from weight and height, what can BMI calculators really capture about your health?
We do know that high BMIs are often linked to multiple chronic conditions, including high blood pressure, diabetes, and heart disease. However, there are plenty of other factors that can affect your risk of these conditions; BMI is simply one among several pieces of the puzzle.
What Is BMI, and How Is It Calculated?
Technically speaking, the formula for BMI is weight in kilograms divided by height in meters squared. (If you’re not one for the metric system or mental math, you can always use an online BMI calculator.)
As for its origins, BMI was first introduced in the early 19th century by a Belgian mathematician named Lambert Adolphe Jacques Quetelet, who created the formula as a simple way to help the government measure obesity rates in the general population—not in individual people—to determine the distribution of health-related resources.
Two centuries later, BMI is still used to screen for weight categories—healthy (18.5-24.9), underweight (below 18.5), overweight (25-29.9), and obese (above 30)—that may be associated with certain health issues. However, according to the Centers for Disease Control and Prevention (CDC), BMI “does not diagnose the body fatness or health of an individual.”
What BMI Metrics Can and Can’t Tell You
While BMI alone can’t be used for diagnosis, some research suggests that BMI is accurate in predicting the risk of conditions such as high blood pressure and diabetes, hence its use as an initial screening measure.
“Those who have a BMI of 30 or higher tend to be at a greater risk of insulin resistance, which can elevate blood glucose,” explains One Drop coach Lisa Graham, a registered nurse (RN) and certified diabetes care and education specialist (CDCES). “When BMI is higher, there are typically additional fat cells [or existing fat cells may be enlarged] in the body, which make it harder to move sugar in the blood into the cells, thus increasing insulin resistance.”
However, while higher BMIs are often correlated with insulin resistance, BMI alone “does not say beyond a reasonable doubt that a person will have more or less insulin resistance” (not to mention the chronic conditions that can result from insulin resistance, such as diabetes), notes One Drop coach Hanna, CDCES, and registered dietitian/nutritionist (RDN). Why? Insulin resistance is most closely associated with visceral fat, which lies in the spaces between your abdominal organs (e.g., you can’t see it with the naked eye), while subcutaneous fat is found between the skin and outer abdominal wall (e.g., it’s easier to see—for example, the fat in our under-arms).
“One of the limitations of BMI is that it doesn’t indicate which part of the body has more fat,” explains Hanna. In other words, two people can have identical BMIs but vastly different distributions of fat in their bodies. And, as a result, “their blood sugars and chronic disease risk will likely be different,” says Hanna.
Generally speaking, “people can be healthy or unhealthy in any of the BMI categories,” adds One Drop coach Julia Dugas, RDN, and certified personal trainer (CPT). “BMI doesn’t take into account age, gender, genetics, or the ratio of muscle vs. fat tissue.”
Some research even suggests that people may actually be healthiest in the “overweight” BMI category instead of the “healthy” category, notes Dugas. A 2013 review published in the medical journal, JAMA showed that, overall, grade 1 obesity (defined as a BMI of 30-34.9) was not associated with a higher risk of death from all causes, and, interestingly, an “overweight” BMI was associated with a significantly lower risk compared to those with a “normal” BMI.
Why? The study authors noted several potential explanations, including something called the obesity paradox—which suggests that, despite the increased risk of heart health issues associated with high BMIs, those with a high BMI actually tend to have a better chance of survival after certain heart-related incidents or procedures—as well as the possibility of bias among the self-reported weight and height data in the research they reviewed.
Anecdotally, though, as someone who’s worked in the fitness industry for several years, Dugas says she’s also seen “many personal trainers fall into the ‘overweight’ and even the ‘obese’ categories of BMI, simply because they have a lot of muscle tissue.”
Regardless, the point is that “there are both healthy and unhealthy people (lots of them) in every category,” says Dugas.
BMI Calculator Alternatives to Estimate Body Fat
“Body fat is similar to hormonal tissue,” explains Ford Brewer, MD, MPH, a board-certified preventative and occupational medicine specialist. “It releases cytokines, which are internal chemicals that drive chronic conditions such as insulin resistance.”
While your BMI is more of a rough estimate of the amount of body fat you have, calculating your body fat percentage tends to be more accurate, as it takes more factors into account than just height and weight to better distinguish fat from non-fat tissue. Below are some common methods for estimating body fat percentage:
- Tape measure. One of the most convenient ways to estimate body fat ratio is with a tape measure. Once you measure your waist, hip, and neck circumferences, for example, you can plug those numbers into a body fat percentage formula created by the U.S. Navy Seals (which also requires your age, gender, height, and body weight). You can also use those measurements to calculate your relative fat mass (RFM):
- Men: 64 - (20 x height/waist circumference) = RFM
- Women: 76 - (20 x height/waist circumference) = RFM
- Calipers. Calipers are a relatively inexpensive, accessible tool that measures body fat by pinching different skin folds. However, accuracy depends on the person using the calipers and their knowledge of how to take each measurement, so if you’re using the tool on your own and don’t have much expertise on the subject, it’s best to take the results with a grain of salt.
- Weight scales. If you’re looking for a more automated option, some weight scales include features that can help you discern muscle mass, fat mass, bone mass, and water weight so you can get a better idea of your actual body fat ratio. (To get a more complete picture of your own body composition, check out One Drop’s Complete Weight for Diabetes and Heart Health packages.)
- Dual-energy X-ray absorptiometry (DEXA). DEXA scans give you a detailed analysis of your fat tissue (including differences in visceral vs. subcutaneous fat), muscle mass, and bone density. The scan itself is quick and painless (you lie on your back on an X-ray table while the machine scans your body for about 15 minutes), and after it’s over, your results show you various pictures and graphs depicting the ratios of your different fat, muscle, and bone tissue. While DEXA is said to be highly accurate in measuring body fat percentage, the scans are very expensive to perform and require interpretation from a radiologist, so they’re not as accessible on a large scale as, say, BMI charts.
Overall, no matter which method you choose, “body fat percentage tends to be a much better indicator of health status and health risk than BMI,” says Dugas. Research shows that waist circumference, for example, can often tell you more about your risk of high blood pressure and high blood sugar than BMI. Similarly, RFM has been shown to be a better predictor of metabolic syndrome than BMI.
However, notes Dugas, “even body fat percentage should be used with caution when looking at what is considered ‘ideal’ or ‘normal.’ I’ve worked with many people (especially women) with body fat percentage goals that were below the threshold of where they would typically maintain a regular menstrual cycle (which is around 22% or higher body fat percentage).”
In other words, while many people assume “the lower, the better” when it comes to weight, body fat, and BMI, that is simply not always true, says Dugas. Just as there are implications for health at a higher weight, “there are health implications to losing too much weight and not maintaining enough body fat,” she explains, including not just menstrual health concerns, but also impaired immune function and osteoporosis, among other issues.
The Bottom Line On BMI
BMI may not exactly be the gold standard it’s cracked up to be, but given its widespread use by primary care doctors and even insurance companies, it’s probably not going anywhere any time soon, says Hanna.
That said, keep in mind the history behind BMI, she adds. Not only was it created by a mathematician (i.e. someone without a medical background), but it was also introduced long before we’d even seen an increase in chronic conditions such as diabetes and heart disease in the general population.
“BMI is not perfect,” says Hanna. “It’s also not an indicator of success or failure, but I have seen some individuals view it that way.”
Instead, when it comes to weight management, Michelle Routhenstein, CDCES, RDN, who specializes in preventive cardiology, recommends focusing on waist circumference, which she calls one of the “strongest determinants” of cardiometabolic risk and insulin resistance.
“In my private practice, I’ve seen people who have a high BMI and a normal waist circumference, as well as people with a normal BMI and increased waist circumference,” shares Routhenstein. Meaning, BMI and waist circumference often don’t correlate with each other. But what we do know, she explains, is that “an increased waist circumference is an indication of visceral fat—or fat around vital organs—which can lead to inflammation that can promote plaque formation in the arteries.”
So, while BMI can be a “good starting point,” says Routhenstein, “it shouldn’t be used as the sole evaluator of weight when it comes to chronic condition management.”
In fact, solely focusing on BMI can sometimes backfire when it comes to weight management and overall health. “As a registered dietitian and certified personal trainer, I’ve fallen into the ‘overweight’ category of BMI since before I graduated high school, and the behaviors I implemented to reach the ‘healthy’ category were extremely damaging to my mental and physical health,” shares Dugas. “I was less healthy trying to obtain ‘normal’ than I was when I just accepted that my weight likes to sit in the ‘overweight’ category.”
Even the National Eating Disorders Association (NEDA) has taken issue with BMI, noting not just its inaccuracies, but also its tendencies to promote unhealthy attitudes and behaviors about body weight and size.
“Our society puts so much pressure on people to stay out of the ‘overweight’ and ‘obese’ categories that, oftentimes, it can actually be more damaging to your health to try so hard to fit into the ‘healthy’ category than it is to just accept being at a higher weight and having healthy behaviors at whatever size you are,” says Dugas. “The irony is that feeling shameful and unsupported about your weight can actually lead to weight gain if you’re using food to cope with your emotions, and could even lead to social isolation and less physical activity (e.g., fear of being judged by others, especially in a place like a gym),” she explains.
If you’re looking for more sustainable, empowering ways to stay on top of your health, work with your One Drop coach, and check out our tips for thinking more holistically about chronic condition management.
This article has been clinically reviewed by Jamillah Hoy-Rosas, MPH, RDN, CDCES, and VP of clinical operations at One Drop.