When you visit your physician for a check-up, they want to determine whether you weigh more than you should and whether your weight places you at risk of health problems. To do this, they calculate a value called the BMI, or body mass index, a measure that looks at weight relative to height. For many years, BMI has been the standard for determining if someone is at higher risk of health problems based on body weight. However, it’s rapidly losing favor among clinicians and fitness trainers alike. In fact, the outcry against using BMI as a measure of body composition is growing stronger.
Why is there growing objection to BMI as a measure of body composition? Many experts believe the BMI should be retired and replaced with a measure that’s more accurate. One argument against BMI is that it only measures body weight and compares it to height. It doesn’t take into account how much of a person’s weight is body fat and how much is lean body mass. Instead, it looks at total body weight. For example, an athlete could be classified as overweight or obese based on BMI alone because they weigh more. But, the extra weight they carry is mostly muscle.
At the other end of the spectrum, an older person might be inaccurately classified as normal or underweight based on BMI, but they still carry too much body fat and weigh so little because they’re lost muscle. In addition, women naturally have a higher body fat percentage and the formula doesn’t take this into account.
A New Approach to Measuring Body Fat & Composition
Is change in the air? Researchers at Cedars-Sinai have come up with a new way to estimate body fat and one that could eventually replace BMI as the “gold standard” measurement of body composition. Their new twist on BMI is called the relative fat mass index, or RFM. Rather than comparing body weight to height, the RFM uses height and waist circumference for the calculations.
We know that a larger waist size is linked with a higher risk of some chronic health problems, including cardiovascular disease and type 2 diabetes. A larger waist measurement is a marker for more visceral abdominal fat, the type you find deep in the pelvic cavity and the kind most strongly linked with health problems. So, it makes sense to include waist circumference in a formula that looks at health risks related to weight and body composition.
How valid and useful is RFM as a measurement? Researchers at Cedars-Sinai used a database of 12,000 adults and tested 300 different formulas for estimating body fat. For 3,500 of the patients, they used the RFM and compared the value to the value obtained from a DXA scan. The DXA scan is the most accurate for estimating body fat percentage as well as distinguishing muscle and bone mass, but it’s not always practical to get a DXA scan as you have to go to a facility that offers this test. Plus, a DXA scan exposes people to a low dose of radiation, around one-tenth of the amount you get from a chest X-ray. In the study, the RFM value most closely agreed with the DXA results.
In fact, in the validation dataset, relative fat mass (RFM) proved more accurate than BMI at estimating body fat mass and percentage of whole-body fat in women. Plus, it held up better across ethnic groups and age categories. Just as importantly, it was less likely to misclassify someone as obese compared to the BMI measurement.
The RFM may be a suitable replacement for the long-standing BMI measurement. Plus, RFM is easy to measure. All you need is a tape measure to determine waist circumference and height. Plus, it makes sense to use RFM as a marker of future disease risk over BMI. We know that central obesity, a high waist circumference, is most strongly linked with cardiovascular disease, hypertension, and type 2 diabetes, so it makes sense that a measurement that predicts health outcomes would take into account waist circumference.
If you’d like the formula, here it is:
RFM for adult males: 64 – (20 x height in meters/waist circumference in meters)
RFM for adult females:76 – (20 x height in meters divided by waist circumference in meters)
The best way to take your waist measurement is to wrap the tape measure just above the hip bone. Then measure your height and plug the values into the equation. One reservation that some experts have about using RFM over BMI is differences in technique when measuring waist circumference. You get different values based on where you place the tape measure, how tightly you pull it to take the measurement and even the condition of the tape measure. Tape measures can stretch over time and give inaccurate measurements.
So, RFM isn’t set to replace BMI just yet. However, keep an eye out for further updates. No doubt, we need a better measure than BMI for evaluating health risks. Also, your physician needs to measure your waist size as it’s an important marker of future health risk. Too many are still only calculating a BMI and not using a tape measure at all.
The Bottom Line
Body mass index is still the measure most clinicians use for determining health risk based on weight. Yet, as you can see, it has significant shortcomings. RFM isn’t ready for prime time yet, but you should still track your waist circumference as studies show it’s a health marker. A waist size over 40 inches is linked with a higher chronic disease risk in men and over 35 inches in women. Ideally, we would track waist circumference as we do blood pressure, blood glucose, and lipids. It’s that important.
If you’re athletic and lift weights, don’t place too much stock on BMI. Get a waist measurement instead. You can also use skin calipers to get an estimate of your body fat percentage. Although it’s not completely accurate and is highly dependent on technique, you can use it to monitor for changes in body fat.
Science Daily. “More accurate measure of body fat developed”
Medscape Family Medicine. “New Equation Bests BMI at Estimating Body Fat Mass”
Scientific Reports. Volume 8, Article number: 10980 (2018)
CDC.gov. “Body Mass Index: Considerations for Practitioners”
Science Daily. “BMI Not a Good Measure of Healthy Body Weight, Researchers Argue”
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