Despite being a flawed measure, BMI, or body mass index, is still the barometer by which we measure whether a person is obese or of normal body weight. For years now, healthcare professionals have used calculators to measure a patient’s BMI and declared them to be of normal weight, underweight, overweight, or obese based on the number that the calculator popped out. But, is our faith in this measure misplaced? BMI as a measure is increasingly being criticized for being inaccurate in a significant portion of the population. Is this a number we can trust?
To measure your own BMI, divide your weight in kilograms by the square of your height in meters. It’s easier to use an online BMI calculator to get the value and there are plenty available. Once you have your calculated BMI, this is what the number corresponds to:
· Underweight – BMI of 18.4 or lower
· Normal weight – BMI of 18.5 to 24.9
· Overweight – BMI of 25 to 29.9
· Obese – BMI of 30 or more
Simple, right? It sounds like a straightforward way to find out whether you’re overweight or obese but most experts agree that the formula is flawed, particularly for athletes and the elderly. Certain athletes, especially strength athletes and bodybuilders, carry lots of weight on their frame but that mass is muscle rather than fat. Still, using the BMI equation, all weight counts. So, a bodybuilder might fall into the obese category based on BMI. When you realize that bodybuilders have a low body fat percentage and the weight they have is muscle, you can see that the formula fails. Similarly, the elderly may fall into the normal range based on BMI but still have a high body fat percentage. Their low body weight, especially if they don’t strength train, may be primarily due to loss of muscle tissue. Based on their high body fat percentage, which BMI doesn’t take into account, they should still be in the overweight or obese range.
So, what is the best measure of whether a person is overweight or obese? You need to know an individual’s body fat percentage. The best measure in a clinical setting is dual-energy x-ray absorptiometry (DXA), a type of scan that directly measures body fat and calculates body fat percentage. This is the gold standard for measuring body fat. There are other ways to measure body fat percentage, including a simple at-home method like using skin calipers that’s reasonably accurate. But, most other methods, like scales that use bioelectrical impedance, are not particularly accurate or reliable.
Is BMI an Inaccurate Measure in All Women after Menopause?
We know that BMI falls short as a measure of obesity in certain athletes and in the elderly, but a new study published in the journal Menopause suggests that BMI may be an inaccurate measure in almost all women after menopause. As part of the study, researchers looked at data from over 1,300 women who had gone through menopause. They compared calculated BMI measurements with the results of DEXA scanning, the gold standard for measuring body fat percentage. What they found was BMI measurements were not accurate for defining obesity in women past menopause.
Why might this be? As you know, menopause is a time of great change – hormones fluctuate and you begin to lose muscle and bone mass at a faster rate. It’s also common for women, particularly inactive ones, to gain body fat. The combination of these factors makes it difficult to say whether a woman truly falls into the obese category, overweight, or normal weight category based on BMI alone. Loss of muscle and bone mass reduces body weight along with BMI, making the BMI appear closer to the normal range even in the setting of a high body fat percentage. So, measured BMIs post-menopausally are likely to underestimate obesity.
Body Fat Distribution Matters Too
Since BMI is a measure of total body weight relative to height, it says nothing about body fat percentage or how that fat is distributed. We now know that all body fat isn’t created equal in terms of health risks. The riskiest type of fat is deep belly fat called visceral fat. Unlike subcutaneous fat that you can pinch, visceral fat is deep within the pelvic cavity and is stored around organs like the liver and pancreas. Visceral fat is pro-inflammatory fat and releases inflammatory chemicals called cytokines that contribute to insulin resistance. As such, it’s associated with a higher risk of type 2 diabetes and cardiovascular disease.
To determine that amount of visceral fat you have requires an imaging study, usually an MRI. But, that’s not always practical. A simpler way to get an idea of how much visceral fat you’re carrying is to measure the size of your waistline. Studies show that we carry around 10% of our total body fat as visceral fat and large waist size is an indicator that you’re carrying too much of this dangerous type of fat. How large is too large? For a woman, greater than 35 inches is too much while the cut-off value for men is 40 inches. So, measuring waist size is just as important, if not more so, than measuring BMI. It also more accurately reflects future health risks.
The Bottom Line
The usefulness of BMI as a measure of obesity is questionable, particularly for athletes, the elderly and, now, women after menopause. Knowing your body fat percentage and your waist size offers more useful information with regard to future health risks. You can easily measure your waist size and follow it to see if it’s changing over time. Monitoring your body fat percentage is a bit harder but skin calipers are a reasonably accurate way to follow how your body fat percentage changes over time and is more accessible than more accurate methods that require a visit to a special facility. So, grab a pair of calipers and a tape measure and put them to work for you.
Menopause. 2017 Nov 13. doi: 10.1097/GME.0000000000000989. [Epub ahead of print]
Harvard T.H. Chan School of Public Health. “Waist Size Matters”
British Medical Bulletin, Volume 97, Issue 1, 1 March 2011, Pages 169–196, https://doi.org/10.1093/bmb/ldr002.