The Skinny on BMI

BMI, or Body Mass Index, is once again receiving some attention questioning its usefulness as an indicator of health. A recent article in Time magazine points out what athletes have known for years, that if you work out regularly, it’s easy to tip the scales into the overweight category according to BMI, even for very fit individuals. So what gives? Why do doctors keep using BMI to assess the health of their patients, and what do you need to know about BMI?

BMI uses a simple calculation based on height and weight to determine the likelihood that someone is carrying too much or too little body fat. Although it isn’t foolproof, it’s a pretty good starting point to determine whether you need to lose a few pounds. You can calculate your own BMI easily using an online calculator, like this one. If you’re a female with a medium to small build, a healthy body fat level, and not unusually muscular, you’ll probably find yourself fitting pretty easily within the healthy range. Once you get outside of those categories, BMI gets a little trickier.

We all know that muscle weighs more than fat and BMI is a classic example of how this bears out in practice. Since bodybuilders and other athletes emphasize gaining muscle in their training, they frequently fall into an unhealthy BMI, while exhibiting a very healthy range of body fat. This is one reason that both male and female athletes benefit from prioritizing performance over a number on the scale (here’s a fun blog on why weight isn’t always the best predictor of health or performance).

This happens across the board for men, women and even children. Since men have a lower percentage of body fat than women, and since BMI doesn’t consider gender, men are even more likely to be considered overweight by the BMI calculator, especially if they work out. So where does that leave you when calculating your goal weight or when talking to your doctor at your next physical?

Basically, BMI is a good starting point for a conversation about your weight. If you know that you’re overweight but you’re also pretty fit, BMI probably isn’t going to be the best way for you to establish your goal weight. We know that for overweight individuals, losing 10 percent of their bodyweight is associated with huge improvements in health indicators, so this might be the best place for you to start if you’re looking for an achievable goal with a big pay-off for your health.

Here are other indications of healthy bodyweight:

  1. Tracking body fat through skin caliper or electrical impedance testing. This isn’t perfect in terms of accuracy, but if you control for hydration and the individual doing the testing, it’s not bad.
  2. Changes in measurements, especially in the hips/thighs (for women) and the abdominal area (for men and women).
  3. The Body Adiposity Index is one tool that’s been thrown around as an alternative to the BMI, as an easy to use indicator of obesity that is based on measurements, rather than body weight. The calculation, like this one, for that index is based on your hip measurement to height ratio and can be a better indicator of health for muscular individuals.

The final word seems to be that, whether you’re male or female, as you gain muscle the numbers on the scale and calculations, such as BMI, based on that number lose some of their usefulness as the best indication of your health. Take a look at your energy levels, your blood test results (you’re getting those taken at your physical, too, right?), how your clothes fit and how your body measurements have changed over time. The best approach uses those, in conjunction with BMI, for a conversation with your doctor about whether your health is heading in the right direction.

Does BMI leave you confused? Have you mastered it? Tell us how you track your health in the comments below.

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