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Given the fact that it doesn’t distinguish between lean body weight and fat weight, why is body mass index so widely used to determine ideal body weight?

When I started seriously exercising a number of years ago (lifting weights and cardio), I gained 20+ pounds and dropped 2 waist sizes in my pants. I'm not muscle bound, but do have a lot more muscle now then when I started. According the BMI chart for my height and weight, I'm borderline obese, yet in reality I'm quite fit. At an annual physical my doctor pulled out the BMI chart, looked at me again and said "well, that obviously doesn't work for you."

What's a better way to measure/guage fitness or appropriate weight? Is it solely by measuring % of body fat?

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Because it is simple and people like simple things. Too bad it sucks. npr.org/templates/story/story.php?storyId=106268439 –  Wayne In Yak Jan 17 '13 at 20:05
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up vote 9 down vote accepted

It's a simple measurement that provides an immediate idea of whether you're overweight, underweight, or neither. It's based on the idea that people who weigh significantly more or less than the average person of their height are probably not a healthy weight. That's true in most cases, not all, and again it has only ever been meant as a general guide. We can only assume that people who took it more seriously simply weren't educated about it very well, or something.

As for other measures, I'm sure everyone has their favourite. Possible options:

  • Waist size compared to chest size
  • Percent body fat, as you mention
  • BMI will still work for a person of "average build"

Keep in mind that none of these mean you are healthy or fit. There are more factors that can be measured with bloodwork and the like, but everything is only an indication. I think the best measure of fitness is your own honest personal assessment, based on:

  • Your personal history
  • How you feel mentally and physically
  • What you are capable of (running, lifting, whatever it may be)
  • What and how you eat
  • What you do (exercise, appropriate rest and sleep, etc.)

Of course, this is not to say that if you're doing well on everything then you should forgo regular checkups, and whatnot. Sometimes things just pop up and we may not know the cause.

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BMI is best suited for analyzing populations, not individuals. A single person might carry more muscle than the average and a BMI calculation would label them as "obese". However, when calculated for thousands of people (which can be done with BMI since it's so quick/cheap/easy to measure), the fluctuations tend to average out, and BMI is accurate enough to be useful. For personal use, measurements like body fat percentage, blood pressure, etc are far more accurate.

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BMI is about as simple as it gets to give a measure that works for a lot of people. But you're absolutely right that it doesn't work for everyone. For someone who isn't fit, I would say the BMI works pretty accurately as a signal they need to do something or face medical problems. For someone who is fit, the BMI isn't useful. But you're already fit so don't worry about your BMI.

We need to be watchful of health insurance companies, and employers, who might use the BMI across everyone as if it were accurate for everyone. BMI could show up in incentive programs, and it would be very wrong to penalize someone who's fit who happens to have a messed-up BMI score.

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One reason that I asked the question was regarding insurance companies who seem to use it blindly with very little other data to get a accurate picture. –  Walter Mar 9 '11 at 13:43
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