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I always see BMI charts posted in various health related places, and I know life insurance companies use it for assessing risk.

I've heard the normal range of the index is supposedly based on what normal was in 1800s Poland. Is this true? In any case, should a reasonably bulky person using cardio/strength training ever expect to hit that normal range in the normal course of events, or might one expect to stay in the 'overweight' category due to new muscle mass?

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BMI has a number of fatal flaws, as noted in my answer here. –  Dave Liepmann Jun 15 '12 at 5:25
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up vote 8 down vote accepted

I'm not sure about the origin, but there was an interesting article in the local newspaper about the All Blacks (the New Zealand rugby team/world number one team) in which it was stated even the lightest member of the team was overweight (based on BMI), and four of the players were obese.

As professional sportsman, these players are all very well built, and very muscular, but hardly what you would think of as obese.

I think measuring your body fat might be more helpful, as being X KGs/X cm tall with 5% body fat vs 25% body fat is a big difference.

You could probably also form your own judgements as to whether or not you are overweight, but I'd say, while BMI might be a useful guide, it's hardly definitive as to what your ideal weight is.

Hope this helps. Good luck.

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Here's a cool picture that I feel encompasses what the problem with BMI (or any calculation based mainly on weight) is:
muscle versus fat

Muscles are "heavy" (i.e. their density is considerably larger than that of fat). Once you start adding even just a decent amount of muscle mass, you will become heavier and throw the BMI way off-balance. I think most bodybuilders fall at least in the "overweight" category. Pros probably show up as obese.

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+1 for great photo. I really don't like BMI as a quantitative measure for health. –  Rhea Mar 23 '11 at 15:10
    
Most people are not professional or even amateur bodybuilders. For the general populous, and even moderately trained individuals BMI is a valid and relatively accurate measure of body fat. –  Lego Stormtroopr Jun 16 '13 at 23:12
    
@LegoStormtroopr No, BMI assumes that weight is proportional to height squared, which is false. As a result, it's only valid within a fairly limited range of heights. –  Evan Jun 18 '13 at 14:53
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BMI is a rough estimate that is easy to calculate about where you stand for health, however it can be very rough. People with a lot of muscle can easily be overweight or obese, however this does not mean that they're unhealthy. The problem is that it doesn't differentiate between fat and muscle.

The metric was developed in the 1840's by Adolphe Quetlet in Belguim, however the current guidelines that are used for overweight/underweight are based off frequently updated data from the World Health Organization. The most recent update of BMI ranges took place in 2004 and indicate that 18.5 - 25 is considered to be normal weight for individuals with average muscle build.

However, to know the true state of your health you'll want to know your body fat percentage too. A $40 scale can usually do this for you reasonably well. It won't be 100% accurate, for that you'll want something like a BodPod or Dexa measurement, but it will give you an idea of trend. The American Council on Exercise provides some body fat recommendations for both men and women depending on fitness level. For men a fit body should have 14-17% fat and women should have 21-24% fat. If you're a man looking to get six pack abs you'll need to push down to 10-12% fat.

In short, BMI is a tool that can help, but it's not the only tool. If you know what goes into it it will make a lot more sense and be significantly more useful. It's a good starting point for non-athletes.

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