BMI was developed in the 1840's for the field of social physics. It was meant to be an epidemiological scale to look at the health of populations. Because most populations will have a small number of anorexics, athletes and bodybuilders it is actually useful for what is was designed for. Unfortunately it is less useful when applied to an individual because it is a tool for looking at population trends.

It is useful in that it can provide risk factors for averages - for instance having a BMI over 40 is called morbid obesity because there is a strong chance that the person will have morbidity (symptoms) related directly to their weight. And being between 18.5 and 25 for AVERAGE people (not pregnant people, or bodybuilders or many athletes) is associated with least weight-related risk to health.

It's best to take BMI, as with anything else, with a pinch of salt. If you are working out regularly, are healthy (no high BP, type 2 diabetes etc.), eating a good diet, and are still out of the "normal" BMI catergory it probably doesn't mean anything much. Most of us know if we're honest with ourselves when there is a problem and when there isn't. Not that many athletes will look at their BMI and think "oh gosh, i need to lose weight" because just to BE an athlete you are so aware of diet and exercise. Equally there will be some people with a BMI of 21 who are actually carrying a relatively high % of fat because they eat badly and never exercise and just happen to have a metabolism that stops them getting very heavy.

It can actually happen that anorexics go into hospital and have high % body fat because when you crash diet your body uses the fastest-to-access energy supply - your muscles. So they might only weigh 30kg but 9kg (30%) is fat because they have almost no lean mass left at all. Typically people in that state are very weak and can barely stand up because their bodies have used all the muscle just to stay alive.

FWIW for me to qualify as a normal runner in an american road race (rather than an athena, the catergory for heavier female runners) i would have to have a BMI of 19.5. As it is i am healthy and can run and my BMI is 23.7-24.1, and i have no desire to be lighter.

It might be worth knowing that the link provided is from the site of a man whose daughter was misdiagnosed as anorexic and as such is a fairly biased view.

Bx