Jumpin said:
I wouldn't believe anyone who says they're 6 foot 4, 230 pounds and claim they're not overweight. I am 6 foot 4, wide framed, large boned, and athletic, and I weigh 195 pounds.
Even professional athletes often fall within BMI ranges, only the huge ones don't; some athletes have extra fat too. Anyway, for an example close enough to 6 foot 4 230; here is Alistair Overeem when he won the Strikeforce heavyweight title, he was 6 foot 5 and weighed 225 pounds:

BMI = 26.7, not far out of the BMI range, but I am sure guys who work out all the time like Overeem are past the point where they would question BMI. This is a very small fraction of the population. Most common critics of BMI are just people who don't want to admit they have fatty pounds that they should lose.
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Professional fighters are probably not the best example to use for the validity of the BMI system since competition is based upon weight classes which invariably means athletes must cut weight before weighing in the day before their bout.
Typically, a fighter cuts anywhere from 5-10% of their body weight the day before a bout (drop water weight in the sauna, emptying GI tract, etc.), often more. So a fighter who weighs in at 225 lbs to make the weight limit actually walks around at 240lbs or more. If a fighter is really competing in the weight class he should be in, there isn't much room for excess weight (fat) if he doesn't want to cut a lot of weight before a fight. Most compete in lower weight classes than they really should to stay competitive. And they do this by dropping a significant amount of water weight before weighing in.
But that's really where the BMI system comes up shortest; it does not take into account body composition.
Unfortunately, a lot of people (and a sizable chunk of people who have used Wii Fit) use this as an excuse to completely disregard the BMI system because they don't want to perceive themselves as being "overweight." So they'll justify this by saying they lift weights, they're an athlete, etc. without even knowing what their body composition happens to be.
Many professional (or high level competition) athletes have body fat percentages in the single digits, almost down to the essential body fat levels (under 5% for men). With that little fat, it takes a lot of muscle tissue to put an athlete into the "overweight" category, even according to the BMI system.
But these are the individuals for which the BMI system does not really apply to, rather than the guy who may lift a lot of weights, but still has a significant amount of body fat (over 20% is considered unhealthy). It's still possible to be "normal" weight under the BMI system and still have an unhealthy percentage of body fat if you have very low muscle mass.
A simple tape test comparing the neck, waist and chest measurments is one way, electrical resistance scales will also estimate body water percentage and body fat percentage, which are better indicators of fitness than a simple height to weight ratio which is all the BMI system is.