My beef with BMI

I had my yearly checkup today. Overall it went over very well, with the usual problem areas: because I'm hyperglycemic my blood sugar always is borderline high, and according to my BMI I am almost overweight. While I choose to manage the first one through exercise and diet, the second one annoys me to no end because, as anyone who follows @fitnick knows, I work out a lot! So what gives?

The Body Mass Index (BMI) is defined as follows (thanks Wikipedia!)



The table to interpret the result is as follows:

Category	BMI range – kg/m2	BMI Prime	Weight of a 5 ft 11 in person with this BMI
Severely underweight	less than 16.5	less than 0.66	under 53.5 kilograms (8.42 st; 118 lb)
Underweight	16.5 to 18.4	0.66 to 0.73	118 and 130 lb
Normal	18.5 to 24.9	0.74 to 0.99	130 and 180 lb
Overweight	from 25 to 30	from 1.0 to 1.2	180 and 210 lb
Obese Class I	from 30.1 to 34.9	from 1.21 to 1.4	210 and 250 lb
Obese Class II	from 35 to 40	from 1.41 to 1.6	250 and 290 lb
Obese Class III	over 40	over 1.6	over 290 lb

At 5'7'' and 158 lbs I come in at 24.7, which from my engineering perspective barely in the normal range (Wii Fit agrees, always showing me as borderline overweight. It also likes to puff my avatar out like a marshmallow). My friends who do marathons, bike races, and other generally "fit" people also end up in the high-normal to overweight range. Why is that? Reading from the NIH

BMI is a reliable indicator of total body fat, which is
related to the risk of disease and death. The score is valid for both men and
women but it does have some limits. The limits are:

• It may overestimate body fat in athletes and
  others who have a muscular build.
• It may underestimate body fat in older persons and
  others who have lost muscle mass.

BMI assumes that all people have the same distribution of muscle and fat. Because muscle is heavier than fat, having more of it (from, say, exercising) throws the BMI numbers off skew. When I first heard that I thought that you'd have to be a bodybuilder to have any real effect, but even muscle from long distance running can skew your result. The BMI is trying to find a way to make judgments about a large population, but people come in all forms - tall, short, big boned, skinny - and they defy this sort of grouping.

So keep in mind that if you exercise a lot, trust yourself over the BMI (or Wii Fit) if you are overweight or not. The BMI is a metric to help you find health, and that's it.