I know skinny guys who smoke, drink and have cholesterol levels and blood pressures through the roof (in other words, walking strokes about to happen). I also know a guy who weighs more than 300 pounds but he runs the Honolulu Marathon every year in about 4-5 hours. I also know an Iditarod musher who is about 5-foot-7 and more than 250 pounds, but he’s completed the 1,000-mile-plus race about 20 times and he’s usually in the top half of the field (most distance mushers actually run the equivalent of a marathon every day for about 9-15 days during the race, they don’t just ride the sled’s runners). The two guys who both have BMIs in the 40 range, BMIs that would list them in the morbidly obese class, I’m sure have better cholesterol and blood pressure levels than the skinny guys I mentioned.
BMI is one measure of a person’s health, but it’s only one of many measurements and should not be the final determination of a person’s real health condition. People come in all body types and sizes, and we shouldn’t try to be clones. I’m a larger guy with a football-lineman-gone-to-seed body type (6-foot-1, over 300 pounds), but I commute by bicycle every day (I use studded tires in the winter) and walk about every place else. Even though I have asthma, I have the endurance to go on a 20-40 mile bike ride if I needed to.