bmi < 18.5 is "underweight" ???

BMI is misused when it is applied as a one-off measure of individual health. There are too many other variable involved to simply say "your BMI is X, so you are overweight/underweight."

BMI is best used as a statistical tool to measure changes across a large population. While you can't say that a particular individual is overweight because of their BMI, you can make some inferences from the fact that the average BMI of students at a particular college has gone up, for example.

So, yeah, a particular person could have a BMI that is considered low and still be healthy. On the other end of the spectrum, an NCAA running back could be considered overweight despite not having an ounce of excess fat on their body.

being underweight has disadvantages as well as being overweight. Not enough muscle and fat is dangerous for the body. Too much muscle limits mobility and too much fat wreaks havoc on the joints and insides. Runners have very low bmi's and do not have much muscle or fat or overall decent body strength for that matter.

Careful about that NCAA running back statement. Many, MANY obese people use that argument to explain that they themselves are simply casualties of a broken BMI system. They argue "I just have a ton of muscle, I lift" when in reality they have a ton of body fat in addition to some muscle. The only people who are obese by BMI standards without having fat are Professional football players and professional bodybuilders.

Misapplication wrote:

BMI is misused when it is applied as a one-off measure of individual health. There are too many other variable involved to simply say "your BMI is X, so you are overweight/underweight."

BMI is best used as a statistical tool to measure changes across a large population. While you can't say that a particular individual is overweight because of their BMI, you can make some inferences from the fact that the average BMI of students at a particular college has gone up, for example.

So, yeah, a particular person could have a BMI that is considered low and still be healthy. On the other end of the spectrum, an NCAA running back could be considered overweight despite not having an ounce of excess fat on their body.

Overall wrote:

being underweight has disadvantages as well as being overweight. Not enough muscle and fat is dangerous for the body. Too much muscle limits mobility and too much fat wreaks havoc on the joints and insides. Runners have very low bmi's and do not have much muscle or fat or overall decent body strength for that matter.

So much this.

halt right there fatties wrote:

The only people who are obese by BMI standards without having fat are Professional football players and professional bodybuilders.

Umm, and some (no, I didn't say all) t&f throwers. Example: Brian Oldfield was 6-5 and 276 pounds, well into the "obese" category, and he probably had a lower bodyfat percentage than I do (at 6-2+/154). BTW for a person who's 6-2 the "normal" range varies from 144 to 194:

Here's what I don't understand, though. We know that body mass varies as the cube of height (because bodies are three-dimensional). So say that I'm 1.90m and 70kg (both pretty accurate); an exactly proportional person who's 1.70m (~5-7) would weigh 50kg (110lb). But the BMI calculator says that my weight is in the normal range, and that the exactly-proportional shorter person would be significantly underweight. How do you figure?

because you two aren't exactly proprortional. to a slight degree, you're much more "stretched out" along the z-axis than he is. If he were blown up to your size, he'd probably be a lot wider than you. (there are exceptions to this but generally really tall people have this stretched along z effect).

Also, the 'formula' for BMI was determined by regression to the mean for various heights so it's accounted for. People aren't spherical and don't live in a vacuum.

insurance companies came up with those numbers...

No, actually I fell into this category in high school. I used to bulk up for football in the fall, then spend the winter cutting a that weight for track season. When I'd go in for my spring sports physical, I would basically be in 800m shape, but I'd still have a lot of the muscle weight from spending the summer and fall doing lots of lifting. My doctor once informed me that I was technically overweight despite being one of his fittest patients. I was probably 5'10" and 185 lbs or so at that point. Even running 80 mpw, I never dropped much below 180lbs, so I may well have even been "overweight" while I was running sub 2 minute 800s.

You d better be one pound heavier than your racing weight than one pound lighter. Trust me, I was there myself and running will eventually suffer when you are not eating enough.

But yes, a BMI of 25 is not healthy either.

It's definitely over the edge for a typical person. Ritz is about 17.8, but he's also a professional runner with all kinds of resources to keep him on the right side of healthy. A normal person without all that could very easily go over the edge and see his health suffer.