BMI Obese Bunk

I just read the answer made to the guy inquiring about BMI. I have the impression that a few years ago the Overweight-Obese line was moved to the left on the BMI chart instantly creating (by medical definition) millions of newly obese people. Is this true? Assuming it is, when I am told by my physician “I am sorry to say that you are obese,” I feel like responding “Move that line a bit more to the left and you, too, will be obese.” Obese has a very different meaning to non-physicians. Obese is the 400-lb man or woman, not the person who does not even look fat but is defined obese by the BMI chart.

well , I just want to say wow !!

3 posts in 6 years…

Don’t you know the old Kazakh proverb: When he who speaks little speaks, better listen up.

I’m 30 years old 6’ and 195lbs. My BMI is 26.4 and accordingly I’m classified as overweight. I don’t work out (so no big muscles), have a 32in waist, and can run 100m in about 13sec (about the equivalent of high school girl who is competitive at the one A level [you’re smirking, try and beat me]).

BMI is a good way at quantifying height to weight, but the level that is defined as obese or overweight is inaccurate.

The link for this column is Is the body mass index (BMI) chart an accurate indicator of obesity?

I’ve always been suspicious of the units: kg/m[sup]2[/sup].

It seems to me that if you scale a person up, the m in the denominator should be cubed. Someone with the exact same shape of a body, but 10% taller than the smaller person, would weigh 33% more. If the BMI formula had the height cubed, then their index would be the same.

But as it is, the person who’s 10% taller and scaled proportionally would have a BMI that’s 10% higher.

Take these two hypothetical people who have the same shape but one is 10% taller:

A: 2m 100kg BMI:25
B: 2.2m 133kg BMI: 27.5

It seems like those two people would be equivalently overweight (or not). But their BMI is markedly different.

Well, you know, it’s important to keep one’s post count slim.

In reply to the query about the dimensions of BMI, I want to add the annoyance I feel at using kg for weight. The kilogram is a unit of mass, whereas force is measured in Newtons. A person with a mass of 50 kg has a weight of about 110 pounds, which is the same as 389 Newtons. A person weighing 389 N on Earth will weigh about 1/6 that, i.e., about 65 N on the Moon. However, he will still have a mass of 50 kg.

In the US system, the pound is the unit for force, so it is appropriate to use it for weight. (The unit for mass in the delightful US system is the slug.)

I definitely need to drop a few slugs.

I think you’re complaining about how people use the word “weight” when describing what the BMI is, right? Because mass in the Body Mass Index seems to correctly be used in the calculation itself.

Your annoyance is that people say “I need to lose some weight” when they should be saying “I need to lose some mass.”

Or in my case, “I need to lose some ass.”

Of course, the BMI also doesn’t reflect the healthy/non-healthy qualities of the mass, either, as someone with lots of muscles are going to be heavier in proportion to their height than someone who’s just average fitness for their height, and yet might be a good deal healthier (or not–muscles obviously aren’t the only factor determining healthiness.)

The BMI isn’t supposed to be anything more than a rough way to gauge how much people ought to weigh. Think of it like you think of the speedometer on your car. It’s not supposed to tell you anything but how fast you’re going. It may be a bit imprecise, but it’s usually pretty close. And, no, how fast you’re going isn’t the only factor in determining safe driving, but it’s a major factor, and the speedometer isn’t meant to be used to the exclusion of all other measures or considerations. And yeah, there are a few exceptions where the posted speed limit doesn’t matter, but most people should use it as a rule of thumb most of the time.

The cutoff at 30 represents the best estimate using the somewhat crude tool of BMI of the point at which your weight actuall begins to significantly affect your health. Thus, the cutoff at 30 represents a statistical determination of a health issue. It is, like all such determinations, not necessarily accurate for a given person.

And it has nothing to do with how fast you run the 100-yd. dash. :dubious:

That’s as it may be scientifically, but for those countries using the metric system (e.g. Canada), people invariably talk about weight in grams and kilograms whether you are talking about bathroom scales, buying groceries, or whatever. You can if you wish tell Canadians that they are all wrong when they talk about how much they weigh, but I don’t think you’ll get very far.

The question then becomes, do you want something that measures someone’s proportions, or something that measures someone’s health risks? There are a lot of square-cube scalings in the human body which could affect health: The amount of blood you need to circulate through the body is proportional to volume, but the cross-section of blood vessels to circulate that blood is an area. The weight of the body is proportional to the volume, but the cross-section of the bones that support that weight are an area, and so on. So, it actually doesn’t sound theoretically unreasonable that BMI woud correlate better with health risks than would a metric which used height cubed. A larger person with the same proportions as a smaller person would, in fact, be expected to be at a greater health risk.

Kilograms per cubit meters is mass per unit volume, which is density. That’s what the hydrostatic bodyfat test is for. You’re correct in that density is a better measurement than BMI for determining fatness, but you’re not going to calculate it accurately just on weight and height cubed. Using just those 2 numbers, BMI is the best approximation you can get. And I’m not sure you’re correct about the scaling either, I think by cubing the height you’re further ignoring body shape and compounding that error by an order of magnitude.

If you took a “Body Shape Index” that’s the height in meters divided by the cube of the mass in kilograms, that index would accurately reflect shape - two people of different heights, but with the exact same proportions, would have the same BSI.

As to why this isn’t the best predictor of health, that’s what I was asking, and Chronos provided at least part of the answer. I was also thinking that if person A is 10% taller than person B, ideally A would not also be 10% wider and 10% front-to-back deeper. The ideal width and depth for A would probably be less than that. If the answer is that the ideal would be 7% in those other two dimensions, that correlates to a kg/m[sup]2[/sup] measure like BMI.

Ok, but the skinfold caliper test takes maybe 2-10 minutes (IIRC), which isn’t too much of a hassle, though it does involve a trained professional. That said, Wikipedia seems to agree with Cecil’s take on the relative ease of the BMI: see the Classification section of their “Overweight” article.

Indeed, the Wikipedia article on the BMI notes that the expected increase in BMI for taller people at least partially offset by the fact that taller people are generally skinnier in profile, etc.

Perhaps the word obese means something different in our thinking from what it means to a physician. What we normally think of as obese, a doctor would probably describe as morbidly obese. I’m just grasping at strawmen here.