Why does the BMI account for height at the square rather than the cube?

[MichaelEmouse]

The body mass index accounts for an individual's weight by increasing the height divided at the square, rather than the cube.

Do tall and short people not roughly have the same relative dimensions as normal-sized people?

The surfare area of a shape increases at the square of the dimensions but the volume increases at the cube. Presuming that density is roughly maintained, that means that weight should increase at the cube of dimensions as well.


A 6' man who weighs 184 pounds would be considered overweight, which just sounds wrong.

[Bartman]

Originally Posted by MichaelEmouse

A 6' man who weighs 184 pounds would be considered overweight, which just sounds wrong.

I'm 6'4" and 205. I have a significant gut and could easily lose 20 lbs. In fact I'm working on that and have a goal weight of about 185.

The reason is that while some proportions do increase with height, a lot doesn't. At my height I am not significantly wider or deeper (I guess that is the best way to express that) than a man in similar shape who is a foot shorter than I am. My waist is similar to that of shorter men. As is my chest and shoulder width. My shoe size is pretty standard for men (11). Although I am 10% taller than the average. I'm not 10% larger in any other dimension or body part.

[Loach]

BMI is just a ballpark figure. It doesn't really take into account different body shapes and especially those who have a decent amount of muscle. But it's quick and easy to calculate. Percentage of body fat is a better number to use but it's harder to calculate.

[Napier]

Bartman, aren't you just saying that you have a slim build? Loach, aren't you implying that it isn't important to calculate it right?

This has bothered me for years. Little Buddha statues would have low BMIs and basketball pros fairly high ones, the way BMIs are calculated now. I take the op to imply it should be the cube and I agree. The only thing the current method should do well is predict floor loadings for crowded rooms.

[AaronX]

I thought BMI is an empirical method (find a formula that works and use it), not one based on weight-volume ratio. So you have 2 easy measurements: weight and height. Find a simple formula that gives a range of numbers if an average person is fat, and another range if they're thin.

[drewtwo99]

I am a 6' man and I can say for sure that anything at 185 or so pounds is definitely overweight. I'm at 175 or so right now and trying to cut down on some of my flab.

There have been some recent studies, however, that show that people in the "overweight" category (not obese though) are actually just as healthy if not healthier than "average" BMIers.

So, medical research tends to indicate that if you're average, or on the lighter side of overweight, you're not at higher risk for certain diseases. But if your BMI is on the higher side of overweight, and into obese, then you are at a higher risk for certain diseases.

That's really all there is to it.

That being said, I've heard (though I don't have a cite) that the whole "body types" thing is more or less a myth. There aren't a lot of different body types that will skew the results of a typical BMI calculation for the vast, vast majority of people. People with a LOT of muscle (you really do have to have a lot) will have a high BMI, but not necessarily be at risk for certain diseases. From what I've heard, that's about it.

[hajario]

Originally Posted by Loach

Percentage of body fat is a better number to use but it's harder to calculate.

I don't think that it's calculated at all. It's measured. Right? That's why it's not used.

[Loach]

Originally Posted by hajario

I don't think that it's calculated at all. It's measured. Right? That's why it's not used.

Harder to measure. As far as I know the most effective way of measuring it is an immersion test.

[even sven]

Originally Posted by drewtwo99

I am a 6' man and I can say for sure that anything at 185 or so pounds is definitely overweight. I'm at 175 or so right now and trying to cut down on some of my flab.

There have been some recent studies, however, that show that people in the "overweight" category (not obese though) are actually just as healthy if not healthier than "average" BMIers.

So, medical research tends to indicate that if you're average, or on the lighter side of overweight, you're not at higher risk for certain diseases. But if your BMI is on the higher side of overweight, and into obese, then you are at a higher risk for certain diseases.

That's really all there is to it.

That being said, I've heard (though I don't have a cite) that the whole "body types" thing is more or less a myth. There aren't a lot of different body types that will skew the results of a typical BMI calculation for the vast, vast majority of people. People with a LOT of muscle (you really do have to have a lot) will have a high BMI, but not necessarily be at risk for certain diseases. From what I've heard, that's about it.

This. BMI is not about what "should" or "shouldn't" be overweight. It's about delineating a point where excess weight starts to correlate with health problems. They didn't set the "overweight" line where it is out of some sort of moralizing. That really is the point where weight starts to be associated with health risks, and it really isn't that high. Human are designed to be lean, active creatures.

IMHO, we have a very skewed idea of what a normal weight is. A normal weight is actually quite actively skinny, it's not just "not full of rolls and obviously grossly obese." People who have weights correlated with the best health outcomes are have almost visible ribs, no little belly stashes, and trim hips. Your body doesn't care if you look nice in clothes or you are slimmer than your outright chubby coworkers. That ten pounds that you could probably stand to lose isn't a free bonus just because it doesn't make you look obese. If you have any real amount of extra fat beyond some very slight padding, you would probably be healthier without it. We just don't have a lot of wiggle room for putting on weight before it starts to affect us.

[gatorslap]

BMI seems awfully meaningless, IMO. Two people could have the same height, waist, body fat percentage, muscle tone, etc. and not weigh the same. Maybe one has broader shoulders. Is there some reason people with broad shoulders should be thinner than people with more narrow frames?

[even sven]

Originally Posted by gatorslap

BMI seems awfully meaningless, IMO. Two people could have the same height, waist, body fat percentage, muscle tone, etc. and not weigh the same. Maybe one has broader shoulders. Is there some reason people with broad shoulders should be thinner than people with more narrow frames?

That's why they offer a range, rather than saying "everyone who is 5"6' should weigh exactly 120 pounds." And these ranges are not all that narrow. The "healthy" range for a 5'6" female is 117-143, which takes me from something I can only reach when I am working out every day, to my comfortable weight, to being outright chunky with a potbelly and plenty of jiggle on the high end.

Different body types will probably have different ideals within the ranges, but it's safe to say that even a husky, big-boned women is going to be medically overweight if she is 5'6" and clocking in at 150. If you shoot for somewhere in the "healthy" range, you are probably not facing increased health risks due to weight. If you are outside of the healthy range, it might be a good idea to think about getting closer to the healthy range.

[Bartman]

Originally Posted by Napier

Bartman, aren't you just saying that you have a slim build?

Not what I was trying to say. But it is on the right track. I am decidedly overweight, but my basic build is average for a man of my height. But I suppose you caught something I wasn't aware that I was implying, as men increase in height, builds tend toward slimmer. And that is true. So the BMI reflects that.

The OP's question "Do tall and short people not roughly have the same relative dimensions as normal-sized people?" has a pretty clear answer... at least to me. No they don't. Taller people tend to have slimmer builds and shorter people tend to have stockier builds. To the degree that BMI is a helpful measurement, it does properly map this trend. But as mentioned BMI is only a useful approximation anyway.

[Ambivalid]

..

[Napier]

Originally Posted by Bartman

.

The OP's question "Do tall and short people not roughly have the same relative dimensions as normal-sized people?" has a pretty clear answer... at least to me. No they don't. Taller people tend to have slimmer builds and shorter people tend to have stockier builds. To the degree that BMI is a helpful measurement, it does properly map this trend. But as mentioned BMI is only a useful approximation anyway.

Really? I thought people tended to be scaled copies of the same proportions. Do you know of any research on this?

[DSeid]

Humans are not built in the shape of a cube with sides the measuring the height, therefore it does not follow "that weight should increase at the cube of dimensions."

The origins of the BMI go to Adolphe Quetelet in 1832, based on empirical cross-sectional data in an attempt to fit body builds into a normal distribution. Later it was found to be a reasonable and easily applied correlate of weight related health risks and popularized

Adolphe Quetelet (1796–1874) was a Belgian mathematician, astronomer and statistician, who developed a passionate interest in probability calculus that he applied to study human physical characteristics and social aptitudes. His pioneering cross-sectional studies of human growth led him to conclude that other than the spurts of growth after birth and during puberty, ‘the weight increases as the square of the height’, known as the Quetelet Index until it was termed the Body Mass Index in 1972 by Ancel Keys (1904–2004). ...

... As the relation between body weight and mortality, particularly cardiac disease and diabetes, gradually became a medical concern following the Second World War, the quest for a reliable and practical index of relative weight began to assume increasing importance in the epidemiological and clinical studies that were being initiated. In exploring various indices combining weight and height, it became evident in the 1960s that, in adults, normal body weight in kilograms was proportional to the square of the height in meters, as originally proposed in 1832 by Adolphe Quetelet (1796–1874) (Figure 1), generally known in the then small circle of experts as the Quetelet Index [4,5].

One of the first studies to confirm the validity of the Quetelet Index in epidemiological studies comprised data gathered during the fourth examination of the Framingham study [6]. In a subsequent comparative study of available indices of relative weight and obesity published in 1972, Ancel Keys (1904–2004) confirmed the validity of the Quetelet Index and named it the Body Mass Index (BMI) [7]. Since then, as evidence of the association of obesity with various diseases continues to accrue, the BMI has been used as an expression to report the link of excess relative weight to morbidity and mortality. Primarily derived from data obtained on Anglo-Saxon populations, the generalizibility and applicability of the BMI and its cut-off points to other populations has been questioned and its sensitivity as a measure of excess fat queried. Nevertheless, it remains a dependable value and the basis of much of the associations reported heretofore with obesity, including its detrimental effect on normal kidney function and the course of chronic kidney disease [1].

[Quercus]

Originally Posted by DSeid

Humans are not built in the shape of a cube with sides the measuring the height, therefore it does not follow "that weight should increase at the cube of dimensions."

It doesn't matter if an object is cube-shaped. As long as the proportions stay the same, if it's scaled up, the volume will increase with the cube of the length; whether it's cube-shaped, spherical, donut-shaped, or shaped like a pixie riding a unicorn.
What matters is that as humans get taller, their proportions don't tend to stay the same. Taller people are typically proportionally skinnier.

[Bartman]

Originally Posted by Napier

Really? I thought people tended to be scaled copies of the same proportions. Do you know of any research on this?

Sure. Here is one that looks at several metrics (leg bone size, stability, heat loss, and actual measurements from Singapore and the UK) to estimate that the body scales between 2.3 and 2.8. Here is one that suggests 2.6 is an appropriate number for children 2-19 in the US. So BMI understates scale as you go up in height because it only squares. But cubing would overstate it.

[doorhinge]

BMI does not take into account age, gender, bone mass, or muscle mass. It can't distinguish between lean body mass and fat mass. BMI could best be considered a poor estimation of someone's overall health and further investigation IS REQUIRED before someone could be considered over or under weight.

Heavily muscled athletes will have a high BMI and a low percentage of body fat. Old folks may appear "BMI normal" even though they have lost muscle as they age. During his prime playing days, Michael Jordan's BMI was 27-29, which classifying him as overweight when in fact he simply carried a lot of muscle.

[Mijin]

As others have pointed out though, you have to be very muscular for that to be an issue in using your BMI. For most of us, it's a useful rough measure. Like the fuel gauge in your car.

I think a lot of the hate about BMI is just not liking the result. More to the point, I think there are a lot of people who are approximately average weight and therefore think of themselves as healthy, and then are horrified to be classified as overweight. But the thing is, the average american is on the hefty side by many measures.

[Fuzzy Dunlop]

Originally Posted by Mijin

As others have pointed out though, you have to be very muscular for that to be an issue in using your BMI. For most of us, it's a useful rough measure. Like the fuel gauge in your car.

Well here's a real world example that I think helps illustrate why people find BMI frustrating. About a year ago was the last time I met with my bariatrician (MD specializing in weight loss)

According to BMI, my healthy weight range as a 5'10" man is 149 - 183 lbs.

At the time I was 200 pounds with about 160 pounds of lean mass and 40 pounds of fat. That's a reasonably healthy 20% body fat. I was in no way muscular, in fact I lost about 20 pounds of lean mass while dieting under the doctor's supervision.

According to BMI I needed to lose 17 pounds to break into the very top of the healthy weight range.

If I lost 17 pounds of fat to get down to 183 (the absolute minimum to be healthy under BMI), I would be down to 12.5% body fat. 12.5% body fat is not at all realistic for most adults given their fitness level. And that's the absolute top of the healthy weight range.

Obviously the other option is to lose more lean mass as well as fat. I could've lost 17 pounds as long as I didn't have to lose 17 pounds of fat only. But why would i want to? Why would anyone deliberately lose lean mass just to fit into some calculated guideline when they're already at a healthy body fat percent?

So the bottom line is it's absolutely false that only extremely muscular people are foiled by misleading BMI judgments.

[Mijin]

Originally Posted by Fuzzy Dunlop

So the bottom line is it's absolutely false that only extremely muscular people are foiled by misleading BMI judgments.

I said very muscular, not extremely (IOW I'm not saying BMI works for everyone but mr universe; if you're stacked you're probably aware that BMI is not going to be that accurate for you. For the rest of us, it's a fine guide).

Does that mean I'm implying that you must be stacked? Well, I had some difficulty parsing some of the numbers. Did you lose the 20 pounds prior to, or after, weighing 200 lbs?

[Surreal]

Originally Posted by Fuzzy Dunlop

Well here's a real world example that I think helps illustrate why people find BMI frustrating. About a year ago was the last time I met with my bariatrician (MD specializing in weight loss)

According to BMI, my healthy weight range as a 5'10" man is 149 - 183 lbs.

At the time I was 200 pounds with about 160 pounds of lean mass and 40 pounds of fat. That's a reasonably healthy 20% body fat. I was in no way muscular, in fact I lost about 20 pounds of lean mass while dieting under the doctor's supervision.

According to BMI I needed to lose 17 pounds to break into the very top of the healthy weight range.

If I lost 17 pounds of fat to get down to 183 (the absolute minimum to be healthy under BMI), I would be down to 12.5% body fat. 12.5% body fat is not at all realistic for most adults given their fitness level. And that's the absolute top of the healthy weight range.

Obviously the other option is to lose more lean mass as well as fat. I could've lost 17 pounds as long as I didn't have to lose 17 pounds of fat only. But why would i want to? Why would anyone deliberately lose lean mass just to fit into some calculated guideline when they're already at a healthy body fat percent?

So the bottom line is it's absolutely false that only extremely muscular people are foiled by misleading BMI judgments.

160 lbs of lean body mass on a 5'10" frame seems awfully high to me, especially for someone that isn't muscular. How did you arrive at that figure?

Here's an empirical formula for lean body mass:

http://www.ncbi.nlm.nih.gov/pmc/arti...473290/?page=3

[Fuzzy Dunlop]

Originally Posted by Surreal

160 lbs of lean body mass on a 5'10" frame seems awfully high to me, especially for someone that isn't muscular. How did you arrive at that figure?

Here's an empirical formula for lean body mass:

http://www.ncbi.nlm.nih.gov/pmc/arti...473290/?page=3

I was tested by a medical doctor specializing in weight loss. I was a patient under her care for about a year and she checked and logged my BMI and body composition at each appointment.

[Fuzzy Dunlop]

Originally Posted by Mijin

I said very muscular, not extremely (IOW I'm not saying BMI works for everyone but mr universe; if you're stacked you're probably aware that BMI is not going to be that accurate for you. For the rest of us, it's a fine guide).

Does that mean I'm implying that you must be stacked? Well, I had some difficulty parsing some of the numbers. Did you lose the 20 pounds prior to, or after, weighing 200 lbs?

I went from 265 to 200 while under the care and supervision of this medical doctor. She checked my body composition with each appointment so I know that of the 65 pounds I lost, approximately 20 was muscle. 65 pounds is a lot of weight so naturally it was a mix of fat and lean mass.

So I only brought it up to say I was actually significantly less muscular than I had been previously. And I was never a body builder or athlete.

My point is that I think we should start with the axiom that if you are at a healthy body fat percent, you shouldn't need to lose lean mass to reach a "healthy" weight. And if a metric suggests that to average people then it's not very useful.

So sure, of course a 250 pound body builder with very little fat doesn't do much to discredit BMI. But when it tells a very average 5'10" guy at around 20% body fat that he can't realistically reach a 'healthy' net body weight without losing lean mass, it's no longer a useful tool for determining healthy body weight.

[septimus]

OP's question ("isn't x³ rather than x² the proper way to scale a linear dimension to mass?") has intrigued me also, but it wasn't until this thread that I was sufficiently concerned to Google it.
Google took me to Wikipedia:

However, many taller people are not just "scaled up" short people, but tend to have narrower frames in proportion to their height. Nick Korevaar (a mathematics lecturer from the University of Utah) suggests that instead of squaring the body height (as the BMI does) or cubing the body height (as the Ponderal index does), it would be more appropriate to use an exponent of between 2.3 and 2.7 (as originally noted by Quetelet).

[Mijin]

Originally Posted by Fuzzy Dunlop

I went from 265 to 200 while under the care and supervision of this medical doctor. She checked my body composition with each appointment so I know that of the 65 pounds I lost, approximately 20 was muscle. 65 pounds is a lot of weight so naturally it was a mix of fat and lean mass.

So I only brought it up to say I was actually significantly less muscular than I had been previously. And I was never a body builder or athlete.

My point is that I think we should start with the axiom that if you are at a healthy body fat percent, you shouldn't need to lose lean mass to reach a "healthy" weight. And if a metric suggests that to average people then it's not very useful.

So sure, of course a 250 pound body builder with very little fat doesn't do much to discredit BMI. But when it tells a very average 5'10" guy at around 20% body fat that he can't realistically reach a 'healthy' net body weight without losing lean mass, it's no longer a useful tool for determining healthy body weight.

For comparison, I'm 6 ft 3, and though not ripped I'm muscular enough that sometimes people compliment me, ask me what sets I'm doing etc.
I weigh 185 pounds.

It's got to be unusual for a 5 ft 10 guy to be packing almost that in lean mass alone. Even now with 160 + 40, you must be pretty stacked. Built enough to suspect that BMI is not going to be a very accurate measure for you.

[Fuzzy Dunlop]

Originally Posted by Mijin

For comparison, I'm 6 ft 3, and though not ripped I'm muscular enough that sometimes people compliment me, ask me what sets I'm doing etc.
I weigh 185 pounds.

It's got to be unusual for a 5 ft 10 guy to be packing almost that in lean mass alone. Even now with 160 + 40, you must be pretty stacked. Built enough to suspect that BMI is not going to be a very accurate measure for you.

I guess it's just a question of degree and at what point you're outside the norm and what point you'd expect to be within the norm. For instance, I still feel that if I'm not normal or average then it's not a good metric.

I have to admit I was doing experiments with hypothetical women (just 5'5" arbitrarily and a woman's body fat % guideline) and I wasn't able to find a body weight @ healthy fat composition that was outside the BMI healthy scale for a net body weight, in marked contrast to how my own real numbers worked out.

[Nava]

Originally Posted by hajario

I don't think that it's calculated at all. It's measured. Right? That's why it's not used.

Originally Posted by Loach

Harder to measure. As far as I know the most effective way of measuring it is an immersion test.

It should be measured, but now there are many scales in the market which "calculate " it by asking for your height and gender and assuming a certain value for lean mass. Those numbers are about as trustworthy as the Enron auditors...

[nate]

I'm 6'0, 225 lbs and I wouldn't consider myself fat. My BMI calculation puts me into the obese catagory, but just barely. It does make me concerned about my weight even though it's gotta be mostly muscle as I don't have a gut or anything.

[Chronos]

I think there are also some scales which use body electrical conductivity as a proxy for fat percentage. Which is better than making assumptions based on height and gender, but I don't know by how much.

[Loach]

Originally Posted by Chronos

I think there are also some scales which use body electrical conductivity as a proxy for fat percentage. Which is better than making assumptions based on height and gender, but I don't know by how much.

I always wondered how accurate that is.

[DSeid]

Originally Posted by Loach

I always wondered how accurate that is.

Not bad but it vaies by ethnic group some.

[dtilque]

Originally Posted by nate

I'm 6'0, 225 lbs and I wouldn't consider myself fat. My BMI calculation puts me into the obese catagory, but just barely. It does make me concerned about my weight even though it's gotta be mostly muscle as I don't have a gut or anything.

If you're in real good shape, then the BMI becomes less useful. Get your body fat percentage measured to get a better idea of how much excess weight you have.

[Gukumatz]

I'm a hair above 193 centimetres and weigh 68 kilograms, which Google tells me is about 6'4" and 150 pounds. If this BMI calculator is correct, that means I've got 18,3 on the BMI scale and am in the 6th percentile of weight for my height. I wonder, does anyone have any sources for the kind of problems I may develop by being underweight?

I occasionally jog, when the mood takes me, but mostly I do a lot of walking and standing straight for hours at work. I've never been in the habit of going to the gym. Are there any particular muscle groups that I should be paying attention to? I mean, I can understand a man with a solid gut should pay attention to his back and knees, but I can't think of anything I might be harming muscle or skeleton-wise.

[timmillea]

BMI is a very rough indicator used by the medical profession and the diet industry. It gives an approximate measure for most people of average muscular build, average height and activity level. The BMI calculation would term many elite athletes obese because they have greater musculature than average.

There is a misconception that tall people are not scaled up versions of short people on average. Actually if you measure bones sizes they are. What society tends to do is over-feed short people and under-feed tall people. Since when did portion sizes in school or at a restaurant vary according to the CUBE of person's height?

Then the everyday objects we have to move - books, shopping, furniture etc. all weigh the same regardless of a person's height meaning that the shorter person is getting a more intensive workout than the tall person in everyday life (muscular strength is proportional to the square of one's height, on average).

Tall people tend to have a lighter build than the scaled up short person's build, on average, because of these factors. Hence when devising and promoting the BMI system, the square is used rather than the cube because 1) it is simpler 2) it is good enough in most cases and 3) because of the factors above, measurement suggests that the power is actually around 2.6 and who is going to be working that out?

The BMI's singe biggest weakest is that it takes no account of muscular build. Hence a well-built, healthy lean person can appear obese according to the BMI, while the BMI figure can hide an ultra-light muscular build which is actually obese (in terms of very high proportion of body fat).

So please stop these "I'm 6ft x and weigh x lbs and I need to lose some" because without the weight of your muscles, it is practically meaningless to other people. Someone of the same height could easily be 20 lbs heavier and need to gain weight to be healthy!

[Mijin]

Originally Posted by timmillea

BMI is a very rough indicator used by the medical profession and the diet industry. It gives an approximate measure for most people of average muscular build, average height and activity level.
...
So please stop these "I'm 6ft x and weigh x lbs and I need to lose some" because without the weight of your muscles, it is practically meaningless to other people. Someone of the same height could easily be 20 lbs heavier and need to gain weight to be healthy!

Huh...really not the way I thought you were going to go with that post.

BMI is a rough, back-of-the-envelope figure and is useful in the same way that other rough measures are useful.
If you're 5 ft 9, buy XXL clothes and are not a bodybuilder... chances are you could do with losing some weight. BMI is a measure on that sort of level of precision.

[tim-n-va]

As far as the OP goes, BMI is still just a variation on height-weight tables. It can be useful as a rule of thumb and is probably sufficient as a quick screening tool.

The biggest advantage to the squaring part of the formula is that is puts the result in a wide range of integer values which makes for a nice chart. You could easily just say divide weight in pounds and divide by height in inchs and have a cut off for underweight, normal, overweight and obese. The values would just be in range of 1 - 5 and you'd need a couple of values after the decimal versus BMI where you get values in the 14 - 60 range and you can round to whole numbers.

[Chronos]

I'm not really sure that ease of calculation is relevant... How many people actually calculate their BMI? Most folks will either look it up in a table, or go to some website that calculates it for them, and in either case an exponent of 2.6 would be just as easy to implement as 2.

[echoreply]

If you're going to change BMI, there is good research suggesting that waist to height ratio (WHtR) is a better predictor of health outcomes than BMI. WHtR is calculated as waist circumference (measured midway between the lower rib margin and the iliac crest) in centimeters divided by height in centimeters. It avoids everybody's favorite pitfall of BMI, "but I'm a huge muscle man, that's why my BMI is 30!" (even though I can guarantee that 95% of people with a BMI of 30 do not have it because of extreme musculature). WHtR emphasizes excess adipose tissue (fat in the gut) which is more highly associated with poor health outcomes than other fat, such as on the hips.

I do always find these BMI discussions a bit funny, because they always bring out all of the people complaining that BMI isn't perfect, and it doesn't apply to them or their cousin, etc. What is completely lost is that the US (with Europe and Australia close behind) are looking at a huge obesity epidemic regardless of how it's measured.

[DSeid]

Exactly right. It is an easy to use screening tool and both the tendency to use it as the gold standard definition of adiposity and the tendency to totally dismiss it because of its imperfections are equally misguided.

Yes, someone with normal BMI can still have a high level of adiposity and some who are overweight by BMI have very little body fat. But across a population it will flag those who have excess adiposity relatively well.

[Michael63129]

Originally Posted by Gukumatz

I'm a hair above 193 centimetres and weigh 68 kilograms, which Google tells me is about 6'4" and 150 pounds. If this BMI calculator is correct, that means I've got 18,3 on the BMI scale and am in the 6th percentile of weight for my height. I wonder, does anyone have any sources for the kind of problems I may develop by being underweight?

Here's a partial list, which includes disorders normally associated with being overweight, such as heart disease and diabetes. However, that appears to mainly apply if you have a gene associated with being underweight. Even so, having a BMI under 18.5 appears to be as bad as being obese (the lowest risk of death occurs near the high end of the normal BMI range, higher for men than women, according to this study).

[nate]

What I'm curious about is does a muscular/fit person with a BMI of 30 have the same potential for health problems associated with weight as a fat/sloth with a BMI of 30? In other words, do health problems that are correlated to an obese BMI care what kind of weight it is, whether it is fat or muscle?

[DSeid]

Originally Posted by nate

What I'm curious about is does a muscular/fit person with a BMI of 30 have the same potential for health problems associated with weight as a fat/sloth with a BMI of 30? In other words, do health problems that are correlated to an obese BMI care what kind of weight it is, whether it is fat or muscle?

The sloth has great health risks; the athletic high BMI indidual has low risks.

Not only does it matter whether the weight is fat or muscle, it matters where the fat is as well. Exercise preferentially reduces the higher risk visceral (aka central) fat. Thus even with the same percent body fat someone who exercises has less health risk.