Monday, May 12, 2008

Whither BMI?

Body Mass Index is a much-maligned measure of fatness, yet it remains the standard measure of weight-related health risk among the medical and scientific community. In this post, I'm going to try to explain why this is the case.

Before BMI standards were widely adopted in the 1980s, fatness was largely determined by height/weight charts. Those early BMI standards considered men with a BMI >=27.8 and women with a BMI >=27.3 to be "overweight." Then in 1998, the National Institutes of Health lowered their BMI cutoffs to match the World Health Organization standards. Under the new guidelines, a BMI of 25 or more was considered "overweight," and 30 or more was considered "obese." It sounds a little convenient that these categories happen to fit such nice round numbers, doesn't it? But this is pretty much the basis for these categories:

And also, to a lesser extent, this:

(These images courtesy of Obesity Online).

The relative risk of diabetes and heart disease goes up at a BMI of 25, and the line of BMI vs. risk seems to change slope at around BMI = 30 (references are on the images). And BMI is cheap and easy to measure, all you need is a scale and a yardstick, or even just a telephone--you can just do a survey and ask people their height and weight. In contrast, to actually measure fat mass accurately, you need a machine called a DEXA scanner which costs $20K-$80K depending on how fancy your machine is.

So those are the main reasons why BMI is so popular among researchers and clinicians. Now, on to the criticisms.

The main criticism I hear of BMI is that it is simply a height/weight ratio and does not take into account muscle mass. While this is true, most people (women especially) are not going to be much affected by this unless they are body builders or professional-caliber athletes. Take, for example, Cruiserweight boxing champion David Haye, pictured here:

A very muscular guy, obviously, yet his fighting weight BMI clocks in right at 25, the border between normal and overweight (he's 6'3" and the Cruiserweight limit is 200 lbs. I chose a boxer for this example, because I figure their weights must be pretty accurate). The point here being that while muscle mass is one of the more common criticisms people have of BMI, I think that for the most part it's not that big a confounding factor in most cases.

One element that does seem to be a major confounding factor is race. Several studies have shown that people of Asian descent tend to be fatter than Caucasians with the same BMI, and consequently, they tend to develop weight-related illnesses at lower BMIs (1-4). Latinos also seem to be more susceptible to diabetes at lower weights, although less so than Asians (4). For this reason, some people now think that waist circumference is a better indicator of risk of overweight-associated illness, and other researchers suggest that using the two measurements in tandem provides the best estimate of risk (5-7).

In addition to racial differences, some investigators feel that BMI cutoffs should be different for women and men, as men tend to have more muscle mass due to their higher testosterone levels. As you can see in the graphs above, men have a lower relative risk of diabetes vs. BMI compared to women, but the risks for cardiovascular disease are similar.

Another major problem with using BMI as a determinant of health risk is the fact that several recent studies have shown that people with overweight BMIs have a lower risk of mortality than people with normal BMIs (8-9). (Of course, everyone's risk of mortality is 100% eventually, but this was as measured within the period of the study). This finding was perplexing to researchers, as it would seem to counteract the information in the graphs above, findings which had been replicated many times. Some have suggested that these findings could be a result of some people in the "normal" group experiencing weight loss due to undiagnosed illness that later contributed to death. Others pointed to the so-called "obesity" paradox: the finding that while overweight people are more likely to be diagnosed with heart disease and renal failure, they also have a survival advantage over normal-weight people with this disease (10). (There's a lot more to say about the obesity paradox, but I'll save it for another post!) At any rate, the overall picture is still quite murky.

In summary, I would say that BMI is a somewhat useful tool for determining whether a person is statistically at heightened risk for certain complications of obesity. But it should be kept in mind that this increased probability is not at all a predetermined fate. After all, even a 45-year old woman with a BMI of 30-35 still has less than a 50% chance of developing type II diabetes (11). So, as they say on the internet, YMMV.

1. WHO Expert Consultation.
Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet. 363: 157-163, 2004.

2. Deurenberg-Yap M., Deurenberg P. Is a re-evaluation of WHO body mass index cut-off values needed? The case of Asians in Singapore. Nutrition Reviews 61:S80-S87, 2003.

3. Huxley R, James WP, Barzi F, Patel JV, Lear SA, Suriuawongpaisal P, Janus E, Caterson I, Zimmet P, Prabhakaran D, Reddy S, Woodward M, Obesity in Asia Collaboration. Ethnic comparisons of the cross-sectional relationships between measures of body size with diabetes and hypertension. Obesity Reviews 9:53-61, 2008.

4. Shai I, Jiang R, Manson JE, Stampfer MJ, Willett WC, Colditz GA, Hu FB. Ethnicity, obesity, and risk of type 2 diabetes in women: a 20-year follow-up study. Diabetes Care 29: 1585-1590, 2006.

5. McCarthy HD. Body fat measurements in children as predictors for the metabolic syndrome: focus on waist circumference. The Proceedings of the Nutrition Society 65: 385-392, 2006.

6. Deurenberg P, Deurenberg-Yap M. Validity of body composition methods across ethnic population groups. Forum of Nutrition 56: 299-301, 2003.

7. Koster A, Leitzmann MF, Schatzkin A, Mouw T, Adams KF, van Eijk JT, Hollenbeck AR, Harris TB. Waist Circumference and Mortality. American Journal of Epidemiology April 15 Epub ahead of print, 2008.

8. Flegal KM, Graubard BI, Williamson DF, Gail MH. Excess deaths associated with underweight, overweight, and obesity. Journal of the American Medical Association 293: 1861-1867, 2005.

9. Flegal KM, Graubard BI, Williamson DF, Gail MH. Cause-specific excess deaths associated with underweight, overweight, and obesity. Journal of the American Medical Association 298: 2028-2037, 2007.

10. Schmidt DS, Salahudeen AK. Obesity-survival paradox--still a controversy? Seminars in Dialysis 20: 486-492, 2007.

11. Narayan KMV, Boyle JP, Thompson TJ, Gregg EW, Williamson DF. Effect of BMI on Lifetime Risk for Diabetes in the U.S. Diabetes Care 30: 1562, 1566, 2007.


Jodith said...

One problem you didn't talk about is the difference in bone size. I have a huge bone structure (really, it's not just wishful thinking). My sister and I are the same height, and when we wear the same size, I weigh 20-25 pounds more than she does. By the same token, someone with a small bone structure weighs less than she does at the same size and height. BMI doesn't take that into consideration at all.

Dr. LaWade said...

I would put bone size in the same category as muscle mass because bone size is predominantly determined by weight (bone is constantly remodeled, so if you gain weight, you gain bone, and if you lose weight you lose it Certain individuals might have extraordinarily high or low bone mass for their body weight, but on a population basis, I don't think it's that huge a factor.

Anonymous said...

The 1998 NHLBI report says: "In this report, overweight is defined as a BMI of 25.0 to 29.9 kg/m2 and obesity as a BMI of ³ 30
kg/m2. The rationale behind these definitions is based on epidemiological data that show increases in mortality with BMIs above 25 kg/m2. 28-32" That is, their rationale is about mortality, not about diabetes or heart disease risk, as in the data you showed. One of their citations is the WHO report you mentioned, and in my reading of that report, they also seem to have been thinking in terms of mortality.

Any comment on the mortality data that those committees were working with at the time? Why are they apparently different from some of the more recent studies? And any personal thoughts on whether it makes sense to define a group of people as "unhealthy," when it seems the current best data actually show they're at higher risk (than the reference "normal" group) for certain diseases and lower risk for others?

Dr. LaWade said...

Good points, anonymous. Both at the time of the NHLBI report and since, there have been several papers showing increased mortality among overweight people. The Flegal et al. paper which was the first (as far as I know) to show decreased mortality in overweight people explains the discrepancy by saying that they adjusted confounding factors differently from previous reports. Epidemiology is not my area of expertise, so I wouldn't care to comment on which data are "better" but there definitely seems to be something interesting going on with overweight and mortality.

And I wouldn't say that people with an overweight BMI are being defined as "unhealthy," just that they are considered to be at increased risk for certain diseases which are among the most common causes of death. Although certainly many people, both inside and outside the medical profession, do unfairly make that leap of logic to tar all overweight people as "unhealthy."

The Honorable Samantha Grace said...


Are you interested in doing a guest blog either as a one time deal or as a series? I absolutely love your blog! It is so well researched and substantive and helpful. I have a new series on my blog called The Thin Line of Fat that explores the interconnections between the Health At Every Size/Fat Acceptance movements and chronic pain and illness. I think you'd be perfect for this, if you're willing. Let me know.

And keep up the great work!


vbsmith said...

I like to read your take on the calorie restriction movement for increasing longevity.

Anonymous said...

"The Flegal et al. paper which was the first (as far as I know) to show decreased mortality in overweight people explains the discrepancy by saying that they adjusted confounding factors differently from previous reports."

The Flegal paper was by no means the first paper to report this. If you look at the 1998 NHLBI Clinical Guidelines on Overweight and Obesity report, it actually cites this information (p. 24 of the full report) 10 years ago. The Flegal paper cites some other literature and there actually have been many studies since then that report exactly the same thing. So the interesting question is why so many people still don't realize that this is a common finding.

Dr. LaWade said...

My knowledge of the subject is far from comprehensive, so I can believe that there might be prior studies out there showing a general protective effect of overweight, but I haven't been able to find them. I actually went through all of the references in the Flegal paper and while there were a few studies that hinted at such a finding, they were all either not statistically significant or used non standard BMI groupings (e.g. Troiano et al., who found that having a BMI 23-28 was better than BMI < 23). And the report cited in the Clinical Guidelines only looked at people 55-74 years old, and so not a representative sample. So, I think these limitations, along with the fact that popular opinion and certain studies have indicated that overweight is indeed unhealthy, explains why this is still considered to be controversial.

Girth Watching Matt said...

On the whole no mater what you feelings about BMI there is one cutting question that will tell you most of what you need to know. Can you jog (and I do mean run not run hell for leather) for five minuets without getting out of breath or breaking a sweat?

Staci said...

Your blog is so informative. I hope you continue on as this is July and I don't see any past May. Are you still out there?

Sharon said...

It's interesting you posting the cardiovascular & diabetes curves. If I recall correctly, the mortality rate curves usually show the lowest point of the curves at the 25/26 BMI mark, with category 25-30 pretty much looking as much risk as the 20-25 category is.

Anonymous said...

I think a little bias is showing up here! You have dramatic looking graphs from completely non-representative samples. E.g. one is from a group of male health professionals 40-75 years of age. Then you say (apparently following the misleading Harvard line) that the Flegal study is the only one to find overweight not so terrible. Someone else points out that exactly the same finding was reported in 1998 on a specific page of the NHLBI clinical guidelines report. You counter with "And the report cited in the Clinical Guidelines only looked at people 55-74 years old, and so not a representative sample." Actually it is far more representative than the samples that you display in your graphs, which are not at all representative. So you seem to be using rather different standards for different studies.

Dr. LaWade said...

Anonymous, I chose to present the particular data I did because there were already some nice graphs of it on the internet. That particular data may not be from a representative sample, but it is in accord with scores of other studies from varying populations. I think it's entirely reasonable to be picky about the sample population when you're talking about one or two papers that go against what has been previously shown in a large body of literature, don't you?

elife said...

Just found this site and love it. I have a question, wondering if you have an answer:

We all know if takes +3,500 cals to gain a pound. However, I keep reading that studies say people who were once overweight will gain weight faster than people who never were, because the once-fat folks have extra fat cells (which were "emptied" during the weight loss).

So what does this mean, does it take a formerly fat person fewer than 3,500 cals to gain a pound? Confused...thanks.

Anonymous said...

To keep your body healthy you have to reduce your body weight and ensure that it is with in the BMI range (BMI between 18.5 and 24.9). Exercise and well balanced diet are the best ways to prevent many diseases caused by over weight. Weight control methods can help you to keep your weight within normal BMI range.

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Anonymous said...

"I think it's entirely reasonable to be picky about the sample population when you're talking about one or two papers that go against what has been previously shown in a large body of literature, don't you?"

Yeah, but what you dont seem to realize is that in fact most of the literature actually supports the Flegal papers in showing that overweight (BMI 25-29) isn't associated with any higher mortality than is normal weight. The Harvard group's studies actually show the same thing - before they delete about 90% of their data to come up with an answer they like better.

Anonymous said...

The BMI is maligned for one very good reason. It is seriously flawed. My weight/height puts me well into the obese category. But despite this, I can touch my toes without bending my knees, tie my shoes, and other things obese people cannot do. Overweight, or fat? Sure, no doubt about it. But I'm 6', and 270 pounds. I just have more upper body development than most people. Even without the gut I would need to wear xx large shirts due to the width of my shoulders. Supposedly I should be about 170 pounds. But I used to weigh that,(prior to being placed on cortico steroids) and I looked like a cancer patient. The BMI sucks. A rough "yardstick" is worse than none at all when there are too many variables to be taken into account.

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MonaVie said...

Are you saying that the BMI for diabetic patients are different?