BMI - How Accurate Is it?

Posted on Apr 9th 2009 3:00PM by Mary Kearl

We all know that calculating body mass index (BMI) can give a good indication of weight range (underweight, normal weight, overweight and obese), but just how good is that indication?

It turns out, depending on your race, it may not be so great.

Consistent with earlier findings that have shown BMI to be inexact, authors from a recent study in the British Medical Journal of Nutrition found "that the number used to indicate weight category does not reflect the same amount of body fat for some races compared to others."

If not BMI, what should you use?

Researchers compared the effectiveness of BMI to dual-energy X-ray absorptiometry (DXA) determination of percentage body fat. Their findings?

"Right now non-Hispanic white women are not considered obese until they have a BMI of 30 or above. Based on our data for Hispanic women the number would be around 28," study co-author Molly Bray, associate professor of Pediatrics - Nutrition at the USDA/ARS Children's Nutrition Research Center, said in a press release. "For African American women the number to cross is around 32." The results for men were similar.

OK, so where can you get a DXA evaluation?

"DXA would most likely not be available to most people," study co-author Andrew Jackson, professor of Health and Human Performance at the University of Houston, explains in an e-mail. "It would be possible in a medical setting, likely not in a doctor's office. It would more likely be available at a medical center."

So if BMI is out and DXA isn't widely available, what should you use?

Jackson considers DXA to be the gold standard compared with BMI, which is less accurate because weight is not partitioned into its fat and fat-free components. The next best option? A "skinfold [test] is more accurate than BMI," Jaskson says. "I suspect the use of DXA will grow, but I am not sure how readily access it will be. It will be used more for research and important measurement needs."

6 Comment

Tags: BMI, body-mass-index, calculate-bmi, obesity, skinfold-test

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Reader Comments (Page 1 of 1)

BB 4-09-2009 @ 4:59PM

BMI is a joke it is obsolete and does not apply to people that are fit and heavy. I am 6'3 and 223 lbs my BMI is 27.9 almost obese? The number might mean something if it took in to account body fat % and fat weight, my most recent fat test was 2 weeks ago and a skin fold test put me at 13% I also have a 48" chest with a 34 inch waist. I am very fit I lift 5 days per week and do 5 cardio sesions a week How can my BMI show me at borderline obese??

Inge 4-09-2009 @ 7:23PM

BMI has its limitations but is certainly not useless.
For total health control I can recommend further exploration at http://www.ckbody.com as the "Portal to your Health"

Enjoy!
***********************

naima 4-26-2009 @ 1:38PM

I agree with the first commenter, BB. (BMI is certainly misleading, not only in terms of race but tribal differences--particularly in a population as pluralistic as the United States. Although I grew up in the U.S., for years I had the typical fat to muscle/bone density ratio for members of my father's tribe in West Africa--"lean, mean & muscle to bone"--I was always being referred to as skinny (later thin, despite my obvious bone density & muscle, in my adult years) even though I had always been strong, not a weakling, & from as early as the age of ten, could lift at least 15 pounds in excess of my own body weight, effortlessly. I was a sprinter and sprint cyclist for years & had excellent lung power from years of chopping wood/hauling, lifting, stooping/bending daily year round often in hot, humid temperatures of up to 100 degrees, and later walking up to five hours a day for a period of about five years. Perhaps, in terms of fat percentage, I thin, but that is deceiving, generally weighed more than most of my American peers who appeared to be of the same size visually and in clothing size. At 24, I took a BMI, with a total wgt. of 138 lbs., 118 lbs. was skeletal weight, 9.75% was body fat (a figure which, if stated on its own, usually elicited comments about how I was under the minimum for women). Which group of women? It was just that I was of a West African body-type, one generation removed. I was told I had excellent lung power, which I already knew; my heart rate was above average, and blood pressure were ideal for all aerobic sports. Like most tests & test results, the BMI is Eurocentric--catering to a particular ethnic group & most probably a particularly class--of women & is therefore misleading much of the time. Even though I had no ribs showing, had considerable definition to my physique from years of chopping wood, etc. & being a cyclist, women who were clearly just skin & bone, with little fat on them were regularly calling me skinny. Oftentimes, before it was in vogue for women to have biceps or strong quads or hamstrings--unless they were professional athletes or professional dancers--there seemed to be an obssession with my physique. And overweight women, clearly in need of a gym or truly obese had a field day. Fit & perfectly healthy people, physically capable & strong have been being tyrannized by fat, overweight, lazy people (often on one diet or another) for years. Now they're appropriating tests like the BMI to further confound & confuse the issue. I am glad to see that a discussion questioning the veracity of the BMI tests has begun, so that maybe someday this tyranny of women who are naturally thin, or whose weight is genetically (or ethnically) comprised of bone dense and muscle with naturally low body fat. There are even regional differences, right here in the States, which I didn't realize until I left the eastern seaboard for the midwest, where there were large communities of tall & lean Scandinavians, not the typical English, Irish, Italian, Eastern European and Asian body types that I had grown accustomed to seeing in the Northeast. For me, having grown up in an area of the country in the South, where it was not uncommon for women to proudly carry 35lbs. (or more) in excess weight, while extolling its virtues, when I met the spear side of my family, it answered a lot of questions.

Laurel Yvonne 5-19-2009 @ 11:39PM

BMI is a usesful indicator in clinical settings to estimate degree of malnutrition.

Also, a person with a higher BMI has more body mass, regardless if it is fat or muscle, and therefore more nutrients to break down in periods of hypermetabolic activity; and therefore, can survive hypocaloric feedings or several days without food if complete bowel rest is necessary or hemodynamic stability cannot be achieved.

But agreed, for the general population, BMI is not the best assessment of weight status.

liltruthie 12-22-2009 @ 3:03PM

I'm really glad to have found this article. I just tested my BMI via the NY Times calculator.

I'm happy to say (or indifferent) that I'm at a normal weight. But I played with the calculator to see what I'd have to weigh to be underweight. Currently at 5'3 and 136.5 lbs…I'm doing alright…normal even. But, I could go as low as 105 lbs. and NOT be considered underweight!? On my frame…currently I look very skinny…

I think this is dangerous information. I think this is how so many modeling agencies can get by saying that their clientele is healthy. I think this is how so many young women (and men to some extent) are guiding their health—dangerously!

I'd love to see our understanding of health, appearance, acceptance change…but I don't see it if we rely on this. The food guide pyramid does not take into account cultural idiosyncracies…when will medicine look to the individual as their own measurement for health etc.?

ivan 1-13-2011 @ 4:21PM

bmi is a joke!!! it takes nothing into consideration for example bone mass, muscle mass, body fat.
im 35 years old was a competitive strength athlete, i still workout with weights 4x a week and aerobics 6x a week, my cholestrol is 140 overall, my triglycerides is 40 and ive been following a high fat,high protein diet for the last 18 years.
im 5'11 weigh 225lbs with a bf of 14% but yet according to the highly ignorant and outdated bmi im obese.
of course my bmi is high been building muscle my whole life.
im going to sum everything up its no wonder America has one of the fattest populations in the world along w/ the highest heart disease and diabetic rate in the world.
doctors in other countries use CoenzymeQ10, Green Tea, Fish oil to treat heart conditions however in America we dont because it isnt marketable or you cant patent it so what im saying is doctors and pharmaceutical companies cant line their pockets using healthier and better alternatives.
as far as diet goes most countries eat fish, pork, eggs, cheese, red wine.
in America we dont.
something is seriously wrong with our health system when the rest of the world does the opposite of what we are doing and our obesity/ diabetes rate is through the roof in compared to the rest of the world.