Why BMI Isn’t The Best Measure for Weight (or Health)

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Body Mass Index (BMI) provides an easy way to measure obesity, but more doctors are questioning its accuracy and usefulness.

BMI is supposed to estimate the amount of body fat a person carries based on height and weight, and categorizes people based on what is appropriate for their size. BMI readings under 18.5 mean you are underweight, and could put on a few pounds. If you fall between 18.5 and 24.9, you’re considered normal, while a BMI of 30 or higher qualifies as overweight.

But in recent years, more researchers argue that it’s not the most accurate way to measure body weight. For years, scientists have said that BMI can’t distinguish between fat and muscle, which tends to be heavier and can tip more toned individuals into overweight status, even if their fat levels are low. In the journal Science, the latest data from University of Pennsylvania shows that BMI also doesn’t tease apart different types of fat, each of which can have different metabolic effects on health. BMI cannot take into consideration, for example, where the body holds fat. Belly fat, which is known as visceral fat, is more harmful than fat that simply sitting under the skin. Visceral fat develops deep among muscles and around organs like the liver and by releasing certain hormones and other agents, it disrupts the body’s ability to balance its energy needs. Even relatively thin people can have high levels of visceral fat, which means they might be considered healthy by BMI standards, but internally they may actually be at higher risk of developing health problems related to weigh gain.

(MORE: Five Things You’re Getting Wrong About Weight and Weight Loss)

In April of last year, a study published in the journal PLoS One documented such inconsistencies and questioned the accuracy of using BMI to classify weight status of 1,400 men and women. As TIME reported:

Among the study participants, about half of women who were not classified as obese according to their BMI actually were obese when their body fat percentage was taken into account. Among the men, in contrast, about a quarter of obese men had been missed by BMI. Further, a quarter who were categorized as obese by BMI were not considered obese based on their body fat percentage. Overall, about 39% of participants who were classified as overweight by their BMI were actually obese, according to their percent body fat.

(MORE: 6 Weight Loss Tips Straight from the Nation’s Premier Spas)

So why is BMI still the preferred way to measure weight and evaluate obesity? For one, it’s a relatively easy measurement for doctors to take during an office visit. Taking a person’s height and weight and plugging it into an equation produces a number that informs doctors about whether their patients are at high, low or no risk when it comes to weight-related health problems.

But there may be better ways to measure body fat that provide more useful readings on how likely a person’s weight will contribute to chronic health problems. CT scans and MRIs can provide a clearer glimpse at the body’s make-up by separating out fat from muscle, for example. But these are expensive and involved compared to stepping on a scale. Other types of scans, including dual-energy X-ray absorptiometry (DEXA) images, which are normally used to measure bone density, can also distinguish between fat from bone and muscle mass, but are also costly.

On the more practical level, some researchers have been pushing for using waist circumference or even wrist circumference to gauge potentially harmful weight gain and fat depots, but the evidence supporting this measurement and its ability to predict future health problems isn’t definitive enough yet.

So without a viable way to change how we measure body fat, for now, BMI is the best option. The study authors argue that perhaps doctors should rely on not just assessing body composition but measuring hormones and biomarkers in the blood or urine, for example, to get a better handle on abnormal processes that may contribute to obesity and chronic disease. And until such tests become available, BMI may still prove useful yet — if doctors combine BMI with a comprehensive evaluation of their patients’ medical history and lifestyle habits to get a meaningful, if not yet perfectly precise picture of their weight-related health.

52 comments
lucy201568
lucy201568

that is like saying a pound of feathers is lighter than a pound of rocks...a pound is a pound.Casquette OBEY

AAKDJ
AAKDJ

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jessicaa5980
jessicaa5980

I hate the BMI. Nurse tried to tell me I was underweight 2 weeks ago based on that measure, but I'm perfectly normal. My family is naturally small plus I run track. Maillot Marseille

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BrendanJamesMarcel
BrendanJamesMarcel

I agree with you saying BMI means very little, as very muscular people tend to be categorized as obese by BMI....but the statement that muscle weighs more than fat?...that is like saying a pound of feathers is lighter than a pound of rocks...a pound is a pound.

activistinspireharmony
activistinspireharmony

Way Beyond Weight is a documentary about obesity, the biggest epidemic in the history that affects children.

For the first time children have the same disease symptoms as adults: heart and breathing problems, depression, and type 2 diabetes. All of them are based on obesity.

Worldwide, kids are heavier than they should be. And unhealthy. From Brazil to Kuwait, childhood obesity is becoming very common.

Why are kids carrying this extra weight? The industry, the marketers, the parents, the governments. Who is responsible for raising a healthy child?

Beyond Weight is a movie that seeks to answer those questions in depth. It interviews families, kids and specialists from all over the world.

"Way Beyond Weight" is a movie that seeks to answer those questions in depth. It interviews families, kids and specialists from all over the world.

www.cultureunplugged.com/documentary/watch-online/play/11987

rwicks
rwicks

It's the best thing for the Insurance company.

I'm 6 feet 2 inches.  According to the BMI, my maximum healthy weight is 194lbs.  That's the high level for "healthy".  Guess what the minimum would be for "healthy"?  144 pounds.

144 lbs is considered a healthy weight for a 6 foot 2 male.  If I weighed that much, I would be so weak, I don't think I could move - I'd be ripped though, and you could see every rib and every bump in my spinal cord.  But I would be "healthy".

The BMI is simply designed so the insurance companies can classify most of the population as "overweight" of even "obese".  That's it.

bobandweaver
bobandweaver

The FDA and Food makers decided to poison the people with food chemicals(processed foods)

Did we ask for hormones leading to obesity in our chickens?

Did we ask for Steroids in the beef? 

Did we ask for High fructose corn syrup in "everything" 

We have been poisoned for money, a thousand diets will not reverse the FDA poisons. We have inflammation related obesity which has made the food makers rich

herehttp://tryingtoloseweightgirl.blogspot.com/2013/08/i-need-to-lose-weight-why-most-diets.html

jcastro
jcastro

If you are an athlete then you are ok, you know part of you are muscles. 

But the 90% of people is not, then this a guide, a reference. 

It´s common sense. No need of studies for this.

MTDoug
MTDoug

@TIME @TIMEHealthland this question still being asked? Is ridiculous cause it only takes to factors into consideration

tosynolu
tosynolu

@TIME @TIMEHealthland I agree with this. BMI shouldn't be the best measure of weight. I am overweight according to 'BMI' & my weight is 82kg

JulietteSaintFleur
JulietteSaintFleur

I hate the BMI. Nurse tried to tell me I was underweight 2 weeks ago based on that measure, but I'm perfectly normal. My family is naturally small plus I run track. I eat a lot but for some reason I can never gain weight. I don't think I'm meant to be above 115 lbs. 

You don't need to be a genius to know that BMI is BS.

spot60spot
spot60spot

@TIME Agree, when I get down to the 'normal' range, I'm skinny and look quite sickly. Always thought BMI was odd.

Alissa_Hill
Alissa_Hill

@TIME BMI doesn't measure weight, a scale does. BMI is calculated by weight and height. Did you mean its not an accurate measure of obesity?

Xalanii
Xalanii

@TIME this has been common knowledge for sometime.

duckinspokane
duckinspokane

@TIME thanks for the breaking news. In other current events, the earth actually goes around the sun!

carolgina17
carolgina17

@TIME So...my doctor and I can stop having this conversation?

DrRMunoz_PhD
DrRMunoz_PhD

This a very important topic. For instance in bariatric surgery, the current guidelines to identify patients that can benefit from surgery are currently based on BMI criteria. Often we see non-morbidly obese patients (BMI <35) in which the physiological impact of increased fat depots shows as insulin resistance, dislipidemia, and glucose imbalance such us type 2 diabetes mellitus. At the same time we see patients even with greater BMI and no associated comorbidities. Until such better iand practical indicator becomes available we should pay attention not only to BMI but also - as suggested at the end of this note - the patient and its diseases.

rwicks
rwicks

@jcastro I bike 150 miles a week, and I can do a handstand pushup.  I weigh 205 lbs.

My doctor considers me overweight, and that's what the insurance premiums say too. 

This has nothing to do with common sense.  It has to do with insurance premiums and that is is.  According to the BMI chart, I would be on the low end of a healthy BMI if I weighed 144 lbs.  Do you think any male that is 6"2' that weighed 144 lbs is healthy?  I'd be on the edge of starvation if that was my weight.

rwicks
rwicks

@JohnAtlanta @vallie @TIME It's trivial to calculate your body density.  You just place somebody in a sealed chamber, increase the air pressure by about 5%.  You measure how much air was placed into the chamber to get to 5% and from that you can calculate exact density.

That's an ACCURATE calculation.

If you workout at all, good luck not being "overweight" if you're male.

vrome
vrome

@DrRMunoz_PhD Pardon me for assuming the role of the Greek prefix spelling police, but that's dyslipidemia.

cccliveyourbest
cccliveyourbest

@FitFluential that's unfortunate....as a muscular woman who is fit and strong I've always been 'overweight' in the BMI. Not a good gauge

FitFluential
FitFluential

@cccliveyourbest my son is a swimmer, muscular as well, & has a BMI warning every time. He gets it, but I worry about how other kids react.