Body mass index (BMI) has been getting slammed in the media. For example, this recent article from the LA Times claims that "BMI mislabels 54 million Americans as 'overweight' or 'obese,' study says." Or, as this article highlights, "more than 50 million 'quite healthy' Americans have been mislabeled as obese or overweight thanks to an over-reliance on BMI as a measurement of health..."
What is BMI?
BMI is simply a measure of body mass. More specifically, it is the ratio of body weight (in kilograms) to height (in meters).
For example, Beverly weighs 165 pounds (75 kilograms) and is 5'5" tall (1.65 meters). To calculate her BMI (or use this calculator):
74.84 kg / (1.65 meters)2 = 27.5
A BMI of 27.5 kg/m2 puts Beverly in an 'overweight' category, based on the common classifications.
18.5 - 24.9
25.0 - 29.9
30.0 - 34.9
obese (class I)
35.0 - 39.9
obese (class II)
obese (class III)
You notice that the BMI is not telling us where or what any excess body weight is coming from (fat, muscle, bone, organs, body hair). It never has. Thus, the idea that BMI is limited in its assessment is nothing new.
So, why do we still use BMI?
Well, BMI is a simple, initial test to detect 'obesity', which is an excess amount of body fat that adversely affects health. As you might know, if we keep storing excess fat in our body, it begins to creep up around our hearts and other organs, making it difficult to function properly. Plus, the excess fat causes a chain reaction of other issues, usually detected in the blood (e.g. cholesterol, high blood sugar) that contribute to other diseases, disability, and premature death.
So, you can see why we would be so concerned with 'excess' body fat. Well, if you take a snapshot of most Americans, why would their BMI be high?
HINT: It is not excess muscle, big bones, or exceptionally heavy organs.
Remember, we live in the 'obesogenic environment'. Most Americans are gaining weight, because of body fat – stemming from unhealthy lifestyle choices. However, BMI was never considered to 100% accurate, rather an estimate of the probability of having health issues related to excess fat. I am not sure at what point people started believing that BMI was a perfect estimation of health risk.
The study that is being referred to by the lay media looked at a large, national data set, called the National Health and Nutrition Examination Survey (NHANES).10 They looked at a large sample of 40,420 adults from 2005 to 2012. Examining blood work, such as cholesterol, glucose, and c-reactive protein, they found that 29% of those classified as 'obese' by BMI were metabolically healthy.
In other words, based on their blood work, 29% were not unhealthy, but were classified as 'obese'. Remember, BMI is supposed to be a predictor of health when we do not have the luxury of other health markers.
Despite BMI only measuring weight and height, it actually did okay in this study, as it classified 71% correctly – the majority.
Again, although it is getting attention now, this issue of BMI is nothing new.9 Visually, take the following image of 8,550 men from the NHANES 1994 Data.11
Using a BMI of 25 kg/m2 and body fat of 25% – 1410 men (16%) were classified as 'overweight' by BMI, but were not overfat by body fat %. Also, 695 men (8%) were 'finny' (see below), where BMI said they were 'normal weight', but their body fat was high.
So, in the study of interest, why did some (29%) get classified as obese, yet were healthy? Here are some quick possibilities for this 'obesity paradox':
The big confounding variable here might not necessarily be BMI, but rather aerobic fitness. Research supports that the more aerobically fit we are, the lower our risk of relevant diseases and premature death - irregardless of our body mass.5 There still can be health issues that come from excess body fat, despite being 'fit', and we need to keep up with our fitness over time.1,8
So, the goal is to be both aerobically fit AND maintain a normal weight status.
Earlier I said that "most" Americans have an increased BMI, because of excess body fat.
However, take Jimmy, an ex-athlete, current exerciser, and healthy eater who has low body fat. He is 5'10" and 180 lbs. His BMI is 25.8, putting him in the 'overweight' category.
Or, Johnny, a professional football player, who is extremely fit and lean. He is 6'4" and 260 lbs. His BMI is 31.6, which puts him in the 'obese' category.
Thus, do Jimmy and Johnny weigh more (have more body mass) than an average man their height? Yes, but because their excess weight is due to muscle mass, and not excess fat mass, they could subsequently be at lower risk of having obesity-related diseases.
Where the Fat is Stored
Fat storage is commonly divided between the abdominal region (android) and the hips/buttocks region (gynoid).
We now have several studies that have shown the waist circumference measurement (around the belly button) to be a good and possibly better predictor of disease risk, especially cardiovascular disease, than BMI.7
In other words, too large of a belly is associated with an increased health risk, even if you are normal weight. The 'beer belly' is no joke!
A healthy, waist circumference should be less than 40 inches for men, and less than 35 inches for women. And, don't suck in your belly when taking the measurement!
On the other hand, fat stored around the hips and buttocks (gluteofemoral area) is not associated with increased health risk, rather might even play a protective role in our health.6 In women, gluteofemoral fat is also positively associated with better neurodevelopment and greater cognitive ability of their children - which is a good thing.4
Since 'obesity' is about excess fat, especially as a percentage of one's total body weight, it is possible for us to be 'normal weight', but have too much fat. The slang term I have heard to describe this situation is 'finny' = fat, but skinny.
Research supports that these individuals, even though 'normal weight', are at higher risk of disease from the excess body fat.2
In graduate school, I did a class project on 3 'finny' undergraduate women who were 'normal weight' by BMI. However, after running a valid body composition analysis, they were all found to be 'over fat' by a common body fat % cutoff, ≥ 33%.3 In other words, more than 33% of the body was just fat.
Why? For these women, at least, only 1 of the 3 was physically active (aerobic), while all 3 did not get any resistance exercise (for muscle fitness) and ate very poor diets.
So, someone should not skip living a healthy lifestyle, just because they are 'normal weight'.
Just to clarify, being 'big boned' is not a cause of our weight issues. Of the thousands of body composition scans that I have done over the past 10 years, most skeletons are estimated to weigh about 3-6 pounds. The heaviest skeleton I have ever seen was a large, ex-professional football player who weighed 350 pounds. His skeleton only weighed 13 pounds.
Unlike what the media might have you believe, BMI is not worthless, but has its limitations. For many (71% of adults, based on the study of interest) BMI does just fine is assessing an initial risk for disease. However, it does not correctly predict one's health 100% of the time, and never was supposed to.
In the end, for most of us, the focus should not be placed as much on body weight, rather on our healthy lifestyle choices - our behaviors - especially being physically active, eating a healthy diet, not smoking, getting quality sleep, reducing stress, and loving others.
- Dankel, S. J., Loenneke, J. P., & Loprinzi, P. D. (2016). Does the fat-but-fit paradigm hold true for all-cause mortality when considering the duration of overweight/obesity? Analyzing the WATCH (Weight, Activity and Time Contributes to Health) paradigm. Preventive Medicine, 83, 37-40.
- Deurenberg-Yap, M., Schmidt, G., Van Staveren, W. A., & Deurenberg, P. (2000). The paradox of low body mass index and high body fat percentage among Chinese, Malays and Indians in Singapore. International Journal of Obesity, 24(8), 1011-1017.
- Gallagher, D., Heymsfield, S. B., Heo, M., Jebb, S. A., Murgatroyd, P. R., & Sakamoto, Y. (2000). Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. The American Journal of Clinical Nutrition, 72(3), 694-701.
- Lassek, W. D., & Gaulin, S. J. (2008). Waist-hip ratio and cognitive ability: is gluteofemoral fat a privileged store of neurodevelopmental resources?. Evolution and Human Behavior, 29(1), 26-34.
- Lavie, C. J., De Schutter, A., & Milani, R. V. (2015). Healthy obese versus unhealthy lean: the obesity paradox. Nature Reviews Endocrinology, 11(1), 55-62.
- Manolopoulos, K. N., Karpe, F., & Frayn, K. N. (2010). Gluteofemoral body fat as a determinant of metabolic health. International Journal of Obesity, 34(6), 949-959.
- National Institutes of Health. According to waist circumference. Guidelines on overweight and obesity: Electronic textbook. http://www.nhlbi.nih.gov/health-pro/guidelines/current/obesity-guidelines/e_textbook/txgd/4142.htm
- Ortega, F. B., Cadenas-Sánchez, C., Sui, X., Blair, S. N., & Lavie, C. J. (2015). Role of fitness in the metabolically healthy but obese phenotype: A review and update. Progress in Cardiovascular Diseases, 58(1), 76-86.
- Romero-Corral, A., Somers, V. K., Sierra-Johnson, J., Thomas, R. J., Collazo-Clavell, M. L., Korinek, J., ... & Lopez-Jimenez, F. (2008). Accuracy of body mass index in diagnosing obesity in the adult general population. International journal of obesity, 32(6), 959-966.
- Tomiyama, A. J., Hunger, J. M., Nguyen-Cuu, J., & Wells, C. (2016). Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. International Journal of Obesity. doi:10.1038/ijo.2016.17
- Warren, Mark (2010). Correlation between BMI and %BF for men in NHANES 1994 data. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Correlation_between_BMI_and_Percent_Body_Fat_for_Men_in_NCHS'_NHANES_1994_Data.PNG