Is BMI a Good Index of Our Health?

It started as a joke at home.  My husband has been diligently exercising and weighing himself.  He often calculates his BMI, but finds it too easy to slide to the wrong side of the threshold for overweight.  BMI is just not a good indicator for muscular people, he says, you may not want to use that.  I laughed at his denial attitude every time, until I decided to write on this topic to conclude a debate.  Turned out, I have learned something interesting and useful for myself.

How is BMI calculated?

We use BMI (Body mass index) every day when evaluating patients and discussing weight control.  BMI can be easily and quickly calculated for a large population without special instruments and laborious tests.

BMI is calculated as body weight divided by height squared (kg/m2):

The National Institutes of Health provides a handy online calculator where you could input weight in pounds and height in inches without having to convert by yourself.  They also provide interpretation of the result.

Why is it “squared”?

Why is the body height squared in this formula, instead of any other exponent number?  I had this silly question, and found answer in the 1967 article published by two Welsh scholars 1.  It was a pleasant read.  The authors in fact tested indices calculated with height raised to different power (1, 2, 3, …, n).  They analyzed data from groups of men with various heights and body weight.  The index with squared height is best correlated with weight without being biased by height. 

The “obesity problem” of NFL players

Even in this 1967 article, the authors clearly stated the limitations of BMI.  When BMI indicates a person is overweight, it does not tell you whether the extra weight is from fat, or from muscle or bone.

In a study published in 2005, researchers evaluated BMI of 2,168 National Football League (NFL) players and reported that 97% of them were overweight (BMI>25) and 56% obese (BMI>30) 2.  This study gained broad new coverage.  The study raised the concern over health consequences of the increasing body mass in NFL players.

However, a recent study in 2018 suggested the obesity problem has been overestimated 3.  1,958 athletes who competed in NFL Scouting Combine between 2010-2016 were included in this study.  In addition to BMI, body fat percentage was obtained using a test called Air Displacement Plethysmograph (ADP).  Based on BMI, 98% of these athletes were overweight and 53.4% were obese, similar to the observation in 2005. 

BMI is to blame for the overestimate?

ADP is one of the reliable methods for body fat measurement.  For men, body fat percentage of 8.0-19.9% is the normal range.  Body fat of 20.0-24.9% is considered overweight, >25% obesity 4.  According to this criterion, 22.7% of the players in NFL Scouting Combine are overweight, and 8.9% are obese.  Yes obesity still exists, but to a much less extent.  There remains a possibility that these collegiate athletes will develop more obesity later on in their career.  However, the discrepancy between BMI and body fat percentage in the same people suggested otherwise.

More interestingly to me, among these collegiate athletes, those who were drafted have similar BMI compared to those who were undrafted, whereas the drafted athletes have significantly lower body fat percentage than the undrafted.  This supports body fat percentage is a better prediction of their athletic performance than BMI.

BMI is still the best index for most of us

BMI is not a good indicator of body mass for those who are overly muscular or who lost muscle mass.  For most of us, BMI is still recommended by NIH guideline as the most practical method to assess body fat. 

Its value in assessing risk for various diseases and overall mortality rate is well supported by research5,6.  More accurate methods have been developed, such as CT or DEXA, but those are expensive and not readily available. 

Waist circumference is as important

BMI does not take into account fat distribution in our body.  Waist circumference provides an estimate of visceral fat, an independent predictor for disease risk.  Even among people with normal weight and same BMI, larger waist is associated with higher risk for high blood pressure, type 2 diabetes, high blood cholesterol, heart disease and strokes 7,8.  Men with waist circumference > 40 inches and women > 35 inches have higher mortality rate.

Conclusion

It is easy to do and does not require special instruments to monitor BMI and waist circumference at a primary care clinic or at home.  By far they are still the best tools for us to monitor our body fat and gauge diet and exercise accordingly.  

References

1.         Khosla T, Lowe CR. Indices of obesity derived from body weight and height. Br J Prev Soc Med 1967;21(3):122–8.

2.         Harp JB, Hecht L. Obesity in the National Football League. JAMA 2005;293(9):1058–62.

3.         Provencher MT, Chahla J, Sanchez G, et al. Body Mass Index Versus Body Fat Percentage in Prospective National Football League Athletes: Overestimation of Obesity Rate in Athletes at the National Football League Scouting Combine. The Journal of Strength & Conditioning Research 2018;32(4):1013.

4.         Gallagher D, Heymsfield SB, Heo M, Jebb SA, Murgatroyd PR, Sakamoto Y. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. Am J Clin Nutr 2000;72(3):694–701.

5.         Berrington de Gonzalez A, Hartge P, Cerhan JR, et al. Body-mass index and mortality among 1.46 million white adults. N Engl J Med 2010;363(23):2211–9.

6.         Barroso I, Luan J, Middelberg RPS, et al. Candidate gene association study in type 2 diabetes indicates a role for genes involved in beta-cell function as well as insulin action. PLoS Biol 2003;1(1):E20.

7.         Chan JM, Rimm EB, Colditz GA, Stampfer MJ, Willett WC. Obesity, fat distribution, and weight gain as risk factors for clinical diabetes in men. Diabetes Care 1994;17(9):961–9.

8.         Kuczmarski RJ, Carroll MD, Flegal KM, Troiano RP. Varying body mass index cutoff points to describe overweight prevalence among U.S. adults: NHANES III (1988 to 1994). Obes Res 1997;5(6):542–8.

About the Author Isabel Wang, MD/PhD

I received my MD from PUMC in Beijing China and my Ph.D. in Biochemistry from Stony Brook University on Long Island. Over the years, I have worked in the fields of genetic research and clinical medicine in different parts of the US, including PA, MO, CT, FL, NY and MI. My research has been published in multiple scientific journals. Currently I live in Ann Arbor, MI with my husband and our children and Mango the orange tabby. I love hiking, running, baking, cooking and biking.

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