Is BMI Actually a Useful Tool in Determining Health Status of Children?

School nurses have long been using the BMI (Body Mass Index) scale to determine risk for childhood obesity. Each year we check heights and weights of our students and then send home a letter to parents reporting that their child is over or under weight based on the BMI scale. But is this really the right thing to be doing based on current evidence based research? Personally, I think not.

What is BMI? Body mass Index, or BMI is a supposed measure of a person’s percentage of total fat mass or percentage of body fat. It is an equation based on a person’s height and weight that determines a person’s level of body weight related health.

Some history of the BMI scale: It was a (flawed, somewhat nonsensical) mathematical equation invented in the early 1800’s by a mathematician named Lambert Adolphe Jacques Quetelet as a way of determining the “fatness or fitness” of a collective population of males (not an individual or female). The formula, BMI=k/m2, made little sense from the beginning. Why did we have to square the height in meters but not the weight? No one knows. It also didn’t take into account lean muscle vs body fat, waist size, or bone density. It became a frequently used tool in the 1970’s and forward by doctors as a measure of an individual’s weight related health status as it was easier to use than the other methods such as skin fold caliper testing and underwater weight displacement testing despite the fact that it was not meant for individual statistic analysis nor did it have any basis in actual health. So basically some guy with no health background, albeit very intelligent, made up an equation to evaluate statistics of a health issue and we have just run with it for literal centuries out of convenience.

So why should we change this practice? As mentioned, the BMI does not take into account lean muscle, waist size or bone density vs body fat. As most of us know, muscle and bone are more dense than fat therefore a person with more muscle mass or bone mass is bound to have a higher overall weight. A person could have low body fat, lots of lean muscle and bone tissue and still fall into an “obese” category according to BMI. It’s not measuring the fat percentage of the person, only the weight to height ratio which isn’t helpful in this setting. We’ve all had plenty of students who come up as “obese” when we can see that they are solid muscle. A 17 year old football player who is 6’4″ and 250 lbs. and works out every single day might be a brick wall of solid muscle but according to BMI he is obese and must make changes to their health habits. This is obviously not an accurate assessment.

Should we continue to be concerned about students being over and underweight? Absolutely yes. Depending on the reason for the weight (fat) gain of the person, they aren’t always “unhealthy” however, chronic obesity can lead to diabetes, heart disease, lung disease, cardiovascular issues, infertility, pregnancy and childbirth complications, arthritis, sleep apnea (which can be deadly!), body pain, fatigue, surgical complications, and anxiety and depression, and bullying. As school nurses we are also aware of the fact that a student who is gaining weight rapidly may have some social issues at home going on like abuse or neglect, food insecurity, eating disorder, or mental health issues that would also need to be addressed.

The effects of being chronically underweight are also numerous including anemias, malnutrition, vitamin deficiencies, infertility, pregnancy and childbirth complications, surgical complications, growth and development issues – especially in developing children, fatigue, hair loss, poor immune system thus frequent illness, or poor learning. Weight loss or a child who is severely underweight can also be a sign of other health issues that need to be evaluated such as many cancers, HIV, eating disorders or other psychiatric issues, abuse or neglect, poverty and food insecurity, or even drug use.

We absolutely need to continue monitoring the height and weight of students and comparing to previous measurements so we are able to notice if there is an issue happening that should be addressed.

What are some reasons that it would be difficult to change this practice? BMI is still used by the CDC and WHO as the best assessment tool (despite the known fact that it is not actually best practice). It is also tied to Medicaid billing which is tied to school districts receiving funding for health services. I believe that this can be changed though if the powers that be are all on board with managing this in a different more evidence based way.

What should we do instead of using BMI? The BVI (body volume index) is a slightly different method of determining body fat percentage but unfortunately not possible for school nurses to do as it deals with water displacement measurements. You can see more about how this works here.

The University of Alabama reports that studies using the Tri-ponderal Mass Index (TMI) give a more accurate estimation of body fat percentage in children ages 8 years to 17 years. It is a similar formula to BMI but it cubes the height instead of squaring the height. More studies are needed to determine accuracy in varying ethnicities and age groups. University of Alabama Study Journal of Pediatrics article.

RFM (relative fat mass index). This is a method of using tape measure measurements to measure the distance around the waist in comparison to height of the patient. This does not rely on weight at all. This can absolutely be done in a school setting. I would want to do it with one student at a time privately. As with all of the methods, this would also require educating the students about why we are doing it and what it means as we are being slightly invasive (ie: touching their body). Relative Fat Mass is a better measurement of body fat percentage than Body mass Index. To perform this assessment you will measure around the waist then measure your height and divide waist measurement by height measurement. A normal result should be approximately 0.5 or lower.

These results can be incorrect however when it comes to younger children or the elderly therefore not necessarily a good option for elementary school. This is possibly not the best method to use in a school setting due to the aforementioned inconsistencies for younger children and also it may be embarrassing or too invasive for the student.

Skin fold Calipers is another way to measure body fat. This is a tool that you use to gently pinch the skin folds in certain areas of the body and it has a measurement that tells the percentage of fat. You need to take a measurement from a number of different specified areas and then do the equation to figure out body fat percentage. Many of the calipers come with a chart to help you figure out the numbers. Personally I find this to be both invasive to the student and embarrassing for them to have someone “pinching their fat” then talking about it so I wouldn’t want to do this. I know I would be mortified if they wanted to do this to me at school.

BIA (Bioelectrical Impedance Analysis): this is a great method that is performed using a machine that can measure body composition including body fat, lean muscle, and water content of the body. The handheld machine sends an undetectable electrical current through the body which slows based on what kid of tissue it is going through therefore it can easily measure adipose, water, and lean muscle. It can be done using a simple handheld machine, or even many scales come equipped to measure this information. These machines can be purchased for anywhere from $40 – $300+ on Amazon and are very simple to use. According to research, BIA can be accurately used in children 8 years old to 20 years old. This would be an excellent alternative to using BMI in school if the district has the budget to purchase these machines.

Growth chart tracking without the BMI component. Just as they do at the pediatrician, Students heights and weights can be plotted on a growth chart for a visual picture of increased or decreased weight. We always said at the pediatrician that we don’t care what percentile the child is in (because that is BMI) but we are interested in seeing if they are following a curve appropriately. Are they dropping drastically or going up drastically? It gives us a picture of which students need to be addressed for unexpected weight gain or loss. This wont give a picture of fat percentage or overall health but then neither does calculating a BMI. This would be a simple, noninvasive, reasonable method in a school setting for evaluating a student’s weight.

Link to CDC growth charts: https://www.cdc.gov/growthcharts/clinical_charts.htm#Set2

  • Financial responsibilities for making these changes:
  • BIA machine: $40
  • Converting BMI setting in computerized charting system to reflect changes.
  • Training nurses to use BIA machine and documentation of new procedure: 1-2 hours salary
  • Measuring tapes: $10

TL/DR

  • BMI is an outdated tool that does not benefit us as much as previously thought.
  • Alternatives to using BMI may include Relative Fat Mass Index (RFI), skin fold calipers, Bioelectrical Impedance Analysis (BIA), growth chart tracking (without BMI component).
  • Tracking weight/growth in students is important to continue to do in order to prevent or manage weight related health issues and/or monitor for concerns such as abuse, neglect, or mental health issues.
  • This is a difficult issue to reform as CDC and WHO continue to use BMI and it is also linked to Medicaid reimbursement for school funding.

What do you think as a school nurse? Should we fight to change this practice and go with evidence based research or leave it be?

sources used:

https://biography.yourdictionary.com/lambert-adolphe-jacques-quetelet

https://www.npr.org/templates/story/story.php?storyId=106268439#:~:text=The%20BMI%20was%20introduced%20in,the%20government%20in%20allocating%20resources.

https://www.sciencealert.com/new-accurate-rfm-body-fat-measure-could-end-bmi-body-mass-index

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5559202/

https://www.livestrong.com/article/288241-how-to-calculate-your-total-body-volume/

https://www.health.pa.gov/topics/school/Pages/Growth-Screen.aspx

https://www.health.pa.gov/topics/school/Pages/SHARRS.aspx

https://www.uab.edu/news/health/item/8313-better-than-bmi-study-finds-more-accurate-way-to-determine-adolescent-obesity

Published by Emme Mauer M.Ed., BSN, RN, CSN

Mom to two preemies, anxiety sufferer, postpartum depression survivor, and school nurse extraordinaire.

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