What is BMI?

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Body Mass Index (BMI) is used to determine whether you are in the healthy weight

range for your height and is often used in conjunction with your waist circumference

to assess the risk of the fat you carry around the middle. It is often used as an

inexpensive way and easy-to-perform way of screening for weight category such as

underweight, normal, overweight, obese and extremely obese (see graphic ).

To calculate your BMI, you divide your weight in kilograms by the square of your

height in metres.

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You can calculate your BMI here: 

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https://www.heartfoundation.org.au/your-heart/know-your-risks/healthy-
weight/bmi-calculator

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Considerations to take with BMI

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Although the same formula is used for both adults and children when calculating BMI,
the results need to be interpreted in a slightly different way. Children and teenagers
BMI need to be both age and gender specific due to the amount of body fat changes 
that occur with age and the difference in body fat between girls and boys during these
years. The CDC BMI-for-age growth charts take these changes into account and show
the BMI as a percentile ranking instead (see below).

 

 

How good is BMI an in indicator of body fatness?

 

In general, the correlation between BMI and body fatness is considered to be fairly strong (Flegal & Graubard, 2009; Freedman, Horlick & Berenson, 2013 & Wohlfarht-Veje, et al, 2014). However, even if two people do have the same BMI, they could have very different levels of body fatness.

For example, if man and woman have the same BMI, the woman will tend to have more body fat than the man due to gender differences.
Ethnicity can also impact the accuracy of BMI on body fat. An African American person will tend to have less fat than a Caucasian and someone of Asian ethnicity will tend to have more body fat than a Caucasian.
Someone who is elderly with the same BMI as someone younger will also tend to have more body fat. An athlete will also have less body fat than a non-athlete with the same BMI. This is generally due to the fact athletes usually have more muscle mass than a non-athlete, meaning they often weigh more and their BMI will be higher.

So should we still consider BMI?

According to the BMI scale for adults, someone who is between 25 and 29.9 would be classed as overweight and anyone with a BMI of over 30, will be considered obese. However, this will mean that athletes and anyone who engages in physical activity that has a higher amount of lean muscle mass will have a higher BMI than those who do not exercise and have an increased body fat mass.

For example, someone who is 156cm tall and weighs 60kg will have a BMI of 24.7 according to the BMI calculator (the very top of the “Healthy” range). It is recommended that for someone of this height, the healthy body range is 45-60.6kg.
Now, what if we do skin folds on the individual and it is recorded that they have a body fat percentage of 21% which is considered as in the “Fitness” range? This would indicate the person has more lean muscle mass than the BMI accounted for and is at no health risk!

Therefore, this shows that if you are dealing with someone who exercises regularly, it is important to undertake further testing to assess as to whether the person is actually at health risk or not.

 

Why is BMI not accurate? What tests should be used instead?

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Essentially, BMI can be used as an indicator of high body fatness but only as a screening tool, not a diagnostic of the health of a person. BMI is commonly used to determine overweight and obesity however, it does not measure body composition and therefore, may misclassify an individual. For example, someone may be classed as “overweight” when they are in fact in the normal range, simply based on their BMI (Carlson, 2015).

To determine if a high BMI actually puts a person at a health risk, the practitioner would have to perform further tests. These tests could include taking skin folds, physical activity, family history or evaluations of diet.

Additionally, the practitioner may refer to other methods such as underwater weighing, bioelectrical impedance or DEXA scans. However, these methods are not always readily available and can either be too expensive or need to be conducted by a highly experienced individual to avoid human error in the measurements.

Bioelectrical impedance analysis is considered a popular tool to use for measuring body fat percentage based on its accessibility, ease and portability (Shim, Cross, Norman & Hauer, 2014). However, there has been inconclusive evidence on just how good this measure can be depending on the type of scale used, whether the scales are calibrated correctly and how accurate they can be when compared to other measures of body fat percentage.

So, it is suggested that in order to ensure the technique used is inexpensive whilst still providing quick and accurate results, skin callipers are used to determine skin folds and body fat percentage (Ostojic, 2006 & Zando, 1987). This technique also doesn’t not require a lot or space of equipment and can be performed on a large group of people quickly (Williams & Bale, 1998).

 

 

References

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Carlson, K. A. (2015). The relationship between BMI and body composition in collegiate athletes. Illinois State University.

 

Flegal, K.M. & Graubard, B.I., (2009). Estimates of excess deaths associated with body mass index and other anthropometric variables. American Journal of Clinical Nutritional. 89(4), 1213–1219.

 

Freedman, D.S., Horlick, M. & Berenson, G.S. (2013). A comparison of the Slaughter skinfold-thickness equations and BMI in predicting body fatness and cardiovascular disease risk factor levels in children. American Journal of Clinical Nutritional. 98(6), 1417–24.

 

Ostojic, S.M. (2006). Estimation of body fat in athletes: skinfolds vs bioelectrical impedance. Journal of Sports Medicine & Physical Fitness. 46(3): 442-446.

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him, A., Cross, P., Norman, S. & Hauer, P. (2014). Assessing various body composition measurements as an appropriate tool for estimating body fat in national collegiate athletic association division 1 female collegiate athletes. American Journal of Sports Science and Medicine, 2(1); 1-5.

 

Williams C.A. & Bale, P. (1998). Bias and limits of agreement between hydrodensitometry, bioelectrical impedance and skinfold calipers measures of percentage body fat. European Journal of Applied Physiology &Occupational Physiology. 77(3): 271-277.

 

Wohlfahrt-Veje, C. et al. (2014). Body fat throughout childhood in 2647 healthy Danish children: agreement of BMI, waist circumference, skinfolds with dual X-ray absorptiometry. European Journal of Clinical Nutrition. 68(6), 664–70.

 

Zando, K.A. (1987). The validity and reliability of the Cramer Skyndex Caliper in the estimation of percent body fat. Journal of Athletic Training. 22(1): 23-25.

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