Bells Broadcast: Is Body Mass Index (BMI) a good indicator of body mass or obesity?

BMI is a widely used measure in determining obesity and skin fold thickness measures are a tool commonly used to determine percentage of body fat.

Today, Annabel and the teams at Longevity Exercise Physiology Drummoyne, Edgecliff, Marrickville, Bella Vista, Randwick, Pymble, Balmain, Neutral Bay, Coburg – Melbourne, and Barrie, Ontario – Canada will delve into the reasons why health practitioners use BMI as an indicator for body mass and the advantages and disadvantages this can bring for individuals with different goals.

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. To calculate your BMI, divide your weight in kilograms by your height squared in metres. Proponents of BMI suggest that the benefits (little equipment required, no tester expertise needed, it is inexpensive) outweigh the errors.

 

“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.”

 

In general, the correlation between BMI and body fatness is 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 a man and woman have the same BMI, the woman will tend to have more body fat than the man due to gender differences. 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.

 

“Even if two people do have the same BMI, they could have very different levels of body fatness.”

 

Studies have found that BMI is significant in being a poor predictor of body fat, and this measurement has been found to be slightly accurate when predicting health risk. When reviewing health risk, BMI was not the only measurement used to predict mortality risk. Reviewing the studies surrounding this topic where they use BMI as the primary body composition measurement, and measuring weight loss due to training, diet or a combination of both, BMI is shown to be very misleading.

Essentially, BMI can be used as an indicator of high body fatness but only as a screening tool, not a diagnostic of a person’s health. BMI is commonly used to determine being overweight or obese, 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 puts a person at a health risk, the practitioner will have to perform further tests. These tests could include taking skin folds, physical activity, family history or evaluations of diet. Additionally, the practitioner may utilise 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.

 

“To determine if a high BMI puts a person at a health risk, the practitioner will have to perform further tests. These tests could include taking skin folds, physical activity, etc…”

 

Research on body fat percentage has improved in recent years, and in some cases, this measurement may provide a better picture of an individual’s risk of weight-related diseases than BMI. Body fat percentage distinguishes fat from muscle and calculates the percentage of body fat present in the body. While BMI is a rough estimate of body fat, body fat percentage is a more accurate number. Using body fat percentage, we might find that a highly trained athlete is overweight, according to the BMI, but not overfat. Likewise, a person of normal weight on the BMI scales may still be overfat and at risk of weight-related diseases. The downfall of body fat percentage as a measure is that the methods of determining a person’s body fat percentage are not all equal, and most accurate methods are not readily available. 

In conclusion, when using BMI, clinical judgment must be used in applying these criteria to a patient because obesity refers to excess adiposity rather than excess weight, and BMI is a surrogate for adiposity. The results of the numerous studies support the use of body fat percentage over BMI when determining the body composition of the adolescent athletic population.

 

Have you ever been told about your BMI score and are concerned about the result, whether it is too low or too high for your age group?

If this is something you have discussed with your GP or another health professional and want to discuss it more in detail and make a change, give Longevity Exercise Physiology Drummoyne, Edgecliff, Marrickville, Bella Vista, Randwick, Pymble, Balmain, Neutral Bay, Coburg – Melbourne, and Barrie, Ontario – Canada a call on 1300 964 002 today!

 

Written by Annabel Bergman

 

References:

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.

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.