Body mass index (BMI) is a commonly used measurement of how over- or underweight a person is, but its usefulness has been questioned. For personal trainers to be able to explain to clients its drawbacks and usefulness, it is necessary to understand what it is, what it tells us and what it does not. Only then can the important practical question of whether we should use it in client or patient assessment be fully answered.
BMI relates bodyweight to height. If someone is heavy and short, then their BMI is high and, if a client is light and tall, their BMI is low. BMI is calculated by dividing a person’s weight by their height squared.1 This simple mathematical relationship was first described in 1835 by the Belgian mathematician, astronomer and statistician Adolphe Quetelet after the link between bodyweight and mortality was first identified and has since been one of the essential ways in which a person’s health is assessed.
What does BMI tell us? Noting that Obesity II and III are what were formerly referred to as grossly obese, the table shown to the right can be used to classify levels of obesity.2
BMI is an important disease risk indicator. Studies have shown that the higher the BMI, the greater the risk of heart disease, heart attack, stroke and diabetes. It is also a predictor of early death and a review in the Journal of the American College of Cardiology concluded that BMI remains the simplest and most widely accepted measure of obesity.3
However, it needs to be noted that there has been some controversy about whether or not BMI is the best measure of obesity and therefore questions have been raised over its usefulness as a risk indicator. David Haslam, GP and chair of the National Obesity Forum, suggested that measuring waist circumference could be more helpful in identifying patients at higher risk of these problems. A study by the Mayo Clinic4 found that obese patients (according to BMI) had no higher risk of mortality. It was suggested that these findings could be explained by the lack of discriminatory power of BMI to differentiate between body fat and lean mass.
It is usually assumed that a high BMI means that a person has a high body fat content, and it is this and inactivity which is so detrimental to health and life expectancy. However, bodyweight is not just about body fat. Muscle, bones and organs are major contributors to bodyweight. Clearly, if someone’s body composition is 35% fat, then 65% of bodyweight is made up of these other components. It is for this reason that the usefulness of BMI as a health indicator has been questioned. Highly trained individuals – such as elite athletes – with a high muscle mass and low body fat will have a higher BMI.5 Under these circumstances, BMI is not a useful measure of health or risk of disease.
So, should those in fitness and health practices use BMI as a measurement of health? Despite some limitations, it is still a useful measure unless the trainer is targeting competitive athletes. A large majority of a personal trainer’s clients, whether they are those who want to get fit, who have specific health goals or patients in exercise referral practice, will not be fit with a high muscle mass and good muscle definition. In fact, as levels of obesity continue to rise worldwide, they are increasingly overweight with a high body fat content. BMI is a simple way of quantifying this. It is useful for setting goals; when I see a client with a BMI above 25, I tell them about the normal healthy range and we discuss what BMI they could aim to achieve over a given time. This becomes an entrance into the all-important conversation about exercise and nutrition, and BMI then becomes a way of monitoring progress that the client or patient can easily understand.
It is crucial that BMI is measured properly. Error can easily creep into height measurement if the client is not positioned correctly. Furthermore, height will change as posture improves, so it should be measured every time BMI is calculated. Similarly, every attempt should be made to measure weight under standard conditions each time, ideally before a training session starts (weight loss occurs during a high-intensity session), in the client’s normal training kit and without footwear.
It is important to remember that BMI is only part of a health and fitness assessment. Resting heart rate,5 blood pressure,6 waist circumference and, where safe to do so, a measurement of aerobic fitness together with BMI build up the picture of person’s fitness level. Debate remains about the value of measuring body composition and I personally don’t do it routinely. Firstly, because skin fold measurements are difficult to perform accurately and because reliable and repeatable results can only be obtained by the bioelectrical impedance method if the conditions are the same each time the measurement is made. However, accurate body composition measurements are useful in highly trained muscular individuals whose BMI is above normal.
Although BMI is of little use in highly trained individuals with a high muscle mass and low body fat percentage, in untrained, inactive overweight people BMI is an important indicator of health and remains an important part of the toolbox for assessing health and fitness for the vast majority of clients.
- To work out BMI: Weight = 80kg, Height = 1.60m – 1.6 x 1.6 = 2.56 – 80/2.56 = 31.25 = BMI.
- Logue, J. et al. (2010). Management of obesity: SIGN Guideline. British Medical Journal, 340: 474-476.
- Letwin, S.E. (2008). Which measures of obesity best predict cardiovascular risk? Journal of the American College of Cardiology, 52: 616-619.
- McArdle, W.D., Katch, F.I . & Katch, V.L. (2000). Essentials of Exercise Physiology, 2nd edition.Baltimore: Lippincott Williams & Wilkins, 520.
- In the absence of heart disease resting pulse rate of itself provides little information. But heart rate measurements for a given intensity of exercise over a given time are a helpful indicator of aerobic fitness progress.
Source: Fitpro Network