BMI; When Should it Be Used?

Being overweight can greatly increase the risk of high blood pressure, muscle, bone and respiratory disorders and chronic disease including type 2 diabetes, heart disease, stroke and some cancers, and reduces life expectancy. Being overweight can also cause social discrimination, low self-esteem, poor body image and depression. But how do we objectively measure if someone is overweight? Most of the time we are told to use BMI.

#1: WHO Body Mass Index Classification

#2: WHO Body Mass Index Classification - Asian Population

#3: Older Adults and BMI

#4: Alternative Measure

#1: WHO Body Mass Index Classification

BMI can indicate the relative amount of body fat on an individual’s frame but does not directly calculate body fat percentage. Therefore, this classification may not be suitable for all population groups. For instance, BMI tends to overestimate body fat in those with a lean body mass e.g. athletes and bodybuilders.

#2: WHO Body Mass Index Classification - Asian Population

The current WHO levels of BMI that correspond to increasing degrees of risk of chronic  morbidity and of mortality were primarily derived for populations of European origin. Australians of Asian origin have a higher proportion of body fat for the same BMI than Caucasians therefore the above classification may underestimate body fat and comorbidity risk in this population. Therefore, there is modified cut off points for BMI categories in Asian populations as seen below.

#3: Older Adults and BMI

For people who are over 65 years old, it has been shown that being overweight is not associated with increased mortality risk. Instead, there is increased mortality risk for those with a BMI <23kg/m2. Older people commonly have a decrease in skeletal muscle mass and strength, due to the decline in the production of muscle tissue. With ageing there is also a change in spinal shape and intervertebral thickness, making it difficult to determine height, and thus BMI inaccurate. It is more effective to monitor and work towards lowering blood pressure and normalising blood lipids rather than reducing weight in people over 65 years old.

#4: Alternative Measure

Due to BMI not always being an accurate measure of a person’s risk of chronic disease; Waist circumference is starting to be used more and provides a better estimate of risk.