Body Mass Index, or BMI, which is calculated as weight over height, is a commonly used method for assessing weight status, but does it actually give a good assessment of how healthy we are?

The World Health Organization developed a classification system for BMI which is as follows:

• Those with a BMI <18kg/m2 are classified as being underweight.
• Normal weight individuals have a BMI between 18.5-24kg/m2.
• Individuals with a BMI between 25-29kg/m2 are classified as being overweight and
• Those with a BMI>30kg/m2 are obese.

While this BMI classification system is a useful measure to use for assessing your weight status, it's important to highlight that it does have some limitations as a measure of overall health. 

Limitations of BMI

1. BMI does not distinguish between weight associated with muscle and weight associated with body fat or water; therefore, it may not be useful in athletic/muscled individuals. For example, consider a 6ft tall female who’s a CrossFit fanatic and weighs 86kg - her BMI is 25.6, which is classed as ‘overweight’. But in reality, this individual is packed full of muscle and leads a very healthy lifestyle!

2. BMI doesn't take into the account the difference in the metabolic activity of muscle versus fat. Muscle is metabolically very active (which means it uses a lot of energy/calories), while fat tissue doesn’t use as much. This means having more muscle causes our BMR (the minimal amount of energy we use to keep the body functioning) to be higher - which basically means we’ll use more energy/calories doing less!

3. It also means we need more energy/calories through food because our muscles will use a lot of this energy. Whereas, when we eat excess calories and don’t have much muscle needing to use it, it has to be stored (probably as fat). So, higher muscle mass is actually associated with more positive health outcomes but isn’t reflected in a BMI!

4. Another limitation of BMI is that it doesn’t capture body fat location information which is an important variable of overall health and disease risk. For example, carrying too much fat around the stomach area is known to be a risk factor for type 2 diabetes, stroke, heart disease and certain cancers. Waist circumference is a good measure for assessing body fat distribution and can complement BMI as a measure of weight status. Waist circumference for a healthy woman should be <80cm and waist circumference for a healthy man should be <94cm. 

Conclusion

To summarize, BMI is a very useful measure for assessing weight status. However, there are many other ways to assess your overall health. These include how you feel, how much energy you have, your hydration, sleep quality, how your clothes fit, how you enjoy staying active, the type of food you eat, your lifestyle behaviors such as smoking and alcohol intake, and so many more!

Health is certainly not one size fits all!

• - BMI
• - Body Mass Index
• - Health
• - weight
• - weightloss