BMI is an Index measure just like the stock market index or the consumer price index. The index has been shown to be a reliable way to gauge an appropriate weight for an individual based on their height. There is a long list of studies linking an appropriate BMI to better health outcomes. Likewise, a BMI outside of the normal range is linked to a large number of Australia’s biggest killers: cardiovascular disease, ischaemic heart disease, diabetes, certain types of cancer etc. That alone should be enough for you to stand up and pay attention to your BMI.
To calculate your BMI you take your weight in kilograms and divide it by your height in metres, squared. For example I am 190cm tall and weigh about 85kgs. My BMI is 85kg/1.9m2 = 23.55. A normal BMI is considered to be between 18.5 and 25. 25.1 to 30 is considered clinically overweight and 30+ is considered to be clinically obese.
One of the major criticisms of BMI has been that it is useful to gauge the health of large population groups but falls down when analysing the individual. This is usually followed by an example of an elite rugby player or olympic weightlifter who has a BMI well in excess of 25 but a very healthy, low body fat percentage and a high fitness level. This should not serve to refute BMI as a useful measure but simply shows that the more information we can gather about an individual, the better we can assess their current and future health outcomes. Likewise, BMI, body fat percentage and fitness levels are all good measures but if we also measure blood pressure, cholesterol, waist to hip ratio etc. we get an even better picture of an indivdual’s health. However, BMI still holds it’s own as an important health marker.
The brilliance of BMI is in it’s simplicity. As long as you have a set of scales and a calculator (or a clever brain), you can measure your BMI. You don’t need skin fold callipers and embarrassing skin fold measurements, invasive blood tests or even a sphygmomanometer (blood pressure measuring device). Almost every person in the World can obtain some form of feedback on their appropriate weight and therefore their health.
There has also been some recent research suggesting that a BMI of 26-27 may in fact have a higher correlation with longevity than a BMI of 18.5-25. This research is new and there certainly hasn’t been enough evidence yet to change the recommended guidelines. Some experts suggest that a slightly higher BMI in later life can help protect against the bone and muscle wastage associated with ageing. Essentially, the hypothesis is that a little more “money in the bank” allows you to withdraw for longer. There is good logic in this argument but only a small body of evidence thus far. My recommendation is still to aim for a BMI of 18.5 to 25 and if you prefer being a little heavier, you should weigh this information against other health markers and the potential risks associated with each.
So to all of the BMI-bashers out there. I suggest that we let the science dictate our recommendations, not our opinion of what looks good or feels good. Use your BMI, but use it wisely and as always I encourage a healthy experimentation to establish what is right for you. For example, some people find a decrease in injuries when they lose weight and some find a loss of strength is counterintuitive if they losw weight but it all starts with a measure, followed by a plan, followed by action.
Please share this with all of your health-conscious friends.