Whats your BMI?

BMI or body mass index has been used for over 100 years as a measure of how ‘fat’ you are. It is used by doctors up and down the country, it is taught to dieticians and used by many fitness professionals and diet programmes. It is NOT used by me at SavagePT. Here’s why…

Time for a wee history lesson, Lambert Adolphe Jacques Quetelet was a mathematician and statistician who created the Quetelet Index in 1832, now known as the Body Mass Index. This was created to assess the danger of individuals developing heart disease based on their weight, i.e overweight individuals were at higher risk.

This formula takes your weight in kg and divides by your height squared in meters. When this formula was being written averages were taken based on the population and a ‘desirable’ percentage was given for each type of frame i.e small, medium and large men.

The BMI percentages look like this:

>18.5 – 24.9% – Healthy weight

>25-29.9% – Overweight

>30% – Obese

Simple maths, easy to follow. Lets apply it to some fictional women.

• Woman A: is 5ft 3inches and weights 12stone, inactive and has a high body fat percentage. Her BMI is 29.7, she is overweight.

• Woman B: is also 5ft 3inches and weighs 11stone 10lbs. She is very active and lean, and is visibly quite muscular. Her BMI is 29, she is overweight.

We can see here that according to the BMI both women are overweight, yet woman B is clearly fitter, healthier and at less risk of obesity related diseases than woman A. This is because the BMI scale neglected to take into account the difference in density of muscle and fat. 1lb of muscle and 1lb of fat weigh the same, but the muscle will take up less space, therefore you are the same weight, but thinner and at less risk of obesity related diseases.

The other major problem with the scale is that individuals do not fit into neat boxes when it comes to height. The result of this is that the calculations are off, the BMI scale will tell a smaller person that they are ‘thinner’ and a taller person that they are ‘fatter’.

This is why at SavagePT I steer clear of the old BMI scale. It is much more important to focus on how you look and how you feel. If you’re happy, I’m happy.

References:

http://ndt.oxfordjournals.org/content/23/1/47.full

http://www.medicalnewstoday.com/articles/255712.php