Made any New Year resolutions this year? What’s your #1 on the list? I bet it is losing weight. In the Netherlands it is the number one resolution for 2012 and I expect in many other countries as well. Research from ING Banking & Insurance indicates that about 80% of the Dutch have made New Year resolutions this year. By equating the fulfillment of a resolution to an economic value ING was even able to calculate that on average the Dutch would give €450 to keep their resolution, resulting in a total economic value of €4.5 billion in the Netherlands alone. Given the economic crisis we are in, a serious amount of money. To stimulate keeping the resolution, maybe the government could introduce a tax in case of failure. This could be a very interesting idea, since not many of us seem to be able to keep our resolutions. To illustrate, 88% will fail to stop smoking, 95% will keep the same weight or even gain weight.

So you want to lose weight, but how much? One way of making that estimate is to use the Body Mass Index. It is a much used number in the medical profession to measure if you’re overweight. It divides your weight in kilograms by the square of your length in meters. So if you’re 1.86 meter tall and weigh 92 kg, a BMI results of 26.6. A BMI score lower than 18.5 kg/m2 implies underweight, above 25 kg/m2 overweight. Interesting to note is that the BMI would put Schwarzenegger (in his Conan years, 1.83 m tall and about 107 kg) in the serious overweight category. I can’t imagine that to be correct. Let’s take an analyst view at this way of assessing overweight.

To calculate the BMI, only height and weight are required. Would that be enough to decide if a person is overweight? It’s important to know that the density of fat is less than that of muscle which in turn has a smaller density than bone. In other words, the less fat you have, and the more your body is made up of muscle and bone (meaning you’re a fit person), the greater the numerator in the BMI formula, and therefore the higher the BMI. So when you start to exercise to lose weight and build up muscle, you will be worse of according to the BMI. This can’t be right, the BMI model must be wrong.

The concept of BMI is the work of a Belgian mathematician, Adolphe Quetelet . He was one of the first to use statistics to draw conclusions about society. He published his "Quetelet Index" in 1832, later known as BMI. Quetelet had no interest in studying overweight when he developed his index. His main interest was to apply probability calculus to human physical characteristics which led him to develop the BMI formula. He found that during normal growth, weight tends to increase in relation to height in meters squared. So for a population as a whole, the BMI is an easy to calculate number that helps study weight issues. There is no rationale or medical evidence that explains why the model is right on indicating whether you are overweight or not. So the BMI was initially developed to measure a societally trend, not as a diagnostic tool to draw conclusions on an individual level. But that is the way in which it is now used, even by doctors. Statistical speaking, that’s rubbish.

So if not the BMI, than what indicator should you use to determine the weight you should lose? You could take a look in the mirror, or ask your partner. But for sure these are not very objective measures. So maybe the best measure is to put on your favorite jeans and feel if they still fit, not to tight. That probably is the best model to use to optimize your weight. Keeping your resolution will also become easier, because buying a complete new wardrobe will probably cost you more than €450.