BMI (Body Mass Index) has been used for over 100 years in population studies, by doctors, personal trainers, and other health care professionals, when deciding whether their patients are overweight. However, BMI has one important flaw - it does not measure your overall fat or lean tissue (muscle) content.
Body Mass Index, derived from a simple math formula, was devised in the 1830s by Lambert Adolphe Jacques Quetelet (1796-1874), a Belgian astronomer, mathematician, statistician and sociologist. BMI is said to estimate how fat you are by dividing your weight in kilograms by your height in meters squared. However, as mentioned earlier, the measurement is flawed, especially if the person carries a lot of muscle.
Nick Trefethen, Professor of Numerical Analysis at Oxford University's Mathematical Institute, wrote in a letter to The Economist that the BMI formula is flawed and is only a rough guide to helping people judge whether they have a healthy weight.
Body Mass Index, derived from a simple math formula, was devised in the 1830s by Lambert Adolphe Jacques Quetelet (1796-1874), a Belgian astronomer, mathematician, statistician and sociologist. BMI is said to estimate how fat you are by dividing your weight in kilograms by your height in meters squared. However, as mentioned earlier, the measurement is flawed, especially if the person carries a lot of muscle.
Nick Trefethen, Professor of Numerical Analysis at Oxford University's Mathematical Institute, wrote in a letter to The Economist that the BMI formula is flawed and is only a rough guide to helping people judge whether they have a healthy weight.
Trefethen said:
"If all three dimensions of a human being scaled equally as they grew, then a formula of the form weight/height3 would be appropriate. They don't! However, weight/height2 is not realistic either. A better approximation to a complex reality, which is the reform I wish could be adopted, would be weight/height2.5. Certainly if you plot typical weights of people against their heights, the result comes out closer to height2.5 than height2."
The current BMI formula leads to confusion and misinformation, Trefethen believes. The height2 term divides the weight by too much when people are short, and by too little when they are tall. The result is short people being told they are thinner than they really are, while tall people are made to think that they are fatter than they are.
When Quetelet devised the BMI formula, there were no computers, calculators or electronic devices, so he opted for a very simple system. Trefethen does wonder, though, why institutions today on both sides of the Atlantic continue using the same flawed formula.
When Quetelet devised the BMI formula, there were no computers, calculators or electronic devices, so he opted for a very simple system. Trefethen does wonder, though, why institutions today on both sides of the Atlantic continue using the same flawed formula.
Perhaps "nobody wants to rock the boat", Trefethen suggested. Various agencies and institutions have agreed on something, which is comforting.
There are probably other flawed formulae out there. There seems to be an exaggerated respect for measures which depend on mathematics. However, the BMI one probably beats them all, especially as the world population of obese people exceeds one billion.
Trefethen Offers an Alternative Formula to the Current BMI one
Trefethen said "Suppose we changed that exponent from 2.0 to 2.5 and adjusted the constant so that an average-height person did not change in BMI. Suddenly millions of people of height around 5' (five feet tall) would gain a point in their readings, and millions of people of height around 6' (six feet tall) would lose a point."
He proposes a new formula where:
BMI = 1.3*weight(kg)/height(cm)2.5 = 5734*weight(lb)/height(in)2.5.
"In our overweight world, such changes would distress some short people and please some tall people, but the number they'd be using would be closer to the truth and good information must surely be good for health in the long run," Trefethen said.
Quetelet would probably have viewed using the 2.5 exponent favorably, said Alain Goriely, a professor of Mathematical Modelling at Oxford University's Mathematical Institute. Apparently, Quetelet wrote in a Treatise on man and the Development of his Faculties in 1842:
"If man increased equally in all dimensions, his weight at different ages would be as the cube of his height. Now, this is not what we really observe. The increase of weight is slower, except during the first year after birth; then the proportion we have just pointed out is pretty regularly observed.
But after this period, and until near the age of puberty, weight increases nearly as the square of the height. The development of weight again becomes very rapid at puberty, and almost stops after the twenty-fifth year. In general, we do not err much when we assume that during development the squares of the weight at different ages are as the fifth powers of the height; which naturally leads to this conclusion, in supporting the specific gravity constant, that the transverse growth of man is less than the vertical."
But after this period, and until near the age of puberty, weight increases nearly as the square of the height. The development of weight again becomes very rapid at puberty, and almost stops after the twenty-fifth year. In general, we do not err much when we assume that during development the squares of the weight at different ages are as the fifth powers of the height; which naturally leads to this conclusion, in supporting the specific gravity constant, that the transverse growth of man is less than the vertical."
Goriely commented: "So according to Quetelet the scaling is 3 for babies (babies are spheres), 2 for kids (kids grow more like celery sticks, as we know), then 5/2=2.5 for grownups (beefing up so to speak). It seems Quetelet never cared about obesity (not a big issue in the 1840's)."
Many say that waist-to-height ratio is a better measurement than BMI. They say you should keep your waist circumference to less than half your height.
Many say that waist-to-height ratio is a better measurement than BMI. They say you should keep your waist circumference to less than half your height.
No comments:
Post a Comment