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Improving BMI

 February, 20 2009

The Problem With BMI

These days most everyone is familiar with BMI, Body Mass Index. It is supposed to tell you if you are too thin, too fat, or just right. The problem with BMI is; it is inconsistent over the range of heights people come in. It works great to compare people an inch or two apart, but no one needs that anyway. What does not answer clearly answer is, am I at a good weight for my height.

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The reason the BMI formula is inconsistent is it divides weight by the square of height. As anyone who paid attention in high school can tell you, if you increase the dimensions of a body in proportion, its volume and mass grow at the cube of the change in dimension, not the square. Therefore BMI will be higher for taller people of the same approximate body type. This leads to anomalies like American Football and Basketball players being in the obese range of BMI when one look at them shows that they are not. Consider the case of Basketball player Shaquille O’Neal, an extreme case to be sure, but extreme cases are good tests of systems. Shaq is 7’ 1” tall and weighs 325 lbs. (2.16M – 148 Kg) His BMI is 31.7, well into the obese range, yet he is trim, muscular, and healthy.

The solution to this is simple, take the BMI formula and cube height rather than square it. I am going to name this modified formula MH3, an abbreviation of the actual formula, M/H^3. M = Weight in Kilograms, and H = Height in Meters. The BMI formula is M/H^2. This new formula does have some distortion at the extremes, but is a much better indicator across a much wider range of heights. To return to our example, Shaq has a MH3 of 14.7, right in the center of the athletic range.

Interpreting MH3

People’s two basic questions about weight are; is my weight is healthy, is my weight is attractive?

Healthy isn’t a given weight or waist size, it is a measure of how fit you are. Someone who lifts weights and has a lot of muscle and little fat can be very healthy and yet weight a lot more than another of the same height who weighs less but carries a lot of fat, has very little muscle, and no endurance. A good test of fitness is; can you walk up four or five flights of stairs without becoming winded? An alternate test is to run in place for a minute or two. If you can do either of these without becoming exhausted and return to normal breathing and heart rate quickly, you are at least passably fit. If you can’t, you should consider taking steps to improve your fitness, whatever your MH3 or BMI number.

What most really want to know is; what is an attractive weight? Tastes vary, but we can look at the HM3s people who are generally considered to be attractive and see what theirs are.

Wikipedia has an extensive section on Playboy Magazine, including biographies on every Playmate of the Month, which give their weight and height. They also have an informative entry for Victoria’s Secret including bios of their most famous models. Analyzing a random sample of these girls showed that their MH3s ranged from 9.5 for the slender ones to 13 for the curvy ones, with the Victoria’s Secret fashion models being mostly in the 10s and provided all the 9s, while the Playboy centerfold models were mostly in the 11s and provided all the 12s in the sample. It is interesting to note that the Victoria’s Secret models, who were chosen to appeal to their female customers, were generally thinner than Playboy’s models chosen to appeal to men.

Judging from that sample, the MH3 range for looking great in a bikini is 10 – 13. I found the 9s to be too thin, but that is just my opinion. I also have seen women who score 14 that look great. I can’t emphasize enough that being in good physical shape is more important than weight.

It was harder to find data on attractive men, but I did find a site, Sports Reference, which had data on athletes from many sports. The range of MH3 for male athletes was from 12 to 16 with a few exceptions on the high side such as Linemen in American Football and weight lifters. Olympic Decathlon Gold medalists, who are paragons of male fitness and have average builds, were centered around 13, as were top Football (Soccer) players. A fit 12 is a slender male where a fit 16 is heavily muscled.

If you find the models in Playboy and Victoria’s Secret or the male athletes in mainstream sports attractive, and you wish to look like that, then these numbers give you a guideline as to a target weight range that many find attractive. If you find some other body image attractive, then this information won’t be much help. Whatever your body type, getting into good physical shape will improve your attractiveness as much as losing weight and exercise will make it easier to lose weight and keep it off.

I am including links to my MH3 & MBI calculators, one for metric and one for English measure. These calculators are just tools which provide a bit of information you may find interesting and useful.

Go To English Calculator

Go To Metric Calculator

You should consult a physician, dietitian, or professional trainer before embarking upon any course of diet or exercise that is a major departure from your current practices.


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