Some Fun Interpreting BMI and the Curious Milestones

Sep 16, 2011

The holy grail of weightloss surgery is reaching goal weight. It is the veritable chalise from which the half-brother of James enjoyed his last swig. The chalise is far shinier if said goal weight is also within the BMI [mass (m) in kilograms divided by the square of height (h) in metres] range that is regarded as normal, namely 20-to-24.9 kilograms per square metre. If you had to translate this into physics terms, it would be the amount of mass your body is applying to a square area (A) the length of whose sides is your height (h). 

The area of this square whose sides are equal to your height is 

A = h x h 

Everyone's body mass index is given by 

BMI = m/(h x h)

       = m/A

The gravitational force (F), measured in Newtons, of planet Earth on any given body with mass (m) is the product of the mass (m) of that body and the gravitational constant (g), i.e.

F = mg 

[g is a constant acceleration of 9.81 m/s-sq whose absolute size is not really relevant to the outcome of the discussion here but I'll include it just to preserve integrity.] 

If I divided both sides of this equation by the area of a square (A) the length of whose sides are my height, I would have 

F/A = mg/A 

I can re-write this as 

F/A = (m/A) x g

      = 9.81 x (m/A) ... [remember we said g = 9.81] 

Earlier, we saw that BMI = m/A so, in reality

F/A = 9.81 x BMI

Now F/A is a well-known physics formula for pressure (P) measured in Pascals (Pa), or, Newtons per square meter. Many of us can relate to the concept of pressure as pounds per square inch, PSI in your car tires.

Since

P = F/A

then

P = 9.81 x BMI

We can re-write this as

BMI = (1/9.81) x P...roughly 1/10 th of P.

What does all this mumbo-jumbo translate into? Well, quite simply, for each and every one of us, our body mass index is nothing other than about one-tenth of the amount of pressure that our body is exerting on the surface of the earth if our mass was spread out evenly over an area that is defined by our heights. If you doubt me, get your kid to show his physics teacher this write-up.

In a weird way, BMI is almost a measure of fairness. It is as if each of us is allocated a quota of 200 - 249 Pa of pressure to apply to the surface of the earth. If we take one-tenth of that to make the math easy and rename it the body mass index, we get to 20-to-24.9 as the yardstick of fairness. However, if you're muscular, well done, you may apply more pressure. If you're fatter, you're putting too much pressure on earth. If you're taller, it's not your fault so you are allocated more area therefore you can carry more weight, but your overall allocation is the same as everyone else's. Dwarf bodybuilders have it the best.

In the grand scheme of things, whatever difference does it make if my BMI is 45 or 70? Either way I'm sickly obese and need surgery to help me lose weight. I'm sure someone somewhere has done a comparison of this strange pressure measurement that we all know fondly as BMI to various health issues and correlated it to all sorts of maladies like diabetes, hypertension, etc. In my humble opinion, BMI is merely a guideline, a point of departure which should be used in conjunction with other measures of health, like body-fat content, exercising pulse rate and resting blood pressure to determine fitness. 

Someone my height of 1.78 m would have to weigh between 63.4 kg and 78.9 kg (139.4 lb - 173.6 lb) to be considered of normal weight by the current BMI yardstick. To put that into personal historical perspective, I weighed 64 kg at 18 in my final year of high school when I couldn't have been much shorter than I am now. Back then, I was Sportsman Of The Year, had decent body definition with very little fat, easily ran 6 km to warm up before 2 hrs of tennis, opened the bowling for my cricket team and could bench more than my body weight. The point is, I was neither scrawny nor unhealthy. In subsequent years as I became a man, I carried 10 more kilograms without appearing particularly overweight, probably looking more muscular if anything. In fact, by 1991 when I was 27 and probably the fittest I've ever been, I weighed 85 kg (187 lb) when I lost an epic 90-minute encounter at the US Squash Nationals in Portland, OR. At the start of the game, my BMI was 26.8 yet nobody in his right mind would have labelled me overweight. As a matter of fact, I shed 3.6 kg (8 lb) of water weight in that game which reduced my post-match BMI to 25.7, just a notch into the 'overweight' category. As you can see, BMI is not a stone-etched guideline, if you'll excuse another metaphor from Hebrew mythology. Nor is it the last word about fitness. A healthy and fit bodybuilder would have a BMI that is way off the charts, whilst a wheezing chain-smoker could have a BMI in the normal range.

Another popular WLS milestone, the loss of 100 lbs, is particularly meaningless given how arbitrary it is. It is probably the Shroud of Turin of WLS, dubious at best. 100 lb is the same as 45.5 kg so what's so special about that? If, like me, you prefer metric units of measure, there's nothing especially magical about the number 45.5. Further, if you were 500 lb at surgery, 100 lb lost is just scratching the surface compared to someone who started at say, 295 lb. At the end of the day, 100 lb lost is only worth jumping about like a Wheel-of-Fortune winner over if you swear allegiance to lbs as your unit of measure. Far more meaningful is the percentage of the mass still needed to be lost to get to goal. For the record, I was 351 lb pre-op and just weighed in at 250 lb at the 37-week mark. I'll be watching my mailbox for my 100 lb prize. However, before I start patting myself on the back, the fact remains that 8 months later, after all this starvation and hypotension and constipation and stalls, I'm only 2/3 of the way to goal. The sobering reality is that my work is still cut out for me.

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