Body Mass Index (BMI): mine, yours, and Trump’s

The ‘investigative press’ is quite concerned about President Trump’s height.  If he is 6 foot 3 and 239 pounds, he is overweight but not obese, if he is 6 foot 2 he is obese.  All this is a matter of definition by a single number — the body mass index (BMI).

So let’s all calm down and find out what ours actually is.  There is a website which will do it for you. Here’s the link — https://bmicalculator.mes.fm/?gclid=CM66rIG2tc0CFYQ2gQodOdINEg — have at it. You can use pounds feet and inches as well as kilograms and meters.

The current definition of obese is a BMI over 30, overweight between 25 and 30, and normal weight under 25.

Who cares?  Well, you should if BMI’s correlate with mortality and they do.

A great paper 5 years ago by Katherine Flegal analyzed nearly 3 million people with 270, 000 deaths reported in a variety of studies — https://jamanetwork.com/journals/jama/fullarticle/1555137.

The problem is that the lowest mortality didn’t occur in those with normal weight (BMI < 25) but was lowest in the overweight group — not by much (6%), and second lowest in the mildly obese (BMI 30 – 35), over 35 it was 20% higher.

Naturally this did not sit well people who'd staked their research careers on telling people to lose weight.  There is a truly hilarious article describing a meeting at Harvard discussing the paper.  Here's a link https://www.nature.com/news/the-big-fat-truth-1.13039.   It's worth reading in its entirety, particularly for a graph it contains.

One study by a Harvard guy removed 900,000 people from the study leading to the following great comment — “It's hard to argue with data,” says Robert Eckel, an endocrinologist at University of Colorado in Denver. “We're scientists. We pay attention to data, we don't try to un-explain them.”

The Nature paper contains a terrific graph from the following paper — Source: Childers, D.K. & Allison, D.B. Int. J. obesity 34, 1231–1238 (2010).

Look at it carefully.  Mortality vs. BMI is plotted in several curves one for people between 20 – 30, one for 30 – 40, etc. etc.  Under 50 the best BMI to have in terms of mortality is under 25, but over 50 it rises, so that at 70 the low point is around 27 (in the overweight range) and not far from Trump at 6' 3" (29.9) or even at 6' 2" (30.7).

In a way this data fits with the fact that for a long time Americans were getting fatter and fatter, yet living longer and longer.  For details see — https://luysii.wordpress.com/2009/08/20/something-is-wrong-with-the-model/.

Why should the best BMI for you to have rise after age 50?  I've not seen this explanation anywhere else.

The BMI is far from perfect, but to calculate it all you need are two simple measurements that anyone can make — height and weight. It doesn't rely on what people remember.  However the calculation is not a simple ratio of weight divided by height but weight divided by height squared.

People lose height as they age, so the BMI is quite sensitive to it (remember the denominator has height squared).  Well as a high school basketball player my height was 6′ 1”+, now (at age 75) its 6’0″ (God knows what it is several years later). So even with constant weight my BMI goes up.

Well it’s time to do the calculation to see what a fairly common shrinkage from 73.5 inches to 72 would to to the BMI (at a constant weight). Surprisingly it is not trivial — (72/73.5) * (72/73.5) = .9596. So the divisor is 4% less meaning the BMI is 4% more, which is almost exactly what the low point on the curve does with each passing decade after 50 ! ! ! This might even be an original observation, and it would explain a lot.

As long as I’m on the subject of the ‘investigative press’  here is how they glossed over Hillary’s fainting spell during the presidential campaign, calling it a stumble.  Remarkable discipline that they all used the same word.  So take their worries about Trump’s weight with a grain of salt.

“Like the Michigan poll I started out with, most of the latest campaign surveys were carried out during last weekend and earlier this week, when the news was dominated by Clinton’s stumble outside Ground Zero, which prompted the campaign to reveal that she had pneumonia.” John Cassiday New Yorker

“For Orient—and the many media organizations that have recently been circulating her work—Clinton’s stumble looked like proof that they were right.” Wired — https://www.wired.com/2016/09/rogue-doctors-spreading-right-wing-rumors-hillarys-health/?mbid=nl_92316_p7&CNDID=24850134

The Boston Globe had a similar echo of the meme that all Hillary did when leaving the 9/11 ceremony was stumble.

As Richard Pryor famously said when his wife caught him with another woman. He denies anything is going on, and asks his wife, “Who you gonna believe, me or your lying eyes?”

See for yourself — https://www.youtube.com/watch?v=StyKyAFOGLY

 

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Comments

  • neil  On January 17, 2018 at 10:48 pm

    The data suggesting that having a BMI under 25 may be associated with increased mortality vs 25-30 is somewhat flawed. Whilst the reference (s) aren’t at hand, the standard explanation is that the <25 group is tarnished with many ill people who have low bodyweight. The classic is cahexia associated with malignancy. I've heard this debunked at several international cardiology conferences. rds

  • luysii  On January 17, 2018 at 11:00 pm

    Certainly true, but Flegal’s study had nearly 3,000,000 people. Also the graph in the Nature article is worth a look, particularly the curves for the 60 and 70 year olds.
    Another point to keep in mind, is how flat the curves are near the minimum, so we’re only talking about a few % difference in mortality.

    Thanks for commenting. Also the study I cited is 5 years old. Hopefully there is newer and larger data now available.

  • John Wayne  On January 18, 2018 at 11:28 am

    I think your hypothesis that slowly changing height could be the source of the apparent drift of health outcomes vs BMI as people age. This is probably pretty hard to test with existing data; it has been my experience that getting measured at the doctor’s office is very imprecise. Furthermore, one may have to be take into account what time of day you are seen (I think people shrink a bit as the day goes on, but that is something I heard around the water cooler).

  • luysii  On January 18, 2018 at 11:39 am

    You are right about shrinking during the day. As we stand the pressure on our intervertebral discs (25% of spine height) increases, forcing water out and decreasing disc height. It certainly would be easy to find out by how much we shrink by evening (and I don’t know if anyone has the answer). The same process of loss of intervertebral disc height is thought to occur as we age.

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