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Monday, December 10, 2012

Does tallness cause heart disease? No, but sex does

Popular beliefs about medical issues are sometimes motivated by a statistical phenomenon known as “spurious relationship”, among other names. Two variables X and Y are influenced by a third variable C, which leads to X and Y being correlated and thus the impression that X and Y are causally associated.

Take a look at the table below, which I blogged about in a previous post (). This table shows that there is a strong unadjusted correlation between height and arterial stiffness, a marker of heart disease. The likelihood that the correlation is due to chance is lower than one tenth of a percentage point (P<.001).



Interestingly, the authors of the study even use height as a control variable to narrow down the “true” causes of arterial stiffness (column with adjusted results), assuming that height did indeed influence arterial stiffness and what they found to be a key predictor of arterial stiffness, 2-hour postprandial glucose.

But there is no convincing evidence that height causes heart disease, with exception of pathological extremes – e.g., acromegaly. Extremes tend to influence statistical results somewhat, leading to conflicting conclusions that end up being disseminated by the popular media (). This is one of the sources of popular beliefs about medical issues.

Another, more important, source are real confounders. And this takes us back to the issue of height being associated with heart disease. In fact, height will typically be significantly associated with heart disease in almost any study that includes men and women and does not control for biological sex.

One of the reasons is that women overall tend to have a significantly lower incident of heart disease than men. The other is that height is significantly lower among women than men, on average, even though there are several women who are taller than the average man.

The table above was from a study including both sexes. Therefore, the strong association between height and arterial stiffness is a “reflection” of the strong association between being male and increased arterial stiffness. If one were to add a variable coded as 0 for male and 1 for female, and use it in a multivariate analysis of predictor of arterial stiffness, together with height, the effect of height would probably “disappear”.

Biological sex is the control variable, the “confounder”, that the authors should have used to narrow down the “true” causes of arterial stiffness (second column in the table). In the absence of biological sex, controlling for height accomplished something similar, but in a “wobbly” way, leaving many readers scratching their heads in confusion.

11 comments:

Anonymous said...

Ha! I thought you were going to say tall guys had more sex and therefore less heart disease.

George Henderson said...

Would this correlation also influence BMI and heart disease stats? Seeing as height is one of the 2 BMI factors?

Ned Kock said...

Hi Anon. I apologize for the “catchy” title for the post. I prefer to use “sex” to refer to a construct that many people call “gender”. The reason is that the latter is a much broader construct.

Ned Kock said...

Hi George. I am sure about BMI, but body fat percentage is another measure that reflects biological sex in a way that may lead to wrong conclusions in the same way that height can.

Ned Kock said...

For a little discussion on body fat percentages in fit men and women, and how they vary, see the post linked below – which also delves into a controversial topic, namely “how lean should one be”.

http://bit.ly/fWdsPC

rosemike089 said...

Did you try a hypothesis X and Y which is X is refers on the tallness that causes heart disease and Y is the sex that causes heart disease?

Can you prove which is the best answer?

ANd regarding on the blog it was nice and very informative also.herbal incense

Ned Kock said...

Yes, but in other contexts. Multivariate adjustment generally takes care of it. This post is mostly to make a point regarding this type of confounding effect.

soy candle michigan said...

Hi, just to share my opinion. sex, if unprotected can be the source of disease such as STDs but tallness doesn't unless, the person reaches abnormal height growth.

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