Monday, February 11, 2013

The War of the End of the World: The health puzzle posed by its survivors

The War of Canudos took place in Brazil in 1896 and 1897. Canudos was a settlement of several thousand deeply religious Christians, led by a man known as Antonio Conselheiro. They opposed the recent establishment of the Republic of Brazil, particularly the institution of income taxes and civil marriage; the former was considered government-sponsored theft and the latter a sacrilege. The republic had been declared in 1889 following a military coup that deposed Dom Pedro II, an emperor beloved by the common people and under whose rule slavery had recently been abolished.

Canudos was located in the Brazilian sertão, an inhospitable semi-arid region in the northeastern part of the country. The inhabitants of Canudos were the sertanejos. The term jagunço was used to refer to the males, especially the outlaws. Many of the sertanejos lived in semi-starvation, in poor sanitary conditions, and with very limited (if any) access to healthcare. Infant mortality was very high at the time. Those who reached adulthood were typically of small stature, and very thin; not lean, thin – often described as “skin and bones”.

Below is what a typical young jagunço would look like at the time of the War of Canudos. (Some authors differentiate between jagunços and cangaceiros based on small differences in cultural and dress traditions; e.g., the hat in the photo is typical of cangaceiros.) The jagunços tended to be the best fed among the sertanejos. They were also known as cold-blooded killers. The photo is a cropped version of the original one; the grizzly original is at the top of a recent blog post by Juan Pablo Dabove (). The blog post discusses Vargas Llosa’s historical fiction book based on the War of Canudos, the masterpiece titled “The War of the End of the World” ().



Jorge Mario Pedro Vargas Llosa, a Peruvian-Spanish writer and politician, was the recipient of the 2010 Nobel Prize in Literature; “The War of the End of the World” is considered one of his greatest literary achievements. Euclides da Cunha wrote the most famous non-fictional account on the War of Canudos, another masterpiece that has been called “Brazil’s greatest book”, titled “Rebellion in the Backlands” (). The Portughese title is “Os Sertões”. Vargas Llosa’s book is based on da Cunha’s.

Sergio Rezende’s movie, “Guerra de Canudos” (), is a superb dramatization of the War of Canudos. I watched this movie after reading Vargas Llosa’s and da Cunha’s books, and was struck by two things: (a) the outstanding performances, especially by José Wilker, Cláudia Abreu, Marieta Severo, and Paulo Betti; and (b) the striking resemblance of the latter (Betti) to Royce Gracie (), a very nice man whom I interviewed () for my book on compensatory adaptation (), and who is no stranger to Ultimate Fighting Championship and mixed martial arts fans ().

In a nutshell, the War of Canudos went more or less like this. There were four military campaigns against the settlement. The third was a major one, led by one of Brazil’s most accomplished military leaders at the time, Colonel Antônio Moreira César. The jagunços, resorting to guerrilla warfare, fought off the government troops in the first three. The fourth, led by General Arthur Oscar de Andrade Guimarães, saw the jagunços defeated in a war of attrition primarily due to lack of access to food and water, after heavy losses among government troops. At the end, nearly all of the surviving jagunços were executed, by knife – to their absolute horror, and the perverse pleasure of the executioners bent on revenge, as the victims believed that they would not go to heaven if their lives were ended by knife, even against their will.

Ned, what is your point regarding health!?

After going through numerous sources, paper-based and online, academic and non-academic, I am convinced that a significant number of the survivors of the Canudos War lived to their 90s and beyond. This conclusion is based chiefly on comparisons of various dates, especially of interviews with survivors. No single source dedicated to this particular health-related aspect of the War of Canudos seems to exist. There is a video clip that shows some of the survivors (), speaking in Portuguese, with their ages shown in subtitles (“years”, in Portuguese, is “anos”). One of them, a man, is listed as being a supercentenarian.

In modern USA those who live to the age of 90 and beyond are outliers. Less than 2 percent of the population reach the age of 90 (). Most of them are women. My impression is that among the survivors of the War of Canudos, the 90+ percentage was at least 5 times higher; even with access to sanitation and healthcare in modern USA being much better at any age.

If my impression is correct, how can it be explained?

I think that some of the readers of this blog will be tempted to explain the high longevity based on calorie restriction. But the empirical evidence suggests that poor nutrition, in terms of micronutrients and macronutrients, is associated with increased mortality, not the other way around (, , ). Mortality due to poor nutrition is frequently from infectious diseases, in the young and the old. Degenerative diseases are widespread among the overnourished, not the well nourished, and kill mostly at later ages. It is not uncommon for infectious diseases to “mask” as degenerative diseases – e.g., viral diabetes ().

Often people point at hunter-gatherer populations and argue that they are healthy because of their low calorie intake. But mortality from infectious diseases among hunter-gatherers is very high, particularly in children. Others point to the absence of industrial foods engineered for overconsumption, which I think is definitely a factor in terms of degenerative diseases. Some say that a main factor is retention of lean body mass as one ages, referring mostly to muscle tissue, a hypothesis to which the case of the sertanejos poses a problem – what lean body mass!? And, on top of all of their problems, the sertanejos regularly faced long droughts, which may be why they typically had a “dry” look.

Yet others point to low stress. It is reasonable to think that stress is a mediating factor in the development of many modern diseases. Still, the sertanejos living in Canudos have had to endure quite a lot of stress, before and after the War of Canudos. In fact, the depictions of their lives at around the time of the War of Canudos suggest very stressful, miserable lives, prior to the conflict; which in part explains the early success of a religious settlement where life was marginally better.

By the way, the traditional Okinawans have also endured plenty of stress (), and they have had the highest longevity rates in recorded history. Food scarcity has frequently been combined with stress in their case, as with many other long-living groups. Causality is complex here, probably changing direction in different subsets of the data, but I have long suspected that the combination of stress and overnourishment is a particular unnatural one, to which humans are badly maladapted.

A main factor is almost always forgotten: the effective immune systems of those who have been subjected to starvation, poor sanitation, lack of healthcare, and other challenges – especially in childhood – and survived to adulthood. And here some counterintuitive things can happen. For example, someone may be very sickly early in life and barely survive childhood, and then become very resistant to infectious diseases later, thus appearing to be very healthy, to the surprise of relatives and friends who remember “that sickly child”. Immunocompetence is something that the body builds up in response to exposure.

As they say in northeastern Brazil, in characteristic drawl: “Ol’ sihtaneju ain’t die easy”.

Monday, January 28, 2013

How much alcohol is optimal? Maybe less than you think

I have been regularly recommending to users of the software HCE () to include a column in their health data reflecting their alcohol consumption. Why? Because I suspect that alcohol consumption is behind many of what we call the “diseases of affluence”.

A while ago I recall watching an interview with a centenarian, a very lucid woman. When asked about her “secret” to live a long life, she said that she added a little bit of whiskey to her coffee every morning. It was something like a tablespoon of whiskey, or about 15 g, which amounted to approximately 6 g of ethanol every single day.

Well, she might have been drinking very close to the optimal amount of alcohol per day for the average person, if the study reviewed in this post is correct.

Studies of the effect of alcohol consumption on health generally show results in terms of averages within fixed ranges of consumption. For example, they will show average mortality risks for people consuming 1, 2, 3 etc. drinks per day. These studies suggest that there is a J-curve relationship between alcohol consumption and health (). That is, drinking a little is better than not drinking; and drinking a lot is worse than drinking a little.

However, using “rough” ranges of 1, 2, 3 etc. drinks per day prevents those studies from getting to a more fine-grained picture of the beneficial effects of alcohol consumption.

Contrary to popular belief, the positive health effects of moderate alcohol consumption have little, if anything, to do with polyphenols such as resveratrol. Resveratrol, once believed to be the fountain of youth, is found in the skin of red grapes.

It is in fact the alcohol content that has positive effects, apparently reducing the incidence of coronary heart disease, diabetes, hypertension, congestive heart failure, stroke, dementia, Raynaud’s phenomenon, and all-cause mortality. Raynaud's phenomenon is associated with poor circulation in the extremities (e.g., toes, fingers), which in some cases can progress to gangrene.

In most studies of the effects of alcohol consumption on health, the J-curves emerge from visual inspection of the plots of averages across ranges of consumption. Rarely you find studies where nonlinear relationships are “discovered” by software tools such as WarpPLS (), with effects being adjusted according.

You do find, however, some studies that fit reasonably justified functions to the data. Di Castelnuovo and colleagues’ study, published in JAMA Internal Medicine in 2006 (), is probably the most widely cited among these studies. This study is a meta-analysis; i.e., a study that builds on various other empirical studies.

I think that the journal in which this study appeared was formerly known as Archives of Internal Medicine, a fairly selective and prestigious journal, even though this did not seem to be reflected in its Wikipedia article at the time of this writing ().

What Di Castelnuovo and colleagues found is interesting. They fitted a bunch of nonlinear functions to the data, all with J-curve shapes. The results suggest a lot of variation in the maximum amount one can drink before mortality becomes higher than not drinking at all; that maximum amount ranges from about 4 to 6 drinks per day.

But there is little variation in one respect. The optimal amount of alcohol is somewhere around 5 and 7 g/d, which translates into about the following every day: half a can of beer, half a glass of wine, or half a “shot” of spirit. This is clearly a common trait of all of the nonlinear functions that they generated. This is illustrated in the figure below, from the article.



As you can seen from the curves above, a little bit of alcohol every day seems to have an acute effect on mortality reduction. And it seems that taking little doses every day is much better than taking the equivalent dose over a larger period of time; for instance, the equivalent per week, taken once a week. This is suggested by other studies as well ().

The curves above do not clearly reflect a couple of problems with alcohol consumption. One is that alcohol seems to be treated by the body as a toxin, which causes some harm and some good at the same time, the good being often ascribed to hormesis (). Someone who is more sensitive to alcohol’s harmful effects, on the liver for example, may not benefit as much from its positive effects.

The curves are averages that pass through points, after which the points are forgotten; even though they are real people.

The other problem with alcohol is that most people who are introduced to it in highly urbanized areas (where most people live) tend to drink it because of its mood-altering effects. This leads to a major danger of addiction and abuse. And drinking a lot of alcohol is much worse than not drinking at all.

Interestingly, in traditional Mediterranean Cultures where wine is consumed regularly, people tend to generally frown upon drunkenness ().

Monday, January 21, 2013

Happy anniversary Rubbish Diet! Five years old today.

Distracted by the snow,  today's school closure and the prospect of sledging with the 8 year old, it almost passed me by that today is the fifth anniversary of when I launched The Rubbish Diet blog. 

So forgive my momentary lapse of celebration as well as my now very loud 'whoop-whoop'!

5 years!  Blimmin' 'eck!

Back then, I didn't think I'd still be blogging about rubbish even a few months later, let alone five years on.  And as for the amount of rubbish I've avoided and the money saved, that's worth celebrating in itself. 

So, thank you to ALL who have inspired me and to everyone who has taken steps to make a positive difference to help reduce the amount of waste our society generates.

I hope this is going to be a very exciting year in the world of bin-slimming and as I turned over the calendar this year, I realised that for the first year ever The Rubbish Diet is actually now bigger than this blog, developing a life of its own out there in the big wide world, like a small child that you wave off to school for the very first time.

Following hot on the heels of the first Rubbish Diet Shropshire trial, which took place last year,  it feels appropriate that today, of all days, Wiltshire Wildlife Trust launches its own Rubbish Diet challenge, engaging 15 households in the process of slimming their bins over an eight week period, following the very same method that I took back in 2008.

I can't wait to find out how they all get on and I look forward to heading down to Wiltshire to help celebrate during their finale, a Zero Waste Week, which takes place in March.

And March will indeed be a hectic month in more ways than one.  Not only is there a fair chance that you could see 'Yours Truly' making an appearance on The One Show, but I am also looking forward to representing the Zero Waste Alliance UK at an International Zero Waste Symposium, which takes place in San Francisco.  I am, as you can probably guess, both nervous and excited!

And that's just March.  Gawd knows what will happen during the rest of the year.... well, I have an inkling, but for now will just cross my fingers and wait, and burst into song only when necessary.

So, thank you all again for your inspiration, support and engagement.  It's you that keeps me going as well as that HUGE pile of unnecessary waste!

I'm now looking forward to the next five years and encouraging even more people to join in and help tackle it together. 

So good luck to everyone who takes up the challenge this year.  I hope it goes well for all involved.  It's really great to have even more people on board and I believe we can make a huge difference.

THANK YOU!!!

Now, I'm off to celebrate by dodging some snowballs and enjoying that sledge before the opportunity disappears...  ah, such imagery!

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For more information about the Wiltshire Wildlife Trust Rubbish Diet challenge, follow their project blog, which will feature all their latest updates. Also Wiltshire blogger, Jen, is joining the WWT challenge and you can follow her progress at Make Do and Mend Year. More news on Rubbish Diet Shropshire's project will be announced soon.

Sunday, January 13, 2013

We've caught the swapping bug! Sustainable Bury launches its first Swap Shop

L-R: Pippa, Fiona & me: Some of the founding members of Sustainable Bury, at our very first Swap Shop. Photo credit: Sandy Jackson. 



I've been to several 'Swap shops' or 'Give & Take' days organised by Transition Town groups around Suffolk. the last one being being held by Sustainable Bungay in September.  I must confess that when I left that event, I was both quite jealous that Bury St Edmunds didn't have such an occasion to get the good town-folk out swapping, but also very much inspired to do something about it.

After all, in Bungay, what I witnessed was not just a great way of motivating people to declutter their unwanted items and let someone else take them home for free, but there was also a fantastic vibrant community spirit.  

Now left to my own devices, that inspiration might have remained a pipe dream due to competing priorities.  However, my visit to Bungay coincided with the gathering of like-minded folk in Bury St Edmunds - mainly in a pub - to organise a new Transition Town community group called Sustainable Bury.  As we were planning our events, I mentioned how much I'd love to bring the Swap Shop idea under our umbrella.  All it took was someone to suggest enthusiastically that we should run one just after Christmas and Bob's your uncle, the first community event for Sustainable Bury was born!  Such is the power of team-work.

The posters went up, leaflets were distributed and we received some great mentions on Radio Suffolk and excellent coverage in the Bury Free Press, but it was still quite nerve-wracking yesterday morning as we got ready for our 'big event'.  Would the good folk of Bury St Edmunds turn up?

Setting out the tables at the St John's Centre.

Thankfully, yes they did!

Including Kim, one of Bury St Edmunds' entertainment organisers.  I never did find out whether she took those funky specs home, but I really hope she did!


The idea was that people could drop off items that they no longer wanted and take anything away that they wished, for free!  If they didn't have anything to bring, they could simply leave a donation at their discretion.  Any items that were left, we simply pledged to donate them to the charity shops, dotted along St John's Street.

As with any Reuse event, the swap shop featured a real eclectic range of stuff.  I loved these little Saki cups.



And this unopened Body Shop gift pack was certainly a great find for its new owner.



As was this Past Times tea cosy. proclaiming "Where there's tea there's hope"!  That might become my new mantra.







I'm really pleased that for our first event we had around 30 people, perhaps even more.  To be honest, we were so busy listening to all the great feedback from the visitors who dropped by that it was really tricky to keep count.   In fact, the feedback that we received was so encouraging, we have decided to bring forward plans for our next Swap Shop, which we will now run in April.

I'm now looking forward to the Swap Shop becoming a regular event that supports the Reuse infrastructure in Suffolk, in a way that is truly fun and brings the community together.   For those who struggle to get stuff into town, it would also be wonderful for satellite events to be hosted by community centres and schools in the the surrounding estates and villages and I would certainly be happy to help get those rolling.

However, Sustainable Bury is not just about encouraging reuse and reducing waste, there are other great plans on the cards too.  There is a real wealth of ideas coming through, so if you have time, do check out this great piece by the Bury Free Press from yesterday's event, which also features a video of our members, sharing some thoughts about what they'd love the group to achieve.

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Sustainable Bury really is in its infancy, so while we wait for its website/blog to be created, more information can be found at the Facebook page or by following @SustainableBury on Twitter.   If you are local to Bury St Edmunds and would like to get involved,  please do get in touch via Facebook/Twitter, email me or alternatively leave a note below and I will be pleased to pass on your details to the team.  The next planning meeting is this Tuesday, 15 January, 7.30pm at the Fox Inn, in Eastgate Street.  All are welcome.

More information about Transition Towns can be found at www.transitionnetwork.org.

Thursday, January 10, 2013

Don't let food waste be next year's shock headline - my two penneth


Well Happy New Year everyone.  I can see it's kicked off in fine style!

Having just got used to my back-to-school routine and having my first day at the laptop, this morning I found my Twitter stream awash with the topic of food waste and the media's shock that statistics published by the Institution of Mechanical Engineers reveal that 30%-50% of the world's food never gets eaten.

These are huge figures to contemplate, whichever way you look at it, with growers, manufacturers, retailers and consumers all playing a key part or being affected.

But this isn't the first time that the headlines have been full to the brim with the horror of food waste.  Here's a random sample, taking us back to 2007, when I first became aware of the issue.

2012 - Unilever calls on companies to tackle food waste mountain

2011 - UK families are wasting less food but are told to do more

2010 - UK Restaurants waste 600,000 tons of food a year

2009 - Elimination of food waste could lift 1billion out of hunger, say campaigners

2008 - Brown urges Britons to cut food waste

2007 - Call to use leftovers and food waste

Despite making inroads into the waste mountain, I am very much concerned that each spell of outrage even when followed by best intentions, dwindles back towards stunted action, as we move on - for whatever reason - to other competing issues in our lives and organisations.

However, with austerity biting at politics, retailers as well as the householder, 2013 could very well be the year that we look back on as being a watershed period in all our efforts to tackle this huge issue.  There is certainly enough expertise around us to help, with WRAP offering a bank of resources for the hospitality and retail sectors as well as its Love Food Hate Waste website for consumers.  And saving money is a bloody good carrot to wave in front of our noses.

But even so, it can be very easy to slip into old routines when hit by everyday pressures, and that, especially for the householder, is one of the greatest problems in tackling food waste.

If that sounds like you, there are simple steps to help you grab the problem by the balls, and which will help to ensure that you're not contributing to next year's food waste horror story.


1. Make yourself accountable.   Tell your family, friends, Facebook pals, Tweeters or blog readers that you are determined to tackle food waste.  If you think it'll help to keep you motivated, get them to sponsor you for a good cause.

2. Set a deadline to help give you a focus.   For example, trying to reduce it over the next couple of months.  It takes a while for new habits to form, so commit to going the distance rather than the novelty of a short sprint.

3. Start a food waste diary, to identify what type of food goes to waste and why.  This will help uncover regular waste habits.  And remember, throwing that mouldy fruit into the compost bin is still a waste, even if you think you're going to get some 'free' compost out of it.

4. Use a separate bin to monitor your food waste.   Then rejoice when you see it reduce.

5. Stop buying things that you regularly throw away.  If you reduce the problem at source, you'll have pounds in your pocket and less food in the landfill heap.  Check out my ornamental melons story as a very good example.  If you have regular leftovers when buying takeaways, just buy less next time.

6. Freeze it!  Food labels now advise that you can freeze certain products up until the use-by date.

7. Visiting www.lovefoodhatewaste.com is a must!  You'll get top advice on portions, recipes for leftovers and food storage.


Of course, if you've got fire in your belly and want to do more to either understand or tackle the wider issues of food waste, there are various avenues available, including

  • Join food waste campaigner Tristram Stuart and his Feeding the 5000 campaign to help bring an end to food waste in the retail and supply chain.
  • Follow Kerry McCarthy MP and her work commitment to drive change through parliament.  See last year's blogpost about her Food Waste Bill.  And of course, you can always lobby your local MP to get behind the issue.
  • Contribute to the WI Great Food Debate, the NFWI's series of debates about Food Security, which reaches at the very heart of the organisation.

And last but not least, if you live in Wiltshire, Shropshire or Suffolk and want to get involved in slimming your waste-line either on your own or as part of a community project, keep your eyes peeled for some fantastic Rubbish Diet initiatives coming up.

For instance Wiltshire Wildlife Trust is kicking off the new year with a county-wide Rubbish Diet, which will mentor householders through slimming their bins over a period of 8 weeks. If we get through to the next round of the Nesta Waste Reduction Challenge competition, Rubbish Diet Shropshire will be hot on their heels in the springtime, with a choir, school and football club all joining in with the bin slimming antics.  And in Suffolk, my own neck of the woods, there'll be a lot of fun and games involving local well-known personalities who will be leading the way too.   Tackling waste will be a key topic for each of these projects.

Even if you're not a resident of any of these counties, hopefully you'll be inspired to take action from a distance and come together in National Zero Waste Week, which will be held in September, where there'll be another chance to work together to focus on reducing the amount of food waste, and other superfluous stuff that ends up in our bins.

Hopefully, this should all make for a great 2013, especially if we can pull together to ensure there are no shock food waste headlines this time next year.

And on that note... I'm off to have another word with my 8 year old, who's busy listening to BBC Newsround about this very topic.  Time to remind him about his bad habit of abandoning bread crusts!



Monday, December 24, 2012

The 2012 Atherosclerosis egg study: More smoking is associated with more plaque, unless you eat more eggs

I blogged before about the study by David Spence and colleagues, published online in July 2012 in the journal Atherosclerosis (). This study attracted a lot of media attention (e.g., ). The article is titled: “Egg yolk consumption and carotid plaque”. The study argues that “regular consumption of egg yolk should be avoided by persons at risk of cardiovascular disease”. It hints at egg yolks being unhealthy in general, possibly even more so than cigarettes.

I used the numbers in Table 2 of the article (only 5 rows of data, one per quintile; i.e., N=5) to conduct a type of analysis that is rarely if ever conducted in health studies – a moderating effects analysis. A previous blog post summarizes the results of one such analysis using WarpPLS (). It looked into the effect of the number of eggs consumed per week on the association between blood LDL cholesterol and plaque (carotid plaque). The conclusion, which is admittedly tentative due to the small sample (N=5), was that plaque decreased as LDL cholesterol increased with consumption of 2.3 eggs per week or more ().

Recently I ran an analysis on the moderating effect of number of eggs consumed per week on the association between cumulative smoking (measured in “pack years”) and plaque. As it turns out, if you fit a 3D surface to the five data points that you get for these three variables from Table 2 of the article, you end up with a relatively smooth surface. Below is a 3D plot of the 5 data points, followed by a best-fitting 3D surface (developed using an experimental algorithm).





Based on this best-fitting surface you could then generate a contour graph, shown below. The “lines” are called “isolines”. Each isoline refers to plaque values that are constant for a set of eggs per week and cumulative smoking combinations. Next to the isolines are the corresponding plaque values. The first impression is indeed that both egg consumption and smoking are causing plaque buildup, as plaque clearly increases as one moves toward the top-right corner of the graph.



But focus your attention on each individual isoline, one at a time. It is clear that plaque remains constant for increases in cumulative smoking, as long as egg consumption increases. Take for example the isoline that refers to 120 mm2 of plaque area. An increase in cumulative smoking from about 14.5 to 16 pack years leads to no increase in plaque if egg consumption goes up from about 2 to 2.3 eggs per week.

These within-isoline trends, which are fairly stable across isolines (they are all slanted to the right), clearly contradict the idea that eggs cause plaque buildup. So, why does plaque buildup seem to clearly increase with egg consumption? Here is a good reason: egg consumption is very strongly correlated with age, and plaque increases with age. The correlation is a whopping 0.916. And I am not talking about cumulative egg consumption, which the authors also measure, through a variable called “egg-yolk years”. No, I am talking about eggs per week. In this dataset, older folks were eating more eggs, period.

The correlation between plaque and age is even higher: 0.977. Given this, it makes sense to look at individual isolines. This would be analogous to what biostatisticians often call “adjusting for age”, or analyzing the effect of egg consumption on plaque buildup “keeping age constant”. A different technique is to “control for age”; this technique would be preferable had the correlations been lower (say, lower than 0.7), as collinearity levels might have been below acceptable thresholds.

The underlying logic of the “keeping age constant” technique is fairly sound in the face of such a high correlation, which would make “controlling for age” very difficult due to collinearity. When we “keep age constant”, the results point at egg consumption being protective among smokers.

But diehard fans of the idea that eggs are unhealthy could explain the results differently. Maybe egg consumption causes plaque to go up, but smoking has a protective effect. Again taking the isoline that refers to 120 mm2 of plaque area, these diehard fans could say that an increase in egg consumption from 2 to 2.3 eggs per week leads to no increase in plaque if cumulative smoking goes up from about 14.5 to 16 pack years.

Not too long ago I also blogged about a medical case study of a man who ate approximately 25 eggs (20 to 30) per day for over 15 years (probably well over), was almost 90 years old (88) when the case was published in the prestigious The New England Journal of Medicine, and was in surprisingly good health (). This man was not a smoker.

Perhaps if this man smoked 25 cigarettes per day, and ate no eggs, he would be in even better health eh!?

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.

 
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