Tuesday, September 6, 2011

Chocolate, heart disease, and selective skepticism

I've recently developed an obsession with 88% dark chocolate that I can find on sale at Sprouts.  It has about as much fiber as it does sugar, and it's bitterness verges on espresso.  I've prescribed myself to a near-nightly dose of it after dinner.  And one night, while I was medicating, I came across a new study and some responses to the study that got me thinking about how easy it is to be critical about data that do not conform to our preconceived notions.

A meta-analysis was recently published on the effects of chocolate consumption and cardiovascular disease (the full text article can be found here).  The analysis only included 7 studies: 6 cohort studies, one cross-sectional study, and no randomized controlled trials.  Needless to say, I was surprised by the relative strength and consistency of the findings.

Source:  Buitrago-Lopez et. al British Medical Journal 2011.

The various levels of chocolate intake were associated with a 37% decrease in the risk of any cardiovascular disease, and a 29% decrease in the risk of stroke.  This is indicated in the figure by the diamond-shaped confidence intervals, which represent the risk of the combined studies, stratified by cardiovascular outcome, hovering around estimated relative risks.  There was no association between chocolate intake and heart failure, as you can see by the respective diamond crossing the relative risk axis of 1.  Unfortunately, none of these studies measured CVD mortality or total mortality, so we don't know if this reduced CVD risk was associated with a longer lifespan.

If you've been following my previous posts, then hopefully you've taken note that all of these studies are observational rather than randomized and interventional.  This makes this type of data prone to confounding variables (although many of these studies corrected for numerous confounders) and bias.  The bias is especially worrisome given that chocolate is somewhat of a luxury that probably associates with discerning palates, and because of all the marketing buzz regarding cocoa's abundance of antioxidants.

Even the authors are dubious of their findings since chocolate has more than just antioxidant-rich cocoa.  "The high energy density of commercially available chocolate (about 500 kcal/100 g) means excessive consumption will probably induce weight gain, a risk factor for hypertension, dyslipidaemia, diabetes, and cardiometabolic disorders in general."  And they didn't even mention sugar, which I'm more concerned about than calories.  And others are quick to point out that the association may very well not be due to the chocolate, as I mentioned above.  Marion Nestle, at FoodPolitics, concurs: "wisely, the authors point out that much more research is needed to confirm these benefits, not least because the studies were observational, not clinical trials."  

I agree with all of these issues.  And personally, I need substantial evidence to convince me that a food is not just food, but is actually therapeutic.  But if we are going to be skeptical about chocolate because of insufficient evidence, shouldn't we be just as skeptical of other foods or nutrients with similar insufficient evidence?

My last post on meta-analyses featured a study by Siri-Tarino et al. that concluded that prospective cohort studies show no association between saturated fat intake and CVD*.  Chocolate, on the other hand, does show a benefit, even in studies with similar design.  Furthermore, the reduction in the risk of stroke appears greater than might be expected from abstaining from processed meat.  Despite these difference, recommendations to reduce saturated fat abound, while poor chocolate only gets criticism.

Again, we have to be skeptical about all of these studies until there is either sufficint clinical trial data, overwhelming epidemiological data, or preferrably, a combination of clinical trials, epidemiology, and laboratory experiments.  It's imprudent to disparage a study because chocolate "just can't be healthy" or quickly agree with an association becuase processed meats "must be bad for you."  We have to be equally critical of all studies of the same methodology, regardless of the outcome. 

As with coffee, I'm not convinced that chocolate is a health food or will prevent heart disease; although if you can restrain yourself, it certainly doesn't appear harmful.  And with more investigation, perhaps chocolate will be definitively healthy.  So until we have more conclusive evidence,  eat chocolate because it is divine, because we can mull over the richness of it will friends at the dinner table, and because it is the perfect end to a meal.  And be equally skeptical of claims for all foods, regardless of how sweet, sweet, delicious they may be.

*As I described in the previous post, and to be accurate, there have been clinical trials to investigate the benefit of saturated fat reduction.  Replacing saturated fat with polyunsaturated fat does indeed lower the risk of cardiovascular disease.  Although it does not appear to reduce CVD mortality or total mortality.

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