Tuesday, February 14, 2012

Chocolate and the Happy Heart

In honor of Valentine's Day, one of our hospitalists, Tom Ormiston, gave a special "Bite-Sized EBM" presentation to the residents, covering a number of recent, thought-provoking papers.  One of them was a meta-analysis of chocolate consumption and the risk of cardiovascular and metabolic outcomes.  Before proceeding further, we must disclose a potential conflict of interest:  Dr. Ormiston passed out chocolate hearts to the audience (although no chocolate-sponsored ballpoint pens), and they were divine.

The meta-analysis included seven large observational studies of chocolate consumption.  The investigators chose to examine cardiometabolic outcomes, because of cocoa's purported "antioxidant, anti-hypertensive, anti-inflammatory, anti-atherogenic, and anti-thrombotic effects."  The analysis compared the participants who reported the highest level of chocolate consumption to those with the lowest. Here were there results:
Should this be called a Black Forest plot?

Remember, a relative risk (RR) less than one means that one group (in this case, the high chocolate group) has a lower rate of events than a comparison group (the low chocolate group).   The high chocolate group had an overall 37% decrease in the risk of cardiovascular disease, a 29% decrease in stroke, and no difference in heart failure.  Only one study, performed in Japan, looked at the risk of diabetes and found that there was a trend towards decreased diabetes in the high chocolate group.  The investigators had planned a subgroup analysis of dark vs. milk vs. white chocolate, but unfortunately, none of the studies collected this data.

All of you EBM acolytes can go ahead and start poking holes in this review:
  • All the studies were observational, and we know that when it comes to meta-analysis, garbage in,  garbage out.  In their defense, most of the studies were higher quality and controlled for confounders, such as body mass index, other dietary factors and education level.  Still, what about the intangibles -- are the lovelorn less likely to receive chocolate, and thus more likely to have a bad cardiovascular outcome?
  • The studies reported chocolate consumption using different scales.  "High" chocolate consumption could mean anything from at least once a week to more than once a day.
  • Some of the studies included only healthy participants, while others included those who already had a cardiovascular diagnosis.  Is chocolate useful only for primary prevention, or can it be used for secondary?
Before we become shills for Willy Wonka, let's recall the disappointing results of anti-oxidant vitamin trials, which held initial promise but may actually increase overall mortality.  And unlike typical dietary supplements, cocoa is usually consumed in high-fat, high-sugar form, so any recommendations for the consumption of chocolate would be premature.  The authors of this meta-analysis concluded appropriately, "Considering the limited data available, any conclusions should be cautious....Corroboration is now required from further studies....to test causation rather than just association."

But since it's Valentine's day, we don't wish to rain on anyone's parade.  Feel free to enjoy your chocolate, as long as you take it with a grain of sugar.

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