Too late for Valentine’s Day

The American Journal of Clinical Nutrition missed an opportunity when they put a meta-analysis of randomized studies of chocolate in the March issue rather than the February issue, but that hasn’t stopped the media coverage.  Stuff has a headline asking “Is chocolate really heart-healthy?”  There’s a law of journalism that states any headline ending in a question mark can be answered by the word “No”, and Stuff does not disappoint us here.

The newspaper story is actually pretty good — very good by the standards of nutrition journalism.  There are four key points, and the story makes three of them clearly and alludes to the fourth

• The research summarised randomized trials of chocolate or cocoa, not observational studies, so it is prima facie at least worth reading
• The studies compared a chocolate/cocoa-based supplement to one identical except for the chocolate. That is, they gave the same number of extra calories to both cocoa and control groups
• The effects were really small
• There was a lot of variation between trials in how they administered the chocolate, and in the effects.

When I say that the effects were really small, I mean really, really small.  The reductions in blood pressure, for example, were about 1.5mmHg.  LDL (bad) cholesterol went down about 1%, blood glucose went down about half a percent.   And this is in a situation where chocolate is being compared to something else with the same calorie content.

The effects were a bit larger for something called ‘flow-mediated dilation’, which is supposed to measure the responsiveness of the endothelium, the inner lining of blood vessels.  However, for flow-mediated dilation it looks like there were probably some negative trials that didn’t get published, and the results were quite sensitive to how the averaging of trial results were done.  The researchers used a method that (in this case) gave all trials about equal weight.  The alternative, giving large trials proportionally greater weight, estimates the benefit only half as big.  Also, the benefit on flow-mediated dilation was mostly seen two hours after the chocolate dose, not in the long term.

Fortunately, cardiovascular benefits have never really been the strongest argument for eating dark chocolate.