Chocolate: the new health food?

A UK cohort study published a paper yesterday with the title “Habitual chocolate consumption and risk of cardiovascular disease among healthy men and women.”  In contrast to the last chocolate study to make headlines, this is actual research, involving 20,000 people followed up for twelve years, and was published in a respectable medical journal.

The study found that people who ate more chocolate back in the mid-1990s had less cardiovascular disease over the period to 2008. Of course, this makes for a great press release and headlines

• Stuff: Eating chocolate every day linked to lower heart disease and stroke
• NZ Herald (from the Telegraph): Two choc bars a day keeps doctor away
• One News: Is chocolate good for you? New study suggests 100g a day may be beneficial

Are the findings of the paper true? Well, that depends on what you mean by ‘true’, which is important to remember when you see claims that 90% of scientific results are false.

On the one hand, it’s true that the EPIC study recruited all these people and asked them questions about their diet in the 1990s, and it’s presumably true that the proportion getting cardiovascular disease was lower among those who ate more chocolate and that the study included people who ate up to 100g/day.  These are historical facts, not health claims.

The conclusion of the research paper was

Cumulative evidence suggests that higher chocolate intake is associated with a lower risk of future cardiovascular events, although residual confounding cannot be excluded. There does not appear to be any evidence to say that chocolate should be avoided in those who are concerned about cardiovascular risk.

This is also probably true: there is a correlation; it could be due to confounding; there doesn’t seem to be any big extra risk from eating chocolate (instead of something else with similar calorie content).

At the other extreme, though, the Herald and One News headlines are misleading: they imply that adding 100g/day of chocolate to your existing diet would be beneficial. First, while the maximum consumption was 100g/day, 95% of the study participants consumed less than 40g/day, and 90% less than 25g/day. Second, and more important, the study looked at people’s normal diet, not at changes in diet.

If you add 100g/day of chocolate to your diet, you either need to cut more than 500 Calories of other foods, or exercise a lot more, to avoid gaining weight. The study participants basically did this: the high-chocolate and low-chocolate groups had similar BMI and waist:hip ratio, and the high-chocolate group exercised more.

Third, there is confounding. The people who ate more chocolate might have been healthier for other reasons. For example, those who ate no chocolate were more likely to have diabetes, which is probably why some of them ate no chocolate. The difference in cardiovascular disease rates was far too small for confounding to be ruled out as an explanation, no matter how carefully the analysis was done (and it was done pretty well).

Fourth, and in some ways most important, is the role of chance. This was a big study, but it still came up with only moderately strong evidence that the correlation was real, and that’s considering the study on its own. We don’t know whether there was any publication bias leading positive results about chocolate to be easier to publish in the scientific journals, but we can be sure there was publication bias in the media coverage. Always, if you see a diet and health study on TV, it must have had unusually interesting results. Even when it’s valuable as a component in the cumulative scientific literature, the biased selection for interesting results usually means you can’t believe it in application to your own life.

I got up at 5:15 today in order to be on breakfast TV to talk about this study. That would never have happened if the results had been different.