Today's newspapers are all running front-page stories about a new study showing the benefits of a Mediterranean diet. However, one of Aaron Carroll's sharp-eyed readers pointed out that Table S5 in the appendix shows that the only truly big difference between the test group and the control group was their consumption of olive oil and nuts, which participants in the test group were given free. Sure, enough, if you read down to the very end of the report, the authors say this:
The interventions were intended to improve the overall dietary pattern, but the major between-group differences involved the supplemental items. Thus, extra-virgin olive oil and nuts were probably responsible for most of the observed benefits of the Mediterranean diets.
I'd add one more thing. The researchers were studying the frequency of "end point" events: heart attacks, strokes, and deaths. Here are the raw numbers: in the test group (Mediterranean diet + nuts), the total number of all those events over the period of the study was 3.8 percent. In the control group it was 4.4 percent.
Now, these are statistically significant and probably represent a genuine result. But don't get too excited. The participants were all over 55 and had either Type 2 diabetes or three major risk factors for cardivascular disease. So you'd expect them to be teetering on the edge of one of these end point events anyway. But even at that, the additional risk over five years from eating whatever you wanted was 0.6 percentage points. That's not really a helluva lot.