Overall, the 13 studies selected for inclusion represented 365 participants, with diets lasting between four and 24 weeks (6 weeks on average) and walnuts providing between 10 and 24 per cent of total calories.

One study allowed 15 g of walnut oil in addition to the 37 g of walnuts (15). Another study incorporated the walnuts into meat products to facilitate participant blinding (16). It was apparent from the remaining studies that patients were given or recommended specific daily amounts of whole walnuts. The control diets were diverse, as outlined in Table 1. Four studies used low-fat diets, with a cutoff of ,30% energy from fat in 3 studies and ,20% energy from fat in the fourth study. A Mediterranean-style diet was used in 3 studies. Three other studies used the average diets for the country: 2 average American and 1 traditional Japanese. One study reported that the control meals were cholesterol lowering, and another specified that control meals were provided but did not elaborate on the composition.

The studies were selected to be controlled trials – the control diets included Standard American, Japanese and Mediterranean. Some of the studies provided the meals.

Studies were accepted for the meta-analysis if they were a controlled trial that evaluated a walnut-enriched diet compared with a control diet.

I have to agree that substituting 25% of calories with something approximately more healthy than the usual choice might be A Good Thing.

Ooops!

In other words, how do we know that the benefit of substituting 25% of your calories with walnuts wouldn’t also be provided by substituting 25% of your calories with anything else vaguely “healthy”?

Did any of these studies have a proper control group?