Francis Stevenson Chambers 2019

Participants were randomized into either a diet change group, or habitual diet control group, based on an Excel-generated randomization schedule. The researchers separated participants into two groups by chance: the diet-change group & the habitual-diet control group.

We hypothesised that engaging in a brief diet intervention which emphasized increasing healthy and decreasing unhealthy food intake would reduce levels of depression symptoms compared to a habitual diet control group. The researchers predicted that participants who ate more healthy food & less unhealthy food for a short period of time would have fewer symptoms of depression than those who continued their usual diet.

This difference was significant, with the DC group having significantly lower CESD-R scores on Day 21 compared to the HD group, controlling for baseline CESD-R scores (F[1,75] = 7.792, p = .007, Cohen’s d = 0.65; see Table 3). After 21 days, the diet-change group had lower depression scores than the group that ate as usual. The researchers also considered both groups’ scores at the start.

The results of this RCT provide support for improving diet as a useful adjunct treatment to reduce depressive symptoms. The authors suggest that improving diet can be used alongside other treatments to help reduce symptoms of depression.

A follow-up study could compare a diet-change group with a usual-diet group for 6–12 months. This would test whether people can keep eating healthier food and whether any improvement in depression symptoms lasts.

I found Cohen’s d = 0.65 difficult to understand. I would like someone to explain what this number means and what it tells us about the difference between the two groups.