Jacka et al. have just published in BMC Medicine (January 30, 2017) the results of a study showing that healthy dietary change leads to significant improvement in depression.
In their "SMILES" study (another astonishing acronym--it stands for "Supporting the Modification of Lifestyle in Lowered Emotional States" !!), they included 67 people with moderate to severe depression, who also had low dietary quality (e.g. they were not eating a lot of vegetables, fiber, fish, and nuts, but were eating a lot of processed foods and sweets). The participants were randomized to receive 7 sessions of either nutritional counseling, or social support. The nutritional counseling aimed to encourage a Mediterranean-style diet, including whole grains, nuts, fish, legumes, vegetables, and fruit, while avoiding sweets and processed foods. The social support involved having neutral conversations or playing games ("befriending").
People in the nutritional counseling group dropped their depression rating scores by about 40%, while scores in the social support group dropped by only 20%, which was statistically different with a p-value of about .03. That is, the difference in outcome was about 97% likely to be due to the treatment effect, rather than to chance.
Reflection and Conclusion
It goes without saying, in my opinion, that healthy diet must be emphasized for everyone, in order to improve all aspects of physical and mental well-being. In depressive states, nutrition tends to be less healthy, for a variety of reasons: people may have less energy to attend to good nutrition; their depression may cause reduced or increased appetite; there may be comorbid eating disorders; there may be medication-induced side effects affecting nutritional behaviour or metabolism; and the depression could be associated with negative economic and social factors, which are in turn related to poor nutrition.
In addition to this, it is possible that there is a direct biological effect of poor nutritional quality causing depression.
So, in order to manage depression, and all other illnesses, it is of course important to encourage healthy dietary change.
But in the case of this study, there are a few ways in which the results may seem to exaggerate the effect of the nutritional change:
First, it seems to me that any substantial positive lifestyle change could have an "active placebo" effect independent of its direct effect on mood. Changing your diet in the ways suggested is, in my opinion, intrinsically healthy. But in order for someone to actually make these changes, they would have to frequently make different decisions than usual in the course of their day (for example, they would eat broccoli and fish instead of a fast food burger). It is clearly healthier to have that broccoli and fish, but the therapeutic impact of this decision may not be exclusively because of the better nutrition--it is also because the person is taking an active, disciplined step to choose something deliberately, in the name of changing their life for the better. It is this positive intention which may be a significant part of the therapeutic impact.
People in the control group, who had conversations and card games during the 7 sessions, probably did not experience much of a sense of their lifestyle substantially changing. I think a more interesting activity for the control group would involve something similarly "lifestyle changing" but not following the same nutritional guidelines.
So I wholeheartedly affirm the practice of making positive dietary changes! I have seen numerous patients who are feeling better as a result of making similar changes themselves. But it is also important to question the conclusion of studies like this one...Here's a reason why: suppose that there was another study which showed that buying an expensive nutritional supplement led to similar improvements in depression scores. The supplement, in reality, could be inert, or could simply be flavoured water. But taking the supplement would require a very active commitment on the part of participants, to alter their dietary behaviour in a consistent, disciplined way, with enthusiastic support from people claiming to be experts, over 12 weeks. In this case, we could agree easily that the therapeutic impact of the apparent nutritional change was due to a change in disciplined habits, combined with positive expectancy, rather than due to the wholesome properties of the supplement itself. If people were to assume, erroneously, that the supplement (and not the discipline involved to take the supplement) caused the benefit, then people would be at risk of being taken advantage of financially by the supplement manufacturers.
In conclusion, I strongly encourage that people make positive, disciplined, healthy dietary change. It quite probably will improve all aspects of health, including mental health. But be careful not to overvalue the results of studies such as this one, since the same reasoning that would allow you to uncritically endorse this study's conclusions could make you vulnerable to the persuasive efforts of marketers selling products which also require disciplined adherence.