Healthy diet is *associated* with reduced mental decline in elderly

Healthy body, healthy mind. A new study reports that elderly people who consume a healthy diet rich in fruits, vegetables, nuts, and fish, are more likely to maintain higher levels of thinking and memory compared to those who consume a less healthy diet (Reuters):

“As our study is observational, we can only say that a healthy diet was associated with a reduced risk of cognitive decline and cannot definitively say there is a causal relationship,” he said.

It is important to note that the study reported an "association" between a healthy diet and reduced cognitive decline. Thus, as the author states, one cannot conclude that there is a causal relationship in which the healthy diet "caused" the reduced risk of mental decline. Indeed, one can argue that the causation might run in the reverse direction (reverse causation) with people possessing greater cognitive faculties being more likely to maintain a healthy diet. However, this association is intriguing especially in light of other evidence that diet influences mental fitness (link1).

The study was performed as follows:

"Smyth and his coauthors used data from two multinational randomized trials of a blood pressure medication. They included more than 27,000 men and women age 55 and older who had a history of coronary, cerebral or peripheral artery disease or high-risk diabetes and who were followed until death, stroke, heart attack or hospitalization. Half the participants were followed for less than five years. 

The researchers used the food frequency questionnaire to estimate how “healthy” people’s dietary habits were, awarding higher scores to frequent consumption of foods like vegetables, fruits, nuts, soy proteins and fish. The top fifth of people with the healthiest diets were about 24 percent less likely to experience cognitive decline during the study than the bottom fifth with the worst diet scores, the study team reports in Neurology."

So the original study was on blood pressure medication, but to control for diet the researcher also had the subjects fill out a questionnaire on diet. A diet was classified as healthy if it contained lots of fruits, vegetables, nuts, soy proteins, and fish. They were then able to use the data from the questionnaire coupled with measurements on cognitive decline to show the association. Quite impressively, the people with the healthiest diet were 24% less likely to experience cognitive decline than the people with the worst diets.

However, one important complicating circumstance was that a healthy diet clustered with a generally healthy lifestyle:

“Our study shows that those with the healthiest diet tended to be more active, were less likely to smoke and had lower body mass index,” Smyth said. “This suggests that the consumption of a healthy diet is likely to be associated with a healthy lifestyle in general.”

Thus, there were multiple factors that associated with reduced mental decline, and disentangling these factors is difficult. One might expect that a healthy lifestyle in general could favor retaining one's mental faculties. When you try to control for other at least some of the confounding variables like physical activity, the protective effect is still present although to a lesser degree:

"About 14 percent of people in the healthiest diet category had cognitive decline compared to 18 percent of those in the least-healthy category after taking physical activity, high blood pressure and cancer history into account."

In summary, association does not necessarily imply causation. One possibility is that multiple factors associated with a healthy lifestyle including diet contribute to reduced mental decline, and these various confounding factors cannot easily be separated from one another despite the best efforts of the researchers (see previous paragraph). Second as mentioned above, there is the possibility of reverse causation.

Finally, there could be a positive feedback effect in which a healthy diet leads to greater mental health which leads to a greater commitment to a healthy diet and so on. In this scenario, a healthy diet is both cause and effect, and so is the reduced mental decline. Such a positive feedback loop makes it difficult to separate cause from effect because both are causes and both are effects.

It should be possible to distinguish these alternative explanations with a randomly controlled trial (RCT). In such a trial, one group would be given a healthy diet whereas the control group would be given a normal, less healthy diet. Each group would contain an approximately equal number of people who possess healthy lifestyle characteristics excluding diet. Then the mental health decline of both groups would be measured and compared.

Figure 1. A healthy diet, and more generally a healthy lifestyle, is associated with greater mental acuity in the elderly.