All of the exploratory studies [43, 45] and some hypothesis- driven studies [26–27] have
been solely cross-sectional studies. These studies have in general suggested that a healthy
diet (e.g. fruits, vegetables, and fish) can be associated with a better cognitive function.
However, although these cross-sectional studies are useful for hypothesis generation, they
are not useful for causal inference, because they cannot establish whether AD or cognitive
decline is a result of dietary habits or a primary event leading to dietary changes. In addition,
most of the studies [26–27, 29, 43] examined the overall cognitive performance, while the
association between DP and specific cognitive domains was explored in only one study