A group made up of researchers based in Australia and Italy evaluated the available findings from both animal and human studies in the hope of formulating recommendations for future research.
Their systematic review, published in Neuroscience and Biobehavioral Reviews, looked at 163 preclinical animal, 16 observational, and 44 intervention articles assessing the relationship between polyphenols and outcomes related to depression.
They concluded that the results of human studies are unclear, with some reporting an inverse relationship between depression and polyphenol intake, and others reporting no association or effect.
Polyphenols may counteract physiological changes in depression and anxiety
Polyphenols – a class of bioactive compounds found in many plant foods that includes flavonoids, phenolic acids, lignans, and stilbenes – are known to have antioxidant and anti-inflammatory properties, which protect against conditions including cancer, cardiovascular disease, and diabetes.
They have also been implicated in healthy ageing, while some studies suggest that higher dietary intake of flavonols such as kaempferol and quercetin may slow the rate of age-related cognitive decline.
And they may work to counteract physiological changes in depression and anxiety, with flavonoids in particular associated with a decreased risk of depression.
In this review, most animal studies were found to demonstrate that exposure to polyphenols alleviated behaviours associated with depression. However, the results from human studies were less clear.
‘Major evidence gaps’ identified in research on polyphenols and depression
The authors identified “several major evidence gaps” in the research that they believe to be worthy of further investigation.
These included the fact that most studies measuring the effects of individual polyphenols and polyphenol-rich foods “did not assess overall consumption of polyphenol types or quantities” and, as such, failed to account for “the multi-dimensionality of dietary polyphenol exposure”.
Dietary polyphenol intake from alcoholic beverages was another consideration that was neglected, they said. One cross-sectional study reported that participants who consumed more than 20g of alcohol per day showed significantly higher urinary concentrations of several polyphenols than those who consumed less than 0.1g of alcohol per day.
“It is important for future observational and whole-of-diet studies to account for alcohol intakes, specifically those that contain high concentrations of polyphenols, such as beer and wine,” the authors wrote.
They also drew attention to the short-term nature of most research: 70% of the intervention studies had a polyphenol exposure period that was less than or equal to three months.
Meanwhile, just eight of the 40 intervention studies reviewed were carried out among populations with diagnosed depression, with no observational studies and only two intervention studies conducted in under-18s.
And, as the gut microbiota facilitate the transformation of polyphenols into their bioactive metabolites, polyphenol bioavailability is heavily dependent on an individual’s gut microbiome composition.
“Variability in participants’ gut microbiome composition may explain, in part, the discrepancies in the results of the studies included in this review,” the authors suggest.
Polyphenols ‘might be a novel adjunctive strategy’ for treating depression
The World Health Organization has identified depression as a leading cause of disability worldwide; it is predicted to be the leading cause of disease burden by 2030. As such, alternative treatment strategies are welcomed.
The authors suggested that the studies reviewed demonstrated that dietary polyphenol intake “might be a novel adjunctive strategy for the prevention and treatment of depression due to the potential ability of polyphenols to modulate several pathophysiological pathways”.
However, future research in humans is required if the existing research gaps are to be addressed – in particular, the impact of polyphenols on younger populations and people who are clinically depressed.
“Considering the burden of depression to individuals and the society, addressing gaps in polyphenols’ potential to improve depression is an urgent necessity,” they concluded.
