Background:
Dementia is a leading public health challenge worldwide with no effective treatment nor proven preventive interventions available. Compelling evidence shows that dietary patterns can slow the rate of cognitive decline suggesting diet is a promising preventive measure in the battle against dementia.

Objective:
The aim of this systematic review is to summarize the evidence of three dietary patterns, the Mediterranean diet, the ketogenic diet and the MIND diet, on the prevention of cognitive decline. These three diets are known to have antioxidant and anti-inflammatory properties and have been associated with reduced dementia risk and slower cognitive decline.

Methods:
A systematic search was conducted in major electronic databases (PubMed, ScienceDirect and Web of Science) up 31 January 2022, using the key search terms “Mediterranean diet”, “ketogenic diet”, “MIND diet”, “dementia”, “cognition” and “aging”. Statistical analysis was performed using RoB 2 and Jadad scale to assess the risk of bias and methodological quality in randomized controlled trials (RCT), respectively.

Results:
Only RCTs were included in this study, and specifically 13 studies (n = 3,523 participants) for the Mediterranean diet, 7 studies (n = 372 participants) for the ketogenic diet and 1 study (n = 37 participants) for the MIND diet. Participants’ cognitive status was normal in 10 studies, 8 studies included patients with mild cognitive impairment, and 3 studies included Alzheimer’s disease patients.

Conclusions:
All three dietary interventions have been shown to slow cognitive decline rate in the studies included in this systematic review. Mediterranean diet was shown to be beneficial for global cognition in the shortest of time, after 10 weeks of adherence, the ketogenic diet had a beneficial effect in patients with diabetes mellitus improving verbal recognition, while the MIND diet showed benefits in obese patients improving working memory, verbal recognition, memory and attention.