Table 4.
Summary of the evidence (from meta-analysis and systematic reviews) deriving from human studies aimed at evaluating the impact of nutrients on cognitive function or gut microbiota.
| Nutrients | Type of Study/Impact on Cognitive Function | Type of Study/Impact on Gut Microbiota |
|---|---|---|
| proteins | Observational studies/no statistically significant differences in protein intake levels in AD patients and healthy controls [73] | No meta-analysis and systematic reviews were identified. |
| fats |
Prospective studies/significantly higher risk of AD and dementia development in patients with higher SFA, while the results regarding unsaturated fatty acids intake were not statistically significant [74,75] RCT/no protective effects of omega-3 supplements in the prevention of cognitive decline, which is contradictory with previous findings [76] |
RCT, observational studies (cross-sectional)/higher intake of fat and SFA was associated with poorer gut microbiota diversity, diets high in MUFA were associated with decreased total bacterial number, while whereas high PUFA intake had no effect on gut microbiota diversity and richness [77] |
| fiber | No meta-analysis and systematic reviews were identified. | RCT/dietary fiber resulted in higher abundance of Bifidobacterium and Lactobacillus spp. but does not affect α-diversity [78] |
| polyphenols | RCT/no definitive confirmation of the preventive effect of polyphenols on cognitive disorders [79] nor of neuroprotective effect of polyphenols supplementation in aging adults [80]. | No meta-analysis and systematic reviews were identified.. |
| vitamin B | RCT/no effect of B vitamins on cognitive function in older adults with or without cognitive impairment[81] | No meta-analysis and systematic reviews were identified. |
| vitamin D |
Observational (cross-sectional and longitudinal cohort) studies/low vitamin D status is related to poorer cognition [82] and increased risk of cognitive impairment [83]. Interventional studies with control group/no significant effect of vitamin D supplementation on cognition [82] |
RCT, interventional and observational studies/vitamin D influences the composition of the gastrointestinal microbiome (varied and limited studies) [84] |
| antioxidant vitamin |
Case–control studies/significantly lower plasma levels of α-carotene, β-carotene, lycopene, lutein, vitamin A, C, and E in AD patients [85] RCT/no good evidence that supplementation can preserve cognitive function, prevent dementia [86] |
No meta-analysis and systematic reviews were identified. |
Abbreviations: AD, Alzheimer’s Disease; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; RCT, randomized controlled trials; SFA, saturated fatty acids.