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Brain Health and Cognitive Function - NED Infobite
BANT's scientific NED InfoBites are designed to provide key elements of the latest research using plain language. They provide quick overviews on particular health issues and nutrition topics for a speedy introduction to the science. Visually attractive and easily shareable with clients and social media followers.
2024
Abstract
With an ageing global population, it is expected that the incidence of age-related disorders such as Alzheimer's disease and cognitive decline is likely to increase. With the absence of effective treatment, focus is turning towards modifiable diet and lifestyle risk factors as a way of prevention. This NED Infobite highlights research on nutritional interventions, time-restricted eating, sleep and probiotics on Alzheimer's and cognitive decline.
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How to reduce brain inflammation: 4 actions you can take today
Dr Ruscio is a Doctor of Natural Medicine, Doctor of Chiropractic, clinical researcher and author. His podcast, Dr Ruscio Radio presents cutting edge information in health, nutrition and functional medicine distilled into practical advice that can be used to improve our health.
2023
Abstract
Dr Ruscio focuses on the link between brain inflammation and neurodegenerative diseases such as Alzheimer’s and Parkinson’s but also brain fog and mood. He summarises how it might be possible to reduce neuroinflammation through diet, looking at the links with dysbiosis and leaky gut but also exercise, sleep hygiene, and stress management. A concise overview of the evidence for diets such as Paleo, Low FODMAP, Ketogenic and Low Carb, as well as the Mediterranean diet for reducing inflammation and supporting brain health is provided. The potential for lab testing via anti and pro-inflammatory markers is also reviewed.
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Mediterranean diet and structural neuroimaging biomarkers of Alzheimer's and cerebrovascular disease: A systematic review.
Gregory, S, Pullen, H, Ritchie, CW, Shannon, OM, Stevenson, EJ, Muniz-Terrera, G
Experimental gerontology. 2023;172:112065
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Nearly a million people in the UK have Alzheimer's disease, the most common type of dementia. Mediterranean diet (MedDiet) adherence is associated with healthy brain ageing, a reduced stroke risk, and a lower incidence of dementia. Seven studies were included in this review to evaluate the effects of MedDiet on hippocampal volume and white matter hyperintensity volume, predictors of stroke and dementia. This systematic review did not reveal any associations between MedDiet adherence and hippocampal volume. However, there was a significant negative relationship between MedDiet adherence and white matter hyperintensity volume in two of the studies included. It is necessary to conduct more robust studies to investigate the associations between MedDiet adherence and structural brain imaging findings and understand the mechanisms behind dementia and other cerebrovascular diseases. This study could provide healthcare professionals with valuable information about the effects of increased MedDiet adherence on brain health, including its potential to delay neurodegenerative disease progression.
Expert Review
Conflicts of interest:
None
Take Home Message:
Due to inconclusive results on the associations between MedDiet adherence and AD and cerebrovascular related structural neuroimaging findings, specific recommendations for the MedDiet cannot be made on the basis of this study until further research has been completed.
Evidence Category:
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A: Meta-analyses, position-stands, randomized-controlled trials (RCTs)
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B: Systematic reviews including RCTs of limited number
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C: Non-randomized trials, observational studies, narrative reviews
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D: Case-reports, evidence-based clinical findings
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E: Opinion piece, other
Summary Review:
Background:
Changes in hippocampal volume (HV) and white matter intensity volume (WMIV) have been identified as structural neuroimaging biomarkers of neurodegenerative disease such as Alzheimer’s Disease (AD) and Cerebrovascular disease (CVD) respectively. Evidence has shown adherence to the Mediterranean Diet (MedDiet) has been associated with reduced risk for strokes. This review evaluated the MedDiet in relation to HV and WMIV.
Methods:
The review followed PRISMA guidelines and was registered on PROSPERO. Literature searching resulted in seven studies published between 2012 and 2022, which met the inclusion criteria. Six studies analysed cross-sectional data and one analysed longitudinal data. The NIH Quality Assessment Tool for Observational and Cross-Sectional Studies was used to assess risk of bias.
Overall, the studies were rated as low-risk of bias with details of the research question, participant group exposure and outcome variables included. Due to moderate to high heterogeneity in some studies, a meta-analysis was deemed unsuitable and narrative synthesis was conducted to present the results.
Results:
Mean participant age ranged from 53.19 to 80.3 years and volunteers were healthy or had subjective cognitive decline and a few participants had dementia (n=46).
Hippocampal Volume:
Four studies included 20,077 participants and found no significant associations between MedDiet adherence and hippocampal volume. All four studies were cross-sectional from larger cohort studies. To establish causative relationships longitudinal and RCT trials are required.
White Matter Hyperintensity Volumes:
Four studies included 1938 participants. Two studies found a significant negative association between MedDiet and WMHV, demonstrating higher Mediterranean Scores were associated with lower level of WHMV. The other two studies found no significant associations.
Although the review methodology with a piloted search strategy was considered a key strength, the lack of meta-analysis as planned in the a priori protocol, due to minimal eligible studies and high heterogeneity was a limitation as well as the restriction of brain imaging outcomes.
Conclusion:
Overall, these results are inconclusive on the associations between the MedDiet and HV and WMHV, and identify a gap in the knowledge, therefore further research such as RCT’s remains a priority to further understand the impact diet may have on neuroimaging markers of AD and CVD.
Clinical practice applications:
There were no significant associations between MedDiet adherence and HV, which was surprising given the evidence stating adherence to the MedDiet is associated with a lower incidence of dementia and stroke. However all four studies were cross-sectional studies and in order to detect causal associations, longitudinal and RCT’s are needed. Two studies did show a significant association between higher MedDiet adherence and lower WMHV, whereas two studies reported no significant associations.
Caution needs to be taken when recommending the MedDiet specifically for a reduction in HV and WMHV until further research has been undertaken.
Considerations for future research:
Future research should consider:
- larger cohorts and participants from the Mediterranean region where lifelong adherence to the MedDiet is more likely.
- looking at other risk factors to include obesity, lack of activity, poor sleep quality and stress.
- evaluating different socio-economic status, which has been shown to impact dietary behaviour.
- alternative imaging outcomes such as cortical thinning, PET amyloid and tau.
- . gold standard for methodology in particular dietary analysis and scanning and outcome derivation.
Abstract
Previous studies have demonstrated an association between adherence to the Mediterranean diet (MedDiet) and better cognitive performance, lower incidence of dementia and lower Alzheimer's disease biomarker burden. The aim of this systematic review was to evaluate the evidence base for MedDiet associations with hippocampal volume and white matter hyperintensity volume (WMHV). We searched systematically for studies reporting on MedDiet and hippocampal volume or WMHV in MedLine, EMBASE, CINAHL and PsycInfo. Searches were initially carried out on 21st July 2021 with final searches run on 23rd November 2022. Risk of bias was assessed using the NIH Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies. Of an initial 112 papers identified, seven papers were eligible for inclusion in the review reporting on 21,933 participants. Four studies reported on hippocampal volume, with inconclusive or no associations seen with MedDiet adherence. Two studies found a significant association between higher MedDiet adherence and lower WMHV, while two other studies found no significant associations. Overall these results highlight a gap in our knowledge about the associations between the MedDiet and AD and cerebrovascular related structural neuroimaging findings.
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Inflammation moderates the effects of lifestyle modification on neurocognition among individuals with resistant hypertension.
Avorgbedor, F, Blumenthal, JA, Hinderliter, A, Ingle, K, Lin, PH, Craighead, L, Tyson, C, Kraus, W, Sherwood, A, Smith, PJ
Journal of clinical hypertension (Greenwich, Conn.). 2023;25(1):106-110
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Hypertension is one of the primary causes of cardiovascular disease, stroke, Alzheimer’s Disease, and Alzheimer’s Disease and related dementias (AD/ADRD). Among individuals with hypertension, those with resistant hypertension (RH) appear to have the greatest risk of cerebrovascular disease and associated cognitive impairment. The aim of this study was to investigate the potential influence of individual differences in pre-treatment inflammatory profiles on changes in cognition following lifestyle modification among RH participants in the TRIUMPH clinical trial. This study is a report based on the TRIUMPH study which was a randomised clinical trial. One hundred forty patients with RH were randomised with 2:1 allocation to either a 4-month Centre-based Lifestyle intervention or Standardized Education and Physician Advice. Results show that basal levels of elevated peripheral inflammation may represent an intermediate phenotype of risk for cognitive decline. In fact, individuals with higher levels of c-reactive protein at baseline demonstrated greater improvements in Executive Function/Learning following participation in an intensive lifestyle intervention. Authors conclude that their findings may help inform targeted treatments to reduce ADRD among middle-aged and older adults with cardiovascular disease risk factors.
Abstract
Individuals with resistant hypertension (RH) have the greatest risk of cerebrovascular disease and cognitive impairment among individuals with hypertension. Elevated levels of pro-inflammatory cytokines may represent a critical yet unexamined factor influencing the impact of healthy lifestyle changes on cognitive function. We explored the influence of inflammation on changes in cognition following lifestyle modification among individuals with RH participating in the TRIUMPH clinical trial. One hundred forty participants with RH completed a battery of neurocognitive tests along with the inflammatory marker C-reactive protein (hsCRP) and were subsequently randomized to an intensive 4-month lifestyle modification intervention or to education and physician advice control. Results indicated that the effects of lifestyle modification on Executive Function and Learning were moderated by pre-intervention hsCRP levels (P = .049), with treatment efficacy increasing across levels of baseline inflammation levels (low: d = 0.12; mild: d = 0.43; moderate: d = 0.81). We conclude that inflammatory profiles may help identify individuals more likely to improve executive functioning resulting from lifestyle modification.
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The effects of multi-nutrient formulas containing a combination of n-3 PUFA and B vitamins on cognition in the older adult: a systematic review and meta-analysis.
Fairbairn, P, Dyall, SC, Tsofliou, F
The British journal of nutrition. 2023;129(3):428-441
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Due to the insidious onset, cognitive impairment often goes unnoticed for several years, with clinical diagnosis being made late into the disease progression. Cognition is critical for functional independence as people age, and intact cognition is vital for humans to communicate effectively. The aims of this study were to (i) investigate whether supplementation with a combination of omega-3 polyunsaturated fatty acids (n-3 PUFA) and B vitamins alone or as part of a multi-nutrient formula can prevent cognitive decline in older adults, and (ii) determine whether the effects of a single nutrient intervention with either n-3 PUFA or B vitamins could be modified by the status of the other nutrient. This study is a systematic review and meta-analysis of fourteen studies of which eleven were randomised controlled trials and the rest were post hoc analysis of randomised controlled trials. Results show a benefit of supplementing with nutrient formulas that contain both n-3 PUFA and B vitamins on global cognition and episodic memory with small to moderate effect sizes. In fact, they can help preserve cognition in the older adults. Authors conclude that more experimental work providing a combination of nutrients including both n-3 PUFA and B vitamins, in healthy older adults or those showing early signs of cognitive decline, is clearly warranted to better explore how nutrition can impact the trajectory of cognition in older adults.
Abstract
There is now evidence to suggest that there may be an interaction between B vitamins and n-3 PUFA, with suggestions that increasing intake of both nutrients simultaneously may benefit cognition in older adults. The aim of this systematic review was to investigate whether supplementation with a combination of n-3 PUFA and B vitamins can prevent cognitive decline in older adults. Randomised controlled trials conducted in older adults that measured cognitive function were retrieved. The included trials provided a combination of n-3 PUFA and B vitamins alone, or in combination with other nutrients. Trials that provided n-3 PUFA alone and also measured B vitamin status or provided B vitamin supplementation alone and measured n-3 PUFA status were also included. The databases searched were The Cochrane Library, EMBASE, CINAHL, Scopus and MEDLINE. A total of 14 papers were included in the analysis (n 4913; age: 60-70 years; follow-up 24 weeks to 4 years). The meta-analysis results found a significant benefit of nutrient formulas, which included both n-3 PUFA and B vitamins alongside other nutrients, v. placebo on global cognition assessed using composite scores from a neuropsychological test battery (G = 0·23, P = 0·002), global cognition using single measures of cognition (G = 0·28, P = 0·004) and episodic memory (G = 0·32, P = 0·001). The results indicate that providing a combination of n-3 PUFA and B vitamins as part of a multi-nutrient formula benefits cognition in older adults v. a placebo, and the potential for an interaction between these key nutrients should be considered in future experimental work.
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Brain-derived neurotrophic factor in diabetes mellitus: A systematic review and meta-analysis.
Moosaie, F, Mohammadi, S, Saghazadeh, A, Dehghani Firouzabadi, F, Rezaei, N
PloS one. 2023;18(2):e0268816
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Brain-derived neurotrophic factor (BDNF) is a key molecule mainly involved in brain cell growth. It is also responsible for the survival of beta cells of the pancreas, which produce a hormone known as insulin, which is responsible for balancing blood sugar. Poor functioning beta cells can result in the development of diabetes. It is unclear as to whether high or low BDNF is present in individuals with diabetes. This meta-analysis and systematic review of 2734 individuals aimed to determine BDNF levels in individuals with diabetes, depression, and diabetic retinopathy. The results showed that serum BDNF levels were low in individuals with diabetes, however plasma levels remained unchanged. BDNF was lower in individuals with type 2 diabetes, diabetes mellitus, depression, and diabetic retinopathy. It was concluded that decreased BDNF may contribute to the development of diabetes and associated complications. This study could be used by healthcare professionals to understand the close links between diabetes and brain disorders such as dementia and Alzheimer’s disease.
Abstract
BACKGROUND Brain-derived neurotrophic factor (BDNF) is a neurotrophic factor expressed in several tissues, including the brain, gut, and pancreas. Activation of the BDNF/TrkB/CREB reduces hepatic gluconeogenesis, induces hepatic insulin signal transduction, and protects against pancreatic beta-cell loss in diabetes mellitus (DM). Several studies have investigated the possible association between BDNF and DM and its complications, but the results have been conflicting. AIM: In the present study, we aimed at systematically reviewing the literature on the serum and plasma levels of BDNF in DM and its subgroups such as T2DM, DM patients with depression, and patients with retinopathy. METHODS A comprehensive search was conducted in PubMed, Scopus, and Web of Science. We identified 28 eligible studies and calculated the standardized mean difference (SMD) of outcomes as an effect measure. RESULTS The meta-analysis included 2734 patients with DM and 6004 controls. Serum BDNF levels were significantly lower in patients with DM vs. controls (SMD = -1.00, P<0.001). Plasma BDNF levels were not different in patients with DM compared with controls. When conducting subgroup analysis, serum BDNF levels were lower among patients with T2DM (SMD = -1.26, P<0.001), DM and depression (SMD = -1.69, P<0.001), and patients with diabetic retinopathy (DR) vs. controls (SMD = -1.03, P = 0.01). CONCLUSIONS Serum BDNF levels were lower in patients with DM, T2DM, DM with depression, and DM and DR than the controls. Our findings are in line with the hypothesis that decreased BDNF levels might impair glucose metabolism and contribute to the pathogenesis of DM and its complications.
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The Effects of Spirulina maxima Extract on Memory Improvement in Those with Mild Cognitive Impairment: A Randomized, Double-Blind, Placebo-Controlled Clinical Trial.
Choi, WY, Lee, WK, Kim, TH, Ryu, YK, Park, A, Lee, YJ, Heo, SJ, Oh, C, Chung, YC, Kang, DH
Nutrients. 2022;14(18)
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Alzheimer’s disease, which involves the most severe level of memory impairment, is mostly irreversible, thus it is critical to prevent deterioration of cognitive function. Spirulina is a microalga known to be high in nutrients and is considered one of the most perfect dietary supplements for humans because it contains many bioactive substances. The aim of this study was to report the effects of spirulina maxima 70% ethanol extract (SM70EE) on cognitive function in older adults (aged over 60 years) with memory impairment. This study is a randomised double-blind placebo-controlled trial. The study recruited eighty participants who were randomised into two equal-sized groups of 40 individuals each. The investigators and participants were blinded to the treatment regimens. Results showed statistically significant differences between the visual leading test and the visual working memory test. Furthermore, the vocabulary results showed a statistically significant difference between groups. SM70EE ingestion was determined to be safe for the human body because no clinically significant adverse reactions or physical changes were observed. Authors conclude that continuous intake of SM70EE could show improvement in memory function through improved visual memory and vocabulary.
Abstract
Spirulina maxima is a marine microalga that has been promoted worldwide as a super food. This study was conducted to evaluate its ability to improve memory in the older adults using Spirulina maxima 70% ethanol extract (SM70EE). This randomized, double-blind, placebo-controlled clinical trial comprised 80 volunteers recruited from Jeonbuk National University Hospital in Jeonju, Republic of Korea, who were randomly assigned to two groups. The participants received either 1 g/day of SM70EE or a placebo without otherwise changing their diet or physical activity. The participants were examined at baseline and after a 12-week interval to determine whether there were changes in their results for visual learning, visual working memory, and verbal learning tests from the Korean version of the Montreal Cognitive Assessment, brain-derived neurotrophic factor and beta-amyloid levels, and total antioxidant capacity. Compared to the placebo group, the treatment group showed a significant improvement in visual learning and visual working memory test results and enhanced vocabulary. SM70EE use was shown to improve memory, with no adverse effects. Its efficacy in alleviating Alzheimer's disease symptoms was verified for the first time through this clinical trial. SM70EE could play a role in the management of patients with dementia. This trial is registered with registration number of clinical research information service (CRIS: KCT0006161).
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Dietary supplements in neurological diseases and brain aging.
Naureen, Z, Dhuli, K, Medori, MC, Caruso, P, Manganotti, P, Chiurazzi, P, Bertelli, M
Journal of preventive medicine and hygiene. 2022;63(2 Suppl 3):E174-E188
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The rate of neurodegenerative diseases (ND) is increasing at a concerning rate. The condition is characterized by the progressive decline of neuron function in the brain, eventually leading to cognitive impairment and motorneuron disorders. This process appears to be mediated by the complex interplay of factors. The brain is extremely sensitive to oxidative stress, and oxidative stress and inflammation of the nervous tissue appear to be a common denominator in all neurodegenerative diseases. One of the challenges of ND for prevention as well as treatment and treatment development, is that the initial disease progression usually goes unnoticed, with symptoms only becoming apparent in the more advanced stages when irreversible damage has occurred. Diet quality has a significant impact on brain health and hence can also influence ND development. For example, the Mediterranean diet (MedDiet) has demonstrated many valuable attributes that can reduce ND incidences and improve cognitive function. This review looked at dietary components, natural compounds and medicinal plants that have shown to be beneficial for brain health in ND. The authors discussed the MedDiet followed by a brief review of dietary supplements, including N-acetylcysteine (NAC), phospholipids (Phosphatidylserine, Phosphatidylcholine), Gamma-aminobutyric acid, melatonin, omega-3 fatty acids, neurotropic vitamin B (B1, B6 and B12), S-adenosyl methionine (SAMe), the amino acid tryptophan, magnesium and various polyphenols. Several medicinal plants are reviewed that have demonstrated positive effects on preventing or alleviating neurological diseases. This includes Withania somnifera (Ashwagandha), Baccopa monnieri (Brahmi), Acorus calamus (Calamus) and Hypericum perforatum (St. Johns Wort). The review concluded that many bioactive compounds and plant constituents that can be obtained from a qualitative diet, as well as certain medicinal plants and supplements, can help preserve and promote brain health and prevent the onset of ND. Large clinical trials are needed to assess their suitability for their wider use.
Abstract
A healthy diet shapes a healthy mind. Diet quality has a strong association with brain health. Diet influences the onset and consequences of neurological diseases, and dietary factors may influence mental health at individual and population level. The link between unhealthy diet, impaired cognitive function and neurodegenerative diseases indicates that adopting a healthy diet would ultimately afford prevention and management of neurological diseases and brain aging. Neurodegenerative diseases are of multifactorial origin and result in progressive loss of neuronal function in the brain, leading to cognitive impairment and motoneuron disorders. The so-called Mediterranean diet (MedDiet) with its healthy ingredients rich in antioxidant, anti-inflammatory, immune, neuroprotective, antidepressant, antistress and senolytic activity plays an essential role in the prevention and management of neurological diseases and inhibits cognitive decline in neurodegenerative diseases such as Alzheimer's, Parkinson's and Huntington's diseases. The MedDiet also modulates the gut-brain axis by promoting a diversity of gut microbiota. In view of the importance of diet in neurological diseases management, this review focuses on the dietary components, natural compounds and medicinal plants that have proven beneficial in neurological diseases and for brain health. Among them, polyphenols, omega-3 fatty acids, B vitamins and several ayurvedic herbs have promising beneficial effects.
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Nutritional intervention for diabetes mellitus with Alzheimer's disease.
Li, Z, Li, S, Xiao, Y, Zhong, T, Yu, X, Wang, L
Frontiers in nutrition. 2022;9:1046726
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Diabetes Mellitus (DM) affects more than 463 million people worldwide. Similarly, the number of deaths related to Alzheimer’s disease (AD) has increased by 145%. There are several common risk factors for Type 2 Diabetes and AD, including obesity, insulin resistance, and ageing, as well as common pathological mechanisms, including altered insulin signalling, oxidative stress, neuroinflammation, mitochondrial dysfunction, formation of glycated proteins and metabolic syndrome. This review aims to summarize the therapeutic effects of different nutritional therapy strategies on the reduction of DM and AD risk. Controlling blood sugar levels and reducing calorie intake is crucial to preventing diabetes and Alzheimer's disease. The low-carbohydrate, ketogenic, and Mediterranean diets have been found to improve glucose control in people with Type 2 diabetes (T2D). In addition, MIND (Mediterranean-DASH Diet Intervention for Neurodegenerative Delay) and a ketogenic diet may improve cognition in AD patients. Lactobacillus, Bifidobacterium probiotics, and prebiotics, such as inulin, may inhibit the progression of T2D and AD diseases by suppressing inflammation and modulating gut microbes. In addition, vitamins A, C, D, E, B6, B12, folate, long-chain polyunsaturated fatty acids, zinc, magnesium, and polyphenols may improve cognitive decline, homocysteine levels, and insulin resistance in AD and T2D patients. Healthcare professionals can use the results of this review to understand the beneficial effects of dietary strategies and multi-nutrient supplementation on DM and AD. However, further robust studies are required to investigate the risk factors and underlying mechanisms behind DM-combined AD progression.
Abstract
The combined disease burden of diabetes mellitus (DM) and Alzheimer's disease (AD) is increasing, and the two diseases share some common pathological changes. However, the pharmacotherapeutic approach to this clinical complexity is limited to symptomatic rather than disease-arresting, with the possible exception of metformin. Whether nutritional intervention might extend or synergize with these effects of metformin is of interest. In particular, dietary patterns with an emphasis on dietary diversity shown to affect cognitive function are of growing interest in a range of food cultural settings. This paper presents the association between diabetes and AD. In addition, the cross-cultural nutritional intervention programs with the potential to mitigate both insulin resistance (IR) and hyperglycemia, together with cognitive impairment are also reviewed. Both dietary patterns and nutritional supplementation showed the effects of improving glycemic control and reducing cognitive decline in diabetes associated with AD, but the intervention specificity remained controversial. Multi-nutrient supplements combined with diverse diets may have preventive and therapeutic potential for DM combined with AD, at least as related to the B vitamin group and folate-dependent homocysteine (Hcy). The nutritional intervention has promise in the prevention and management of DM and AD comorbidities, and more clinical studies would be of nutritional scientific merit.
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Is Air Pollution Associated with Increased Risk of Dementia? A Meta-Analysis of Epidemiological Research.
Dhiman, V, Trushna, T, Raj, D, Tiwari, RR
Neurology India. 2022;70(3):1004-1019
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Several studies have indicated that there may be a link between exposure to air pollution and the development of dementia. This study aimed to perform a systematic review and meta-analysis to determine the risk of developing dementia, cognitive decline, and/or Alzheimer’s disease (AD) following air pollution exposure. The results showed that exposure to small particulate matter (less than 2.5 micro m) was a risk factor for the development of dementia, AD and dementia caused by blockages in the blood vessels leading to the brain. Exposure to ozone was a risk factor for AD. Carbon monoxide, large particulate matter (10 micro m and less), nitrogen dioxide, nitrous oxides, and sulphur dioxide exposure was not a risk factor for any dementia related diseases. It was concluded that exposure to certain types of pollution may be detrimental to brain health and further research is required in developing countries, where pollution exposure can be particularly high. This study could be used by health care professionals to make informed decisions on the control of air pollution in those who are at risk of developing dementia or who are already suffering from this disease. The introduction of plants and air purifiers may be of benefit to those who are vulnerable.
Abstract
BACKGROUND There are prevailing inconsistencies in epidemiological research about air pollution being a risk factor for dementia. OBJECTIVE We performed meta-analyses to calculate the pooled estimates of the risk of developing dementia due to air pollution exposure. METHODS AND MATERIALS We performed a systematic search in PubMed, Google Scholar, The Cochrane Library, and J-GATEPLUS databases for peer-reviewed epidemiological studies reporting the risk of developing all-cause dementia, cognitive decline, Alzheimer's disease (AD), and vascular dementia (VaD) due to exposure to particulate matter with an aerodynamic diameter less than 2.5 μm (PM2.5) and less than 10 μm (PM10), ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), nitrogen oxides (NOX) and sulfur dioxide (SO2) from the beginning until December 2020. Meta-analysis was performed by adopting the random-effects model using Meta-XL. RESULTS In all-cause dementia, the pooled hazard ratio (HR) for PM2.5 and NO2 exposure was 1.03 [(95%CI: 1.01-1.06; I2 = 99% (P < 0.001)] and 1.00 [(95%CI: 1.00-1.01; I2 = 96% (P < 0.001)], respectively. The pooled HR for NOX was 1.00 [(95%CI: 1.00-1.01; I2 = 61% (P = 0.05)]. In AD, the pooled HRs for PM2.5 and O3 was 1.08 (95%CI: 1.01-1.15; I2 = 99% (P < 0.001)]) and 1.02 (95%CI: 0.96-1.08; I2 = 100% (P < 0.001)], respectively. In VaD, the pooled HR for PM2.5 exposure was 1.03 (95%CI: 1.01-1.06; I2 = 91% (P < 0.001)]. In NO2/NOX, the results were found to be equivocal. Meta-analysis could not be performed in cognitive decline because of wide variations in assessments methods. CONCLUSIONS The present study showed exposure to PM2.5 as a risk factor for all-cause dementia, AD, and VaD and exposure to O3 as a possible risk factor for AD. These findings can be used for policy measures and further research.