Cognitive decline, dietary factors and gut–brain interactions Study Notes

Cognitive Decline, Dietary Factors, and Gut-Brain Interactions

Authors and Affiliations

  • Barbara Caracciolo, Aging Research Center, Department of Neurobiology, Care Sciences and Society (NVS), Karolinska Institutet and Stockholm University, Stockholm, Sweden.

  • Weili Xu, Aging Research Center, Department of Neurobiology, Care Sciences and Society (NVS), Karolinska Institutet and Stockholm University, Stockholm, Sweden.

  • Stephen Collins, Farncombe Family Digestive Health Research Institute, Faculty of Health Sciences, McMaster University, Hamilton, Ontario, Canada.

  • Laura Fratiglioni, Aging Research Center, Department of Neurobiology, Care Sciences and Society (NVS), Karolinska Institutet and Stockholm University, and Stockholm Gerontology Research Center, Stockholm, Sweden.

1. Introduction

  • Aging profoundly impacts the brain at various levels (cellular, functional).

  • Declines in sensory, motor, and higher cognitive functions appear with age, highlighting the interaction between normal physiological changes and age-related diseases (Salthouse, 2009; Schaffer et al., 2012).

Cognitive decline spectrum

  • Cognitive decline encompasses a range from intact cognition through mild cognitive impairment (MCI) to dementia:

    • Mild Cognitive Impairment (MCI): An intermediate stage where functional independence is maintained, but neuropsychological tests show impairment (Petersen, 2004).

    • Dementia: Characterized by a significant, progressive decline in multiple cognitive domains, affecting daily functioning (APA, 2013).

    • Alzheimer’s Disease (AD): Most prevalent cause of dementia (60-70% of cases); diagnosed by presence of neurofibrillary tangles and senile plaques (Blennow et al., 2006).

    • Vascular Dementia (VaD): Second-most common cause of dementia, resulting from ischemic or hemorrhagic brain lesions (Roman, 2003).

Prevalence of dementia

  • Age-specific prevalence rates nearly double every five years, climbing from approximately 1.5% in those aged 60-69 to 40% in people aged 90+.

  • Global dementia prevalence is around 3.9% for individuals over 60, with regional variations:

    • 1.6% in Africa,

    • 3.9% in Eastern Europe,

    • 4.0% in China,

    • 4.6% in Latin America,

    • 5.4% in Western Europe,

    • 6.4% in North America (Qiu et al., 2007).

  • In Europe, age-adjusted prevalence rates for dementia are: 6.4% overall; 4.4% for AD; and 1.6% for VaD (Lobo et al., 2000; McVeigh and Passmore, 2006).

  • Estimated 36 million individuals globally suffer from dementia, with 4.6 million new cases occurring annually (Ferri et al., 2005).

  • Incidents of dementia show minimal geographical variation, around 7.5 per 1000 person-years, with age-related spikes (1 in 1000 at ages 60-64 to over 70 per 1000 at 90+ years) (Qiu et al., 2007).

2. Diet: A Key Modifiable Risk Factor for Dementia and Predementia Syndromes

  • Epidemiological research indicates a link between modifiable lifestyle factors and cognitive decline, suggesting prevention avenues (Solfrizzi et al., 2008).

  • Dietary factors have become a core subject in cognitive aging research.

2.1. Brain, Nutrients, and Neuroprotection

  • Nutrients are essential bioactive molecules, most of which must be sourced from food (Morris, 2012).

  • The high metabolism and turnover rate of nutrients in the brain make it especially dependent on optimal nutrient intake.

2.1.1. Oxidative Stress and Vitamins

  • High metabolic rates in the brain result in oxidative stress (Bishop et al., 2010).

  • Regulation of oxidative stress involves:

    • Antioxidant enzymes (endogenous) needing exogenous nutrients including vitamin E, vitamin C, carotenoids (e.g., beta-carotene), manganese, copper, selenium, and zinc.

ACE Vitamins

  • Studies show inconsistent results regarding dietary antioxidant intake and cognitive decline.

    • Vitamin E has correlations with reduced AD risk in some longitudinal studies (Devore et al., 2010; Engelhart et al., 2002b).

    • Contradictory evidence for vitamins C and A regarding dementia and cognitive decline (Corrada et al., 2005; Devore et al., 2010).

2.1.2. Inflammation, Polyphenols, and Unsaturated Fats

  • Inflammation is critical in the pathogenesis of atherosclerosis and neuroinflammation, linking to neurodegeneration (Gorelick, 2010).

    • Increased serum C-reactive protein (CRP) correlates with heightened risk of AD and VaD.

2.1.2.1. Polyphenols

  • Polyphenols (plant secondary metabolites) include flavonoids and are found in fruits, vegetables, tea, spices, and olive oil; they may regulate oxidative stress and enhance vascular health (Stevenson and Hurst, 2007).

  • Studies, such as PAQUID, suggest protective roles of flavonoids against cognitive decline (Commenges et al., 2000).

2.1.2.2. Unsaturated Fatty Acids

  • Two classes: Monounsaturated (MUFA) and Polyunsaturated (PUFA), crucial for brain integrity and cognitive function (Gillette-Guyonnet et al., 2013).

  • Dietary intakes of various FAs, particularly omega-3 (DHA), are being studied for links to dementia prevention.

2.2. Dietary Patterns and Cognitive Decline

  • Evidence suggests dietary patterns, like the Mediterranean diet (MeDi), may be more beneficial than high intakes of individual nutrients (Kesse-Guyot et al., 2012).

  • MeDi emphasizes fruits, vegetables, fish, and olive oil, which correlate with lower cognitive decline rates (Tangney et al., 2011).

3. Vascular Risk Factors/Diseases

  • Key vascular factors associated with dementia risk include hypertension, high cholesterol, obesity, and diabetes (Luchsinger and Gustafson, 2009).

3.1. Hypercholesterolemia

  • Mixed findings on the relationship between cholesterol levels in midlife and late life (Kivipelto et al., 2002).

  • Recent evidence suggests a decline in total cholesterol after midlife may indicate early dementia stages (Solomon et al., 2009).

3.2. Adiposity

  • Obesity in adulthood shows strong correlations with increased dementia risk (Xu et al., 2011).

3.3. Diabetes Mellitus

  • Diabetes has been widely studied for its link to dementia, yielding inconsistent results, particularly concerning AD (Arvanitakis et al., 2006b).

3.4. Hypertension

  • Studies show a relationship between hypertension in midlife and risk of cognitive decline and dementia (Wysocki et al., 2012).

3.5. Serum Homocysteine and B Vitamins

  • B vitamins potentially protect against cognitive decline, but definitive epidemiological links remain weak (Luchsinger and Mayeux, 2004).

4. The Role of Gut Health in Brain Functioning

  • The gut microbiome's composition reportedly correlates with cognitive function.

4.1. Studies in Mice on Microbiome–Brain Interactions

  • Germ-free mice studies indicate the gut microbiome's crucial role in brain function and development.

4.2. Human Studies on Microbiome–Brain Interactions

  • Alterations in gut microbiota in specific conditions, like hepatic encephalopathy, correlate with cognitive impairment (Bajaj et al., 2012).

4.3. The Intestinal Microbiome in the Elderly

  • Changes in the intestinal microbiome with aging correlate with cognitive decline and frailty (Biagi et al., 2010).

5. Conclusion

  • Despite acknowledging dietary relevance in cognitive health, gaps in understanding persist.

  • Future studies should explore big cohort datasets including dietary assessments to clarify the interactions between diet and brain health.