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Evolution of the Human Diet and Its Impact on Gut Microbiota, Immune Responses, and Brain Health.
González Olmo, BM, Butler, MJ, Barrientos, RM
Nutrients. 2021;13(1)
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One explanation for the increased prevalence in chronic disease and mental illness is from the evolutionary perspective. This suggests the rapid shift in diet towards processed foods in the past 200 years has not allowed for sufficient adaptation of the gut microbiome. The gut microbiome plays an important role in the digestive, immune and nervous systems via the gut-brain axis, and may be a key factor in modulating inflammation and disease. The aim of this review is to discuss how what we eat affects the immune system and impacts our brain health. The literature currently shows significant associations between the Western diet and its impact on the health of the gut microbiome and the brain. Increased intake of saturated fats, refined carbohydrates and sugar, coupled with a reduction in fiber, negatively impacts the digestive system and elicits an immune response. This response can lead to neuroinflammation, which is now found to be associated with deficits in learning and memory, as well as increased rates of neurodegenerative disease and depression. Based on the existing literature, the authors conclude the human gut microbiome has not had sufficient time to adapt to many modern foods, thus leading to inflammation and disease. The authors recommend that a diet composed of natural whole foods with minimal processing can help prevent and alleviate some of the burden caused by chronic disease, and suggest future studies focus on improving techniques to evaluate neuroinflammation in humans.
Abstract
The relatively rapid shift from consuming preagricultural wild foods for thousands of years, to consuming postindustrial semi-processed and ultra-processed foods endemic of the Western world less than 200 years ago did not allow for evolutionary adaptation of the commensal microbial species that inhabit the human gastrointestinal (GI) tract, and this has significantly impacted gut health. The human gut microbiota, the diverse and dynamic population of microbes, has been demonstrated to have extensive and important interactions with the digestive, immune, and nervous systems. Western diet-induced dysbiosis of the gut microbiota has been shown to negatively impact human digestive physiology, to have pathogenic effects on the immune system, and, in turn, cause exaggerated neuroinflammation. Given the tremendous amount of evidence linking neuroinflammation with neural dysfunction, it is no surprise that the Western diet has been implicated in the development of many diseases and disorders of the brain, including memory impairments, neurodegenerative disorders, and depression. In this review, we discuss each of these concepts to understand how what we eat can lead to cognitive and psychiatric diseases.
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The Perfect Cytokine Storm: How Peripheral Immune Challenges Impact Brain Plasticity & Memory Function in Aging.
Muscat, SM, Barrientos, RM
Brain plasticity (Amsterdam, Netherlands). 2021;(1):47-60
Abstract
Precipitous declines in cognitive function can occur in older individuals following a variety of peripheral immune insults, such as surgery, infection, injury, and unhealthy diet. Aging is associated with numerous changes to the immune system that shed some light on why this abrupt cognitive deterioration may occur. Normally, peripheral-to-brain immune signaling is tightly regulated and advantageous; communication between the two systems is bi-directional, via either humoral or neural routes. Following an immune challenge, production, secretion, and translocation of cytokines into the brain is critical to the development of adaptive sickness behaviors. However, aging is normally associated with neuroinflammatory priming, notably microglial sensitization. Microglia are the brain's innate immune cells and become sensitized with advanced age, such that upon immune stimulation they will mount more exaggerated neuroimmune responses. The resultant elevation of pro-inflammatory cytokine expression, namely IL-1β, has profound effects on synaptic plasticity and, consequentially, cognition. In this review, we (1) investigate the processes which lead to aberrantly elevated inflammatory cytokine expression in the aged brain and (2) examine the impact of the pro-inflammatory cytokine IL-1β on brain plasticity mechanisms, including its effects on BDNF, AMPA and NMDA receptor-mediated long-term potentiation.
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Lifestyle modifications with anti-neuroinflammatory benefits in the aging population.
Muscat, SM, Barrientos, RM
Experimental gerontology. 2020;:111144
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Abstract
Aging-associated microglial priming results in the potential for an exaggerated neuroinflammatory response to a subsequent inflammatory challenge in regions of the brain known to support learning and memory. This excessive neuroinflammation in the aging brain is known to occur following a variety of peripheral insults, including infection and surgery, where it has been associated with precipitous declines in cognition and memory. As the average lifespan increases worldwide, identifying interventions to prevent and treat aging-associated excessive neuroinflammation and ensuing cognitive impairments is of critical importance. Lifestyle has emerged as a potential non-pharmacological target in this endeavor. Here, we review important and recent preclinical and clinical literature demonstrating the anti-inflammatory effects of lifestyle modifications such as exercise, diet, and environmental enrichment in the context of aging and memory. Importantly, we focus on research indicating that these lifestyle modifications do not need to be lifelong, suggesting that such interventions may be efficacious in the prevention and treatment of aging- and neuroinflammation-associated cognitive impairment, even when initiated in older age.
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The impact of nutrition on COVID-19 susceptibility and long-term consequences.
Butler, MJ, Barrientos, RM
Brain, behavior, and immunity. 2020;87:53-54
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The impacts of Covid-19 are being felt across the world, affecting health, healthcare and economies. Statistics from across the world are showing that the elderly, those with underlying medical conditions and under-represented minority groups are particularly vulnerable to severe complications and have a higher risk of dying of Covid-19. This opinion piece presents arguments for the importance of focusing on diet to support health resilience in general and the immune system in particular, to minimise the impact of this and future viruses. Research is presented on excessive intake of saturated fat leading to chronic activation of the innate immune system (first line, rapid defence against infection), resulting in inflammation, and associated heightened susceptibility to complications of viral infection. The standard western diet (high saturated fat, refined carbohydrates and sugars, low levels of fibre, unsaturated fat and antioxidants) has also been shown to affect the adaptive immune system (second line, delayed defence against infection), depressing its action against infection. The piece also discusses possible long-term, future impacts of those recovered from Covid-19 infection, particularly in relation to neurodegenerative diseases such as Alzheimer’s. The authors call for fresh, healthy wholefoods to be readily available and affordable to everyone in society.
Abstract
While all groups are affected by the COVID-19 pandemic, the elderly, underrepresented minorities, and those with underlying medical conditions are at the greatest risk. The high rate of consumption of diets high in saturated fats, sugars, and refined carbohydrates (collectively called Western diet, WD) worldwide, contribute to the prevalence of obesity and type 2 diabetes, and could place these populations at an increased risk for severe COVID-19 pathology and mortality. WD consumption activates the innate immune system and impairs adaptive immunity, leading to chronic inflammation and impaired host defense against viruses. Furthermore, peripheral inflammation caused by COVID-19 may have long-term consequences in those that recover, leading to chronic medical conditions such as dementia and neurodegenerative disease, likely through neuroinflammatory mechanisms that can be compounded by an unhealthy diet. Thus, now more than ever, wider access to healthy foods should be a top priority and individuals should be mindful of healthy eating habits to reduce susceptibility to and long-term complications from COVID-19.
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Food for thought: how nutrition impacts cognition and emotion.
Spencer, SJ, Korosi, A, Layé, S, Shukitt-Hale, B, Barrientos, RM
NPJ science of food. 2017;:7
Abstract
More than one-third of American adults are obese and statistics are similar worldwide. Caloric intake and diet composition have large and lasting effects on cognition and emotion, especially during critical periods in development, but the neural mechanisms for these effects are not well understood. A clear understanding of the cognitive-emotional processes underpinning desires to over-consume foods can assist more effective prevention and treatments of obesity. This review addresses recent work linking dietary fat intake and omega-3 polyunsaturated fatty acid dietary imbalance with inflammation in developing, adult, and aged brains. Thus, early-life diet and exposure to stress can lead to cognitive dysfunction throughout life and there is potential for early nutritional interventions (e.g., with essential micronutrients) for preventing these deficits. Likewise, acute consumption of a high-fat diet primes the hippocampus to produce a potentiated neuroinflammatory response to a mild immune challenge, causing memory deficits. Low dietary intake of omega-3 polyunsaturated fatty acids can also contribute to depression through its effects on endocannabinoid and inflammatory pathways in specific brain regions leading to synaptic phagocytosis by microglia in the hippocampus, contributing to memory loss. However, encouragingly, consumption of fruits and vegetables high in polyphenolics can prevent and even reverse age-related cognitive deficits by lowering oxidative stress and inflammation. Understanding relationships between diet, cognition, and emotion is necessary to uncover mechanisms involved in and strategies to prevent or attenuate comorbid neurological conditions in obese individuals.