Interleukin-6 and risk of cognitive decline: MacArthur studies of successful aging

Can consuming flavonoids restore old microglia to their youthful state?

Microglial cells, which are resident macrophages in the central nervous system, are "primed" in the aged brain and are hypersensitive to messages emerging from immune-to-brain signaling pathways. Thus, in elderly individuals who have an infection, microglia overreact to signals from the peripheral immune system and produce excessive levels of cytokines, causing behavioral pathology including serious deficits in cognition. Importantly, recent studies indicate dietary flavonoids have anti-inflammatory properties and are capable of mitigating microglial cells in the brains of aged mice. Thus, dietary or supplemental flavonoids and other bioactive agents have the potential to restore the population of microglial cells in the elderly brain to its youthful state. This review briefly describes the immune-to-brain signaling pathways, consequences of microglial cell priming, and the potential of flavonoids to mitigate brain microglia and cognitive deficits induced by inflammatory cytokines.

Rosuvastatin reduces microglia in the brain of wild type and ApoE knockout mice on a high cholesterol diet; implications for prevention of stroke and AD

We have previously shown that a high cholesterol (HC) diet results in increases in microglia load and levels of the pro-inflammatory cytokine interleukin-6 (IL-6) in the brains of wild type (WT) and apolipoprotein E knockout (ApoE-/-) mice. In the present investigation, we analyzed whether treatment with rosuvastatin, an inhibitor of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, would prevent the increases in inflammatory microglia and IL-6 levels in the brain and plasma of WT and ApoE-/- mice. We report that a HC diet resulted in an increased microglia load in the brains of WT and ApoE-/- mice, in support of our previous study. Treatment with rosuvastatin significantly decreased the microglia load in the brains of WT and ApoE-/- mice on a HC diet. Rosuvastatin treatment resulted in lowered plasma IL-6 levels in WT mice on a HC diet. However, in the present study the number of IL-6 positive cells in the brain was not significantly affected by a HC diet. A recent clinical study has shown that rosuvastatin reduces risk of ischemic stroke in patients with high plasma levels of the inflammatory marker C-reactive protein by 50%. The results from our study show that rosuvastatin reduces inflammatory cells in the brain. This finding is essential for furthering the prevention and treatment of neurodegenerative diseases such as Alzheimer's disease (AD) and stroke.

Luteolin inhibits microglia and alters hippocampal-dependent spatial working memory in aged mice

A dysregulated overexpression of inflammatory mediators by microglia may facilitate cognitive aging and neurodegeneration. Considerable evidence suggests the flavonoid luteolin has antiinflammatory effects, but its ability to inhibit microglia, reduce inflammatory mediators, and improve hippocampal-dependent learning and memory in aged mice is unknown. In initial studies, pretreatment of BV-2 microglia with luteolin inhibited the induction of inflammatory genes and the release of inflammatory mediators after lipopolysaccharide (LPS) stimulation. Supernatants from LPS-stimulated microglia caused discernible death in Neuro.2a cells. However, treating microglia with luteolin prior to LPS reduced neuronal cell death caused by conditioned supernatants, indicating luteolin was neuroprotective. In subsequent studies, adult (3-6 mo) and aged (22-24 mo) mice were fed control or luteolin (20 mg/d)-supplemented diet for 4 wk and spatial working memory was assessed as were several inflammatory markers in the hippocampus. Aged mice fed control diet exhibited deficits in spatial working memory and expression of inflammatory markers in the hippocampus indicative of increased microglial cell activity. Luteolin consumption improved spatial working memory and restored expression of inflammatory markers in the hippocampus compared with that of young adults. Luteolin did not affect either spatial working memory or inflammatory markers in young adults. Taken together, the current findings suggest dietary luteolin enhanced spatial working memory by mitigating microglial-associated inflammation in the hippocampus. Therefore, luteolin consumption may be beneficial in preventing or treating conditions involving increased microglial cell activity and inflammation.

Neuroimmune pharmacology from a neuroscience perspective

The focus of this commentary is to describe how neuroscience, immunology, and pharmacology intersect and how interdisciplinary research involving these areas has expanded knowledge in the area of neuroscience, in particular. Examples are presented to illustrate that the brain can react to the peripheral immune system and possesses immune function and that resident immune molecules play a role in normal brain physiology. In addition, evidence is presented that the brain immune system plays an important role in mediating neurodegenerative diseases, the aging process, and neurodevelopment and synaptic plasticity. The identification of these mechanisms has been facilitated by pharmacological studies and has opened new possibilities for pharmacotherapeutic approaches to the treatment of brain disorders. The emerging field of neuroimmune pharmacology exemplifies this interdisciplinary approach and has facilitated the study of basic cellular and molecular events and disease states and opens avenues for novel therapies.

Hyperammonemia induces neuroinflammation that contributes to cognitive impairment in rats with hepatic encephalopathy

BACKGROUND & AIMS

Hyperammonemia and inflammation cooperate to induce neurological alterations in hepatic encephalopathy. Recent studies in animal models suggest that chronic hyperammonemia and neuroinflammation impair learning ability by the same mechanism. Chronic hyperammonemia might induce inflammatory factors in the brain that impair cognitive function. We sought to determine whether hyperammonemia itself induces neuroinflammation, whether ammonia-induced neuroinflammation mediates cognitive impairment, and whether neuroinflammation also occurs in rats with bile duct ligation (BDL rats)-a model of chronic liver injury that results in hyperammonemia and hepatic encephalopathy.

METHODS

Chronic moderate hyperammonemia was induced by feeding male Wistar rats an ammonium-containing diet or performing BDL. Rats that received a standard diet or a sham operation were used as controls. Neuroinflammation was assessed by measuring activation of microglia and inflammatory factors. Brain samples were collected from hyperammonemic and BDL rats; microglial activation was determined by immunohistochemistry and quantification of inflammatory markers (ie, inducible nitric oxide synthase, interleukin-1beta, and prostaglandin E2). Learning ability and motor activity were assessed in hyperammonemic and BDL rats given ibuprofen as an anti-inflammatory agent.

RESULTS

Chronic moderate hyperammonemia or BDL activated the microglia, especially in cerebellum; increased inducible nitric oxide synthase, interleukin-1beta, and prostaglandin E2 levels; and impaired cognitive and motor function, compared with controls. Ibuprofen reduced microglial activation and restored cognitive and motor functions in the hyperammonemic and BDL rats.

CONCLUSIONS

Chronic hyperammonemia is sufficient to induce microglial activation and neuroinflammation; these contribute to the cognitive and motor alterations that occur during hepatic encephalopathy.

Cognitive status and future risk of frailty in older Mexican Americans

BACKGROUND

Because cognitive impairment and frailty share common risk factors (eg, high proinflammatory cytokines), we examined whether poor cognition predicts subsequent risk of frailty in initially nonfrail Mexican Americans aged 67 years and older.

METHODS

Frailty was defined as meeting one or more of the following components: (a) unintentional weight loss of >10 pounds, (b) weakness, (c) self-reported exhaustion, and (d) slow walking speed. Sociodemographic factors, Mini-Mental State Examination, medical conditions (stroke, heart attack, diabetes, arthritis, cancer, and hypertension), and depressive symptoms were obtained. Main outcome measure was risk of becoming frail over 10 years.

RESULTS

Out of 942 participants who were nonfrail at baseline (1995-1996), 57.8% were women and the mean age was 73.7 years (SD = 5.3). In general estimation equation models testing the relationship between Mini-Mental State Examination (<21 vs. ≥21) and the risk of becoming frail over a 10-year period, there was a significant association (odds ratio = 1.09, 95% confidence interval = 1.00-1.19; p = .0310)] between the cognition-by-time interaction and odds of becoming prefrail or frail over time. This association was independent of age, sex, marital status, education, time, and medical conditions, indicating that nonfrail participants with poor cognition had a 9% odds per year of becoming frail over time compared with those with good cognition.

CONCLUSION

Low Mini-Mental State Examination score was independently associated with increased risk of frailty over a 10-year period in older Mexican Americans. Low Mini-Mental State Examination score may be an early marker for future risk of frailty.

Role of lipoproteins and inflammation in cognitive decline: do they interact?

The aim of this study was to examine the associations between high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol, triglycerides, and cognition and focus on the modifying effect of inflammation. Data were collected in the population-based Longitudinal Aging Study Amsterdam and analyzed with mixed linear models. The sample comprised 1003 persons ≥ 65 years with cognitive data on at least 2 occasions over 6 years of follow-up. Cognition was measured with the Mini-Mental State Examination (general cognition), Auditory Verbal Learning Test (memory), and Coding Task (information processing speed). We found an independent association between high HDL cholesterol and better memory performance. In addition, low LDL cholesterol was predictive of worse general cognitive performance and faster decline on information processing speed. Furthermore, a significant modifying effect of inflammation (C-reactive protein, α-antichymotrypsin) was found. A negative additive effect of low LDL cholesterol and high inflammation was found on general cognition and memory performance. Also, high triglycerides were associated with lower memory performance in those with high inflammation. Thus, a combination of these factors may be used as markers of prolonged lower cognitive functioning.

Correlations between peripheral blood mononuclear cell production of BDNF, TNF-alpha, IL-6, IL-10 and cognitive performances in multiple sclerosis patients

The aim of this study was to investigate the role of Brain Derived Neurotrophic Factor (BDNF) and inflammatory factors in the development of cognitive dysfunctions in Multiple Sclerosis (MS). We correlated peripheral blood mononuclear cell (PBMC) production of BDNF, Tumor Necrosis Factor-alpha (TNF-alpha), Interleukin (IL)-6 and IL-10 with performances on specific neuropsychological tasks in a selected series of MS patients. We studied a sample of 30 patients with relapsing-remitting (RR)MS, segregated by gender and matched for age, education, disease duration, type of immunomodulating therapy, degree of disability and overall cognitive status. We found that low BDNF levels were correlated with increased time of execution on a divided attention and visual scanning task whereas high levels of IL-6 were correlated with low Mini Mental State Examination scores. We did not observe any significant correlations between IL-10, TNF-alpha levels and cognitive performances in our patients. In conclusion our study shows a correlation between low BDNF and high IL-6 production by PBMCs and poorer performances in cognitive tasks in RRMS patients suggesting a possible role of these factors in cognitive impairment in MS.

Cerebral FDG-PET scanning abnormalities in optimally treated HIV patients

Background

The long-term neurological consequences of HIV infection and treatment are not yet completely understood. In this study we examined the prevalence of cerebral metabolic abnormalities among a cohort of neurologically intact HIV patients with fully suppressed HIV viral loads. Concomitant analyses of circulating brain derived neurotrophic factor (BDNF) were performed to correlate these abnormalities with potential signs of neuro-regenerating/protective activity, and concomitant analyses of circulating tumour necrosis factor (TNF) α, interleukin (IL) 6, and soluble urokinase plasminogen activator receptor (suPAR) were performed to correlate these abnormalities with potential signs of neurodegenerative processes.

Methods

The study population consisted of HIV-positive patients known to be infected for more than 5 years and on antiretroviral (ARV) treatment for a minimum of three years with no history of virological failure, a CD4 count above 200 × 10⁶ cells/l and no other co-morbidities. The distribution of the regional cerebral metabolic rate of glucose metabolism was measured using fluorine-18-flourodeoxyglucose positron emission tomography (FDG-PET) scanning. The PET scans were evaluated for individual pathology using Neurostat software and for group pathology using statistical parametric mapping (SPM). Circulating levels of BDNF, TNF α, IL-6 and suPAR were measured by ELISA techniques.

Results

More than half (55%) of the patients exhibited varying severities of mesial frontal reduction in the relative metabolic rate of glucose. Compared to healthy subjects, the patients with abnormal FDG-PET scanning results had a shorter history of known HIV infection, fewer years on antiretroviral therapy and higher levels of circulating TNF α and IL-6 (p = 0.08).

Conclusion

A large proportion of optimally treated HIV patients exhibit cerebral FDG-PET scanning abnormalities and elevated TNF α and IL-6 levels, which may indicate imminent neuronal damage. The neuroprotective effect of early ARV treatment should be considered in future prospective follow-up studies.

Interleukin 6-174 G/C promoter and variable number of tandem repeats (VNTR) gene polymorphisms in sporadic Alzheimer's disease

BACKGROUND

Previous studies examining the association between the interleukin 6 (IL-6)-174 C/G polymorphism and Alzheimer's disease (AD) have yielded conflicting results. Furthermore, the C allele of the IL-6 variable number of tandem repeats (VNTR) polymorphism was associated with a delayed onset and a decreased risk of AD.

METHODS

A total sample of 149 AD patients, and 298 age- and sex-matched unrelated caregivers from Apulia, southern Italy, were genotyped for the apolipoprotein E (APOE) polymorphism, the VNTR polymorphism in the 3' flanking region, and the -174G/C single-nucleotide polymorphism (SNP) in the promoter region of IL-6 gene on chromosome 7. Furthermore, we performed a haplotype analysis on these two polymorphisms on IL-6 locus.

RESULTS

IL-6 VNTR and -174G/C allele and genotype frequencies were similar between AD patients and controls, also after stratification for late-onset (> or =65 years) and early-onset (<65 years) or APOE epsilon4 status. Furthermore, there was no evidence of linkage disequilibrium between the VNTR and -174G/C polymorphisms, not supporting a previous reported additive effect of both IL-6 polymorphisms on AD risk.

CONCLUSIONS

Our findings did not support a role of IL-6-174 G/C and IL-6 VNTR polymorphisms in the risk of sporadic AD in southern Italy, suggesting that these polymorphisms of IL-6 gene were at most weak genetic determinants of AD.

Physical frailty is associated with incident mild cognitive impairment in community-based older persons

OBJECTIVES

To test the hypothesis that physical frailty is associated with risk of mild cognitive impairment (MCI).

DESIGN

Prospective, observational cohort study.

SETTING

Approximately 40 retirement communities across the Chicago metropolitan area.

PARTICIPANTS

More than 750 older persons without cognitive impairment at baseline.

MEASUREMENTS

Physical frailty, based on four components (grip strength, timed walk, body composition, and fatigue), was assessed at baseline, and cognitive function was assessed annually. Proportional hazards models adjusted for age, sex, and education were used to examine the association between physical frailty and the risk of incident MCI, and mixed effect models were used to examine the association between frailty and the rate of change in cognition.

RESULTS

During up to 12 years of annual follow-up, 305 of 761 (40%) persons developed MCI. In a proportional hazards model adjusted for age, sex, and education, physical frailty was associated with a high risk of incident MCI, such that each one-unit increase in physical frailty was associated with a 63% increase in the risk of MCI (hazard ratio=1.63; 95% confidence interval=1.27-2.08). This association persisted in analyses that required MCI to persist for at least 1 year and after controlling for depressive symptoms, disability, vascular risk factors, and vascular diseases. Furthermore, a higher level of physical frailty was associated with a faster rate of decline in global cognition and five cognitive systems (episodic memory, semantic memory, working memory, perceptual speed, and visuospatial abilities).

CONCLUSION

Physical frailty is associated with risk of MCI and a rapid rate of cognitive decline in aging.

Aging-dependent changes of microglial cells and their relevance for neurodegenerative disorders

Among multiple structural and functional brain changes, aging is accompanied by an increase of inflammatory signaling in the nervous system as well as a dysfunction of the immune system elsewhere. Although the long-held view that aging involves neurocognitive impairment is now dismissed, aging is a major risk factor for neurodegenerative diseases such as Alzheimer;s disease, Parkinson;s disease and Huntington's disease, among others. There are many age-related changes affecting the brain, contributing both to certain declining in function and increased frailty, which could singly and collectively affect neuronal viability and vulnerability. Among those changes, both inflammatory responses in aged brains and the altered regulation of toll like receptors, which appears to be relevant for understanding susceptibility to neurodegenerative processes, are linked to pathogenic mechanisms of several diseases. Here, we review how aging and pro-inflammatory environment could modulate microglial phenotype and its reactivity and contribute to the genesis of neurodegenerative processes. Data support our idea that age-related microglial cell changes, by inducing cytotoxicity in contrast to neuroprotection, could contribute to the onset of neurodegenerative changes. This view can have important implications for the development of new therapeutic approaches.

Midlife C-reactive protein and risk of cognitive decline: a 31-year follow-up

There is evidence for a relationship between raised inflammatory markers, including high sensitivity C-reactive protein (hs-CRP), measured late in life, and an increased risk of cognitive decline and dementia. This study evaluates the association of midlife hs-CRP concentrations with late-life longitudinal trends in cognitive function. Data are from the Honolulu-Asia Aging Study (HAAS), a longitudinal community-based study of Japanese American men. hs-CRP levels were measured on average 25 years before cognitive testing began in 1991. Subjects were followed from up to three follow-up examinations (mean of 6.1 years). At each exam, cognitive function was measured with the Cognitive Abilities Screening Instrument (CASI). This analysis includes a sub-sample of 691 subjects dementia-free in 1991. With incident dementia cases included, those with the highest quartile of hs-CRP had significantly more cognitive decline than those in the lowest quartile, after adjustment for baseline CASI score, demographic and cardiovascular risk factors. When cases were removed, there was no difference in cognitive decline by CRP quartile. This relationship was not modified by the presence of apolipoprotein E varepsilon4. These findings suggest that inflammatory mechanisms during midlife may reflect underlying processes contributing to dementia-related cognitive decline late in life.

Replication by the Epistasis Project of the interaction between the genes for IL-6 and IL-10 in the risk of Alzheimer's disease

Background

Chronic inflammation is a characteristic of Alzheimer's disease (AD). An interaction associated with the risk of AD has been reported between polymorphisms in the regulatory regions of the genes for the pro-inflammatory cytokine, interleukin-6 (IL-6, gene: IL6), and the anti-inflammatory cytokine, interleukin-10 (IL-10, gene: IL10).

Methods

We examined this interaction in the Epistasis Project, a collaboration of 7 AD research groups, contributing DNA samples from 1,757 cases of AD and 6,295 controls.

Results

We replicated the interaction. For IL6 rs2069837 AA × IL10 rs1800871 CC, the synergy factor (SF) was 1.63 (95% confidence interval: 1.10–2.41, p = 0.01), controlling for centre, age, gender and apolipoprotein E ε4 (APOEε4) genotype. Our results are consistent between North Europe (SF = 1.7, p = 0.03) and North Spain (SF = 2.0, p = 0.09). Further replication may require a meta-analysis. However, association due to linkage disequilibrium with other polymorphisms in the regulatory regions of these genes cannot be excluded.

Conclusion

We suggest that dysregulation of both IL-6 and IL-10 in some elderly people, due in part to genetic variations in the two genes, contributes to the development of AD. Thus, inflammation facilitates the onset of sporadic AD.

The effects of cardiovascular risk factors on cognitive compromise

As life expectancy in the United States continues to increase, the projected numbers of elderly people who will develop dementia will grow rapidly. This paper reviews four well-established cardiovascular risk factors (type 2 diabetes, hypertension, cholesterol, and inflammation), for which there is longitudinal epidemiological evidence of increased risk of dementia, Alzheimer's disease, mild cognitive impairment, and cognitive decline. These risk factors are of special interest because of their potential modif lability, which may affect the course of cognitive compromise. Diabetes is the cardiovascular risk factor (CvRF) most consistently associated with cognition. Hypertension in midlife is consistently associated with cognition, but its associations with late-life hypertension are less clear. Total cholesterol is not consistently associated with cognition, interleukin-6 and C-reactive protein are inflammatory markers relatively consistently associated with cognition. Composites of the CvRFs increase the risk for dementia in a dose-dependent fashion, suggesting a cumulative effect of these factors on neuronal stress. In the relatively few studies that have reported interactions of risk factors, they potentiate each other. The effect of each of these risk factors varies according to apolipoprotein E genotype, it may be that the effect of these risk factors varies according to the presence of the others, and these complex relationships underlie the biological mechanisms of cognitive compromise. This may be crucial for understanding the effects on cognition of druqs and other approaches, such as lifestyle chanqe, for treatinq these risk factors.

Modifiers of cognitive function and brain structure in middle-aged and elderly individuals with type 2 diabetes mellitus

Cognitive deficits and hippocampal atrophy, features that are shared with aging and dementia, have been described in type 2 diabetes mellitus (T2DM). T2DM is associated with obesity, hypertension, dyslipidemia, hypothalamic pituitary adrenocortical (HPA) axis abnormalities and inflammation, all of which have been shown to negatively impact the brain. However, since most reports in T2DM focused on glycemic control, the relative contribution of these modifying factors to the impairments observed in T2DM remains unclear. We contrasted 41 middle-aged dementia-free volunteers with T2DM (on average 7 years since diagnosis) with 47 age-, education-, and gender-matched non-insulin resistant controls on cognition and brain volumes. HPA axis activity and other modifiers that accompany T2DM were assessed to determine their impact on brain and cognition. Individuals with T2DM had specific verbal declarative memory deficits, reduced hippocampal and prefrontal volumes, and impaired HPA axis feedback control. Diminished cortisol suppression after dexamethasone and dyslipidemia were associated with decreased cognitive performance, whereas obesity was negatively related to hippocampal volume. Moreover, prefrontal volume was influenced by worse glycemic control. Thus, obesity and altered cortisol levels may contribute to the impact of T2DM on the hippocampal formation, resulting in decreased verbal declarative memory performance.

Relationship between a frailty-related phenotype and progressive deterioration of the immune system in HIV-infected men

CONTEXT

Immunological similarities have been noted between HIV-infected individuals and older HIV-negative adults. Immunologic alterations with aging have been noted in frailty in older adults, a clinical syndrome of high risk for mortality and other adverse outcomes. Using a frailty-related phenotype (FRP), we investigated in the Multicenter AIDS Cohort Study whether progressive deterioration of the immune system among HIV-positive individuals independently predicts onset of FRP.

METHODS

FRP was evaluated semiannually in 1046 HIV-infected men from 1994 to 2005. CD4 T-cell count and plasma viral load were evaluated as predictors of FRP by logistic regression (generalized estimating equations), adjusting for age, ethnicity, educational level, AIDS status, and treatment era [pre-highly active antiretroviral therapy (HAART) (1994-1995) and HAART (1996-1999 and 2000-2005)].

RESULTS

Adjusted prevalences of FRP remained low for CD4 T-cell counts >400 cells per cubic millimeter and increased exponentially and significantly for lower counts. Results were unaffected by treatment era. After 1996, CD4 T-cell count, but not plasma viral load, was independently associated with FRP.

CONCLUSIONS

CD4 T-cell count predicted the development of a FRP among HIV-infected men, independent of HAART use. This suggests that compromise of the immune system in HIV-infected individuals contributes to the systemic physiologic dysfunction of frailty.

Relationship between inflammation and cognitive function in obstructive sleep apnea

OBJECTIVES

Obstructive sleep apnea (OSA) can have adverse effects on cognitive functioning, mood, and cardiovascular functioning. OSA brings with it disturbances in sleep architecture, oxygenation, sympathetic nervous system function, and inflammatory processes. It is not clear which of these mechanisms is linked to the decrease in cognitive functioning. This study examined the effect of inflammatory parameters on cognitive dysfunction.

MATERIALS AND METHODS

Thirty-nine patients with untreated sleep apnea were evaluated by polysomnography and completed a battery of neuropsychological tests. After the first night of evaluation in the sleep laboratory, blood samples were taken for analysis of interleukin 6, tumor necrosis factor-alpha (TNF-alpha), and soluble TNF receptor 1 (sTNF-R1).

RESULTS

sTNF-R1 significantly correlated with cognitive dysfunction. In hierarchical linear regression analysis, measures of obstructive sleep apnea severity explained 5.5% of the variance in cognitive dysfunction (n.s.). After including sTNF-R1, percentage of variance explained by the full model increased more than threefold to 19.6% (F = 2.84, df = 3, 36, p = 0.05). Only sTNF-R1 had a significant individual relationship with cognitive dysfunction (beta = 0.376 t = 2.48, p = 0.02).

CONCLUSIONS

sTNF-R1 as a marker of chronic inflammation may be associated with diminished neuropsychological functioning in patients with OSA.

IL-6 and IL-18 in blood may discriminate cirrhotic patients with and without minimal hepatic encephalopathy

BACKGROUND AND AIMS

Patients with liver cirrhosis may present minimal hepatic encephalopathy (MHE) that can be unveiled using specific neuropsychologic examination. Evaluation of MHE in cirrhotic patients might have prognostic value. The psychometric HE score (PHES) has been recommended as the "gold standard" in the diagnosis of MHE. It has been proposed that critical flicker frequency (CFF) analysis would be useful for easier detection of MHE. It would also be useful to have some peripheral parameter that could reflect the presence of MHE. It has been recently proposed that inflammation-associated alterations and hyperammonemia may cooperate in the induction of hepatic encephalopathy. The aim of the present work was to assess whether there is a correlation between the alterations in parameters reflecting inflammation, hyperammonemia, and the presence of MHE.

METHODS

We have studied in 55 patients with liver cirrhosis and 26 controls the performance in the PHES battery and the CFF, ammonia, and some interleukins (ILs) as inflammatory markers.

RESULTS

IL-6 and IL-18 were significantly higher (2.5-fold and 2.2-fold, respectively) in patients with MHE than in those without MHE. There were significant correlations between IL-6 or IL-18 levels and PHES score and CFF. Moreover, all patients with MHE had IL-6 levels higher than 11 ng/mL, whereas all patients without MHE had IL-6 levels lower than 11 ng/mL.

CONCLUSIONS

Inflammatory alterations related with IL-6 and IL-18 may contribute to MHE. Serum concentration of IL-6 and IL-18 may be useful to discriminate cirrhotic patients with and without MHE.

The effects of cardiovascular risk factors on cognitive compromise

As life expectancy in the United States continues to increase, the projected numbers of elderly people who will develop dementia will grow rapidly. This paper reviews four well-established cardiovascular risk factors (type 2 diabetes, hypertension, cholesterol, and inflammation), for which there is longitudinal epidemiological evidence of increased risk of dementia, Alzheimer's disease, mild cognitive impairment, and cognitive decline. These risk factors are of special interest because of their potential modifiability, which may affect the course of cognitive compromise. Diabetes is the cardiovascular risk factor (CvRF) most consistently associated with cognition. Hypertension in midlife is consistently associated with cognition, but its associations with late-life hypertension are less clear. Total cholesterol is not consistently associated with cognition. Interleukin-6 and C-reactive protein are inflammatory markers relatively consistently associated with cognition. Composites of the CvRFs increase the risk for dementia in a dose-dependent fashion, suggesting a cumulative effect of these factors on neuronal stress. In the relatively few studies that have reported interactions of risk factors, they potentiate each other. The effect of each of these risk factors varies according to apolipoprotein E genotype. It may be that the effect of these risk factors varies according to the presence of the others, and these complex relationships underlie the biological mechanisms of cognitive compromise. This may be crucial for understanding the effects on cognition of drugs and other approaches, such as lifestyle change, for treating these risk factors.

Neuroinflammation associated with aging sensitizes the brain to the effects of infection or stress

The aging brain is characterized by a shift from the homeostatic balance of inflammatory mediators to a proinflammatory state. This increase in neuroinflammation is marked by increased numbers of activated and primed microglia, increased steady-state levels of inflammatory cytokines and decreases in anti-inflammatory molecules. These conditions sensitize the aged brain to produce an exaggerated response to the presence of an immune stimulus in the periphery or following exposure to a stressor. In the brain, proinflammatory cytokines can have profound effects on behavioral and neural processes. As the aged brain is primed to respond to inflammatory stimuli, infection or stress may produce more severe detriments in cognitive function in the aged. Typically after an immune stimulus, aged animals display prolonged sickness behaviors, increased cytokine induction and greater cognitive impairments compared to adults. Additionally, aging can also augment the central response to stressors leading to exaggerated cytokine induction and increased decrements in learning and memory. This alteration in neuroinflammation and resultant sensitization to extrinsic and intrinsic stressors can have considerable effects upon the elderly's recovery and coping during disease and stress.

Exaggerated sickness behavior and brain proinflammatory cytokine expression in aged mice in response to intracerebroventricular lipopolysaccharide

Age-associated changes in glial reactivity may predispose individuals to exacerbated neuroinflammatory cytokine responses that are permissive to cognitive and behavioral complications. The purpose of this study was to determine if aging is associated with an exaggerated sickness response to central innate immune activation. Our results show that intracerebroventricular (i.c.v.) administration of lipopolysaccharide (LPS) caused a heightened proinflammatory cytokine response (IL-1beta, IL-6, and TNFalpha) in the cerebellum 2h post i.c.v. injection in aged mice compared to adults. This amplified inflammatory profile was consistent with a brain region-dependent increase in reactive glial markers (MHC class II, TLR2 and TLR4). Moreover, LPS caused a prolonged sickness behavior response in aged mice that was paralleled by a protracted expression of brain cytokines in the cerebellum and hippocampus. Finally, central LPS injection caused amplified and prolonged IL-6 levels at the periphery of aged mice. Collectively, these data establish that activation of the central innate immune system leads to exacerbated neuroinflammation and prolonged sickness behavior in aged as compared to adult mice.

Inflammatory markers and cognitive function in middle-aged adults: the Whitehall II study

OBJECTIVES

To assess whether C-reactive protein (CRP) and interleukin-6 (IL-6) are associated with low cognitive performance and decline in middle-aged adults.

DESIGN/SETTING

The Whitehall II study; an ongoing large-scale, prospective occupational cohort study of employees from 20 London-based white-collar Civil Service departments.

PARTICIPANTS

Data from more than 3000 males and 1200 female employees.

MEASURES

Inflammatory makers measured in 1991-1993 and five cognitive tests (short-term verbal memory, inductive reasoning (AH4-I), vocabulary (Mill Hill), and phonemic and semantic fluency) performed in 1997-1999 and 2002-2004. Performance in the lowest sex-specific quintile indicated low cognitive performance or decline. Covariates included sociodemographics, health behaviours and health conditions.

RESULTS

In age-adjusted analyses both CRP and IL-6 were associated with all cognitive measures in 1997-1999, even though the association with memory was not consistent. After extensive adjustment raised CRP levels were only associated with poor cognitive performance on the AH4-I (OR=1.38; 95% CI: 1.05-1.82) and Mill Hill (OR=1.52; 95% CI: 1.14-2.03) and IL-6 on semantic fluency (OR=1.27; 95% CI: 1.14-2.03). Associations were more evident in men than in women. No clear relationship was observed for decline.

CONCLUSIONS

Our results suggest that raised levels of inflammatory markers in midlife are moderately associated with lower cognitive status, but little with cognitive decline.

Inflammation as a potential mediator for the association between periodontal disease and Alzheimer's disease

Periodontal disease (PDD) is associated with increased risk of cardiovascular disease, cerebrovascular disease, and mortality in many studies, while other studies have begun to suggest an association of PDD with Alzheimer's disease (AD). This paper discusses how infectious pathogens and systemic infection may play a role in AD. The roles of infection and inflammation are addressed specifically with regard to known AD pathologic lesions including senile plaques, neuron death, neurofibrillary tangles, and cerebrovascular changes. A testable model of proposed pathways between periodontal infection and AD is presented including three possible mechanisms: a) direct effects of infectious pathogens, b) inflammatory response to pathogens, and c) the effects on vascular integrity. The role of gene polymorphisms is discussed, including apolipoprotein (APOE) varepsilon4 as a pro-inflammatory and pro-infection genotype.

Interleukin-6 covaries inversely with hippocampal grey matter volume in middle-aged adults

BACKGROUND

Converging animal findings suggest that higher peripheral levels of inflammation are associated with activation of central inflammatory mechanisms that result in hippocampal neurodegeneration and related impairment of memory function. We have recently shown, consistent with animal findings, an inverse association between peripheral levels of interleukin-6 (IL-6), a relatively stable marker of systemic inflammation, and memory function in mid-life adults. In the current study, we extend this work to test whether systemic inflammation is associated with reduced grey matter volume of the hippocampus.

METHODS

For this purpose, we used a computational structural neuroimaging method (optimized voxel-based morphometry) to evaluate the relationship between plasma IL-6 levels and hippocampal grey matter volume in a sample of 76 relatively healthy community volunteers ages 30-54.

RESULTS

Peripheral levels of IL-6 covaried inversely with hippocampal grey matter volume, and this relationship persisted after accounting for several possible confounders, including age, gender, race, years of education, percent body fat, blood pressure, smoking, physical activity, hours of sleep, alcohol use, and total grey matter volume.

CONCLUSIONS

To our knowledge, this is the first report of a relationship between a peripheral marker of IL-6 and hippocampal grey matter volume, raising the possibility that low-grade systemic inflammation could plausibly presage subclinical cognitive decline in part via structural neural pathways.

Functional decline in cognitive impairment--the relationship between physical and cognitive function

BACKGROUND

Physical function decline is associated with dementia, which might either be mediated by the coexisting sarcopenia or directly related to the impaired cognition. Our objectives are to examine the relationship between cognitive function and performance-based physical function and to test the hypothesis that cognitive function is related to poor physical function independent of muscle mass.

METHODS

We measured muscle strength, performance-based physical function and muscle mass using dual-energy X-ray absorptiometry and cognitive function using the cognitive part of the Community Screening Instrument of Dementia (CSI-D) in 4,000 community-dwelling Chinese elderly aged >65 years. A CSI-D cognitive score of >28.40 was considered as cognitively impaired. The effect of cognitive impairment on muscle strength and physical function was analyzed by multivariate analysis with adjustment for age, appendicular skeletal mass (ASM), the Physical Activity Scale for the Elderly (PASE) and other comorbidities.

RESULTS

In both genders, the cognitively impaired (CSI-D cognitive score >28.40) group had a weaker grip strength (-5.10 kg, p < 0.001 in men; -1.08 kg in women, p < 0.001) and performed worse in the two physical function tests (in men, 6-meter walk speed, -0.13 m/s, p < 0.001, chair stand test, 1.42 s, p < 0.001; in women, 6-meter walk speed, -0.08 m/s, p < 0.001, chair stand test, 1.48 s, p < 0.001). After adjustment for age, ASM, PASE and other comorbidities, significant differences in grip strength (-2.60 kg, p < 0.001 in men; -0.49 kg, p = 0.011 in women) and the two physical function tests persisted between the cognitively impaired and nonimpaired group (in men, 6-meter walk speed, -0.072 m/s, p < 0.001, chair stand test, 0.80 s, p = 0.045; in women, 6-meter walk speed, -0.049 m/s, p < 0.001, chair stand test, 0.98 s, p < 0.001).

CONCLUSIONS

Poor physical function and muscle strength coexisted with cognitive impairment. This relationship was independent of muscle mass. It is likely therefore that the functional decline in dementia might be related directly to factors resulting in cognitive impairment independently of the coexisting sarcopenia.

Pre-operative inflammatory markers and the risk of postoperative delirium in elderly patients

OBJECTIVE

Pathophysiological mechanisms leading to delirium are not clear. Age is a known risk factor and hypothesised to be accompanied by a low-grade inflammatory state. Previous studies have shown an association between delirium and circulating proinflammatory markers in acutely ill and postoperative patients. In light of the ageing/inflammation theory, we investigated the association of these markers with delirium in not acutely ill, elderly patients.

METHODS

In a prospective nested case-control study levels of C-reactive protein (CRP), interleukin 6 (Il-6), insulin growth factor 1 (IGF-1) were measured pre-operatively in elderly patients admitted for hip-surgery. These levels were compared between patients who later developed a post-operative delirium and patients who did not. Patients were matched for age and disease severity.

RESULTS

Eighteen patients who developed delirium post-operatively were matched with 50 controls. Median APACHE-scores were below 16 in both groups. Pre-operative serum concentrations of CRP, Il-6 and IGF-1 did not differ between groups. IL-6 levels were associated with a measure of cognitive impairment.

CONCLUSION

In the present study no relationship was found between levels of pre-operative circulating pro-inflammatory markers and post-operative delirium in elderly patients, who were free from acute or severe disease.

Association between IL-8 cytokine and cognitive performance in an elderly general population—The MEMO-Study

OBJECTIVES

To investigate the associations between circulating cytokines and specific neuropsychological domains of cognitive functioning (memory, processing speed and motor function) and general cognitive function (MMSE) in healthy elderly individuals.

METHODS

In a cross-sectional study of 369 community dwelling elderly subjects, we examined the relationship between serum IL-1beta, sIL-4R, IL-6, IL-8, IL-10, IL-12 and TNF-alpha concentrations and cognitive performance using an extensive standardized and validated cognitive test battery assessing memory, word fluency, perceptual/cognitive speed, attention and executive functioning, and motor speed.

RESULTS

Multivariate analysis adjusted for various confounders and Bonferroni correction for multiple comparisons demonstrated that increased serum concentrations of IL-8 were associated with poor performance in the memory and speed domains and in motor function. No significant associations were found between the remaining cytokines and domains of cognitive functioning. Global cognitive functioning, as measured with MMSE, was not associated with any cytokine.

CONCLUSIONS

This study suggests an association between circulating IL-8 concentrations and cognitive dysfunction in the elderly. An interaction between this cytokine and glial cells may help explain the pathophysiological mechanisms leading to cognitive impairment in our study group.

Growing old with epilepsy: the neglected issue of cognitive and brain health in aging and elder persons with chronic epilepsy

The purpose of this review is to examine what is known about cognitive and brain aging in elders with chronic epilepsy. We contend that much remains to be learned about the ultimate course of cognition and brain structure in persons with chronic epilepsy and concern appears warranted. Individuals with chronic epilepsy are exposed to many risk factors demonstrated to be associated with abnormal cognitive and brain aging in the general population, with many of these risk factors present in persons with chronic epilepsy as early as midlife. We suggest that a research agenda be developed to systematically identify and treat known modifiable risk factors in order to protect and promote cognitive and brain health in aging and elder persons with chronic epilepsy.

Obesity and central obesity as risk factors for incident dementia and its subtypes: a systematic review and meta-analysis

While dementia affects 6-10% of persons 65 years or older, industrialized countries have witnessed an alarming rise in obesity. However, obesity's influence on dementia remains poorly understood. We conducted a systematic review and meta-analysis. PUBMED search (1995-2007) resulted in 10 relevant prospective cohort studies of older adults (40-80 years at baseline) with end points being dementia and predictors including adiposity measures, such as body mass index (BMI) and waist circumference (WC). There was a significant U-shaped association between BMI and dementia (P = 0.034), with dementia risk increased for obesity and underweight. Pooled odds ratios (OR) and 95% confidence intervals (CI) for underweight, overweight and obesity compared with normal weight in relation to incident dementia were: 1.36 (1.07, 1.73), 0.88 (0.60, 1.27) and 1.42 (0.93, 2.18) respectively. Pooled ORs and 95% CI for obesity and incident Alzheimer's disease (AD) and vascular dementia were 1.80 (1.00, 3.29) vs. 1.73 (0.47, 6.31) and were stronger in studies with long follow-up (>10 years) and young baseline age (<60 years). Weight gain and high WC or skin-fold thickness increased risks of dementia in all included studies. The meta-analysis shows a moderate association between obesity and the risks for dementia and AD. Future studies are needed to understand optimal weight and biological mechanisms.

Psychological, social and biological determinants of ill health (pSoBid): study protocol of a population-based study

BACKGROUND

Disadvantaged communities suffer higher levels of physical and mental ill health than more advantaged communities. The purpose of the present study was to examine the psychosocial, behavioural and biological determinants of ill health within population groups in Glasgow that differed in socioeconomic status and in their propensity to develop chronic disease especially coronary heart disease and Type 2 diabetes mellitus.

METHODS

Participants were selected at random from areas known to be at the extremes of the socioeconomic continuum in Glasgow. Within the categories of least deprived and most deprived, recruitment was stratified by sex and age to achieve an overall sample containing approximately equal numbers of males and females and an even distribution across the age categories 35-44, 45-54 and 55-64 years. Individuals were invited by letter to attend for assessment of their medical history, risk factor status, cognitive function and psychological profile, morbidity, and carotid intima-media thickness and plaque count as indices of atherosclerosis. Anonymised data on study subjects were collected from the General Practice Administration System for Scotland to analyse characteristics of participants and non-participants.

RESULTS

700 subjects were recruited. The response (active participants per 100 invitation letters) in the least deprived group was 35.1% and in the most deprived group was 20.3%. Lowest response was seen in young males (least deprived 22.4% and most deprived 14.1%).

CONCLUSION

This cross-sectional study recruited the planned sample of subjects from least deprived and most deprived areas within Glasgow. As evident in other studies response differed between the most and least deprived areas. This study brought together researchers/academics from diverse disciplines to build a more sophisticated understanding of the determinants of health inequalities than can be achieved through unidisciplinary approaches. Future analyses will enable an understanding of the relationships between the different types of measure, and of the pathways that link poverty, biology, behaviour and psychology and lead to health inequalities.

Immune activation and inflammation in HIV-1 infection: causes and consequences

Thorough research on HIV is progressively enabling us to understand the intricate mechanisms that link HIV-1 infection to the onset of immunodeficiency. The infection and depletion of CD4(+) T cells represent the most fundamental events in HIV-1 infection. However, in recent years, the role played by chronic immune activation and inflammation in HIV pathogenesis has become increasingly apparent: quite paradoxically, immune activation levels are directly associated with HIV-1 disease progression. In addition, HIV-1-infected patients present intriguing similarities with individuals of old age: their immune systems are characterized by a loss of regenerative capacity and an accumulation of ageing T cells. In this review, we discuss the potential reasons for the establishment of sustained immune activation and inflammation from the early stages of HIV-1 infection, as well as the long-term consequences of this process on the host immune system and health. A simplified model of HIV pathogenesis is proposed, which links together the three major facets of HIV-1 infection: the massive depletion of CD4(+) T cells, the paradoxical immune activation and the exhaustion of regenerative capacity.

Cytokines and cholinergic signals co-modulate surgical stress-induced changes in mood and memory

Inflammatory cytokines and the cholinergic system have been implicated in the effects of stressors on mood and memory; however, the underlying mechanisms involved and the potential interrelationships between these pathways remain unclear. To address these questions, we administered neuropsychological tests to 33 generally healthy surgery patients who donated blood samples several days prior to undergoing moderate surgery (baseline), on the morning of the surgery (i.e., a psychological stressor), and one day after surgery. Eighteen control subjects were similarly tested. Serum levels of inflammatory cytokines, acetylcholinesterase (AChE) activity, and the stressor-inducible AChE-R variant were measured. An elevation in anxiety levels, an increase in depressed mood, and a decline in declarative memory were observed on the morning of the surgery, prior to any medical intervention, and were exacerbated one day after surgery. The surgical stressor-induced elevated IL-1 beta levels, which contributed to the increased depressed mood and to the post-surgery increase in AChE-R expression. The latter increase, which was also predicted by pre-surgery AChE-R and post-surgery mood disturbances, was associated with exacerbated memory impairments induced by surgery. In addition, elevated levels of AChE-R on the morning of the surgery predicted the post-surgery elevation in IL-6 levels, which was associated with amelioration of the memory impairments induced by surgery. Taken together, these findings suggest that exposure to a surgical stressor induces a reciprocal up-regulation of AChE-R and pro-inflammatory cytokines, which are involved in regulating the surgery-induced mood and memory disturbances.

Oxidative protein damage is associated with elevated serum interleukin-6 levels among older moderately to severely disabled women living in the community

BACKGROUND

Elevated interleukin (IL)-6 is associated with adverse outcomes. Our objective was to determine whether serum protein carbonyls, an indicator of oxidative protein damage and oxidative stress, were associated with IL-6.

METHODS

Serum protein carbonyls and IL-6 were measured in 739 women, age > or =65 years, in the Women's Health and Aging Study I.

RESULTS

Geometric mean of protein carbonyls was 0.082 nmol/mg. After adjusting for age and smoking status, log(e) serum protein carbonyls were associated with log(e) IL-6 (beta = 0.143, standard error [SE] = 0.048, p =.003) in linear regression analyses and with elevated IL-6 (> or =2.5 pg/mL) (odds ratio = 1.38, 95% confidence interval, 1.02-1.86, p =.037) in logistic regression analyses. CONCLUSION. Oxidative damage to proteins is independently associated with serum IL-6 among older women living in the community. Increased oxidative stress may be a factor involved in the pathogenesis of the proinflammatory state that occurs in older adults.

Association between genetic variants of IL-1beta, IL-6 and TNF-alpha cytokines and cognitive performance in the elderly general population of the MEMO-study

This study is to investigate the associations between specific polymorphisms in three cytokine genes and domains of cognitive functioning in a population based study in the elderly. In a cross-sectional study of 369 community dwelling elderly subjects we examined the relationships between the polymorphisms IL-1beta-1418C-->T, IL-6-572G-->C and TNF-alpha-308G-->A and the cognitive function domains memory, processing speed and motor function using an extensive neuropsychological test battery. Linear regression models were used in the analysis and results adjusted for multiple comparisons. A significant association between the IL-1beta-1418C-->T polymorphism and memory performance was found with carriers of the T allele (dominant model) having worse memory performance than those with the C allele. In addition, a significant association between the TNF-alpha-308G-->A polymorphism and processing speed was observed, indicating better performance for heterozygous or homozygous carriers of the A allele. These results remained significant after adjustment for known confounders of cognitive function and additional Bonferroni correction for multiple comparisons. Our study provides first results on detrimental effects of the IL-1beta-1418C-->T polymorphism on memory performance and neuroprotective effects of the TNF-alpha-308G-->A polymorphism on processing speed in elderly individuals. Further research is needed to prospectively examine changes in cognitive performance in relation to cytokine genotypes.

Biomarkers related to aging in human populations

Biomarkers are increasingly employed in empirical studies of human populations to understand physiological processes that change with age, diseases whose onset appears linked to age, and the aging process itself. In this chapter, we describe some of the most commonly used biomarkers in population aging research, including their collection, associations with other markers, and relationships to health outcomes. We discuss biomarkers of the cardiovascular system, metabolic processes, inflammation, activity in the hypothalamic-pituitary axis (HPA) and sympathetic nervous system (SNS), and organ functioning (including kidney, lung, and heart). In addition, we note that markers of functioning of the central nervous system and genetic markers are now becoming part of population measurement. Where possible, we detail interrelationships between these markers by providing correlations between high risk levels of each marker from three population-based surveys: the National Health and Nutrition Examination Survey (NHANES) III, NHANES 1999-2002, and the MacArthur Study of Successful Aging. NHANES III is used instead of NHANES 1999-2002 when specific markers of interest are available only in NHANES III and when we examine the relationship of biomarkers to mortality which is only known for NHANES III. We also describe summary measures combining biomarkers across systems. Finally, we examine associations between individual markers and mortality and provide information about biomarkers of growing interest for future research in population aging and health.

Inflammation and rate of cognitive change in high-functioning older adults

BACKGROUND

Inflammatory proteins including interleukin-6 (IL-6) and C-reactive protein (CRP) have been associated with incident cognitive impairment, but little research has addressed their effects on the rate of cognitive change, and findings are mixed. The purpose of this study was to examine the relationship between serum levels of IL-6 and CRP and the rate of cognitive change across a range of cognitive domains in a sample of healthy older persons.

METHODS

Growth curve analysis was performed on data from the MacArthur Study of Successful Aging, a longitudinal cohort study of high-functioning older adults aged 70-79 years at baseline in 1988 and reinterviewed in 1991 and 1995 (N = 851). Individual growth curve parameters were derived from baseline and follow-up performance in abstraction, language, spatial ability, verbal recall, spatial recognition, and global cognitive function based on age, IL-6, CRP, and covariates.

RESULTS

Cross-sectionally, there is a generally linear negative relationship between inflammation and cognition, such that higher levels of inflammation are associated with lower levels of baseline cognitive function. After controlling for potential confounders, there was no effect of inflammation on baseline cognitive function or the rate of longitudinal cognitive change. However, persons in the top tertile on IL-6 were at an increased risk of incident declines on the Short Portable Mental Status Questionnaire (SPMSQ).

CONCLUSIONS

Although high levels of inflammation are associated with incident cognitive impairment, these results do not generalize to the full range of cognitive changes, where the role of inflammation appears to be marginal.

Blood inflammatory markers and risk of dementia: The Conselice Study of Brain Aging

Incidence studies of blood inflammatory markers as predictors of dementia in older age are few and did not take into account hyperhomocysteinemia, although this condition is associated with both inflammation and increased risk of dementia. We investigated the relationships of baseline serum C-reactive protein (CRP), serum interleukin 6 (IL6), plasma alpha-1-antichymotrypsin, and hyperhomocysteinemia (defined as plasma total homocysteine>15 micromol/L) with risk of incident Alzheimer's disease (AD) and vascular dementia (VaD) in a dementia-free Italian population-based elderly cohort (n=804, 53.2% women, mean age 74 years) with 4 years of follow-up. No inflammatory marker, alone or in combination, predicted AD risk whereas the combination of high CRP and high IL6 was associated with risk of VaD (HR, 2.56; 95%CI, 1.21-5.50) independently of socio-demographic confounders, traditional risk factors and hyperhomocysteinemia. By contrast, in the same model, hyperhomocysteinemia was independently associated with AD (HR, 1.91; 95%CI, 1.02-3.56) but not VaD risk. Blood inflammatory markers are associated with increased VaD risk but do not predict AD, which seems selectively associated with hyperhomocysteinemia.

Type 2 diabetes and risk of cognitive impairment and dementia

Diabetes is a major public health burden. Even a modest effect of diabetes on cognitive function has significant public health implications. Several lines of mechanistic evidence implicate a role of insulin and glucose metabolism on risk of developing dementia, including Alzheimer's disease. Population-based studies have shown that those with type 2 diabetes mellitus have an increased risk of cognitive impairment, dementia, and neurodegeneration. There are many mechanisms through which diabetes could increase risk of dementia, including glycemia, insulin resistance, oxidative stress, advanced glycation endproducts, inflammatory cytokines, and microvascular and macrovascular disease. This paper presents a review of the evidence on diabetes and increased risk of dementia and cognitive impairment, a discussion of different possible mechanisms, and remaining gaps in our knowledge.