Are tobacco and alcohol use related to Alzheimer's disease? A critical assessment of the evidence and its implications

Thirty Risk Factors for Alzheimer's Disease Unified by a Common Neuroimmune-Neuroinflammation Mechanism

One of the major obstacles confronting the formulation of a mechanistic understanding for Alzheimer's disease (AD) is its immense complexity-a complexity that traverses the full structural and phenomenological spectrum, including molecular, macromolecular, cellular, neurological and behavioural processes. This complexity is reflected by the equally complex diversity of risk factors associated with AD. However, more than merely mirroring disease complexity, risk factors also provide fundamental insights into the aetiology and pathogenesis of AD as a neurodegenerative disorder since they are central to disease initiation and subsequent propagation. Based on a systematic literature assessment, this review identified 30 risk factors for AD and then extended the analysis to further identify neuroinflammation as a unifying mechanism present in all 30 risk factors. Although other mechanisms (e.g., vasculopathy, proteopathy) were present in multiple risk factors, dysfunction of the neuroimmune-neuroinflammation axis was uniquely central to all 30 identified risk factors. Though the nature of the neuroinflammatory involvement varied, the activation of microglia and the release of pro-inflammatory cytokines were a common pathway shared by all risk factors. This observation provides further evidence for the importance of immunopathic mechanisms in the aetiopathogenesis of AD.

Impact of Alcohol Abuse on Susceptibility to Rare Neurodegenerative Diseases

Despite the prevalence and well-recognized adverse effects of prenatal alcohol exposure and alcohol use disorder in the causation of numerous diseases, their potential roles in the etiology of neurodegenerative diseases remain poorly characterized. This is especially true of the rare neurodegenerative diseases, for which small population sizes make it difficult to conduct broad studies of specific etiological factors. Nonetheless, alcohol has potent and long-lasting effects on neurodegenerative substrates, at both the cellular and systems levels. This review highlights the general effects of alcohol in the brain that contribute to neurodegeneration across diseases, and then focuses on specific diseases in which alcohol exposure is likely to play a major role. These specific diseases include dementias (alcohol-induced, frontotemporal, and Korsakoff syndrome), ataxias (cerebellar and frontal), and Niemann-Pick disease (primarily a Type B variant and Type C). We conclude that there is ample evidence to support a role of alcohol abuse in the etiology of these diseases, but more work is needed to identify the primary mechanisms of alcohol's effects.

Association between alcohol and Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by dense deposition of amyloid-β (Aβ) protein in the brain, failure of the memory and dementia. At present, there is no cure for AD and current treatments only provide a temporary reduction of symptoms. Thus, there is a need for effective preventive/curative strategic approaches. Accordingly, epidemiological studies have reported a reduction in the prevalence of AD in individuals ingesting low amounts of alcohol, while a moderate consumption of ethanol may protect against Aβ. These data are conflicting with other observations that assigned detrimental effects of heavy alcohol use on brain function, which are apparently similar to those observed in AD. These discrepancies questioned whether or not alcohol is a protective agent against the development of AD, whether the probable protective effects are influenced by the quantity and/or frequency of drinking. These issues are addressed in this review with the aim to suggest the real risk of alcohol for developing or preventing AD.

Beyond Ischemic Heart Disease: Are There other Health Benefits from Drinking Alcohol?

Evidence is growing that alcohol consumption confers health benefits beyond protection from ischemic heart disease. We review this evidence with regard to cerebrovascular disease, peripheral vascular disease, diabetes, cholelithiasis (gallstones), cognitive functioning, and stress reduction and subjective psychosocial benefits. Other possible benefits are briefly considered. The weight of evidence suggests that low-level alcohol consumption offers some protection against ischemic stroke. The evidence that moderate alcohol consumption protects against diabetes and gallstones is also fairly strong. The possibility of other health benefits cannot be dismissed. For all the conditions considered, more research is indicated. The application of more appropriate statistical techniques, studies of patterns of drinking, and experimental approaches to delineating underlying mechanisms should enable firmer conclusions to be drawn. A better understanding of both the benefits and the risks of alcohol use for individuals and populations will facilitate the development of appropriate program and policy interventions to promote health.

Smoking and cognitive impairment among older persons in Malaysia

INTRODUCTION

Previous studies have shown conflicting results on the association between smoking and cognitive function. This study aims to examine the relationship of smoking with cognitive function.

METHODOLOGY

Data for the study, consisting of 2553 older adults aged 60 years and older, were drawn from a nationwide household survey entitled "Determinants of Wellness among Older Malaysians: A Health Promotion Perspective" conducted in 2010.

RESULTS

Current smokers had lower rates of cognitive impairment compared to never smokers (17.4% vs 25.9%), while cognitive function in former or ex-smokers was almost similar to that of the never smokers. Findings from multiple logistic regression analysis showed that current smokers were 37% less likely to be cognitively impaired, compared to the never smokers (odds ratio [OR] = .63; 95% confidence interval [CI]: .46-.86) while controlling for potential confounders. No difference in cognitive function was observed between former smokers and never smokers (OR = .94; 95% CI: .71-1.25).

CONCLUSION

Although the findings indicated a negative association between cigarette smoking and cognitive impairment, we are unable to conclude whether this relationship is causal or affected by other unmeasured confounding factors, especially survival bias.

Alzheimer disease: epidemiology, diagnostic criteria, risk factors and biomarkers

The global prevalence of dementia is as high as 24 million, and has been predicted to quadruple by the year 2050. In the US alone, Alzheimer disease (AD) - the most frequent cause of dementia characterized by a progressive decline in cognitive function in particular the memory domain - causes estimated health-care costs of $ 172 billion per year. Key neuropathological hallmarks of the AD brain are diffuse and neuritic extracellular amyloid plaques - often surrounded by dystrophic neurites - and intracellular neurofibrillary tangles. These pathological changes are frequently accompanied by reactive microgliosis and loss of neurons, white matter and synapses. The etiological mechanisms underlying these neuropathological changes remain unclear, but are probably caused by both environmental and genetic factors. In this review article, we provide an overview of the epidemiology of AD, review the biomarkers that may be used for risk assessment and in diagnosis, and give suggestions for future research.

Effects of long-term moderate ethanol and cholesterol on cognition, cholinergic neurons, inflammation, and vascular impairment in rats

There is strong evidence that vascular risk factors play a role in the development of Alzheimer's disease (AD) or vascular dementia (vaD). Ethanol (EtOH) and cholesterol are such vascular risk factors, and we recently showed that hypercholesterolemia causes pathologies similar to AD [Ullrich et al. (2010) Mol Cell Neurosci 45, 408–417]. The aim of this study was to investigate the effects of long-term (12 months) EtOH treatment (20% v/v in drinking water) alone or long-term 5% cholesterol diet alone or a combination (mix) in adult Sprague–Dawley rats. Long-term EtOH treatment (plasma EtOH levels 58±23 mg/dl) caused significant impairment of spatial memory, reduced the number of choline acetyltransferase- and p75 neurotrophin receptor-positive nucleus basalis of Meynert neurons, decreased cortical acetylcholine, elevated cortical monocyte chemoattractant protein-1 and tissue-type plasminogen activator, enhanced microglia, and markedly induced anti-rat immunoglobulin G-positive blood–brain barrier leakage. The effect of long-term hypercholesterolemia was similar. Combined long-term treatment of rats with 20% EtOH and 5% cholesterol (mix) did not potentiate treatment with EtOH alone, but instead counteracted some of the EtOH-associated effects. In conclusion, our data show that vascular risk factors EtOH and cholesterol play a role in cognitive impairment and possibly vaD.

Epidemiology of Alzheimer disease

The global prevalence of dementia is estimated to be as high as 24 million, and is predicted to double every 20 years through to 2040, leading to a costly burden of disease. Alzheimer disease (AD) is the leading cause of dementia and is characterized by a progressive decline in cognitive function, which typically begins with deterioration in memory. Before death, individuals with this disorder have usually become dependent on caregivers. The neuropathological hallmarks of the AD brain are diffuse and neuritic extracellular amyloid plaques-which are frequently surrounded by dystrophic neurites-and intracellular neurofibrillary tangles. These hallmark pathologies are often accompanied by the presence of reactive microgliosis and the loss of neurons, white matter and synapses. The etiological mechanisms underlying the neuropathological changes in AD remain unclear, but are probably affected by both environmental and genetic factors. Here, we provide an overview of the criteria used in the diagnosis of AD, highlighting how this disease is related to, but distinct from, normal aging. We also summarize current information relating to AD prevalence, incidence and risk factors, and review the biomarkers that may be used for risk assessment and in diagnosis.

Effect of smoking and time on cognitive function in the elderly without dementia

OBJECTIVE

To examine the association between smoking and changes in cognitive function over time in elderly persons without dementia.

METHODS

The results of neuropsychological tests grouped into domains of memory, abstract-visuospatial, and language, from several intervals over a 5-year period in 791 elderly patients without dementia or cognitive impairment. Smoking history was categorized as never, current, or past smokers and related to the slope of performance in each cognitive domain using generalized estimating equations.

RESULTS

Performance in all cognitive domains declined over time. Memory performance declined more rapidly among current smokers aged >75 years than in nonsmokers similar in age, including those who never smoked or had quit smoking. The effect was stronger among those without an APOE-epsilon4 allele. There was no association between smoking and performance in any cognitive domain in persons aged <75 years, and there was no association between past smoking and performance on any of the three cognitive factors at any time interval in either age group.

CONCLUSION

Current smokers aged >75 years perform more poorly on cognitive tests and appear to decline in memory more rapidly than their peers who do not smoke, especially if they lack the APOE-epsilon4 allele. Smoking does not affect cognitive performance in those persons aged <75 years.

Effect of vitamin E on memory retention in rats: possible involvement of cholinergic system

This study concerned effects of vitamin E and the cholinergic system on memory retention of passive avoidance learning in rats. Post-training intracerebroventricular (i.c.v.) injections were carried out in all experiments. Administrations of vitamin E (10, 25 and 50 microg/rat), nicotine (0.1 microg/rat) and pilocarpine (0.5 microg/rat), the muscarinic receptor agonist increased memory retention, while mecamylamine (0.01, 0.1 and 0.5 microg/rat), the nicotinic receptor antagonist and scopolamine (0.1, 1 and 5 microg/rat), the muscarinic receptor antagonist decreased memory retention. The combination of vitamin E with nicotine or pilocarpine showed potentiation. Effects of mecamylamine or scopolamine were attenuated by vitamin E. It is concluded that vitamin E has a close interaction with cholinergic system in memory retention process.

The effects of chronic nicotine and alcohol use on neurocognitive function

Despite the extensive literatures on the independent effects of chronic nicotine and alcohol use on neurocognition, little is known about their combined impact. The purpose of this paper was to examine the effects of chronic nicotine and alcohol use on neurocognition in participants who took part in Project MATCH, a study of the efficacies of three behavioral treatments for adults diagnosed with alcohol abuse or dependence. Multiple regression and ANCOVA analyses were conducted to determine the relationship between lifetime weeks of tobacco use and years of alcohol use problems and neuropsychological test performance. Results showed that although years of chronic alcohol use was significantly inversely related to neuropsychological test scores, and chronic nicotine use showed an additive effect, substance use accounted for little variance in neuropsychological functioning. These findings suggest that some protective factors may have helped to attenuate the impact of chronic substance use on neurocognition. The importance of considering individual differences in such research is highlighted. Additional studies on the combined effects of chronic nicotine and alcohol use on neuropsychological function are warranted.

Alcohol and tobacco consumption as risk factors of dementia: a review of epidemiological studies

The association between dementia and smoking or alcohol use has been examined in several epidemiological studies. In many case-control studies, a decreased risk of dementia or Alzheimer's disease (AD) was observed among smokers. However, when this association was analysed in prospective studies, an increased risk of AD was observed. In addition, in the PAQUID study, we showed that the decreased risk disappeared after adjustment for educational level and occupation. These factors are strong confounders in the association between dementia and tobacco use. We also showed that moderate consumption of wine was associated with a lower risk of developing AD. This result remains unchanged after adjustment for many potential confounders. The association between moderate alcohol consumption and risk of developing a dementia or AD was recently confirmed by prospective studies. In some studies, wine consumption was more specifically associated with a decreased risk, whereas beer or spirit consumption was not associated. These results suggest that tobacco consumption is not associated with a lower risk of dementia and that moderate alcohol intake does not increase the risk of developing dementia.

Alzheimer’s Disease and Vascular Dementia

This chapter explores the frequency with which dementing illnesses occur in populations, their distributions by personal characteristics, and what is known about their causes and potential protective factors. The primary focus is on the most common forms of dementia: Alzheimer's disease and vascular dementia. The chapter summarizes clinical and pathologic features of Alzheimer's disease and vascular dementia, and highlights recent theories of how risk factors affect brain reserve. With the evolution of the epidemiology of dementia from case-control studies to prospective cohort studies has come a new set of methodological challenges. These include identification of representative populations, enhancing subject participation and retention in studies, the need to include institutionalized as well as community-dwelling populations, the non-standardized use of cognitive tests to screen for dementia, and the complexities of the diagnostic process itself. Finally, the chapter summarizes risk and protective factors for disease expression, including cardiovascular risk factors, lifestyle factors, and factors that affect brain reserve.

Mid-life smoking and late-life dementia: the Honolulu-Asia Aging Study

We studied the association between mid-life smoking and late-life dementia in the Honolulu Heart Program (1965-1971) and follow-up assessment for dementia (1991-1996) of 3734 Japanese-American men (80% of survivors). Neuropathologic data were available for 218 men. Adjusting for age, education and apolipoprotein E (APOE) genotype, the risk of Alzheimer's disease (AD) in smokers increased with pack-years of smoking at medium (odds ratio (OR)=2.18, 95% confidence interval (CI)=1.07-4.69) and heavy (OR=2.40; 95% CI=1.16-5.17) smoking levels. Very heavy smoking was not associated with AD (OR=1.08; 95% CI=0.43-2.63). Findings were similar when AD cases included those with cerebrovascular disease and for all dementias combined. Adjustment for cardiovascular and respiratory factors or stratification by apolipoprotein E genotype did not change these associations. In an autopsied subsample, the number of neuritic plaques increased with amount smoked. This study suggests that amount smoked is associated with an increasing risk of AD and Alzheimer-type neuropathology up to heavy smoking levels. The lack of association in very heavy smokers may be due to a hardy survivor effect.

Defining alcohol-related fatal medical conditions for social-cost studies in western societies: an update of the epidemiological evidence

OBJECTIVES

To elaborate a state-of-the-art list of alcohol-related fatal medical conditions for future social-cost studies in Western societies.

METHODS

Three major social-cost studies were compared with regard to their respective section on fatal health effects attributable to long-term as well as short-term use of alcohol. On discordant conditions, a systematic literature search was conducted in the Medline and ETOH databases.

RESULTS

There is no consensus between social-cost studies with respect to alcohol-related causes of mortality. Based on the recent epidemiological evidence on alcohol and health, this paper suggests an up-to-date list of fatal medical conditions for which the causal relationship has been established with sufficient scientific evidence. A further investigation is needed, however, to reestimate relative risks by meta-analysis.

CONCLUSIONS

Evaluating new epidemiological evidence regularly is necessary for the purpose of up-to-date social-cost studies.

Risk factors for Alzheimer's disease: a population-based, longitudinal study in Manitoba, Canada

BACKGROUND

Current knowledge of risk factors for Alzheimer's disease (AD) is limited. Data from a longitudinal, population-based study of dementia in Manitoba, Canada were used to investigate risk factors for AD.

METHODS

Cognitively intact subjects completed a risk factor questionnaire assessing sociodemographic, genetic, environmental, medical and lifestyle exposures. Five years later, 36 subjects had developed AD and 658 remained cognitively intact.

RESULTS

Older subjects or those who had fewer years of education were at greater risk of AD. After adjusting for age, education and sex, occupational exposure to fumigants/ defoliants was a significant risk factor for AD (relative risk [RR] = 4.35; 95% CI : 1.05--17.90). A history of migraines increased the risk of AD (RR = 3.49; 95% CI : 1.39--8.77); an even stronger effect was noted among women. Self-reported memory loss at baseline was associated with subsequent development of AD (RR = 5.15; 95% CI : 2.36--11.27). Vaccinations and occupational exposure to excessive noise reduced the risk of AD.

CONCLUSIONS

Some well-known risk factors for AD were confirmed in this study and potential new risk factors were identified. The association of AD with a history of migraines and occupational exposure to defoliants/fumigants is of particular interest because these are biologically plausible risk factors.

Is smoking associated with the risk of developing Alzheimer's disease? Results from three Canadian data sets

PURPOSE

To determine whether smoking is associated with Alzheimer's disease (AD).

METHODS

Analyses were conducted using three Canadian data sets: the University of Western Ontario Dementia Study (200 cases, 163 controls), the Canadian Study of Health and Aging (258 cases, 258 controls), and the patient database from the Clinic for Alzheimer Disease and Related Disorders at the Vancouver Hospital and Health Sciences Centre (566 cases, 277 controls). The association between smoking and AD was investigated using bivariate analyses and multiple logistic regression models adjusted for the potential confounders age, sex, educational level, family history of dementia, head injury, and hypertension.

RESULTS

The results of bivariate analyses were inconsistent across the three data sets, with smoking status a significant protective factor, a significant risk factor, or not associated with AD. The results of multiple logistic regression models, however, were consistent: any association between smoking status and AD disappeared in all three data sets after adjustment for confounders.

CONCLUSIONS

Smoking status was consistently not associated with AD across all three data sets after adjustment for confounders. Failure to adjust for relevant confounders may explain inconsistent reports of the influence of smoking on AD. Any protective effect of smoking may be limited to specific AD subtypes (e.g., early onset AD).

Does an interaction between smoking and drinking influence the risk of Alzheimer's disease? Results from three Canadian data sets

Investigation of the relationship of smoking and drinking to Alzheimer's disease (AD) may advance research on the cause of AD and provide a basis for treatment. Pharmacological mechanisms for an involvement of smoking and drinking are plausible but epidemiologic reports are inconsistent. Evidence of behavioural and physiological interactions suggests that tobacco and alcohol use may not only individually affect AD, but may also modify each other's effects. A modelling strategy was developed to examine the interaction between smoking and drinking on the risk of AD. Three Canadian data sets were analysed: the University of Western Ontario Dementia Study (UWODS) (n=363); the Canadian Study of Health and Aging (CSHA) (n=516), and the database from the Clinic for Alzheimer Disease and Related Disorders at the Vancouver Hospital and Health Sciences Centre, University of British Columbia site (UBC) (n=843). Multiple logistic regression models were adjusted for the potential confounders age, age squared, sex, education, family history of dementia, head injury and hypertension. Analysis of the CSHA provided evidence consistent with the hypothesis that smoking and drinking influence each other's effects on AD, with smoking reducing the risk of AD among drinkers. A similar interaction was marginally significant (p=0.052) in the UWODS data set, but not significant in the UBC data. Extension of these analyses, particularly in longitudinal studies and within genetic risk groups, is needed to determine whether this interaction can be replicated. If so, research on the biological interactions of nicotine and alcohol may provide a basis for the development of therapeutic interventions as well as providing clues to the cause of this disorder.

Nicotinic systems in central nervous systems disease: degenerative disorders and beyond

Advances in the understanding of the structure, function, and distribution of central nervous system (CNS) nicotinic receptors has provided the impetus for new studies examining the role(s) that these receptors and associated processes may play in CNS functions. Further motivation has come from the realization that such receptors are changed in degenerative neurologic diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Ongoing investigations of the molecular substructure of CNS nicotinic receptors and their pharmacology have begun to open up new possibilities for novel CNS therapeutics with nicotinic agents. Exploiting these possibilities will require understanding of the role(s) that these receptor systems play in human cognitive, behavioral, motor, and sensory functioning. Clues from careful studies of human cognition and behavior are beginning to emerge and will provide direction for studies of potentially therapeutic novel nicotinic agents. Modulation of these receptors with the ultimate goal of producing therapeutic benefits is the goal of these investigations and drug development. This paper will review studies from our laboratory and others that point to the importance of CNS nicotinic mechanisms in normal human cognitive and behavioral functioning as well as their role in disease states. In addition, this paper will examine potential clinical applications of nicotine and/or nicotinic agonists in a variety of CNS disorders with particular emphasis on structural brain disease including: movement disorders such as Parkinson's disease and Tourette's syndrome, cognitive/behavioral disorders such as Alzheimer's disease, attention deficit/hyperactivity disorder, and schizophrenia, and other more speculative applications. Important results from early therapeutic studies of nicotine and/or nicotinic agonists in these disease states are presented. For example, recent studies with nicotine and novel nicotinic agonists such as ABT-418 by our group in AD patients suggest that nicotinic stimulation can improve the acquisition and retention of verbal information and decrease errors. Preliminary results from a series of studies examining the acute and subchronic quantitative effects of nicotine on cognitive and motor functioning in Parkinson's disease suggest that acute nicotine administration and stimulation improves some aspects of cognitive and motor performance and may improve the processing speed of more complex tasks. The most likely near-term applications of novel nicotinic agonists in CNS disorders are likely to be in those disorders that are degenerative in nature, e.g. Parkinson's disease and Alzheimer's disease, or other movement disorders such as Tourette's syndrome. The most likely direct therapeutic role for nicotinic agonists is as augmentation therapy in combination with other agents rather than as monotherapy, except early in disease states or as a prophylactic or preventative treatment.

Five-year incidence and prediction of dementia and cognitive decline in a population sample of women aged 70-79 at baseline

OBJECTIVE

To determine the incidence of dementia and cognitive decline and their predictors in a population sample.

SETTINGS

A rural geographically defined population served by a single health centre.

SAMPLE

75% of the 70-74-year-old, 100% of the 75-79-year-old women registered with the practice.

METHOD

Interview the total sample using an augmented version of CAMDEX, including informant interview, and reaction times at baseline and 5 years later. IQ was estimated at baseline only.

RESULTS

365 women were interviewed at baseline, 237 at follow-up. Incidence of dementia of all severities was 3.1% per annum (95% CI, 2.2-4.3). CAMCOG declined by 12.6 points in those who became demented, MMSE by 3 points. Observed, self-reported and informant-reported cognitive difficulty at baseline all predicted later incidence of dementia. The additional use of estimated IQ and reaction times did not increase the efficiency of either cross-sectional detection of dementia or prediction of dementia at 5 years.

CONCLUSIONS

The incidence of dementia was in the range reported by other studies; slight cognitive decline was noted in the total population, increasing with age and marked in incident dementia. Of clinical importance was the finding that both the individuals and the informants of those who subsequently demented were aware of decline 5 years earlier. Simple and relatively easily administered cognitive tests were found to be the most efficient method of detection of dementia.