Aluminum and Neurodegenerative Diseases (original) (raw)

Aluminum (Al) is a common element found in large amounts in the earth's crust (Priest et al., 1988). Aluminum-containing minerals are present in relatively inert rock types, especially in igneous formations, such as granite and quartz. Laterization of various silicate rocks weathering into finer particles results in the formation of sedimentary bauxite, where together with iron, Al is present largely as the oxide. It is as bauxite that Al is generally mined and second only to iron, Al is the most widely used metal (Hetherington, 2007). Despite its commonality, Al has no known beneficial biological roles and is not an essential element for any organism. Aluminum-containing minerals are rather unreactive, and this is also true for metallic aluminum, as this is quickly oxidized in air and thus coated by a very thin but robust layer of the oxide. This apparent inertness has led to the concept that aluminum may not constitute a health hazard. Consequently a wide range of Al compounds have been added as stabilizers in many processed foods. Alum, which is any trivalent Al-containing salt, is the oldest and most commonly used vaccine adjuvant. Recent findings indicate that the effectiveness of the adjuvant relies on both its immunomodulatory as well as inflammatory properties. Al salts have also found utility in water clarifying processes by effecting precipitation of organic particulate matter. Growing incidence of acidic rain has led to greater solubilization of aluminum salts from their insoluble form in rocks. This has led to an elevated Al content in many water reserves used for residential supply. Thus, human exposure to more soluble forms of Al in water and foodstuffs has grown. Reports from both biological laboratories and from study of human population health indicate that prolonged aluminum ingestion can result in neurological abnormality. Accumulating indications strongly suggest that Al can further the onset and development of neurodegenerative disorders, principally Alzheimer's disease (AD). There are many reports suggesting that Al can provoke excessive inflammatory events in the brain. Superfluous immune reactivity that is not an obvious response to a trauma such as injury or infection is a distinguishing feature of the elderly brain and appears exacerbated in nervous system abnormalities. Most neurodegenerative diseases have no obvious cause and do not have a clear genetic basis. Thus, it is probable that the origin of such diseases lies in unknown environmental influences that interact with the progression of aging. The nature of most of such factors is unknown, but there is growing evidence, indicating that Al is likely to be one of these environmental factors. In this review, reports that point to the conclusion that aluminum are able to speed up the worsening of brain function with age, and potential mechanisms are discussed. It should be noted that acceleration of this process would inevitably increase the prevalence of those specific neurological disorders where age is a concomitant risk factor. 2 Growing Bioavailability of Aluminum in the Environment Metallic aluminum was first made by Hans Oersted in 1825 by heating aluminum chloride with elemental potassium (Sigel and Sigel, 1988). Al-containing chemicals have many uses. Mixing aluminum sulfate and lime together in water leads to formation of colloidal aluminum hydroxide, and this can bring about precipitation and removal of waterborne organic material. This method for water clarification is widely used. Al-containing additives are also found in many foodstuffs. They are used as emulsifying agents in preparation of processed cheese, as crisping agents in pickles, in baking powder, and in a variety of food colorings. Aluminum-containing compounds are also found in cosmetics. Commercial preparations of infant formula can contain significant amounts of the metal (Burrell and Exley, 2010; Dabeka et al., 2011). High concentrations of soluble Al can be found in the juice resulting from boiling of acidic fruit in aluminum cookware (Fimreite et al., 1997). The aluminum content of city water supplies is variable, but on occasion, concentrations as high as 0.4-1 mg/L have been reported in drinking water. Although the health effects of these levels of the metal on humans are uncertain, the Joint Food and Agriculture Organization/World Health Organization Expert Committee on Food Additives in 2007 recommended a maximum intake of Al less than 1 mg/kg body weight per week. This corresponds to 63 mg per week for a 140-pound adult. Some commercial pastry products contain Al