One of the most publicized and controversial theories concerns aluminum, which became a suspect in Alzheimer's disease when researchers found traces of this metal in the brains of patients with Alzheimer's disease. Many studies since then have either not been able to confirm this finding or have had questionable results.
Aluminum does turn up in higher amounts than normal in some autopsy studies of Alzheimer's patients, but not in all. Further doubt about the importance of aluminum stems from the possibility that the aluminum found in some studies did not all come from the brain tissues being studied. Instead, some could have come from the special substances used in the laboratory to study brain tissue.
Aluminum is a common element in the Earth's crust and is found in small amounts in numerous household products and in many foods. As a result, there have been fears that aluminum in the diet or absorbed in other ways could be a factor in Alzheimer's. One study found that people who used antiperspirants and antacids containing aluminum had a higher risk of developing Alzheimer's. Others have also reported an association between aluminum exposure and Alzheimer's disease.
On the other hand, various studies have found that groups of people exposed to high levels of aluminum do not have an increased risk. Moreover, aluminum in cooking utensils does not get into food, and the aluminum that does occur naturally in some foods, such as potatoes, is not absorbed well by the body. On the whole, scientists can say only that it is still uncertain whether exposure to aluminum plays a role in Alzheimer's disease.
Manganese is naturally found in the environment and is necessary as a cofactor for many enzymes and is important in several physiological processes that support development, growth, and neuronal function. But excess manganese in the brain can be neurotoxic and has been implicated in sporadic Alzheimer's disease.
Recent studies have shown links between brain high levels of manganese and the development of Alzheimer's or Alzheimer-like symptoms, although the data are mixed as to the nature of the relationship.
Zinc has been implicated in Alzheimer's disease in two ways. Some reports suggest that too little zinc is a problem. Others suggest that too much zinc is at fault. Too little zinc was suggested by autopsies that found low levels of zinc in the brains of Alzheimer's disease patients, especially in a certain region.
On the other hand, a recent study suggests that too much zinc might be the problem. In this laboratory experiment, zinc caused beta amyloid from cerebrospinal fluid -- the fluid that bathes the brain -- to form clumps similar to the plaques of Alzheimer's disease. Current experiments with zinc are pursuing this lead in laboratory tests that more closely mimic conditions in the brain.
Toxins in foods have come under suspicion in a few cases of dementia. Two substances found in seeds of certain legumes in Africa, India, and Guam may cause nervous system damage. Both enhance the action of a substance called glutamate, which has also been implicated in Alzheimer's disease.
In Canada, an outbreak of a nervous system disease similar to Alzheimer's occurred among people who had eaten mussels contaminated with domoic acid. This chemical, like the legume substances, enhances glutamate. While these toxins may not be a common cause of dementia, they could eventually shed some light on the mechanisms that lead to nerve cell damage.
In some nervous system diseases, a virus is the culprit, lurking in the body for decades before a combination of circumstances stir it to action. For years, researchers have sought a virus or other infectious agent in Alzheimer's disease.
This line of research has yielded little in the way of hard evidence so far, although one study in the late 1980s did provide some data that have kept the possibility alive. A larger investigation is currently under way.