Aluminum is a naturally occurring element found throughout our environment. Given its widespread presence, understanding how aluminum interacts with the human body, especially its entry into brain tissue, is a significant scientific inquiry. This article explores the pathways and mechanisms through which aluminum can access the brain.
Common Routes of Exposure
Humans encounter aluminum daily through various sources. Food and drinking water are primary routes, with an average adult consuming about 7-9 milligrams of aluminum per day through food. Vegetables, breads, cakes, pastries, and some infant formulas are significant dietary sources. Aluminum can also be introduced during food processing and from cooking in aluminum utensils.
Drinking water contains low levels of aluminum (below 0.1 mg/L), though some areas have concentrations as high as 0.4-1 mg/L. Beyond food and water, exposure occurs through consumer products like antiperspirants, cosmetics, and certain medications. Airborne aluminum, primarily from dust and industrial activities, represents another route of exposure.
Absorption and Circulation in the Body
Once encountered, aluminum can be absorbed into the human body through several pathways. The gastrointestinal tract is the main route for ingested aluminum, though only a small fraction (0.04% to 1.0%) is absorbed. This absorption rate varies with aluminum’s chemical form and the presence of substances like citrate.
Inhaled aluminum particles can enter the bloodstream through the lungs and respiratory epithelium, with approximately 1.5-2% of inhaled aluminum being absorbed. Dermal absorption through the skin is limited, with studies indicating that only about 0.012% of applied aluminum reaches the bloodstream. After absorption, aluminum enters the bloodstream, where it circulates throughout the body. A significant portion, about 90%, binds to serum transferrin, a protein also responsible for transporting iron, while smaller amounts bind to albumin and citrate.
Breaching the Brain’s Defenses
The brain is protected by a specialized structure called the blood-brain barrier (BBB), which regulates the passage of substances from the blood into the brain tissue. Despite this defense, aluminum can breach the BBB through several mechanisms. The most significant pathway involves transferrin-mediated transport, where aluminum mimics iron and uses the transferrin receptor system to cross the barrier. This system facilitates iron transport into brain cells.
Aluminum can also enter the brain via the choroid plexuses and the nasal cavity. Some research suggests that aluminum bound to citrate may be transported across the BBB by the system Xc- exchanger. Additionally, factors such as increased permeability of the intestinal mucosa or the presence of compounds like citric acid can promote aluminum entry into the brain. While high concentrations of aluminum can potentially affect BBB integrity, studies examining this have used aluminum levels much higher than those typically found in human blood plasma.
Presence and Persistence within Brain Tissue
Once aluminum enters the brain, it can distribute across different brain regions. Unlike many other substances, aluminum tends to accumulate within brain tissue and is cleared slowly. Estimates for the half-life of aluminum in the human brain can range from 20% of a person’s lifespan to even longer.
This slow clearance means aluminum persists and accumulates over time. The ability of aluminum to enter and remain in brain tissue underscores the importance of understanding these entry mechanisms for brain health research.