The brain’s blood-brain barrier (BBB), a densely packed layer of cells lining the interior of cerebral blood vessels, is capable of protecting brain tissue from viruses, bacteria, parasites, and many chemical toxins. But the BBB is not perfect: It is vulnerable to influxes of heavy metals such as lead, iron, copper, and mercury—molecules that represent a serious potential source of toxicity associated with neurological dis-ease and cognitive impairments.
“Heavy metals can enter the body from a variety of sources—the diet, drinking water, breath-ing contaminated air, and absorption through the skin,” says David Mischoulon, MD, PhD, Director of Research at MGH’s Depression Clinical and Research Program. “In small quantities, some metals—such as iron, copper, and zinc—are needed to promote the health and proper functioning of cells in both the body and brain. But others have no place in the brain, and their presence there causes damage and disruption.”
Arsenic and Stroke
One recent 30-year study has raised concerns about the effects of low-to-moderate levels of the heavy metal arsenic on the brain. Researchers followed nearly 3,600 adults who were regularly exposed to low-to-moderate levels of arsenic in their drinking water or foods such as flour, rice and other grains. After controlling for other factors, the scientists found that participants with the highest levels of arsenic in their systems had a 50 percent increased risk of death by heart attack or stroke and a significant increase in risk of developing cardiovascular disease compared to those with the lowest levels, according to a report published Sept. 24, 2013 in the journal Annals of Internal Medicine.
The researchers recommended that people vary their food sources and day-to-day eating patterns to reduce the concentration of arsenic in their diets, and consider having their drinking water—especially ground water—tested for arsenic. Other sources of arsenic include certain seafoods, paints, rat poison, fungicides, pesticides, and wood preservatives.
Other Heavy Metal Dangers
Although many heavy metals besides arsenic have the potential to injure the brain, five in particular are addressed here because they are commonly encountered in the environment.
WHAT YOU CAN DO
To minimize exposure to unsafe levels of heavy metals, make an effort to:
- Consume a nutritious, low-calorie diet with plenty of fresh (organic if possible) fruits, vegetables and leafy greens, nuts and seeds, beans and other healthy foods. Antioxidants contained in green tea and dark-colored berries can help prevent damage from free radicals.
- Read labels and avoid products that contain toxic metals—such as household cleaning agents, hobby supplies, fertilizer, pesticides, or lead-based paint.
- See your doctor if you feel you have been exposed to toxic levels of heavy metals.
Iron is needed for the transport of oxygen via red blood cells, energy production, and the synthesis of both the chemical neurotransmitters used to relay messages among brain cells and the fatty myelin coating of axons that connect brain cells to one another. Excessiveiron has been linked to higher levels of free radicals and oxidative damage that contributes to the death of brain cells. Accumulations of iron in affected brain regions are a key facet of Parkinson’s disease (PD), Alzheimer’s disease (AD), and certain other neurodegenerative diseases, as are changes in iron-related proteins. Sources: Iron cooking utensils, drinking water, red meats, chicken, fish, liver, legumes, fortified cereal, dark leafy greens. Recommended intake: 8 mg a day for males and females over 50 years of age.
Copper is necessary for the synthesis of neurotransmitters and numerous essential enzymes. It helps the body absorb iron, is involved in the transport of oxygen, and is important for nerve function, tissue and bone formation, and a healthy immune system. Excessivecopper appears to be involved in the production of cell-damaging free radicals, and may prevent the brain from ridding itself of beta-amyloid plaques, one of the hallmarks of AD. Copper is implicated directly or indirectly in pathological processes associated with numerous neurological diseases, including AD, Lou Gehrig’s disease (ALS), PD, and prion disease. Sources: Drinking water, seafood such as oysters and shellfish, organ meats such as liver, nuts, legumes, leafy greens, whole grains, and chocolate. Recommended intake: 2 mg daily for adults.
Zinc helps regulate communication between brain cells and is thought to play a role in learning and the formation of short-term memories in the hippocampus, a key brain memory region. It is generally concentrated in brain cells and is involved in neuronal activity. Zinc deficiency: An estimated 40 percent of older Americans may be zinc deficient. Too little zinc is thought to impair the immune system, lower defenses against free radicals and contribute to impaired memory and mental lethargy. An animal study published online October 2013 in the Journal of Biological Chemistry suggests that zinc deficiency may cause protein alterations that contribute to the buildup of toxic proteins and plaques characteristic of diseases such as AD and PD. Sources: Oysters, crabs, and lobster, red meat, poultry, beans, nuts and seeds, whole grains, leafy greens, chocolate and mushrooms. Recommended intake: 11 mg for adult males and 8 mg for females.
This heavy metal has no natural function in the brain and nervous system, and can be highly toxic, especially in large amounts. It is associated with disruption of neurotransmitters, impairment of energy metabolism within cells, brain cell death, impairments of vision, hearing, and speech, lack of coordination, hypertension, and lowered intelligence. In addition to blocking the action of important enzymes, it makes cells particularly vulnerable to oxidative damage. Sources: Likely sources of mercury in humans are contaminated shellfish or fish. See EPA advisories and guidelines on fish that are safe to eat at http://fishadvisoryonline.epa.gov/General.aspx.
Lead serves no useful purpose in the brain and can be toxic. High lead intake is associated with greater risk for AD and other forms of dementia. One study found that people exposed to lead on the job faced three times the normal risk of developing AD. Lead also impairs the production of myelin, alters the bio-activities of cells, disrupts the BBB, blocks the action of certain neurotransmitters, and interferes with the formation of synapses, among other effects. Sources: Lead is generally inhaled or swallowed. Common sources include lead-based paint dust or chips, drinking water (from lead pipes), and vegetables grown in lead-contaminated soil.