Heavy metals such as lead, cadmium, and mercury are natural elements but can be harmful to health even at low doses, especially over a long period of time. Aluminium, while not strictly a heavy metal, behaves as one in the human body and is therefore included.

Heavy metals are pervasive in many aspects of modern life including food, water, air, tobacco smoke, alcoholic beverages and home products (1,2). Table 1 shows the common sources of heavy metal exposure.

Low dose concurrent exposure to a number of individual heavy metals can result in a heavy toxicity load in an individual (1). The International Agency for Research on Cancer (IARC) has classified cadmium compounds and inorganic arsenic as carcinogenic for humans, and methylmercury and inorganic lead as possibly carcinogenic for humans (3).

Table 1. Heavy metal sources

Metal exposure and excess are associated with a large variety of pathologies (Table 2). The toxic effects of metals depend on the type and form of the element, route and duration of exposure, and to a greater extent, on a person’s individual susceptibility (5,19).

High concentrations of toxic metals during pregnancy represent a serious concern as the foetus is vulnerable to influences and may accumulate toxic metals (5).

Toxic metals are environmental factors in the aetiology of autism spectrum disorder (ASD), especially in developing countries. In developed countries, environmental factors other than toxic metals greatly contribute to the aetiology of ASD (20).

Table 2. Health risks of chronic heavy metal exposure

Traditional Understanding

The standard treatment for heavy metal poisoning is chelation therapy with the most commonly used chelating agents shown in Table 3. Chelating agents remove toxic metals by forming a stable, non-toxic complex which can be transported across physiological barriers, facilitating excretion from the body (44).

Conventional chelation therapies carry the following limitations (12,45,46,47):

• Adverse effects, including hepatotoxicity, nephrotoxicity, malaise, nausea, vomiting, skin reactions, poor clinical recovery.
• Their tendency to increase the neurotoxicity of several metals.
• Progressive deficiencies of copper, zinc and other essential trace nutrients.
• Inconvenience of parenteral administration.

Table 3. Commonly used chelation agents (12,21,48,49)

Latest Research

Seafood

Exposure to arsenic and mercury through increased shellfish consumption could favour a pro-atherogenic lipid profile and elevated levels of oxidised LDL (50).

Selenium and fish rich in omega-3 PUFAs antagonise mercury toxicity. It is necessary to determine the risks (toxic effects of mercury) and benefits (from omega-3 PUFAs) of fish consumption (21). Most fish in Australia are low in mercury. Long living, predatory species such as shark, broadbill, marlin and swordfish contain higher levels than smaller fish. Canned tuna from Australia are usually a smaller species and are caught when they are less than 1 year old, and have lower mercury levels than other tuna (51,52).

Complementary medicines

The presence of heavy metals, including arsenic, in herbal, ayurvedic and Chinese medicines is attributed to contamination during cultivation of plants, accidental cross-contamination during processing, and the introduction of metalloids as a therapeutic ingredient (13). This highlights the importance of sourcing a reputable, quality controlled supplier of complementary medicines.

Antioxidants

Daily vitamin E and vitamin C (400 IU/ 1g) for 12 months can reduced oxidative damage in lead exposure, however it does not change blood lead levels or clinical signs and symptoms (53). Vitamin C may affect lead absorption especially at high doses when lead is ingested via drinking water at low concentrations (53).

Sulphur containing amino acids (methionine, cysteine, taurine) and their derivatives - glutathione and N-acetylcysteine (NAC) are powerful antioxidants and may offer a chelating site for heavy metals. Supplementation during chelation might provide beneficial effects in eliminating toxic metals (54).

Rice

The concentration of arsenic in rice grains depends on a number of factors including rice cultivars, arsenic concentration in the irrigation water and irrigation management practices (13). Rice also contains cadmium and lead. Washing of rice reduces concentrations of these metals, and cooking (normal, high-pressure or microwave) reduces the bioaccessibility(55). Rice based infant foods could pose a problem given infant vulnerability to metal toxicity (56). Australian grown rice has arsenic levels below the allowable limit (57)and it would be wise to check the origin of dietary rice, as well as ingredients in rice products such as infant foods and protein powders.

Alzheimer’s disease (AD)

Elevated levels of aluminium, mercury and cadmium may play a role in the progression of AD (58). Regular drinking of silicon-rich mineral waters results in significant excretion of aluminium and lowering of body burden in individuals with AD and in those with MS (41,59).

Breastfeeding

Intake of arsenic, lead and mercury by infants through breastfeeding is a health concern in most regions of the world (60). The proportions of breastfeeding infants who exceed the reference values of respective metals, especially lead, are alarming (61).

• In lactating mothers in Korea, mercury is detected in 100% of breastmilk and lead in 77% of samples. Concentrations are similar in other countries (61).
• The number of dental amalgam fillings in the mother correlates with total mercury concentrations in colostrum (62).
• Proteins in breast milk have a high binding capacity to toxic metals. Lactose and fat may enhance and facilitate the absorption of these heavy metals (63).
• Breast milk is a pathway of maternal excretion of cadmium (64).

Lead

The harmful effects of lead during pregnancy are ameliorated somewhat by increased selenium levels, but are magnified by low selenium or elevated cadmium levels (34).

Mercury

Metabolism and/or toxicity of methylmercury is modulated by intake of dietary nutrients including those concomitantly eaten with fish. These include omega-3 polyunsaturated fatty acids (PUFAs), selenium, iodine, tomatoes, fruit and antioxidants (7). The mechanisms underlying the potential protective effect of foods on mercury exposure and toxicity are not fully understood (7).

Maternal exposure to mercury and/or methylmercury from various sources contributes to the metal body burden of their infants. Even low mercury exposure induces high oxidative stress in mothers and infants, which might play a role in infant neurodevelopmental delays (65).

Aluminium

Low levels of plasma magnesium and iron, as well as enhanced vitamin D status may increase aluminium absorption (17).

Table 4. Summary of recent experimental studies