Heavy metals are ubiquitous in our modern-day environment, with arsenic, cadmium, lead, and mercury comprising the bulk of our exposure. These metals have long been known to have adverse health effects, and a growing body of research indicates that they contribute to the chronic disease epidemic we are currently facing both in the United States and around the world. Read on to learn about the causes and consequences of metals toxicity and why metals detoxification is essential for creating optimal health.
Sources of toxic metals
Mercury is perhaps the most well-recognized toxic heavy metal. Industrial plants, coal burning, incinerators, and chlor-alkali facilities have released copious amounts of mercury into our oceans over the past few centuries, resulting in widespread contamination of seafood. Mercury is also a primary component of dental amalgams. The act of breathing and eating is enough to release mercury vapor from amalgams, allowing it to enter the lungs and GI tract. Interestingly, exposure to non-native EMFs from Wifi increases mercury release from dental amalgams.1
For decades, mercury was used as a preservative (thimerosal) in pediatric vaccines, and comprised a significant source of exposure for children; today, it has been removed from most pediatric vaccines but remains in more than half of flu shots.2 It is readily absorbed through the skin and mucous membranes, and its vapor can be inhaled, diffusing into the lungs.
The four forms of mercury
The four types of mercury have different environmental sources, bioavailabilities, and toxicities within the body. Together, they contribute to the total body burden of mercury.
Mercury exists in the atmosphere primarily in its elemental form (Hg0) as a liquid or gas released through mining and burning processes, runoff from landfills, and erosion of natural depots. Elemental mercury vapor is also the primary form of the metal released from dental amalgams.
Inorganic mercury (Hg2+) is released from the surface of corroding dental amalgams. The toxic consequences of inorganic mercury are much greater than elemental mercury; however, it also has more limited mobility in the human body.
Methylmercury (MeHg) is a form of organic mercury that is lipophilic in nature, meaning it readily crosses lipid-based cell membranes. It also binds to cysteinyl residues, structurally mimicking the amino acid methionine and binding to the specific amino acid transporter, thus gaining entry into the central nervous system.3 This property of methylmercury makes it highly mobile, particularly across the blood-brain barrier and intestinal barrier, where it induces toxic effects.
Over 95 percent of the mercury in fish is highly toxic methylmercury. However, this doesn't mean you need to avoid seafood altogether; because methylmercury biomagnifies in aquatic food chains, which eating seafood at the low end of the food chain, such as wild salmon and sardines, significantly limits your mercury exposure. Methylmercury is also formed in the gut when gut bacteria react with the metal.
Ethylmercury was used as a preservative, thimerosal, in vaccines for decades. While it is no longer used in pediatric vaccines, it remains in many flu shots. The administration of repeated doses of thimerosal is linked to neurodevelopmental disorders in children due to the metal’s effect on neural connectivity and immune function.4
Cadmium exposure occurs primarily through inhalation of cigarette smoke and car exhaust. However, consumption of leafy greens grown near highways and exposure to traffic area runoff are also significant, but underappreciated, sources of exposure.5,6
Arsenic contamination of drinking water is widespread in the United States.7 Rice and shellfish also constitute significant sources of exposure, with U.S.-grown rice demonstrating particularly high levels of arsenic.8,9
Drinking water, old houses with lead paint, and cosmetics represent the most significant sources of lead exposure in our modern-day society.10 While leaded gasoline was phased out decades ago, lead remains in our soils as a result of its previous use.
The symptoms of metals toxicity
Symptoms of Mercury Toxicity
Chronic exposure to mercury results in a blockage of enzyme activity, particularly in the mitochondria, dysregulation of glutamate/GABA balance in the central nervous system, heightened sympathetic nervous system activity, and oxidative stress. Together, these mechanisms produce symptoms in diverse body systems.
Methylmercury accumulates in the central nervous system, causing neuropsychiatric effects including anxiety, depression, disturbed senses of taste and smell, headaches, fatigue, irritability, insomnia, impaired concentration, memory loss, tingling of the extremities, tremors, unexplained burning sensations, and tinnitus. Chronic mercury toxicity is also linked to the development of autism spectrum disorder, Alzheimer’s disease, and Parkinson’s disease.11,12,13
Chronic mercury toxicity challenges the immune system, resulting in persistent or recurrent viral, bacterial, and fungal infections, autoimmunity, and allergies/hypersensitivities.14,15
Mercury adversely impacts the cardiovascular system, inducing abnormal heart rhythm, high blood pressure, and cardiomyopathy.16
Skin manifestations of mercury toxicity include eczema and allergic dermatitis.17
Mercury disrupts multiple aspects of the endocrine system. It contributes to hypothyroidism, with an inverse association observed between mercury and thyroid hormone levels.18 It is also linked to metabolic syndrome and infertility.19,20
Chronic mercury exposure may lead to food sensitivities due to immune imbalances in the GI tract, bacterial or fungal overgrowth, recurrent parasitic infections, abdominal cramps, and IBS.21 The gastrointestinal effects of mercury exposure are mediated in part by its impact on the composition of the gut microbiota; exposure to the metal increases mercury and antibiotic-resistant bacteria, causing significant dysbiosis.22
Mercury toxicity also several systemic effects caused by its oxidative and endocrine-disrupting properties, including premature aging, fibromyalgia, and perpetually low body temperature.23
Symptoms of Arsenic Toxicity
For centuries, arsenic was a popular “medicinal” agent for treating syphilis and whitening the skin. However, its long-term use was soon recognized as harmful, causing thickening of the skin, heart disease, and cancer.
Today, the list of harmful effects of arsenic has lengthened. Arsenic promotes the pathogenesis of type 2 diabetes by impairing pancreatic beta cell dysfunction.24 In fact, arsenic contamination of rice has been implicated as a contributing factor in the diabetes epidemic in Southeast Asia.25 Arsenic exposure is also linked to anxiety, depression, and amyloid-beta deposition in the brain, a pathognomonic process in Alzheimer’s disease.26,27
Symptoms of Lead Toxicity
Lead is perhaps most famous for affecting the brain and nervous system development in infants and children; prenatal and early life exposure to lead is linked to reduced IQ, and the CDC states that "No safe blood lead level in children has been identified."28 In adults, lead exposure has similar consequences, reducing brain volume and producing a “dumbing down” effect.29
Lead has well-established toxic effects beyond the brain. High lead levels are associated with reduced fertility in males, increased allergic food sensitization, and osteoporosis by inhibiting osteoblastic bone-forming activity. 30,31,32 Importantly, lead and other heavy metals, such as mercury, appear to exert additive toxic effects.
Symptoms of Cadmium Toxicity
Cadmium toxicity is known to disrupt DNA integrity, producing mutations and chromosomal deletions.33 Cadmium exposure also lowers bone mineral density and is associated with an increased risk of osteoporosis and bone fractures due to its adverse effects on osteoblastic differentiation and enzymes involved in the mineralization process.34, 35
Genetic Differences in Susceptibility to Harm from Metals
Genetic variants contribute significantly to individual susceptibility to harm from heavy metals. Variants in the MT gene that codes for metallothionein, a category of low-molecular-weight proteins that bind to heavy metals, is associated with differential responses to mercury exposure in children.36 Single-nucleotide polymorphisms in glutathione-S-transferase genes, selenoprotein genes, and ATP-binding cassette transporter genes, involved in the detoxification of xenobiotics such as heavy metals, also influence individual susceptibility to mercury toxicity.37
Similarly, the toxic effects of arsenic are also affected by single-nucleotide polymorphisms, including variants in arsenite 3-methyltransferase (AS3MT), an enzyme that methylates arsenic as part of the detoxification process, and glutathione-S-transferase, involved in glutathione antioxidant activity.38
Detoxification Systems for Metals
Fortunately, the human body is armed with biochemical intracellular detoxification systems for eliminating heavy metals. The glutathione system regulates intracellular mercury detoxification.39 The multidrug resistance-associated protein 2 (Mrp2) system plays an intrinsic role in mercury detoxification in the kidneys and enterocytes lining the intestine.40
Once intracellular detoxification systems have processed heavy metals, the metals are mobilized into the bile and intestine via the enterohepatic circulation. Promoting healthy bile flow and subsequently binding up the metals released into the intestine is critical for heavy metals detox; if bile production stagnates or binders are not used, the metals will continue to recirculate between the intestine, liver, and systemic circulation. Interestingly, heavy metal exposure alters the gut microbiota in such a way as to change the composition of bile acids, thereby creating a vicious cycle of impaired bile production and heavy metal recirculation.41 This finding further demonstrates the significance of supporting healthy bile production during the detoxification of heavy metals.
Leaky Gut Worsens Heavy Metal Toxicity
Increased intestinal permeability, also referred to as “leaky gut,” is a common problem in modern-day society that underlies numerous chronic diseases, including autoimmunity, cardiovascular disease, and cognitive dysfunction. Research indicates that leaky gut potentiates heavy metal toxicity by impairing the activity of cellular transporters that facilitate heavy metal detox and by augmenting the harmful effects of endotoxin, metabolites from Gram-negative bacteria that incite a systemic inflammatory response.42 Based on these findings, leaky gut and heavy metal toxicity must be addressed simultaneously to optimize the metal detoxification process.
Arsenic, cadmium, lead, and mercury may be ubiquitous in our environment, but that doesn’t mean you have to fall prey to their harmful effects! Stay tuned for parts 2 and 3 of this blog series, in which we’ll discuss testing methods for assessing heavy metal body burden and how to successfully detox heavy metals from the body.
- Mortazavi G, Mortazavi SM. Increased mercury release from dental amalgam restorations after exposure to electromagnetic fields as a potential hazard for hypersensitive people and pregnant women. Rev Environ Health. 2015; 30(4): 287-292.
- Geier DA, et al. Thimerosal: Clinical, epidemiologic and biochemical studies. Clinica Chimica Acta. 2015; 444(15): 212-220.
- Zalups RK. Molecular interactions with mercury in the kidney. Pharmacol Rev. 52(1): 113-143.
- Hooker B, et al. Methodological issues and evidence of malfeasance in research purporting to show thimerosal in vaccines is safe. Biomed Res Int. 2014; 2014: 247218.
- Igwegbe AO, et al. Effect of a highway’s traffic on the level of lead and cadmium in fruits and vegetables grown along the roadsides. J Food Saf. 1992; 13(1): 7-18.
- Huber M, et al. Critical review of heavy metal pollution of traffic area runoff: Occurrence, influencing factors, and partitioning. Science Total Environ. 2016; 541: 895-919.
- Frost FJ, et al. Identifying US populations for the study of health effects related to drinking water arsenic. J Expo Sci Environ Epidemiol. 2003; 13: 231-239.
- Lai PY, et al. Arsenic and rice: Translating research to address health care providers’ needs. J Pediatr. 2015; 167(4): 797-803.
- Taylor V, et al. Human exposure to organic arsenic species from seafood. Sci Total Environ. 2017; 580: 266-282.
- Zietz BP, et al. Lead in drinking water as a public health challenge. Environ Health Perspect. 2010; 118(4): A154-A155.
- Kern JK, et al. The relationship between mercury and autism: A comprehensive review and discussion. J Trace Elem Med Biol. 2016; 37: 8-24.
- Lee HJ, et al. Pathogenic mechanisms of heavy metal induced-Alzheimer’s disease. Toxicol Environment Health Sci. 2018; 10(1): 1-10.
- Hsu YC, et al. Association between history of dental amalgam fillings and risk of Parkinson’s disease: A population-based retrospective cohort study in Taiwan. PLoS One. 2016; 11(12): e0166552.
- Vas J, Monestier M. Immunology of mercury. Ann N Y Acad Sci. 2008; 1143(1): 240-267.
- Silbergeld EK, et al. Mercury and autoimmunity: implications for occupational and environmental health. Toxicol Appl Pharmacol. 2005; 207(2 Suppl): 282-292.
- Genchi G, et al. Mercury exposure and heart diseases. Int J Environ Res Public Health. 2017; 14(1): pii: E74.
- Weidinger S, et al. Body burden of mercury is associated with acute atopic eczema and total IgE in children from southern Germany. J Allergy Clin Immunol. 2004; 114(2): 457-459.
- Chen A, et al. Thyroid hormones in relation to lead, mercury, and cadmium exposure in the National Health and Nutrition Examination Survey, 2007–2008. Environ Health Perspect. 2013; 121(2): 181-186.
- Moon SS. Association between blood mercury level and visceral adiposity in adults. Diabetes Metab J. 2017; 41(2): 96-98.
- Bjorklund G, et al. Mercury exposure and its effects on fertility and pregnancy outcomes. Basic Clin Pharmacol Toxicol. 2019; [online].
- Rice KM, et al. Environmental mercury and its toxic effects. J Prev Med Public Health. 2014; 47(2): 74-83.
- Summers AO, et al. Mercury released from dental "silver" fillings provokes an increase in mercury- and antibiotic-resistant bacteria in oral and intestinal floras of primates. Antimicrob Agents Chemother. 1993; 37(4): 825-834.
- Stejskal V, et al. Metal-induced inflammation triggers fibromyalgia in metal-allergic patients. Neuro Endocrinol Lett. 2013; 34(6): 559-565.
- Carmean CM, Seino S. Braving the element: Pancreatic β-cell dysfunction and adaptation in response to arsenic exposure. Front Endocrinol (Lausanne). 2019; 10: 344.
- Hassan FI, et al. The relation between rice consumption, arsenic contamination, and prevalence of diabetes in South Asia. EXCLI J. 2017; 16: 1132-1143.
- Chang CY, et al. Subchronic arsenic exposure induces anxiety-like behaviors in normal mice and enhances depression-like behaviors in the chemically induced mouse model of depression. Biomed Res Int. 2015; 2015: 159015.
- Nino SA, et al. Chronic arsenic exposure increases Aβ(1–42) production and receptor for advanced glycation end products expression in rat brain. Chem Res Toxicol. 2018; 31(1): 13-21.
- “Lead.” CDC website. 2019. https://www.cdc.gov/nceh/lead/default.htm.
- Vlasak T, et al. Blood lead levels and cognitive functioning: A meta-analysis. Science Total Environ. 2019; 668(10): 678-684.
- Vigeh M, et al. How does lead induce male infertility? Iran J Reprod Med. 2011; 9(1): 1-8.
- Beier EE, et al. Heavy metal ion regulation of gene expression: Mechanisms by which lead inhibits osteoblastic bone-forming activity through modulation of the Wnt/β-catenin signaling pathway. J Biol Chem. 2015; 290(29): 18216-26.
- Mener DJ, et al. Lead exposure and increased food allergic sensitization in U.S. children and adults. Int J Allergy Rhinol. 2015; 5(3): 214-220.
- Rahimzadeh MR, et al. Cadmium toxicity and treatment: An update. Caspian J Intern Med. 2017; 8(3): 135-145.
- Eom SY, et al. Changes in blood and urinary cadmium levels and bone mineral density according to osteoporosis medication in individuals with an increased cadmium body burden. Hum Exp Toxicol. 2018; 37(4): 350-357.
- Rodriguez J, Mandalunis PM. A review of metal exposure and its effects on bone health. J Toxicol. 2018; Article ID 4854152.
- Woods JS, et al. Modification of neurobehavioral effects of mercury by genetic polymorphisms of metallothionein in children. Neurotoxicol Teratol. 2013; 39: 36-44.
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