As often as oral exposure to heavy metals seems to occur, a healthy individual will rarely get near toxic levels. In general, the overall health and dietary habits of the individual will help to dictate the absorption and toxic effects, if any. A healthy person, consuming a varied and balanced diet, should have the key nutrients needed to help defend against heavy metals. Additionally, the content of heavy metals in a food does not mean that is the quantity that will be absorbed or retained by the body. Here are several reasons why:
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The type of metal being ingested and the chemical form of it: The World Health Organization (WHO) and the Agency for Toxic Substances and Disease Registry (ATSDR) agree that only 2-6% of cadmium ingested is taken into the body (1). This is a significantly smaller absorption than when cadmium is inhaled (30-64%). Similarly, only 10-20% of ingested lead is absorbed (2). Certain chemical forms of metals and particle sizes are absorbed more easily than others.
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The food which it is consumed in or with: Metal absorption seems to be higher during fasting (empty stomach) rather than with food ingestion. Certain nutrients within the food which heavy metals are consumed (or with the food they are consumed), can help to reduce the metal absorption or increase its excretion (see below). For example, the presence of dietary fiber can help to decrease the transit time of a meal, thereby limiting the absorption of metals (and certain nutrients) due to lack of contact time in the intestines. However, this can impact the absorption of both beneficial minerals and heavy metals.
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Vitamin and mineral status: Extensive research has indicated that the presence of trace metals/minerals such as calcium, iron, zinc, selenium and magnesium may greatly reduce metal toxicity, by interfering with their absorption(3). This occurs mainly with lead (but also cadmium and mercury). Many of the metals have similar chemical structures as the minerals, so they end up competing for the same “binding sites” in the intestines. Additionally, some of these minerals (like zinc and selenium) as well as vitamins (A, C, and E) provide antioxidant activity which can help to offset any cellular oxidative stress that would occur from accumulation of the metals. Some minerals and protein substances (e.g. metallothionine) can even bind to the heavy metals or rather “scavenge” them and aid in their excretion (4). However, if the body is deficient in some of these vitamins/minerals, it can lead to increased absorption of, and less protection from heavy metals.
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Dietary habits: The more balanced a diet and the more variety of nutrients, ultimately the “better ammunition” the body has to buffer the absorption and retention of heavy metals. For example, foods high in dietary fiber may help to reduce absorption and accumulation of metals. Leafy vegetables and plant-based foods are reported to be a major source of heavy metals, but their health benefits far outweigh any consequences.
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Smoking status: Tobacco smoke is a source of inhaled cadmium and cigarette smoking is a habit which can more than double the average person's daily cadmium intake. It has been reported that one cigarette contains about 1 - 2 micrograms of cadmium (1)
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Stage of life (age): The body is likely to have increased absorption of nutrients during growth periods, such as infancy, youth, or pregnancy, but it also likely to have increased excretion of such offending substances.
References:
- "Cadmium exposure and human health." International Cadmium Association. Available at: http://www.cadmium.org/pg_n.php?id_menu=5
- Lead toxicity: What is the biological fate of lead? Agency for Toxic Substances and Disease Registry. 2007. Available at: http://www.atsdr.cdc.gov/csem/csem.asp?csem=7&po=9
- Flora SJ. Review: Nutritional components modify metal absorption, toxic response and chelation therapy. J Nutr Environ Med. 2002; 12: 53-67.
- Klaassen CD, Liu J, Chouhuri S. Metallothionein: An intracellular protein to protect against cadmium toxicity. Annu Rev Pharmacol Toxicol. 1999. 39: 267–94.
