Current events, safety concerns, and analytical challenges of heavy metal detection in foods and dietary supplements.
Heavy metals have long been a safety concern in food and feed products. With the growing popularity of nutraceuticals, increased focus has been placed on the presence of these earth elements in raw materials that are central in the formulations of vitamins, botanicals, functional foods, and other dietary products.
Heavy metals are defined as chemical elements with a specific gravity at least five times that of water. They are naturally present in relatively low amounts in the earth’s crust, and they cannot be degraded or destroyed. But a number of these minerals, such as manganese, copper, selenium, and zinc-commonly referred to as trace metals-are essential to maintaining the metabolism of the human body, and possess a broad spectrum of documented health benefits.
For several years, physicians and nutritionists have heralded the ability of zinc to promote immune function, support healthy cell growth, and ensure a proper sense of taste and smell. In men, zinc plays a role in maintaining prostate health, testosterone levels, and overall sexual health. Selenium improves the immune system against bacterial and viral infections, and it fights cancer cells. One of the major nutritional benefits of selenium is increasing the ratio of HDL cholesterol to LDL cholesterol for a healthy heart.
According to a University of Maryland Medical Center study, copper works with iron to form hemoglobin in the blood.1 Copper also aids in the production of collagen and can decrease total cholesterol and LDL levels in blood. Manganese helps the human body utilize key nutrients such as biotin and thiamin, synthesize fatty acids and cholesterol, maintain normal blood sugar levels, and promote optimal function of the thyroid gland. Manganese is also important for normal brain function and nerve areas of our body.
Trace metals are present in low levels in a variety of everyday and uncommon food types. Nuts and shellfish are a great source of selenium; roasted pumpkin and squash seeds contain zinc; liver and sunflower seeds are good sources of copper; and wheat germ and bran are high in manganese. Despite the presence of these beneficial trace elements in scores of foodstuffs, many individuals supplement their diets (and trace metal intake) with products ranging from multivitamins and enzyme tablets to nutritional drinks. The Centers for Disease Control and Prevention estimates that half of American adults take some type of dietary supplement regularly, and women are more likely than men to take dietary supplements.2
To maximize the benefits of dietary supplements and other nutraceuticals, consumers are urged to take them accordingly and maintain a well-rounded and healthy diet. Some trace metals, however, are highly toxic, and there is a risk of them entering the food system. Long-term exposure to these elements may result in slowly progressing physical, muscular, and neurological degenerative conditions, including cancer. Arsenic, cadmium, lead, and mercury are among the leading trace metals of concern in both food products and nutraceuticals.
Levels of arsenic are naturally high in fish and seafood. Cadmium, found in soil, can contaminate agricultural food products. Mercury is an industrial pollutant, as well as a by-product of volcanic emissions.
Currently, arsenic in apple juice may best illustrate public concern over the prevalence of heavy metal contamination in the food chain. Arsenic is present in the environment in two general forms, organic and inorganic. Because organic arsenic passes through the body quickly, FDA states that it is essentially harmless; however, inorganic arsenic, which is usually found in pesticides, can be toxic and pose health risks if consumed at high levels over an extended period.
FDA currently uses 23 parts per billion (ppb) as the threshold limit of arsenic to determine whether apple juice is contaminated. A number of consumer groups believe the agency’s legal limit for arsenic content is too high and have criticized the FDA for not even stringently enforcing the 23 ppb limit.
In 2010, Consumer Reports magazine published an investigation of the presence of heavy metals in protein shakes, launching a heated and prolonged debate in the mainstream media among supporters and detractors of these type of products.3 The examination concluded that regular consumption of the kinds of protein shakes analyzed in the study can, in some cases, create the risk of chronic exposure-even at low levels-to heavy metals such as cadmium and lead. Consumer Reports stated that the most vulnerable people exposed to health problems from the consumption of these products were children under the age of 18, pregnant women, and people with diabetes or chronic kidney conditions.
Over the past few years, a small number of U.S. herbal dietary supplements were found to contain trace amounts of lead, cadmium, and arsenic. Publicity surrounding these incidents helped to raise consumer awareness over the presence of these toxic elements in nutraceuticals and spurred calls for increased regulatory oversight.
Dietary supplements fall under the jurisdiction of FDA, along with a host of more conventional food products. Under the Dietary Supplement Health and Education Act (DSHEA) of 1994, dietary supplement and ingredient manufacturers are responsible for ensuring their products are safe prior to marketing, and FDA is responsible for taking action against any unsafe dietary supplement product after it reaches the market.
Within industry GMPs, dietary supplement manufacturers must follow certain good manufacturing practices to ensure the identity, purity, strength, and composition of their products. If FDA finds a product to be unsafe or otherwise unfit for human consumption, it may take enforcement action to remove the product from the marketplace, or work with the manufacturer to voluntarily recall the product.
The EU has established maximum allowable concentrations for heavy metals in foodstuffs. FDA enforces action levels for poisonous or deleterious substances, including many heavy metals, and Congress is increasingly urging the agency to establish maximum allowable levels on some of these heavy metals (including arsenic) in foods and beverages. Due to the growing concern worldwide, demands are increasing to accurately test for heavy metals at low concentrations in raw materials.
A wide range of methods for the analysis of heavy metals are commercially available. One of the oldest and best known methods is United States Pharmacopeia (USP) <231>. USP method <231> is a color development test that reacts to 10 metals (arsenic, antimony, bismuth, cadmium, copper, lead, mercury, molybdenum, tin, and silver) and is quantitated against a lead standard. Time-consuming and labor-intensive, the method cannot qualify individual elements-only groups-and the aggressive sample preparation employed in the method can bring about significant losses of volatile elements, like antimony and mercury, resulting in false negatives. Currently, USP <231> is not accepted by all jurisdictions. Going forward, its application is likely to be more limited due to the development of more effective test methods.
Today, Inductively Coupled Plasma (ICP) is widely used for the detection of heavy metals in foods and nutraceuticals. ICP is a method for simultaneous multi-element analysis, and it exists in two platforms: the older Optical or Atomic Emission Spectrometry (ICP-OES, ICP-AES) and the more recent ICP Mass Spectrometry (ICPMS). Both ICP-OES and ICPMS have been commercially available for decades.
ICP-OES and ICPMS platforms share many similarities, as the front ends of the instrument is virtually identical. Sample introduction and the argon plasma constitute the front end of the system. Both systems have a high dynamic linear range, or range of concentrations covered by the calibration curve. In ICP-OES, the working range is from low ppm to low percent levels, while the working range for ICPMS is from low ppb to low parts per million.
ICP-OES is an optical system that gathers the light (photons) emitted when the atoms of each element “cool down” after their electrons have been excited in the argon plasma. The relaxation of these electrons from their high-energy, excited state is associated with the release of light energy of very characteristic wavelengths. These photons are refracted and focused onto a Charge Couple Device detector, not unlike the detection system used in most digital cameras.
In the plasma, a fraction of the introduced atoms are ionized. In ICPMS, these ions are extracted in a mass spectrometer where they are separated by their mass to charge ratio and focused onto an electron multiplier detector. The two main advantages of ICPMS are increased sensitivity resulting in lower detection limits and a more simple spectra resulting in less interferences. This is especially important for complex matrices, such as in foods and nutraceuticals, where low levels of toxic elements must be measured in a matrix that often contains high levels of nutrition minerals such as calcium, magnesium, potassium, or iron.
ICP-OES has an advantage in measuring the nutritional minerals that are commonly found at quite high levels. For ICPMS, the detection limit in liquids and solids is 1 ppb and 5 ppb for most matrices, respectively. ICP-OES limits are 0.05 ppm and 5 ppm in liquids and solids, respectively.
Other common methods for the determination of heavy metals include flame atomic absorption and furnace atomic absorption. Some lesser known methods include stripping voltammetry, amperometry, and polarimetry. A number of wet chemistry methods, such as colorimetry and titrations, are also utilized for the analysis of heavy metals.
Metals are prevalent in the environment, and the risk of toxic metal contamination from air, water, food, nutraceuticals, and other sources is real on a global scale. Manufacturers should take great care to follow GMPs and implement comprehensive testing programs to assure product safety. The ongoing development and refinement of analytical methods have made possible the detection of heavy metals at extremely low levels in foods.
1. University of Maryland Medical Center, "Complementary and alternative medicine guide," Medical Reference Guide, http://umm.edu/health/medical/altmed/supplement/copper (accessed Juny 27, 2013).
2. J Gahche et al., "Dietary supplement use among U.S. adults has increased since NHANES III (1988–1994)," NHCH Data Brief, no. 61, April 2011.
3. "Alert: Protein drinks: You don't need the extra protein or the heavy metals our test found," Consumer Reports, July 20. http://www.consumerreports.org/cro/magazine-archive/2010/july/food/protein-drinks/overview/index.htm