Heavy Metals in Pet Food

Minerals are naturally occurring inorganic elements that together make up the “ash” content of food – the material that remains after combustion. Minerals play many vital roles in the body, including involvement in numerous metabolic reactions, maintaining pH balance, functioning of the nervous system, muscle contraction, and providing structure for bones and teeth. Some minerals are nutritionally essential for the health of animals and have well-defined roles in the body. Other minerals are found in trace amounts in foods and body tissues, but do not play a known role in health.

Deficient or excessive intake of all minerals has the potential to negatively impact an animal.¹ The Association of American Feed Control Officials (AAFCO) reports minimum inclusion levels for minerals in dog and cat foods for calcium, phosphorus, potassium, sodium, chloride, magnesium, iron, copper, manganese, zinc, iodine and selenium.² Additional minerals that may provide benefits, but for which essentiality has not been determined, include cobalt, fluoride, molybdenum, sulfur, arsenic, boron, nickel, rubidium, silicon and vanadium.¹ Maximum intake levels have only been set by AAFCO for calcium, phosphorus, iodine and selenium.²

Heavy metals are minerals with a high atomic weight and a density at least five times greater than water.³ They are naturally present in many foods and widely distributed in the environment due to their many uses in industry, agriculture, medicine and technology.³ Arsenic, cadmium, chromium, lead and mercury are considered to be particularly important from a public health perspective because of their high degree of toxicity in people.³ The adverse effects of minerals depends on many factors, including the amount consumed, length of exposure, route of exposure, as well as the age, gender, genetics and nutritional status of the exposed animal.^1,3 In fact, chromium, arsenic and nickel are suspected to be required in miniscule amounts in the diets of animals even though minimum intake levels have not been established. It is critical to provide nutrients within the optimal range of intake to ensure animal health. A diet devoid of a required mineral fed over a long time period could be as detrimental as a diet that contains excessive levels of the mineral.⁴

One of the challenges with determining safe upper limits of minerals for dogs and cats is the limited data that is available for these species. However, it is critically important to compare mineral levels in pet foods to the appropriate reference intakes values. A report published by the United States Food and Drug Administration (FDA) stated that it is incorrect to make a comparison of the trace mineral levels in pet foods to values set by the Environmental Protection Agency and the World Health Organization for people. Instead, the FDA stated that the suitable reference levels are those published by the National Research Council (NRC) Committee on Minerals and Toxic Substances in Diets and Water for Animals (MTSA Committee) in the Mineral Tolerance of Animals Second Revised Edition, 2005.¹ The values reported by the MTSA Committee are based on results from studies in a wide variety of animals, including dogs and cats. Thus, these values are the most relevant available for comparison of the mineral levels found in animal diets.⁴

The MTSA Committee defines the maximum tolerable level (MTL) of a mineral as “the dietary level that, when fed for a defined period of time, will not impair animal health or performance”.¹ The FDA provided recommendations for the appropriate MTL for heavy metals specifically for dogs and cats in the Target Animal Safety Review Memorandum.⁴ The MTL values are based on the information published by the NRC¹ and adjusted if necessary for each mineral depending on the animal species for which the MTL was established. No correction or additional safety factor was applied to an MTL if data was available that was specific for dogs and cats (e.g. cadmium, mercury and lead), or if the tolerance was set based on data from mammalian species known to be particularly sensitive to the mineral (e.g. nickel and arsenic).⁴ For values based on data from species not known to be particularly sensitive to the heavy metal, the MTL was divided by 10 as a safety factor (e.g. chromium).⁴ The MTL values for arsenic, mercury, cadmium, chromium, nickel and lead are shown in Table 1.

At Petcurean, we approach the quality and safety of our pet foods with utmost care. As part of our quality assurance program, we test our ingredients prior to receiving them at the plant as well as testing finished products for both nutritionally essential minerals and potentially toxic heavy metals. This paper reviews the maximum tolerable levels of heavy metals (arsenic, cadmium, chromium, lead, mercury and nickel) in relation to pet food and pet health, and report the results of our systematic testing of these metals in Petcurean foods as determined by an independent laboratory (Table 1).

Table 1. Average heavy metal content in Petcurean dry foods compared to the NRC/FDA maximum tolerable limit

Abbreviations: NRC, National Research Council; FDA, Food and Drug Administration; SD, standard deviation

^aDry matter basis

Arsenic

Arsenic is generally considered to be a non-essential nutrient for animals, though it may provide benefits in ultra-low concentrations.¹ Arsenic is found in over 200 different forms that vary widely in their degree of toxicity.

For dogs and cats, the FDA reported the MTL to be 12.5 mg/kg on a dry matter (DM) basis.⁴ In comparison, the MTL for domestic animals set by the MTSA Committee is 30 mg/kg DM.¹ The average (± SD) arsenic content in a selection of Petcurean dry kibble foods as tested by an independent laboratory was found to be 0.39 ± 0.41 mg/kg on an as fed basis (0.43 ± 0.46 mg/kg DM or 3.4% of the MTL).

Cadmium

Cadmium is not considered an essential nutrient for animals, though studies in rodents, chickens and livestock have shown increased weight gain with the addition of low levels of cadmium to the diet.¹ Absorption of cadmium by the intestines depends on the dose, with higher doses being absorbed more readily. Cadmium seems to increase its own bioavailability by disrupting the intestinal cell barrier. However, with chronic exposure, absorption of cadmium decreases due to the development of protective mechanisms.¹

Chronic cadmium consumption in food or water affects almost all major organs, with the kidney and liver being the target organs for most animals.¹ The toxicity of cadmium is affected by the nutritional and physiological state of the animal.¹ Dogs fed cadmium chloride at concentrations of 1, 3, 10 and 30 mg/kg diet for three months were found to be clinically normal.⁵ Dogs have been shown to tolerate cadmium at a level of 10 mg/kg DM for eight years with no adverse effects, but 50 mg/kg DM over the same time period resulted in renal atrophy, renal tubule degeneration and diminished renal function.¹

The MTL for cadmium for mammalian species was set by the MTSA Committee at 10 mg/kg DM,¹ which is the same as the level reported by the FDA for dogs and cats.⁴ The average (± SD) cadmium content in a selection of Petcurean dry kibble foods as tested by an independent laboratory was found to be 0.09 ± 0.05 mg/kg as fed (0.1 ± 0.06 mg/kg DM or 1% of the MTL).

Chromium

Chromium has been recognized as an essential nutrient since the mid-twentieth century when it was recognized as being essential for glucose metabolism by increasing the effectiveness of insulin.^1,6 Chromium deficiency in humans results in glucose intolerance, weight loss and disorders of the nervous system.¹

Chromium is abundant in water and soil, though uptake by plants and absorption by animals is low.⁷ Most forms of chromium are poorly available, except for organically bound chromium which is better absorbed.⁷ Chromium toxicity is a concern in humans exposed to chromium in occupational and industrial settings.¹

Dogs have a chromium requirement estimated to be less than 12 μg/kg body weight per day.⁶ Diabetic dogs treated with insulin showed no beneficial or harmful effects with supplementation of chromium picolinate at a dosage of 20-60 mg/kg/d.⁸ In obese and non-obese cats, supplementation with 100 μg/d chromium picolinate for six weeks was found to be safe, but had no effect on glucose tolerance.⁹ No definitive intake recommendations have been made for dogs or cats due to insufficient evidence.⁶

The MTL for mammalian species was reported by the NRC to be 100 mg chromium/kg DM from soluble sources such as chromium chloride.^1,4 However, none of the species with an MTL were identified as being particularly sensitive to chromium and there was a lack of dog- or cat-specific data.⁴ Therefore, a safety factor of 10 was applied to the general mammalian MTL, resulting in a value of 10 mg chromium/kg DM for dogs and cats.⁴ The average (± SD) chromium content in a selection of Petcurean dry kibble foods was found to be 1.24 ± 0.72 mg/kg as fed (1.38 ± 0.8 mg/kg DM or 13.8% of the MTL).

Lead

Lead is a significant environmental pollutant with a high degree of toxicity and is not known to be essential for animals.¹ The bioavailability of lead is affected by the chemical form, the level of other dietary constituents, and the age and physiological state of the animal.¹ Lead acts as a toxin by interacting with body proteins and changing their function, inhibiting or mimicking the action of calcium, replacing zinc as a cofactor in enzymes, and inducing oxidative stress.

Animals can tolerate considerably higher daily lead exposure when it is consumed in the diet compared to when it is delivered in the water.¹ Acute lead poisoning in dogs results in anorexia, salivation, vomiting and diarrhea accompanied by spasmodic colic.¹⁰ Death in dogs from acute exposure occurs at doses of 191 mg/kg body weight for lead acetate, 1300 mg/kg body weight for lead oxide and 1366 mg lead/kg body weight for lead sulphate.¹ Dogs have been shown to tolerate 10 mg lead/kg diet without any changes in red blood cell production or kidney function.1In a two year feeding study with dogs, lead at a concentration of 50 mg/kg diet was tolerated without hematological changes, though some changes were observed at dosages of 100 and 500 mg/kg diet.¹

The MTL for mammalian species for lead was reported by the NRC to be 10 mg/kg DM,¹ which as noted above, has been shown to be tolerated by dogs. The average (± SD) lead content in a selection of Petcurean dry kibble foods was found to be 0.16 ± 0.11 mg/kg as fed (0.18 ± 0.12 mg/kg DM or 1.8 % of the MTL).

Mercury

Mercury is not considered an essential element for animals.¹ There are several forms of mercury in the environment, with organic and inorganic forms of mercury differing greatly in their potential to become toxic. Inorganic mercury is usually strongly bound and is less available for absorption by animals.¹ However, bacteria can convert inorganic mercury to organic mercury (methylmercury), which can then enter the food chain. Methylmercury is more toxic than inorganic forms because its bioavailability is significantly greater.¹ Animals accumulate organic mercury faster than they can eliminate it, so animals higher up in the food chain generally consume higher concentrations of mercury.¹ Fish products are the primary dietary source of methylmercury, with long-lived predatory fish, such as swordfish and tuna, containing the highest levels.

Cats are more sensitive to methylmercury than most other animal species.⁴ The FDA reports an MTL of 0.267 mg/kg DM for organic mercury in non-reproducing cats. For reproducing cats, the MTL was calculated to be 0.067 mg/kg DM.⁴ The average (± SD) mercury content in a selection of Petcurean dry kibble foods was found to be 0.01 ± 0.01 mg/kg as fed (0.01 ± 0.02 mg/kg DM or 3.7% of the MTL).

Nickel

Nickel is generally not recognized as an essential mineral for animals, though it is essential for other forms of life such as bacteria and plants.¹ Because most dog and cat foods contain some plant ingredients, they will contain varying amounts of nickel.¹ It is estimated that less than 10% of ingested nickel is absorbed, and the small amount that is absorbed is excreted in the urine.¹

Consuming high amounts of nickel (100 mg/kg diet) for extended time periods is needed in order to observe nickel toxicosis in animals.¹ Nickel toxicity is generally not considered a concern for domestic animals, except for a few localized areas where industry has increased environmental nickel levels.¹ Nickel toxicity symptoms are generally displayed as gastrointestinal irritation.⁶

The MTSA Committee set an MTL for nickel of 50 mg/kg DM based on data from sensitive species.¹ However, dogs have been shown to tolerate diets containing 1,000 mg nickel sulphate per kg DM for two years without signs of adverse effects. The average (± SD) nickel content in a selection of Petcurean dry kibble foods was found to be 1.11 ± 0.40mg/kg as fed (1.23 ± 0.44 mg/kg DM or 2.5% of the MTL).

Summary

Nutrition is a balancing act to ensure adequate intake of all essential nutrients while preventing excesses. It can be just as detrimental to consume a diet with inadequate concentrations of a nutrient as it can be to have high intake levels. Minerals, including heavy metals, are naturally occurring elements in all foods – the key is controlling the levels in finished products to ensure correct amounts are present. The National Research Council and United States Food and Drug Administration have published recommendations for the maximum tolerable levels of many minerals, including heavy metals such as arsenic, cadmium, chromium, lead, mercury and nickel. The results of testing Petcurean dog and cat foods at an independent laboratory for heavy metals show that the average levels are well below the MTL for each metal, providing support for our goal of putting pets first in every decision we make.

This blog was originally published on October 12, 2017. Last revised November 2021.