A balancing act: the nutritional seesaw of copper and zinc

Zinc is an important trace element for the development of epithelialization in the skin: the process whereby new skin cells migrate to where they are needed, such as in wound healing. Additionally, it serves as a cofactor in numerous vital enzymes.

Copper is also known to be a component of several enzymes, and it is involved in hemoglobin formation and immune function processes. It is one of the elements required for melanin production and keratin synthesis. However, you can have too much of a good thing, and copper toxicity due to excessive dietary intake is a rising concern for veterinarians.

Copper and zinc: their importance for pets

Zinc carries a vital role in tissues that undergo cell renewal, such as the skin, where healing and reparations are needed. Interestingly, the skin contains approximately 20% of the total body zinc stores, where the highest concentrations are in the nasal planum, tongue and footpads.

In fast-growing young dogs, a deficiency of zinc can be manifested as a condition called zinc-responsive dermatosis, which is characterized by hairless, hyperkeratotic lesions commonly found around the mouth and joints of the puppy. Although it can be quite a scare for the owner, it is a simple condition to treat: all that is needed is a growth-adjusted balanced diet that is highly digestible, and short-term zinc supplementation. Generally, zinc deficiency in both growing and adult cats and dogs is most visible in the skin – causing a rough impression of the hair and dry, itchy skin.

Interestingly, when it comes to copper deficiency, a study in the Journal of Nutrition found no signs reported in adult dogs. Nevertheless, in growing dogs and cats, loss of hair pigmentation and rough, dull hair coat were noted as the main clinical symptoms of copper deficiency in the diet. However, copper deficiency is considered to be unlikely in pets, as commercial diets contain adequate copper levels. The liver is the central organ for copper metabolism, and hepatic concentration of copper is the most reliable reflection of pets’ copper intake and status.

Maintaining the balance: Are we following safe requirements?

The availability of copper and zinc from different foods and supplements can vary, so the requirement for these trace elements is difficult to define. The requirement will depend on the source in the diet (organic versus inorganic form) and the level of other nutrients that can have interactions with both copper and zinc.

Copper and zinc are both known for their sensitive balance, that can be easily thrown off by poor digestibility of the diet, too high concentrations of calcium, or the presence of phytate, found in diets containing high levels of soy. This can ultimately lead to poor absorption and deficiency of zinc and copper in the body.

However, an excess of these elements is also possible. It is hypothesized that the copper contents of many commercial dog foods are greater than the biologic requirement of dogs – and can even exceed the tolerance limit. It has been noted that the mean hepatic copper concentration in dogs has progressively increased over the past decade and this can be correlated to high copper content in commercial dog foods. At the same time, there is a growing body of evidence suggesting an association between high hepatic copper concentrations and the presence of hepatic abnormalities on a microscopic level that could lead to the development of chronic hepatic disease.

Recently, the current guidelines for copper content in commercial dog foods have been critically discussed, and an important question has been raised: Is there any correlation with the increased incidence of copper-associated hepatopathy in dogs that has been noted over the past decade? Clinicians are looking into the possible relationship between the change in the type of copper used in pre-mixes added to commercial dog foods, and increased use of novel protein meats with high organ inclusion rates, such as liver, that could significantly influence the final copper content in the diets.

The dangers of excess

Copper toxicity manifests as copper accumulation in liver tissue, causing damage and negatively affecting liver function. There are 3 causes of hepatic copper accumulations reported in the literature. Firstly, a hereditary defect, as seen in Bedlington terrier dogs. Secondly, the altered biliary excretion of copper due to hepatic inflammation and fibrosis. The third cause of hepatic copper accumulation is excessive intake through the diet.

Excess copper levels lead to a high degree of oxidative damage to lipids and proteins in the liver tissue, resulting in chronic hepatitis. Veterinary diets are formulated to counteract excessive copper deposition in liver tissue, using high concentrations of zinc salts. These are effective in preventing copper accumulation in the liver, as zinc has properties that help with liver integrity. Additionally, zinc ions induce the synthesis of metallothionein, which binds copper making it unabsorbable from the intestine and possibly detoxifying it in the liver. Zinc acetate or zinc gluconate is recommended as the preferred source of zinc added to hepatic diets, since the sulfate form may be associated with gastric irritation and vomiting.

Expert recommendations

Evaluating the copper content of pet food is considered challenging and it cannot be reliably done by reading the pet food label – as the value on the label would represent added copper, and not the sum of supplements and native copper already found in feedstuffs in the diet. Another challenge lies in the fact that there is a wide bioavailability of copper from organic and inorganic sources, such as different salts (copper oxide versus copper sulfate).

Some veterinary clinicians believe that the current recommendations for copper content in adult maintenance canine diets are too high, and may exceed the upper tolerability limit for some dogs, resulting in hepatic disease. They are calling for reconsideration of copper requirements for dogs set by regulatory bodies.