Why Care about Copper?

The mineral copper is an essential dietary nutrient for dogs. It is needed for the formation and activity of red blood cells, acts as a cofactor in numerous enzymatic reactions, and is necessary for normal skin and hair pigmentation. Copper deficiency can lead to impaired skeletal growth and anemia, but is very rare in dogs today.

Conversely, copper toxicity – too much copper – may be of greater concern. Copper is stored primarily in the liver. As a result, excess copper accumulation in the body manifests as a form of liver disease, called copper-associated hepatopathy. While treatable if caught early, copper toxicosis is a serious disorder and can be fatal.

Genetics Matter

Copper toxicosis may have several underlying causes. Influencing factors include genetics, metabolic/homeostatic anomalies, and diet. For example, Bedlington Terriers have an inherited disorder of copper metabolism that results in an inability to normally use and excrete copper. Copper toxicosis in the breed was one of the first genetic disorders to be studied by the Canine Genome Project and for which a diagnostic genetic marker was found. In this case, a simple autosomal recessive gene is responsible for the disorder.

Other breeds that may have genetically influenced disorders of copper metabolism are the Doberman Pinscher, West Highland White Terrier, American Cocker Spaniel, and Labrador Retriever. Less is understood about the genetics of the problem in these breeds and how copper homeostasis may be influenced by both genetic and environmental factors.

Role of Diet?

In recent years, the role of diet on the incidence of copper-associated liver disease in dogs has received increased attention. In early 2021, a group of six veterinarians from five veterinary teaching colleges collaborated to produce a review paper regarding changes in liver copper levels in dogs and how these changes may be related to pet food (1).

The paper covers a lot of ground and provides detailed analyses of past and recent studies of copper status in dogs and copper-associated liver disease. I will do my best to summarize the paper’s most important points.

Changes in Hepatic Copper Concentrations

Only a few studies have surveyed mean (average) liver concentrations in large groups of pet dogs. However, the data that are available span a period of almost 100 years. They tell a disturbing story:

1. 1929: The first study, published in 1929, reported mean hepatic liver levels of less than 10 micrograms (μg) per gram of liver (dry weight) in the dogs studied.
2. 50 years later: In 1982, following the introduction and widespread use of commercial pet foods, a second study found that average liver values had increased to 200 μg/g, an increase of about 20-fold.
3. 1995: Between 1982 and 1995, the average concentration increased again – this time doubling to 453 μg/g.

Most recently, the role of diet in liver copper concentrations was examined in a small study that compared liver copper concentrations in 9 free-ranging (feral) dogs in Malaysia that survived on human food scraps with those of 9 healthy laboratory dogs fed a commercial dog food containing 23.4 mg/kg copper (within AAFCO recommendations). The laboratory-raised dog had significantly higher liver copper concentrations compared with the free-ranging dogs. On average the healthy laboratory dogs had more than 3-fold greater concentrations of copper in their livers, with a range of 199 to 997 μg/g. For some perspective to health, dogs may develop signs of hepatic injury with liver levels as low as 600 μg/g, and current recommendations are to treat for copper-associated liver disease when levels reach this concentration or greater.

Increased Incidence of Copper-Associated Liver Disease

The authors note that collectively, their veterinary colleges have observed an increased incidence of copper-associated liver disease in dogs over the last 15 to 20 years. For example, one author published a 2017 biopsy study comprised of over 2000 liver samples taken from dogs between the years 2010 and 2015. More than 50 percent of the dogs had liver copper concentrations that were classified as high – greater than 400 μg/g – and many of the samples showed signs of inflammatory liver disease. Another author found that 60 percent of liver biopsies conducted at her center had high copper levels, almost all of which also showed signs of inflammatory liver disease. By any standards, these numbers are worrying.

Role of Commercial Pet Foods?

There is no easy way to state this. The authors of this paper, six veterinary experts from different academic institutions, believe that the changes in average liver copper concentrations and in the incidence of copper-associated liver disease that they report are unlikely to be due to genetic factors or to changes in dogs’ ability to handle/excrete copper. Rather, they theorize that diet is the most important underlying factor influencing liver copper accumulation in dogs in recent years.

Sources of Copper in Pet Foods

Most pet food companies use a vitamin/mineral “premix” in their food formulations. These provide, among other vitamins and minerals, a source of copper, and function to properly balance foods to meet AAFCO Nutrient Profiles. Prior to the late 1990’s most premixes included copper in the form of copper oxide. However, evidence that copper oxide had a very low bioavailability in dogs (i.e. they could not absorb and use it) led to a 1997 AAFCO recommendation that copper oxide should be replaced with copper sulfate, a compound with higher bioavailability.

Certain pet food ingredients are also enriched sources of copper. These include organ meats (in particular liver), fish, some legumes, and even sweet potatoes. The authors speculate that the increased popularity of high protein (meat) foods and perhaps also the use of alternate carbohydrate sources such as sweet potatoes, may be another contributor to elevated copper levels in pet foods.

To summarize: It is possible that three changes in pet food formulations over the last several decades are influencing dietary copper levels. These are (listed in decreasing order of expected impact):

1. Change from using copper oxide to copper sulfate in pet food vitamin/mineral premixes (1997)
2. Increased use of organ meats in high protein dog foods (and increased popularity of these foods)
3. Increased use of non-grain vegetables that are enriched sources of copper (sweet potatoes).

What IS the Dog’s Copper Requirement?

An exact dietary requirement for copper (as for most trace minerals) is difficult to determine because of the body’s self-regulating response to need/dietary levels, to the wide range in availability among copper sources, and because of the influence of other minerals in the diet (nutrient interactions).

Given the data presented here, the minimum level of copper in dog foods appears to be less of a concern than is determining what is a safe upper limit for copper in dog foods. It is important to note that AAFCO removed their safe upper limit for copper in 2007 and has not yet provided a new value. Prior to 2007, the maximum upper limit for copper was 250 mg/kg dry diet.

Authors’ Proposal

The authors of this study propose that, in light of the evidence that dogs’ liver copper levels have been increasing steadily and that current copper intakes may be too high for a substantial number of dogs, that current data and studies be used to define a new safe upper limit for copper in commercial dog foods. In addition, they suggest that current recommendations for copper content in adult maintenance foods are too high and should be reconsidered (and reduced). They respectfully request that AFFCO reconsider the current levels and should also reestablish an upper limit for this mineral.

Take Away for Dog Folks

First, do not panic. While the information provided in this paper is certainly concerning and is important, the take-away is not that dogs are all experiencing copper toxicity. However, if you are concerned, there are few things that you can do:

• Check Copper Levels: Reputable dog food companies provide complete nutrient analyses of their foods. While this information is not included on the pet food label, companies can provide it on their website. The minimum copper concentration for adult maintenance foods is currently 7.3 mg/kg (dry matter basis) or 1.8 mg per 1000 kcal. The prior upper limit was 250 mg/kg. Ask about the concentration in the food that you feed and compare the value to these lower and upper limits.
• Consider Limiting Foods with High Copper Ingredients: Foods that are formulated with organ meats may contain naturally higher levels of copper. If you are feeding a high meat food, check levels and if you are worried, ask the company about how they are monitoring copper levels in their foods.
• Talk to your Veterinarian: If you have a breed that may be at increased risk for copper accumulation in the liver or are worried about your dog’s copper intake and health, first and foremost talk to your veterinarian. Ask about options for monitoring liver health and possible diagnostic procedures.
• Stay Tuned: Hopefully, AAFCO will respond to this important and timely paper and will move to ensure both sufficient minimums and safe upper limits of copper in pet foods. Many thanks to the six authors for this publication.

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Center SA, Richter KP, Twedt DC, Wakshlag JJ, Watson PJ, Webster CRL. Is it time to reconsider current guidelines for copper content in commercial dog foods? Journal of the American Veterinary Medical Association 2021; 258:357-364.

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