Not the Bruce Willis movie.

Rather, mercury levels in pet foods. Are they too high?

Unlike the movie, these worries are not fiction. However, neither is there reason for widespread panic. Let’s look at what we currently know about mercury in pet foods and whether or not these levels should be of concern to dog owners.

What is mercury? Mercury (Hg) is a naturally-occurring metal that is found everywhere; it is in the soil, in water and in the atmosphere. Terrestrial plants take in mercury from the atmosphere, while aquatic plants and microorganisms absorb it from water. Certain fish species are considered to be “bioaccumulators” because they concentrate mercury in their tissues. Species that have unusually high levels of mercury include tuna, mackerel, marlin, swordfish, and shark. Conversely, fish and shellfish such as salmon, sardines, scallops, and crab usually contain lower levels.

Mercury is not a required mineral and is toxic to humans and other animals. Several forms exist in the environment. The organic form, methylmercury, is considered to be the most toxic of these because it is more readily absorbed into the body. Although methylmercury is the form that predominates in the fish meals that are used in pet foods, its actual bioavailability (i.e.  the proportion that is absorbed and used) is not known. Regardless, dogs, like other animals, are susceptible to mercury toxicity. A dose of 500 micrograms/day is acutely toxic to dogs and leads to rapid illness and death. Clinical signs of mercury poisoning include gastrointestinal ulcerations and hemorrhaging, kidney damage, and neurological damage. Because mercury accumulates in tissues, chronic effects of lower levels of consumption over long periods of time can occur.

No Safe Upper Limit for Dogs: There is no question that excessive levels of this metal should not be present in foods. Unfortunately, a safe upper limit in pet foods has not been established by either the Association of American Feed Control Officials (AAFCO) or the National Research Council (NRC). Nor does there exist a compulsory requirement for pet food companies to test their products for mercury (or any other heavy metal). In Europe, the European Commission Directive has set a maximum level of total mercury at 0.4 mg/kg of diet for dry dog foods (12 percent moisture). However, there does not exist empirical support for setting that level.

How Much Mercury is in Dog Foods?  In recent years, several groups of researchers, from different institutions, have published studies measuring mercury levels in commercial dog and cat foods.

Here is a summary of their findings:

• Study 1 (2011): This study was published by a group of mineral scientists. They examined the heavy metal content of 58 brands of dog and cat food and published a comprehensive examination that included comparisons to recommended maximum levels of mercury in human foods. Cat foods had higher mercury contents than dog foods, but none of the foods exceeded 100 micrograms/kilogram of food. The highest level in a dog food was 26.8 micrograms/kg, found in a dry dog food (see below)
• Study 2 (2016): Two researchers with the Department of Natural Resources and Environmental Science at the University of Nevada measured mercury concentrations in 54 brands of dog food and 47 brands of cat food.  The 101 tested brands had Hg concentrations ranging between 1.0 and 604 micrograms/kilogram of food (mean value of 43). Foods with the highest concentrations (>100 mg/kg) contained fish as a primary protein source and canned cat foods generally had higher levels. No foods exceeded the maximum level established for human foods, but several were higher than maximum levels for kingfishers and river otters (fish-eating species). The  highest reported Hg concentration for a dog food was ~ 60 micrograms/kg (see below).
• Study 3 (2018): Joseph Wakshlag and a group of researchers at Cornell University tested 51 dry dog foods. They divided the foods into three categories, based upon the primary protein source in the food. Fish-containing dog foods contained significantly higher levels of mercury when compared with poultry and red meat-containing foods. The median level was 0.0083 mg/Mcal. (Note: A Mcal is equivalent to 1000 kcals). When compared with reported human consumption patterns, the authors reported that dogs consuming the median range of mercury in fish-based foods would be consuming 5 times the maximum recommended levels of mercury for humans.  The  highest reported Hg concentration for a dog food was 13.9 micrograms/1000 kilocalories (see below).
• Study 4 (2019): Most recently, a group of researchers at the University of California (Davis) measured the elemental mercury and methylmercury content in 24 commercial dog foods. In contrast to previous studies that detected mercury in almost all sampled foods, this paper reported that mercury was non-detectable in 21 out of the 24 samples that were tested. Of the three foods that contained Hg, the highest concentration was 490 micrograms/kilogram of food (see below).

So, Let’s Do a Bit of Math: 

The U.S. Environmental Protection Agency (EPA) currently recommends that an adult human should consume no more than 0.1 micrograms of mercury per kilogram of body weight per day. If we apply this recommendation to a person who weighs 175 lbs, this is equivalent to 8 micrograms of mercury per day, as the maximum safe level.

So, let’s look at some of the reported levels that we have for dogs. As our example dog, I will use a 36-lb (16.4 kg), young, adult dog who has an estimated daily caloric requirement of 1000 kilocalories (kcals). We will assume that this dog is consuming a dry food that contains 4000 kilocalories per kilogram (about an average adult maintenance food). On a weight basis, he would need 250 grams of food per day to meet his energy needs.

To make this a bit more personal, let’s call this fellow Stanley.

For each of the four papers that are summarized above, I selected the food that had the highest reported level of mercury and calculated the number of micrograms of mercury that Stanley would be ingesting daily if he was fed that food (Note: The papers reported Hg in several different ways [units], which required slightly different calculations for each situation):

1. Study 1: The highest concentration of Hg in this study was 26.8 microgram/kilogram of food. If fed this food exclusively, Stanley would ingest 6.7 micrograms of mercury per day.
2. Study 2: The highest concentration of Hg in this study was ~ 60 microgram/kilogram of food. If fed this food exclusively, Stanley would ingest 15.0 micrograms of mercury per day.
3. Study 3: The highest concentration of Hg in this study was 13.9 micrograms/1000 kcals. Stanley would ingest 13.9 micrograms of mercury per day.
4. Study 4: The highest concentration of Hg in this study was 490 micrograms/kilogram of food. Stanley would ingest 122.5 micrograms of mercury per day.

In three of these four cases, Stanley would be ingesting more than the EPA upper recommended limit of mercury (8.0 micrograms per day) for a human subject weighing 176 lbs (not to put too fine a point on this, but almost 5-times his weight).

Up on My Box: Collectively, these studies suggest that the levels of mercury in most commercial pet foods is very low and will not put dogs at risk. However, a small number of foods had higher than expected concentrations of mercury. These levels were usually (not always) found in fish-based foods. For each of the four papers reported above, I selected the food that contained the highest level of mercury and calculated how much mercury Stanley would ingest if he was fed that food exclusively. These calculations revealed that, in three of the four foods, levels exceeded the upper limit set by the EPA for a human subject. Although these concentrations were always much lower than the levels known to be acutely toxic to dogs, it is important to note that we do not know what level of mercury is safe for dogs over long periods of time (chronic consumption). There is some evidence of neurotoxic effects of methylmercury poisoning occurring at lower levels than those found to cause acute toxicity, but safe upper limits for dog foods have not been established. 

So here are the facts:

1. A safe upper limit for mercury in dog and cat foods is not known.
2. There is no mandatory requirement that companies test their products for mercury, even when fish is a primary ingredient.
3. Some foods (albeit a very small number) contain levels of mercury that, if fed exclusively, would result in a dog consuming daily levels of mercury that exceed recommended EPA limits for humans.

A Few Recommendations: On a practical level, a reasonable approach is to avoid feeding only foods that contain fish as the primary protein source as a dog’s lone food source. Second, these data provide additional support for the recommendation to avoid feeding a single brand of food to dogs over long periods of time. It is time to robustly reject the outdated advice that dogs do best when fed one brand of food for their entire lives. Just as humans are advised to avoid frequent consumption of tuna because of the risk of excessive mercury exposure, so too is it prudent to avoid feeding dogs a single food, given that we do not have good information regarding mercury levels in all foods. Rather, select a group of foods (and food types, if possible) that you trust and upon which your dog thrives. Rotate these foods over several weeks or months and/or mix foods. Last, if you are concerned about mercury in your dog (or cat) food, contact the manufacturer and inquire if they are testing for mercury and other heavy metals. Although there is no reason to panic, the data presented in these studies do inform us that mercury levels may be too high in a small number of foods.

Cited Studies:

1. Atkins P, Ernyei L, Driscoll W, et al. Analysis of toxic trace metals in pet foods using cryogenic grinding and quantification by IPC-MS, Parts 1 and 2. Spectroscopy 2011; 26:46-56; 57-68.
2. Luippold A and Sexauer Gustin M. Mercury concentratnions in wet and dry cat and dog food. Animal Feed Science and Technology 2016; 222: 190-193.
3. Kim HT, Loftus JP, Mann S, Wakshlag JJ. Evaluation of arsenic, cadmium, lead and mercury contamination in over-the-counter available dry dog foods with different animal ingredients (red meat, poultry, and fish). Frontiers in Veterinary Science 2018: October, doi: 10.3389/fvets.2018.00264.
4. Sires RA, Fascetti AJ, Puschner B. Determination of total mercury and methylmercury concentrations in commercial canine diets. Topics in Companion Animal Medicine 2019; 6-10; doi.org/10.1053/j.tcam.2019.02.002

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