The art of establishing mineral tolerances of dogs and cats

Abstract

For over six decades, nutritional science has provided well-developed, peer-reviewed nutrient recommendations to support the health of dogs and cats. These guidelines are updated based on new scientifically valid research and appropriate peer-review. Recent regulatory and scientific positions around health issues have resulted in strong opinions and desires for rapid regulatory action surrounding mineral nutrition, but with limited and conflicting scientific evidence.

Pet Food Institute nutrition experts have come together to jointly author an article on the complexities of establishing mineral tolerances of dogs and cats to illustrate the limitations in defining mineral tolerances. This discussion covers how mineral requirements were determined, including the opportunities and pitfalls encountered. Scientific councils must review and clarify any proposed changes in conducting mineral nutrition research that might impact complete and balanced foods and surrounding regulations. It is important to clarify the multiple issues in mineral nutrition research and the necessity for thorough evaluation of data while avoiding arbitrary and potentially harmful guidelines.

Establishing nutrient tolerances for dogs and cats is a complex science. This article outlines the challenges and necessities animal scientists must consider when establishing mineral tolerances for both species.

Technical Brief

A goal of nutritional science is to use sound science to develop data and subject those data to peer review to best support optimal health of dogs and cats. Current nutritional guidelines have come from multiple technical committees, including the National Research Council (NRC), the Association of American Feed Control Officials (AAFCO), and the European Pet Food Industry Federation (FEDIAF). The understanding of nutritional requirements of dogs and cats is far from complete, with opportunities to continue advancing the knowledge of nutrition, health, and performance of these species.

The purpose of this review is to illustrate the complexities and limitations in defining mineral requirements and tolerances. For some nutrients, a large and robust database exists that can be easily accessed by scientists charged with defining and updating nutrient requirements. Such is not the case for all minerals.

Scientific panels with representation from industry, academia, and government developed guidelines for the measures of mineral nutriture and adequate intake. The NRC subcommittees often, but not always, inform the development of nutritional profiles for AAFCO and FEDIAF. The first NRC publications were released in 1962 (dog) and 1976 (cat), which were revised in the mid-1980’s and revised again in 2006. During the period between the publications, new research contributed to the advancement of dog and cat nutrition. Robust scientific data regarding safety, bioavailability, mineral interactions, fiber binding, and consistent impact of processing have not always been available in these reviews. The main implication of this lack of data is that AAFCO and FEDIAF guidelines often are based on the NRC nutrient profiles, and the guidelines outlined in these publications are used by several countries’ regulatory agencies to control pet food manufacturing.

National Research Council (NRC) (2006) Perspective:

The NRC is one of the operational units of the National Academies of Sciences, Engineering, and Medicine whose mandate is to provide updated, unbiased scientific information on a broad range of topics important to the betterment of society. National Research Council publications often are considered to be THE authoritative documents in specific subject matter areas. The “Nutrient Requirements of Dogs and Cats” was last published in 2006 and is used by federal and state regulatory agencies to craft nutrient requirements and nutrient profiles that pet food manufacturers must follow to ensure that the dog and cat are being properly nourished.

As determined by NRC, twelve minerals are essential for dogs and cats, with several others assumed to be essential because of their requirements by other animal species. Different techniques were used to establish mineral requirements. They include:

• (a) growth assays

• (b) plasma and milk mineral concentrations

• (c) serum concentrations of mineral-requiring enzymes

• (d) apparent absorption measurements

• (e) mineral balance

• (f) total body mineral analysis

• (g) muscle biopsy

• (h) evidence of disease states (clinical signs; histopathologic findings; skin lesions)

• (i) reproductive performance (impaired fertility; increased perinatal mortality)

• (j) glycemic control (glucose metabolism, insulin sensitivity)

• (k) immune function

• (l) thyroid gland function

• (m) hair mineral concentrations and hair growth

• (n) kinetic approaches

• (o) epidemiologic studies

• (p) breakpoint analysis; dose response curves

• (q) factorial analysis to determine mineral allowances for gestation and lactation.

Regardless of technique used, an estimate of mineral bioavailability in the mineral sources being evaluated is necessary. Four measures of nutrient requirements of healthy animals are presented in NRC (2006) for these physiological states: growth of puppies and kittens after weaning, adult dogs and cats at maintenance, and bitches and queens in late gestation and peak lactation. These measures are:

• (a) minimal requirement - the minimal concentration or amount of a bioavailable nutrient that will support a defined physiological state

• (b) adequate intake - the concentration in the diet or amount required by the animal that is presumed to sustain a given life stage when no minimal requirement is demonstrated

• (c) recommended allowance – based on the minimal requirement and, where applicable, includes a bioavailability factor; if no minimal requirement is available, this value is based on the adequate intake

• (d) safe upper limit – the maximal concentration or amount of a nutrient that has not been associated with adverse effects

Values are expressed as amount/kg DM (assuming a dietary energy content of 4,000 kcal metabolizable energy (ME)/kg), amount per 1,000 kcal ME, and amount/kg body weight^0.75 (dogs) or per kilogram body weight^0.67 (cats).

Recommended allowance values are presented for each macro-mineral and trace element. Adequate intake values are available for most. For many minerals, a minimal requirement value cannot be established because no dose-response study has been conducted. Similarly, few safe upper limit values are available because of the lack of empirical data.

To properly formulate a canine diet, the nutrient requirements of the dog, the nutrient composition of the dietary ingredients, and the bioavailability of the nutrients in the dietary ingredients must be known. In addition, processing of the complete and balanced diet derived from the various ingredients will impact bioavailability. Bioavailability of minerals is altered by the presence of numerous substances in the complete diet that, in turn, may affect utilization of dietary minerals. In many cases, requirements have been established in experiments where nutrients were supplied in purified or semi-purified ingredients for which digestibility and bioavailability values are high. This has a major impact on how NRC requirement figures are used. Nutrient bioavailability in purified or semi-purified ingredients is usually much higher than in ingredients used in commercial dog foods. This led the pet food industry and regulators to conclude that the NRC recommendations should be examined as to their ability to support nutritional adequacy claims in natural ingredient-containing commercial pet foods. To address this issue, AAFCO established Canine and Feline Expert Subcommittees whose charge was to translate the recommendations of NRC into a set of guidelines that could be applied to the practical formulation of pet food and the regulation of nutritional adequacy.

In addition to the bioavailability issue, the number of assumptions that must be made in establishing mineral requirements is striking (e.g., energy density of the diet; form of the mineral; test diet ingredient information; breed, size, and body condition of the animal; intact vs. neutered status; activity level; caloric intake; number of offspring in a litter; etc). Oftentimes, these are not reported in scientific publications, leaving those responsible for defining mineral requirements the task of developing estimations of key inputs used to arrive at requirement figures.

Association of American Feed Control Officials’ Perspective:

The AAFCO is an independent trade association comprised of state feed control officials in the USA. Local, state, or federal laws charge AAFCO members with regulating the sale and distribution of animal feeds and animal drug remedies within the USA.; however, AAFCO itself has no regulatory authority.

Another important function of AAFCO is the establishment of dog and cat food nutrient profiles that are used for the substantiation of nutritional adequacy of dog and cat foods for their intended use. The intent of these profiles is to establish practical maximum and minimum nutrient ranges for dog and cat foods formulated using commonly used complex combinations of ingredients.

In 2007, AAFCO Canine and Feline Nutrition Expert Subcommittees were formed with the task of reviewing dog and cat nutrient profiles using the information in the updated NRC (2006). The recommended allowance values from NRC (2006) became the standard that the 2007 AAFCO Nutrition Subcommittees used for the evaluation of the dog and cat nutritional profiles.

The 2007 AAFCO Nutrition Subcommittees also utilized supplemental references for their evaluation, and specific nutrient concentrations were modified based on additional data from FEDIAF, recent research, practical experience, and unpublished data.

The AAFCO dog and cat nutrient profiles have two life-stage categories (growth and reproduction and adult maintenance) for which minimum and, sometimes, maximum nutrient concentrations have been established. Maximum nutrient concentrations were set when a particular nutrient could reach toxic levels if over-supplemented, or if attention is not paid to the amount contributed by all ingredients. This does not imply that the absence of a maximum concentration for any specific nutrient equates to safety at any concentration; rather, there may be insufficient information in dogs and cats to support an actionable maximum concentration. Establishing maximum concentrations without robust scientific data supporting the amount could result in unwarranted implications of safety if too high or become unjustifiably restrictive if too low. The current dog nutrient profiles contain maximum mineral recommendations for calcium (Ca), phosphorus (P), iodine (I), and selenium (Se). For cats, a maximum for only I is presented. The limits for these minerals were presumably due to the availability of a sufficient body of research information to support a maximum safe upper limit with a reasonable level of certainty. It appears that the maximum limits were established considering the more available forms of minerals (sulfates vs. oxides) to provide safeguards when reporting a single upper limit.

The maximum concentration of iron (Fe) for dogs was removed with the 2007 AAFCO Official Publication. This was based on NRC (2006) stating that information to set a safe upper level does not exist and the 2005 Mineral Tolerance of Animals stated that the previous tolerance level of 3,000 ppm (DM basis) that was established in swine needed to be confirmed in long-term feeding studies. In that publication, tolerance values for other species were less than 500 ppm, bringing into question such a high maximum value. Lastly, given the inherently large variation in the apparent digestibility of Fe from different sources and ingredients, setting a maximum level provided an unwarranted level of implied safety. High analytical concentrations contributed from iron oxide could lead to inadequate available Fe concentrations. Conversely, those same concentrations contributed from an available source could be so high as to be unsafe. This methodology also was utilized to justify the removal of the copper (Cu) maximum concentration, also in 2007.

European Pet Food Industry Federation Perspective:

This organization published the “Nutritional Guidelines for Complete and Complementary Pet Food for Cats and Dogs” most recently in 2021. Recommendations are provided for minimum and maximum nutrient concentrations in commercial pet foods for healthy dogs and cats. In addition, ensuring adequate and safe nutrition is a priority of FEDIAF. The nutrient guidelines are similar in scope to those published by AAFCO (2024), providing recommendations and limits expressed on a DM basis and on an ME basis per 1,000 kcal (or megajoule) of ME. As for adult dog ME requirements, values were determined using either 95 kcal/kg^0.75 or 110 kcal/kg^0.75 taking age and activity level, as well as breed, health, and environmental factors, into account. It is recognized that this approach to establishing nutrient profiles by life stage or age can be different across regulatory bodies. For example, in the FEDIAF (2021) published guidelines, there are separate recommendations provided for early growth of the dog (<14 weeks of age) and reproduction and late growth (>14 weeks of age), whereas AAFCO has a single guideline for growing puppies and reproduction. Both approaches are appropriate and based on currently available science. Each organization (AAFCO, FEDIAF) has independent and unique rationale for their guidelines and, therefore, should be interpreted based only on the entirety of their publication.

FEDIAF (2021) nutrient recommendations for Ca, P, sodium, magnesium, Cu, I, and Fe were increased by 20% compared to NRC (2006) values to compensate for the lower energy requirement of household dogs compared to the energy intake assumed by NRC (2006). Further, it was cautioned that bioavailability of minerals should be carefully considered in diet formulation where the concentration of these nutrients is close to FEDIAF’s recommended amounts (e.g., in high-fiber diets and in formulas where plant-based raw materials rich in phytate are used as the main source of P).

There are two unique attributes pertaining to FEDIAF’s published maximum nutrient values, legal maximum and nutritional maximum. Per the guidelines, a “legal maximum” applies only when the particular trace element is added to the recipe as an additive but relates to the “total” amount present in the finished product. This regulation is laid down in the Community Register of Feed Additives pursuant to Regulation 1831/2003/EC of the Parliament and the Council concerning additives in feeding stuffs. These “legal maximum” concentrations are set for environmental rather than nutritional considerations, the reason being that these concentrations aim to prevent potential environmental contamination from excessive use of certain additives. The legal maximums apply only when nutrients are added in the formulation of the feed, not if the nutrient comes from feed ingredients. When the nutrient comes from ingredients, the “nutritional maximum” should be considered. FEDIAF (2021) defines a “nutritional maximum” as the maximum level of a nutrient in a complete pet food that, based on scientific data, has not been associated with adverse effects in healthy dogs and cats. When sources of a nutrient come from both supplementation and natural sources of an ingredient (s), the legal maximum is of greater importance to ensure adherence to safety regulations and environmental standards. The FEDIAF (2021) guidelines provide a legal maximum for Cu, I, Fe, manganese, Se (wet foods), Se (dry foods), and Zn, whereas nutritional maximum concentrations are listed for Ca and P.

Pet Food Formulation – Mineral Adequacy and Fortification of Dog and Cat Foods

Building nutritional products for pets with adequate mineral balance and fortification must start with knowledge of the capabilities of the manufacturing facility and the impact of the processes employed (e.g., canning vs. gelation; extrusion vs. baking; smoking vs. freeze-drying; injection molding vs. co-extruding). Further, each process varies in mixing capability, process temperature, pressures, coating systems, meat inclusion, stability and oxidation-reduction, and ingredient availability. Taking all these factors into account is critical to begin to formulate the nutritional profiles (including mineral profiles) that are targeted in the final product.

In the USA, it is paramount that the food product meets the nutrient profiles in the AAFCO (2024) Official Publication. Other countries (e.g., Mexico, Chile, Thailand) adopt AAFCO guidelines while others (e.g., European countries) adopt their own guidelines.

Based on these processing and regulatory targets, the following factors must be considered when developing a balanced and well-fortified mineral supplementation program for dogs:

• (a) supplemental mineral sources used; concentration of mineral in the source

• (b) mineral concentration and variability in all dietary ingredients used in specific formulas

• (c) mineral bioavailability of the primary ingredients that contribute to total dietary mineral concentration as well as determining bioavailability of the complete diet since processing and/or the presence of antagonists such as phytate and fiber can reduce mineral bioavailability

• (d) anti-nutritional factors used in the formula and their potential impact on mineral binding and bioavailability

• (e) the impact of processing on mineral form and availability

• (f) calories in the complete and balanced food and their impact on mineral consumption

• (g) palatability of the food and consumption data (if the food is not palatable, the nutritional goal built for each mineral is not met)

• (h) impact of food ingredients on acid-base balance

• (i) the impact of other edibles offered beyond a single complete and balanced food that potentially could lead to unforeseen imbalances and over-fortification

• (j) the formulation must consider the impact of one mineral upon another (e.g., too much Ca will impact absorption of several other minerals)

• (k) particle size (especially as related to Ca and P) can affect solubility that then can impact bioavailability not only of Ca and P but trace minerals as well (due to pH and acid-base effects)

Once mineral concentrations are chosen (considering the impact of each of the factors listed above), some reasonable amount of overage must be built in. One choice is to allow inorganic forms of the mineral in the premix to provide a base level of mineral nutrition (e.g., 25 to 100% of the requirement) while being aware of the minerals contributed by other ingredients and acknowledging the variation in bioavailability of organic minerals, such as Cu, among ingredients derived from plants or animals. Moreover, the factors that may negatively affect bioavailability within plants (e.g., phytate binding of Cu) or animal (e.g., high Zn concentrations antagonizing Cu absorption) ingredients must be considered (Aoyagi et al., 1993).

“Chelation of minerals” is a broad term encompassing a variety of compounds and binding processes with proteins, amino acids, yeasts, and other potential chelating agents and organic materials. The absorption mechanisms for chelated minerals are different from those for inorganic mineral sources. There is much yet to be learned about the role of chelated minerals in dog and cat nutrition.

The Path Forward: Do Opportunities Exist to Further Establish Mineral Requirements of Dogs and Cats?

Currently, there are proven methodologies that enable precise and minimally invasive assessment of mineral requirements of humans and agricultural animals. Despite the availability of these techniques to researchers, the incentive to fund such research in dogs is questionable as it is unknown whether the research findings would be recognized by NRC and AAFCO to justify potential changes in mineral requirements of dogs should the evidence point in that direction. Indeed, one of the criticisms regarding the assessment of many minerals in pet foods was that the current recommendations were made using older data that may not reflect the biological requirements of present-day dogs at different life stages (Butterwick et al., 2011). Furthermore, laboratory dog breeds (e.g., beagle) fed semi-purified diets have been used in establishing many of the mineral requirements, and neither are reflective of present-day animal characteristics (e.g., life stage, breed size, health status, activity level, etc.) or consumer desires regarding dog food formats (e.g., dry kibble, wet food, high meat inclusion, freeze-dried, raw, etc.). Evaluating new techniques and models (e.g., isotope labeling, factorial calculations) or modifying current/traditional techniques (e.g., mineral balance) to study mineral nutrition of dogs and cats present opportunities, but there is still much to elucidate regarding their viability and practical applications.

The goal in reassessing mineral requirements of dogs and cats would be to utilize safe, reliable, non-invasive methods to understand mineral bioavailability and metabolism. Methods that meet these criteria have been utilized in human, agricultural animal, and in select instances, companion animals (Kienzle, 1998; Kastenmayer et al., 2002; Turnland, 2006; Böswald et al., 2019); however, the advantages and drawbacks of these methods, which include stable isotopes and factorial calculations, must be carefully considered.

Over the past two decades, stable isotope tracer technology has been used to study mineral requirements of humans to include mineral uptake and utilization (Turnland, 2006). Use of stabilized isotopes is minimally invasive and allows for a highly sensitive assessment of the bioavailability and metabolism of minerals. For example, several studies conducted in the 1980’s used stable isotope tracers to assess Zn, Fe, and Cu absorption in young and elderly humans (Turnland et al., 1986, 1988). They found that absorption of Zn (Turnland et al., 1986), but not Cu or Fe (Turnland et al., 1988), was lower in the elderly than in younger adult males. Stable isotopes also have been used in dogs to assess absorption of Ca, Fe, Cu, and Zn from dry dog foods, with reported absorption of Ca, Fe, and Zn being markedly lower than that of Cu (Kastenmayer et al., 2002). In terms of safety, stable isotopes are not radioactive and no adverse health effects have been reported at the low levels used in human studies (Davies, 2020). However, this methodology requires considerable financial, time, and labor inputs, not to mention the fact that the isotopically enriched forms of the minerals being evaluated may not necessarily represent the forms of the minerals supplied in the diet, thus potentially exemplifying different kinetics during assimilation. Moreover, the data obtained are only representative of the specific experimental settings and subject characteristics. Recent work on amino acid requirements of dogs using indicator amino acid oxidation methods where small-, medium-, and large-breed dogs were tested showed that depending on the amino acid in question, requirements may differ by breed size (Mansilla et al., 2020; Sutherland et al., 2020). Furthermore, depending on breed size, estimated amino acid requirements differed from those established by the NRC (2006) and from recommendations proposed by AAFCO (2023) for adult dogs at maintenance. This suggests that dog and cat mineral studies utilizing isotope techniques must control for factors that may impact differences in mineral utilization, including age, sex, and breed.

Alternatively, factorial calculations may also be applied, representing another safe, established, non-invasive method for estimating mineral requirements. While factorial calculations have been more commonly utilized for the estimation of mineral requirements of agricultural/food animals, they have been used to estimate mineral requirements in companion animals as well (Kienzle, 1998; Böswald et al., 2019). By accounting for retention and obligatory endogenous losses (e.g., skin, hair, urine, feces) of the mineral in question and considering the mineral’s expected availability, factorial calculations can allow for the estimation of the minimum mineral requirement for maintenance necessary to compensate for those losses. A primary advantage of this factorial approach is that it allows for the estimation of dynamic requirements. For any performance-related factor that may be at play (e.g., growth, lactation, gestation), the additional amount of the mineral in question that may be lost or gained is calculated and added to the estimated maintenance requirement (Kienzle, 1998). Factorial calculations have been applied to estimate the requirements of Ca and P in lactating queens (Kienzle, 1998) and in growing puppies (Böswald et al., 2019). In the more recent study, requirements estimated using the factorial approach were compared with those extrapolated via the NRC (2006) ME requirement. Here, agreement between NRC extrapolation and factorial approaches was observed in very select situations (e.g., estimated Ca requirements for giant breed puppies less than 7 months of age); however, deviations up to ~60% between the estimates were otherwise apparent (Böswald et al., 2019). Accuracy of estimates derived from factorial models depend on the availability of sufficient and robust data. Appropriately capturing and quantifying factors such as basal losses and/or availability of the minerals in question will have a significant impact on the accuracy of this method. For select minerals, particularly trace minerals, data in dogs and cats may be limited. Moreover, caution should be taken when attempting to extrapolate older data considering that these inputs may be influenced by a myriad of animal-related factors, such as breed, age, sex, and genetics, as well as diet-related factors, like ingredient selection, mineral form, nutrient interactions, and processing.

In order to address these hurdles and establish accurate and updated mineral requirements for dogs and cats, perhaps the complementary application of modern day empirical (e.g., stable isotopes) and factorial methods should be considered. A complementary approach would allow for the generation of sufficient and robust data to input into a model that could accurately estimate requirements of dogs and cats in different conditions and of different sexes, ages, breeds, and life-stages. However, despite the availability of suitable methods to assess mineral requirements of dogs and cats, several challenges remain that must be critically evaluated and accounted for before these techniques can be used for this purpose:

• (a) There are numerous factors, such as animal health status and life-stage, source and form of the mineral being assessed, and concentrations of macro-minerals and trace elements that impact mineral bioavailability that complicate use of newer techniques.

• (b) True assessment of mineral requirements must be conducted in healthy dogs and cats while using animals, ingredients, and complete and balanced diets that are representative of the present-day dog and cat populations and commercial diets available in the market.

• (c) The methods must be designed and described in detail to allow for appropriate verification of results across laboratories to determine if future updates or changes in mineral requirements are needed.

• (d) Even though method safety has been established in humans and select animal species, animal welfare issues will always arise when experimenting on dogs and cats.

• (e) Data collected using new techniques may not be accepted by relevant regulatory bodies (i.e., AAFCO takes their lead from NRC; no updated NRC publication is planned; AAFCO will not accept new mineral requirement information absent its review by NRC).

Summary and Conclusions:

As noted by Butterwick et al. (2011), “highlighting the limitations and dilemmas that were faced in determining the requirements... of nutrients and, in this specific case, minerals... will encourage further study and help to make current and future recommendations of more practical use”.

Mineral nutrition of pet animals is a complicated subject. It is not surprising that pet animal nutritionists and veterinarians sometimes question the reliability of the published data in this area of nutritional science. Initiating changes in mineral requirements will require multiple studies to demonstrate the need for change. Most nutrient requirements should be re-examined in light of present-day dog and cat feeding practices and the fact that the types of diets formulated today are very dissimilar to those fed when NRC (2006) was prepared. A new NRC publication is past due given the substantial amount of dog nutrition data published in the last 20 years but, better yet, a system that allows continual updating of nutrient recommendations and profiles as new data on this topic emerge would be ideal. Substantial funding would be necessary to devise and implement such a robust system, but this would be money well spent for the good of the pet, the pet parent, the veterinarian, the pet food manufacturer, and those who regulate pet food production.

Glossary

Abbreviations

AAFCO

Association of American Feed Control Officials

FEDIAF

European Pet Food Industry Federation

NRC

National Research Council

DM

Dry Matter; kcal, kilocalorie

ME

Metabolizable Energy

USA

United States of America

Ca

Calcium

Cu

Copper

P

Phosphorus

I

Iodine

Fe

Iron

Se

Selenium

Zn

Zinc

kg

kilogram

Authors are members of the Pet Food Institute Nutrition Subcommittee (PFI-NSC) Working Group; * is a member at large without member affiliation.

The Pet Food Institute (PFI) serves as a united voice of the pet food industry, coming together as one and advocating for its members interests, sharing best practices and collaborating to ensure the quality, safety, and nutritional adequacy of pet foods manufactured and sold across the United States are of the highest standard.

Disclosures

Authors are primarily employed by the institutions listed as associated with their names and are respective Pet Food Institute member company affiliates with the exception of (*) who is a Professor Emeritus and member at large. None of the authors received financial reimbursements from the Pet Food Institute for contributing to this document and genuinely believe their responsibility is to safeguard the well-being and safety of pets. Participation in the Pet Food Institute Nutrition Subcommittee is voluntary and delegates are from member companies.