Calcium and phosphorus – getting the balance right

Introduction

Developmental orthopedic diseases (DODs) such as osteochondrosis, joint dysplasia, osteodystrophy or bone deformities are frequently seen in dogs, with fast-growing large and giant breeds being at particular risk. The maturation phase is a major life stage for all species, but is of critical importance in fast-growing young animals that have significant skeletal vulnerability during this period, and dogs come into this category. Feeding a balanced diet that provides adequate amounts of energy and nutrients is the basis for a healthy musculoskeletal system, as deficiencies can exacerbate some conditions that are not primarily nutrition-related. Signs of DOD can vary from mild and transient lameness through to severe movement disorders, and can also involve painful deviation of one or more limbs and swelling of the joints. Malnutrition in the first few months of life can lead to lifelong impairment of an animal´s health.

How much is too much?

A major problem when feeding a growing puppy is to determine the optimal amount of energy required for that individual. Excessive energy intake leads to an animal being acutely overweight (i.e., a growth rate that is faster than recommended), which places an additional burden on the growing skeleton 1. Even with some orthopedic disorders that have a hereditary basis, for example hip dysplasia (Figure 1), energy restriction at the puppy stage can reduce the prevalence of these conditions 2. It is now known that a puppy which grows rapidly due to excess energy intake has a much greater risk of being overweight or obese in adulthood 3. Several factors, such as breed, activity level, housing and health status, will influence daily energy requirements, so the required correct calorie level can therefore vary considerably between individuals. In adult dogs, the recommended way to monitor if excess or insufficient energy is being supplied is to use body condition scoring (BCS); identifying an adult dog with a high BCS and excess body fat will indicate that the animal’s diet has a disproportionately high energy content. This is not the case during the puppy stage, where a high-calorie diet can lead to an increased growth rate, and the dog can be overweight without having additional fat deposits. In fact, such puppies may even appear thin and undernourished, yet still weigh too much. The littermates in Figure 2 impressively demonstrate the impact of energy supply during growth: both dogs have the same body condition score, but the animal on the left, which received a higher energy intake from the age of 8 weeks onwards, is clearly larger yet still lean in appearance.

An increased growth rate and the extra weight this brings represent additional stress for the developing skeleton, so the gold standard for determining an optimal growth rate is to monitor a puppy’s body weight through regular weighing (e.g., weekly) and compare with a growth chart 4. Optimal growth curves will differ depending on the anticipated ideal weight of the adult dog, therefore it is crucial to estimate this as accurately as possible. It is important to note that on-pack pet food recommendations for daily energy allowances can be rather high, and in addition calculating the calorie requirements using the 2006 equation from the US National Research Council (NRC) 5 will tend to give an overestimate. Recent studies have demonstrated significantly lower energy requirements in both growing colony dogs and household puppies 4,6 and the latest updated recommendations take this into account *. The average daily energy requirement (metabolizable energy, ME) for growing dogs can be estimated as follows 4:

ME intake [MJ] = (1.063-0.565 x [actual BW/expected mature BW]) x actual BW^0.75

* https://europeanpetfood.org/pet-food-facts/fact-sheets/nutrition/nutritional-needs-of-cats-and-dogs/

Dietary calcium and phosphorus

The two major constituents of bone, calcium and phosphorus, are linked by a functional and regulatory relationship, and are best considered together. The absolute requirements of both minerals, as well as the ratio (the recommended range is between 1:1 and 2:1), should be carefully calculated, as an excess or a deficiency of one or both minerals during the developmental phase can be a major factor in terms of nutritionally induced DODs. In retrospective evaluations of cases seen by the authors at Ludwig-Maximilian University, the majority of growing dogs with signs of DOD were found to have either an excessive or deficient intake of calcium and/or phosphorus, although interestingly the distribution has changed over the years. The first survey (in 1998) revealed that 61% of affected puppies had experienced a calcium excess and only 20% had a deficient intake of this element 1. A second survey in 2018 showed that the majority of DOD patients (58%) were fed a diet deficient in calcium, with only 21% of cases receiving excessive amounts of calcium prior to the orthopedic diagnosis 7.

It can be questioned as to why poor nutrition is such an issue for canine growth when it is much less of a problem in humans? One explanation is that in contrast to humans, maturation occurs much earlier in dogs, with almost the entire growth phase being realized within a single year. Consequently, even limited periods of inadequate nutrient supply pose a disproportionately greater risk for puppies than for children and teenagers. But another difference between dogs and other species such as humans must be taken into account: adaptive mechanisms to compensate for low or excessive calcium and phosphorus supply are virtually non-existent in the canine species 8. Rather than intestinal absorption of both minerals being the major regulatory pathway, the skeletal system is employed to store and mobilize calcium and phosphorus as necessary 9,10. Viewed from the idea that the canine species evolved as a predator, this makes perfect sense: either there was prey available, supplying abundant energy and minerals, or both energy and minerals were scarce, so that energy-dependent up-regulation of intestinal absorption was not feasible. This bone-based regulation is one factor in the development of clinical signs linked to calcium and phosphorus malnutrition in growing and adult dogs.

Deficient calcium intake during the growth phase can cause secondary nutritional hyperparathyroidism, with reduced overall bone formation resulting in various clinical signs, including pathological fractures from normal physical exertion 11. In this context it is important to note that an increased fecal dry matter excretion (due either to a low dietary digestibility or a high food intake) also increases the fecal losses of calcium and phosphorus 12,13,14 and therefore increases the daily requirements for the minerals. As noted above, there is a considerable and increasing number of puppies diagnosed with DOD that are fed a calcium deficient diet. However, calcium excess receives more attention in the literature: several publications report that DOD can be caused experimentally in growing Great Danes by feeding excess dietary calcium 15,16 – although in most of these trials only the calcium supply was increased above the recommended intake; this led to reduced utilization of other elements crucial to skeletal development, such as phosphorus, zinc and copper. Consequently, such DODs may be caused, or at least be aggravated by, a secondary deficiency of these minerals, and in particular phosphorus 17. This tallies with the results of a study that examined the effects of excess dietary calcium when surplus phosphorus was also provided (to allow for a reduced digestibility of this element) in medium and large dog breeds, with no clinical signs of DOD being noted 18. On the contrary, a low phosphorus intake has been shown to have substantial (but reversible) adverse effects on skeletal health 19,20 (Figure 3) and other developmental disorders described in various case reports may also be linked to dietary phosphorus deficiency 21,22. It is also worth noting that, although supplying dietary protein at levels above the recommended daily allowance is often suspected to have a detrimental impact on skeletal development, this can be classified as a myth. Studies have demonstrated that the level of protein fed to growing dogs has no effect on their skeletal development 23,24.

How are daily nutrient requirements calculated?

When recommending daily allowances which will meet the requirements of the majority of the population, the basic approach is to determine the net requirements of each nutrient at various life stages, along with how bioavailability varies depending on the nutrient source. There are several ways to estimate these requirements. Experiments to study the dose-effect relationship of a particular nutrient can elucidate what happens with insufficient, adequate and excessive supply, but such trials need to be highly standardized, with careful selection of suitable target parameters to assess the effect of the nutrient being studied. For calcium and phosphorus, defining the parameters is rather difficult. On the one hand, there may be a prolonged latency period between onset of malnutrition and the appearance of specific clinical signs. The homeostasis of calcium and phosphorus is tightly controlled, so serum concentrations are maintained for a long time, even during severe deficiency. On the other hand, factors such as the ratio of dietary calcium to phosphorus, energy intake, growth rate and any genetic predisposition to skeletal disease are known to exacerbate the clinical signs. The calcium and phosphorus requirements for growing dogs, as defined by the NRC 5, are based on research using large and giant breeds and mainly in puppies younger than 6 months of age, with the trial data then extrapolated to cover the entire growth period of dogs of all sizes. The aim was to “meet or exceed” the requirement of all growing dogs, so it made sense to deduce the recommendations from the group with the highest requirement, i.e., young giant breed puppies. However, this method does not take into account individual differences, and the critical period of maximum growth intensity differs in duration between small and large breeds of dog. While large and giant breeds grow up to 12-15 months of age, small breeds may have reached their final bodyweight by 7-8 months, so extrapolation of data is likely to overestimate the requirement of small breeds. In addition, deducing the requirement with data from the early growth period will overestimate the requirement for the later period of growth, where daily gains have slowed.

Nutritional calculations

Another approach for determining the optimal daily supply of a nutrient is to perform a factorial calculation, which summates the required amounts for maintenance and performance. The advantage with this method is that it takes into account the precise individual requirements for a selected life stage, and it has been used successfully with farm animals, for example to determine the optimal rations for milk production in dairy cattle and for fattening pigs. In growing dogs, the factors to be determined are the tissue gain during growth and the maintenance requirement for the current body weight 25. The maintenance requirement is the amount of a nutrient needed to compensate for inevitable endogenous losses from feces, urine, sweat, skin, fur, etc.; when the endogenous losses are quantified, this gives the “net maintenance requirement” of a nutrient. However, because not 100% of the ingested amount of a nutrient are taken up by the body, the actual availability needs to be taken into account in order to obtain the “gross maintenance requirement” or feeding recommendation 26.

The equation is as follows:

Feeding recommendation for maintenance = e/availability [%] x 100

(where e = the endogenous losses)

Obviously, the overall feeding recommendation is strongly influenced by the availability of a nutrient; if the average availability is reduced by half, the feeding recommendation doubles. Mineral availability differs between sources and overall diet composition, so to ensure adequate intake levels a safety margin is built into the average availability.

Tissue gain during growth requires an additional nutrient allowance to be factored into the equation, as follows;

Net requirement for growth = average daily gain x nutrient concentration in gained tissue

So the feeding recommendation for growth can be calculated from this, as follows;

(e + net requirement for growth)/availability [%] x 100

The canine species shows a uniquely high variation in body size and weight, so to generate the growth curve for an individual puppy it is important to have a good estimate of its future adult body weight (based on the ideal weight of the same-sex parent or, if this is not available, mean weight according to breed standard). There are also differences in growth rate between small, medium, large and giant breeds, so it is necessary to categorize dogs on their adult body weight to better define recommendations for calcium and phosphorus supply. In addition, since growth is not a linear process, using age groups allows further differentiation for the recommendations. The percentage requirement for tissue gain is highest during early growth; this then declines as the dog approaches its adult body weight and the maintenance requirement accounts for most of the total energy and nutrient requirements. The factorial calculation for calcium and phosphorus needs to take both aspects into consideration.

There are different ways to express feeding recommendations. The NRC supplies the values in relation to metabolizable energy (ME). The absolute value calculated using this reference depends on the energy requirement, which may differ considerably from the average for an individual case. It can be predicted, for example, that a Newfoundland puppy will have a lower ME requirement than a Great Dane puppy of the same age and body weight due to differences in temperament and activity. As an alternative to the reference ME, it is possible to relate the recommended daily intake to metabolic body weight (kg^0.75). This has the advantage of calculating the exact requirements – and hence enabling a precisely balanced diet – for an individual dog, but it does not directly inform about the required nutrient content in a diet. This needs to be calculated for each case using the individual ME requirement.

The calcium and phosphorus recommendations for various ages and mature body weight groups required for the calculations are given in Table 1, and a worked example for a young puppy fed a commercial diet can help explain the different steps (Box 1).

Table 1. Requirement for calcium and phosphorus for growing dogs relative to metabolic body weight (kg^0.75) (adapted from 26).

Box 1. Calculating the required calcium (Ca) and phosphorus (P) levels for “Max”, a 22-week-old German Shepherd puppy with a current body weight of 20 kg and an estimated adult body weight of 35 kg.

Should an owner wish to feed a homemade diet, it must be appreciated that most recipes need to be supplemented with minerals and vitamins to meet the daily requirements, and because there are various supplements available, the nutrient supply from all components of the diet must be added together and compared to the requirement to allow selection of a suitable product (Figure 5). An example of a homemade ration is shown in Table 2, from which it can be seen that 20 g of a mineral supplement which contains 22.5% calcium and 10.5% phosphorus will meet the daily recommended allowance for these minerals. Note also that the dietary Ca/P ratio at 1.3/1 is in the ideal range (recommended range 1/1-2/1). However, in order to meet the requirements, the amount of daily supplement offered must be increased if the nutrient concentration is lower, or decreased if the nutrient supply is higher.

Table 2. A potential homemade diet formulated to meet the requirement of a 22-week-old German Shepherd puppy currently weighing 20 kg and with an estimated mature bodyweight of 35 kg.

Conclusion

Metabolizable energy and nutrient requirements change during growth, so a proper overview of a puppy’s nutritional requirements requires regular ration adaptations. Whilst two-monthly reviews are supposedly adequate for dogs growing according to the recommended growth curve, all puppies should be weighed weekly, and the body weight compared to the optimal growth curve to monitor progress. If there is a deviation from the ideal body weight, the diet – and especially the ME supplied – needs to be adapted immediately. For simplicity, this article considers only metabolizable energy, calcium and phosphorus, but in general the requirement for all minerals, trace elements and vitamins should be included when reviewing the nutritional needs of a patient, and whilst the calculations are not difficult, they do illustrate that consideration of all the factors required to achieve a balanced diet is necessary.