A comparison of the bioavailability of three dietary zinc sources using four different physiologic parameters in dogs.

Although there are a number of bioavailability studies reported in the literature involving farm animals for zinc (Zn) as a function of dietary source (Baker and Ammerman 1995, Wedekind et al. 1994), there appear to have been few quantitative bioavailability studies conducted in dogs. The concept of bioavailability is much discussed and defined (Ammerman and Henry 1994). The most common definitions include reference to the proportion of a nutrient that is digested/absorbed and metabolized through normal pathways (Forbes and Erdman 1983). The latter part of this definition, the utilization of the nutrient by the animal in a structural or biochemical capacity, plays a significant part as to the extent of the “availability” as stressed by Fox et al. (1981). Therefore, the “bioavailability” of a nutrient is perhaps better defined as the proportion of intake that is capable of being absorbed through/by the intestine and made available either for metabolic use or storage in animal tissues. This assumes that the bioavailability of the nutrient is a function of the diet per se. However, it is also a response of the animal and of the animal to the diet as a function of its status. Different requirements and bioavailabilities have been found to be dependent upon the physiologic parameter measured (Wedekind and Baker 1990) and on the age and physiologic state of the animal (Lowe, unpublished data). Thus terms that would quantify under what conditions bioavailability had been established, for example, age of the animal, should be stated. If different dietary sources of a nutrient have different bioavailabilities of that nutrient, then a dietary concentration ceases to be an appropriate definition of the requirement for the nutrient. Nutrient requirements should, therefore, always be expressed as daily amounts available for metabolic use. Such complications infer that no one single parameter is appropriate to define the absolute, or relative bioavailability (RB) of a nutrient, and thus no single dietary inclusion rate can be indicated as the “requirement” without qualification. An examination of these data also leads to the recognition that young, or rapidly growing animals and/or tissues exhibit greater responses to differences in the dietary concentrations and expression of bioavailability of various Zn sources. Zinc oxide (ZO) is a commonly used source of supplementary Zn in diets for farm animals (Wedekind and Baker 1990) and for dogs in the UK. Zn from ZO, however, has recently been shown to be of relatively low bioavailability relative to other dietary sources and prone to antagonistic reactions with other dietary nutrients (Baker and Ammerman 1995, Lowe et al. 1994, Lowe and Wiseman 1997). A study was therefore conducted to examine the RB, by slope-ratio assay, of Zn from an oxide source in comparison with Zn from a chelate (ZM), corresponding to 2 mol of amino acids (methionine and glycine) to one of Zn, or a polysaccharide complex (ZP), consisting of zinc sulphate complexed with alkali modified brewers wort. Four physiologic parameters in the adult dog were determined to ascertain to what extent the Zn source influenced the RB values. Materials and methods. Twenty-seven (27) adult Beagle dogs (13.7 kg, SD 1.93) were randomly assigned to nine groups of three. Dogs were housed in 2.4 m (1.7 m 3 1.4 m) concrete block pens with an open-mesh steel gate to the front allowing them visual access to kennel-mates and the central walkway/ exercise area. Bedding was provided in the form of soft-wood shavings. The kennel building was heated and ventilated to maintain a temperature of between 16 and 24°C, 30–70% relative humidity with 12 h of light in a 24-h period. All dogs were monitored throughout the day and pens cleaned once daily. The study protocols were appropriately approved and the animals maintained under the care of a veterinary surgeon for the duration of the study in compliance with the 1986 EC