provided. Other elements that may be required by dogs at trace concentrations include molybdenum, tin, silicon, nickel, vanadium, chromium, lead, and perhaps arsenic.
The dietary requirements of the dog for these elements have not been established. These minerals seem to be widely distributed in the ingredients used to manufacture natural ingredient dog diets, particularly at the low concentrations that appear to meet the requirements of other mammalian species. Therefore, it is highly unlikely that dogs consuming natural ingredient diets would become deficient in any of these trace minerals. Their concentrations may be of concern when dogs are to consume highly purified diets over relatively long periods.
Certain vitamins have been recognized as essential nutrients for dogs for more than 60 years. Despite this long history, precise quantitative requirements have not been established for every vitamin. The recommendations made in Tables 1 and 2 are designed to provide levels that are reasonable based on research with dogs and other species and that have proven satisfactory in practice. The concentrations of most nutrients in Table 2 can be determined from the daily requirements in Table 1 by simple calculation, assuming that the 3-kg growing puppy requires 600 kcal ME per day and that the 10-kg adult dog requires 742 kcal ME for maintenance. Although the vitamin requirements for gestation, lactation, and muscular effort have not been well defined, these needs are generally related to energy intake. As energy intake increases in relation to the extra demands of milk production or exercise, daily intake of vitamins will also increase. The vitamin requirements in Table 2 should be adequate to meet the needs of the entire life cycle of the dog. Since several vitamins are rather unstable, and their destruction may be promoted by light, heat, oxidation, moisture, rancidity, or certain mineral elements, sufficient amounts should be provided to ensure that the recommended concentrations will be present when the diet is consumed. Just as important is the realization that markedly excessive intake of vitamins A and D may be harmful to dogs.
The earliest studies attempting to separate the functions of vitamins A and D on bone growth were carried out in dogs more than 60 years ago (see Mellanby, 1957). The actual dietary requirement for vitamin A in dogs was investigated by Frohring (1935; 1937), who fed a vitamin A-deficient diet to Beagle puppies and calculated that 100 IU vitamin A per kilogram of body weight was lost from the liver each day during growth (3 IU = 1 µg of retinol equivalents). The minimum curative dose of vitamin A equivalents that effected a definite increase in weight was 200 IU (in the form of b-carotene) per kilogram of body weight per day. Crimm and Short (1937), using a similar vitamin A depletion technique, estimated that the minimal daily vitamin A requirement of adult dogs was 22 to 47 IU per kilogram of body weight, or between 8 and 16 µg/kg, which is the universal requirement among those species tested to date. Bradfield and Smith (1938) fed 200, 400, 1,000, or 2,000 IU vitamin A per day per kilogram body weight to growing puppies and measured weight gain and liver vitamin A concentration. To compare the vitamin A activity of carotene sources, other puppies received 200 IU vitamin A equivalents, as carotene in oil or from carrots, per kilogram of body weight per day. While increasing dietary intakes of vitamin A resulted in increases in the vitamin A levels in the liver, 200 IU were adequate to produce maximum gains and slight liver vitamin A storage. In these studies, cod liver oil and carotene appeared to be equally well utilized as sources of vitamin A activity, even though cod liver oil is not an ideal test substance because it contains a variable amount of vitamin D that may interfere with vitamin A absorption if excessive. The data confirmed the earlier observation of Turner (1934) that dietary carotene (from carrots) may be converted to vitamin A and stored in the liver of dogs.
There has never been published a careful assessment of the daily vitamin A requirement for dogs based on the liver storage of the vitamin while feeding an acceptable form of vitamin A esters under controlled dietary circumstances. However, based on available information, the daily vitamin A requirement would be met by 75 IU per kilogram of body weight for adult maintenance and 202 IU per kilogram of body weight for growing puppies. These amounts will be more than provided by a dietary concentration (dry basis) of 3,336 IU per kilogram or 1,112 µg per kilogram of a diet containing approximately 3,300 kcal ME. This is equivalent to 1,011 IU/1,000 kcal ME.
Vitamin A deficiency in the dog was among the first of the vitamin deficiencies to be studied experimentally. It is seldom encountered clinically. Steenbock et al. (1921) reported that dogs deprived of fat-soluble vitamins developed an "ophthalmia." These and other workers (Stimson and Hedley, 1933; Crimm and Short, 1937; Mellanby, 1938; Russell and Morris, 1939; Singh