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Research Needs To make precise quantitative estimates of the vitamin tolerances of animals, one needs a reasonably complete base of experimental and clinical information. The available information base is presently insufficient to support such estimates, however. The toxicities of some vitamins, such as vitamin K, folic acid, pantothenic acid, and vitamin Bit, are so low that hypervitaminosis is very unlikely in practical situations of animal feeding. The lack of more extensive data is, therefore, without conse- quence. For other vitamins that have measurable toxici- ties and may be used in practical situations at levels greater than nutritional requirements, however, the gap in the information is much more serious. The review of the current vitamin literature points to several such gaps. In order to fill them, research should be initiated in the following areas. 1. Vitamin A: The clear potential for hypervitamin- osis A and the small amount of quantitative data de- scribing its safe dietary levels for most species of animals indicates a need to further define those levels, particularly for chronic exposure of domestic species. At a more fundamental level, the biochemical mecha 84 nisms of vitamin A toxicity need to be further eluci- dated. 2. Vitamin D: Further quantitative information is needed to define more clearly dietary levels of vitamin D that are safe for domestic animal species. Studies are also needed to make clear hypercalcemia's role in tissue calcinosis due to hypervitaminosis D. Because of the physiological factors that affect and interspecific differ- ences in circulating levels of 25-hydroxycholecalciferol, the usefulness of this parameter as a clinical indicator of hypervitaminosis D needs to be evaluated. 3. Vitamins E and C: Because of the increasing use of high levels of these vitamins as promoters of immune functions and protectors against stress, the chronic tox- icities of each should be more carefully defined for a variety of domestic animal species. 4. Niacin: The effects of excesses of nicotinamide and nicotinic acid on hepatic and renal function should be elucidated. 5. Choline: The relative toxicities of high levels of the different chemical forms of choline should be re- evaluated in a manner that considers the potential ef- fects of the chloride provided by choline chloride.