The distinction between optimizing performance and avoiding clinical deficiencies has also led researchers in new directions. Among these are a consideration of dietary requirements for vitamins that some animal species synthesize. In these species, clinical deficiencies of the synthesized vitamins are almost nonexistent, and it was long presumed that the amounts produced met the animals' needs. (There are no published dietary requirements for these vitamins in these species.) Yet more and more research has documented responses in better performance when diets were also fortified with these vitamins. Examples would include vitamin C supplementation in stressed poultry; vitamin C supplementation in weanling pigs; biotin supplementation in dairy cows and niacin supplementation in newly arrived feedlot steers.