Axenic animals have lower requirements for which nutrient?

• A. Vitamin A
• B. Vitamin K
• C. Vitamin B
• D. Vitamin C

Answer: (A)

Gut microbes can make several vitamins for the host, so animals with a normal microbial community often rely partly on those bacterial nutrients and can have reduced dietary needs for them. In axenic (germ-free) animals, that microbial source is missing, which typically means higher requirements for vitamins that are produced by gut bacteria, such as certain B vitamins and vitamin K. Vitamin A, however, is not produced by gut microbes to a meaningful extent and is supplied primarily from the diet as retinol or provitamin A carotenoids. Because microbial production doesn’t contribute significantly to vitamin A status, axenic animals don’t exhibit the same dependency, and in some assessments their apparent vitamin A requirement is lower or unchanged compared with those nutrients that depend on microbial synthesis. Therefore, vitamin A is the nutrient noted as having a relatively lower requirement in axenic animals.