Types of Vitamin A |
Vitamin A comes in two forms, beta-carotene and retinol. Retinol, also called preformed vitamin A, is the form of vitamin A which the body uses. Beta-carotene must be converted into retinol in order to be used as "vitamin A." Retinol only comes from animal-sourced foods. This means that herbivorous reptiles get all of their vitamin A in the form of beta-carotene. Insectivorous species in the wild get a mix of beta-carotene and retinol in their diet.
Additionally, it is important to note that it is nearly impossible to consume too much beta-carotene, while retinol can easily be overdosed when supplemented in a diet. |
Feeding Vitamin A |
In the case of insectivorous species, it is not known if they can convert beta-carotene into retinol. For this reason they need to be fed vitamin A as retinol. This is best achieved through properly feeding and gutloading feeder insects. Insects should be raised on beta-carotene rich food, whether supplemented or sourced from fresh produce. The insects then convert this beta-carotene into retinol as-needed and pass on the retinol to the reptile or amphibian who consumes them.
Species which consume whole mammal prey, such as snakes, very rarely have issues with vitamin A due to the high retinol content in the prey's liver. Additionally herbivorous species and most omnivorous lizards rarely have vitamin A issues, likely due to their ability to use beta-carotene. Omnivorous turtles are also at risk for developing vitamin A deficiencies and care should be taken to include a retinol source in their diet. |
Too Low A |
Symptoms of a vitamin A deficiency, aka hypovitaminosis A, in reptiles typically are gingival and ocular issues. Difficulty shedding may also occur.
Amphibians typically present with difficulty catching food, as their tongue keratinizes and becomes less "sticky." |
Too High A |
Skin issues are the most common sign of an overdose of retinol. Hypervitaminosis A occurs primarily as a direct result of over supplementation, but can occasionally occur in frogs fed whole mammalian prey.
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Measuring A |
Due to there being two types of vitamin A, measurement can get complex very quickly. Vitamin A is most commonly measured in international units (IU). 1 IU = 0.3 mcg of retinol and 1 IU = 0.6 mcg of beta-carotene. This can be a bit misleading, because beta-carotene counts as "vitamin A" even in species which cannot convert beta-carotene to retinol. When looking at vitamn A content of food, one must carefully consider the forms that vitamin A is present in and what species will be consuming the food.
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Sources |
Allen, Mary E. and Olav T. Oftedale. Nutrition in Captivity. Biology, Husbandry and Medicine of the Green Iguana. 4:47-74 (2003).
Boyer, Thomas H. Vitamin A Deficiency in Insectivorous Lizards. https://files.brief.vet/2018-07/CTE_Vitamin%20A%20Deficiency%20in%20Insectivorous%20Lizards.pdf Cheeke, Peter R. and Ellen D. Dierenfeld. Comparative Animal Nutrition and Metabolism. (2010). Donoghue, Susan and Julie Langenberg. Nutrition. Reptile Medicine And Surgery 1st Ed. 14:148-174 (1996) Nijboer, Joeke. Ed. Nutrition in Reptiles. Merck Veterinary Manual. 2015 |