Rethinking Reptile Lighting - Part 3

UVB, Vitamin D and Calcium

The process of vitamin D synthesis in the skin of animals upon exposure to UVB is fascinating. Cholesterols in the skin are converted to a molecule known as previtamin D3 when the skin is exposed to UVB radiation. Upon exposure to heat, previtamin D3 undergoes a change and is converted to vitamin D3, which transforms in the liver and kidneys to the active form of vitamin D3. There are other molecules involved, and the process is regulated so that excess UVB exposure will not lead to an overdose of vitamin D, which is fat soluble and carries a risk of overdose, as do all fat-soluble vitamins. In addition to being a major component of bones and eggshells, calcium is involved in countless biological processes at the cellular level, including cell communication, muscle contractions and other functions that are essential to life. Vitamin D3 is responsible for calcium metabolism, and if there is not enough circulating vitamin D3 in the blood, animals are not able to use the calcium in their gut that they get from their food. Dietary calcium would then pass through the gut unused, and animals would be forced to take it from their bones, ultimately leading to a form of metabolic bone disease known as nutritional secondary hyperparathyroidism. Animals can get vitamin D from two different sources: UVB-induced synthesis in the skin, or by ingesting vitamin D in the food that they eat. The livers of vertebrate prey, for instance, such as mice, rats and cod, provide a rich source of vitamin D3. Researchers have found, however, that not all reptiles and amphibians are able to adequately use dietary vitamin D3, and they depend on UVB-induced synthesis of vitamin D in the skin. Some of the reptiles for which this has been shown to be the case include strictly herbivorous species, along with some insectivorous and omnivorous species (and the list is growing). While some reptile species, such as the bearded dragon, could likely not survive without UVB-induced synthesis of vitamin D, others seem to do fine with what they get from their diet. Snakes are commonly kept by breeders in utilitarian enclosures containing a water dish, bedding, a hidebox and subterranean heat to provide a thermal gradient. These enclosures are often less than 6 inches tall, and the snakes are not provided with any lighting except for the ambient room lighting that may pass through the walls of the enclosure. Does this mean that snakes don’t benefi t from UVB? While this type of enclosure may be similar to what the snakes experience in a burrow, eventually they have to come out, and when they do they are often exposed to some level of UVB.