Metabolic bone disease (from calcium deficiency) may be the most common disease of captive chelonians. Vitamin D3 increases intestinal calcium absorption, but UV light exposure may more efficiently increase circulating vitamin D3 than nutritional supplements.

To compare vitamin D3 levels, 18 client-owned Hermann’s tortoises were randomly assigned to 3 groups of UVB exposure: unfiltered sunlight in the natural geographic range, a self-ballasted mercury-vapor lamp, or a fluorescent UVB-emitting lamp. It was hypothesized that patients exposed to the fluorescent lamp would have lower vitamin D3 levels than those in the other groups. All tortoises had equal natural sunlight exposure before the study and were fed the same vegetation throughout the study. At days 0 and 35, plasma vitamin D3 levels in the groups exposed to artificial UVB sources dropped significantly, whereas tortoises exposed to natural sunlight remained stable. There was no significant difference between the groups exposed to artificial UVB.

Adequate synthesis of vitamin D3 may not be achieved via lamp manufacturer recommendations, and tortoises (especially those with metabolic bone disease) should be exposed to unfiltered natural sunlight at a latitude similar to their natural habitat.

Commentary
Reptile husbandry must be tailored to the requirement of a particular species. These animals are highly adapted to their native environments, even in the aspect of UV light, and UV supplementation needs to be based on natural history and be qualitatively appropriate for a given species. Measuring the level, intensity, and physiological effects of provided light is necessary for proper diagnoses and treatment measures. More work is needed in a species-specific manner to understand the requirements of these animals.—Adolf K. Maas, DVM, DABVP (Reptile & Amphibian)

Source
Plasma 25-hydroxyvitamin D3 concentrations in Hermann’s tortoises (Testudo hermanni) exposed to natural sunlight and two artificial ultraviolet radiation sources. Seleri P, Di Girolamo N. AM J VET RES 73:1781-1786, 2012.