Nutrition in Reptiles

Vitamin C synthesis has been reported in many reptile species. It has been suggested that ulcerative stomatitis seen in snakes and lizards may be associated with a vitamin C deficiency, although there is no supportive evidence. In controlled studies with garter snakes (Thamnophis sp) fed supplemental vitamin C, tissue levels and body stores remained stable, although synthesis by the snakes was reduced.

Although most reptiles excrete nitrogen primarily as uric acid, aquatic reptiles typically excrete excess nitrogen as urea or ammonia. The relative proportions of various nitrogenous wastes may depend on the amount and composition of feed, frequency of feeding, and state of hydration. The excessive precipitation of urate crystals in joints, kidneys, or other organs (gout) can be a common condition in some species of captive reptiles. The etiology is not clear, but it is commonly thought that diets high in protein may predispose reptiles to gout. Impaired renal function and dehydration have also been suggested as possible causes.

If poor-quality protein is fed (unbalanced amino acids) or when tissue is catabolized for energy, uric acid excretion increases. Although gout in some reptiles is associated with increased circulating levels, postprandial transient increases in circulating uric acid may be seen in some species and confound the diagnosis. Assuring an adequate state of hydration in a susceptible animal may help prevent uric acid precipitation in joints and organs. Feeding diets low in protein to carnivorous reptiles is unwise, because they are adapted to feeding on high-protein prey.

Most vertebrates can either absorb vitamin D from the diet or synthesize it in the skin from 7-dehydrocholesterol using energy from ultraviolet (UVB) light of certain wavelengths (290–315 nm) in a temperature-dependent reaction. Thus, vitamin D is required in the diet only when endogenous synthesis is inadequate, as develops when animals are not exposed to UV light of appropriate wavelengths.

Many captive basking species appear susceptible to rickets or osteomalacia. Bone fractures, soft-tissue mineralization, renal complications, and tetany can develop. Reptiles frequently show few premonitory signs, although lethargy, inappetence, and reluctance to move are commonly reported. Serum calcium concentrations may not be diagnostically useful. Although blood levels of vitamin D can be measured, normal values for most species are not known. Supplementation with injectable calcium and vitamin D may provide some short-term relief. However, exposure to UV light, or lack of it, may be an important, yet often overlooked, factor in the differential diagnosis. Complicating the diagnosis may be soft-tissue mineralization, seen radiographically or at necropsy.

In green iguanas, metastatic calcification may not result from vitamin D toxicity. Iguanas with both fractured bones and extremely low or undetectable levels of circulating 25-hydroxycholecalciferol also had calcified soft tissues. The etiology of the metastatic calcification is not understood and is contrary to conventional understanding of the signs of vitamin D deficiency and toxicity in domestic species. Dietary sources of vitamin D may not be sufficient to prevent rickets and osteomalacia. Diets with as much as 3,000 IU vitamin D₃/kg did not prevent bone fractures and cortical thinning in green iguanas. Bulbs emitting UVB placed over the lizards at ~12–18 in. for 12 hr/day appeared to reverse the signs in the least severely affected lizards.

Because some lizards seek a warm spot to increase body temperature, placement of a warming bulb, usually incandescent, adjacent to a UVB bulb helps ensure adequate exposure to UVB light. Exposure to unfiltered natural sunlight, depending on latitude, during warmer months and use of UVB bulbs during the rest of the year usually eliminate the risk of bone disease caused by insufficient absorption of calcium (due to a vitamin D deficiency).

Some lizard species may be unable to absorb sufficient dietary vitamin D₃, although the reason is poorly understood. New World primates are believed to have exceptionally high dietary requirements for vitamin D, which may be related to lower numbers of vitamin D cellular receptors than are present in Old World primates. Similar metabolic differences may exist in some basking lizard species, although this has not been established. UVB bulbs are sold in pet stores, but label claims may not be reliable.