Ultraviolet Light, Vitamin D, and Reptile Health
Ultraviolet Light, Vitamin D, and Reptile Health
If you keep reptiles, you have probably learned that they require regular exposure to ultraviolet light (UV), and that UV is essential for the proper use of calcium in the body. UV is involved in converting previtamin D into vitamin D (calciferol), which is then routed to the liver for its final conversion into 25-hydroxyvitamin D (25(OH)D), its active form. In order to maintain health, all vertebrates require calcium in their diets. Calcium, when combined with phosphates, forms the main component of bone, but is also essential for cartilage development and the ability of nerve cells to send messages. The body uses calcium constantly, so it must be constantly replenished. In cases of prolonged hypocalcemia, the bones become weaker and softer (osteoporosis), and nerves dysfunction or shut down. If not treated, severe hypocalcemia can be fatal.
Vertebrates can either absorb vitamin D directly from their food or photosynthetically convert a precursor called 7-dehydrocholesterol into vitamin D in the skin. That conversion requires exposure to the B-band of ultraviolet light (UVB), light at a wavelength of 270-300 nm. Photosynthesis of vitamin D is common across the majority of species of animals and plants.
Diurnal (active in daylight) reptiles, so far as they have been tested, fall into the latter category. Research also shows that geckos (and presumably other nocturnal lizards) also use exposure to light to produce D3, and that their skin is considerably more efficient in running the process. Because of the hypersensitivity of gecko skin, they can produce adequate vitamin levels during their brief exposure to sunlight.
What do reptile keepers need to provide in order to ensure that their animals obtain necessary amounts of vitamin D3 and calcium?
Probably all reptiles require at least a minimal exposure to UVB, though the specific requirements for most species is unknown. The best source of UVB is, of course, natural sunlight, but unless animals are houses outdoors this is rarely possible. An inconvenient property of UV is that it is absorbed or reflected by glass. That means that even if a glass terrarium is placed near a window that is exposed to the sun, most of the UV will be stopped by the window and terrarium glass. Then infrared, however, will penetrate and warm the terrarium, possibly to lethal levels.
There are many ultraviolet lights available to today's terrarium keepers, and these most commonly come as fluorescent tubes that fit into any fluorescent tube-holder. Not all UV light tubes are equal: it is important to get a tube that produces UVB light. More precisely, it should emit at least five percent of its light in the UVB part of the spectrum. Several brands of such lights are available, such as Zoo Med's ReptiSun. Different bulbs have different output levels, so be sure to match the output to the specific needs of your reptiles. Tropical diurnal lizards and tortoises should be provided with lamps that emit 10 percent UVB, while temperate lizards, most snakes, and nocturnal lizards need only a five percent lamp.
How much UVB exposure is sufficient? Data to support any specific recommendation are limited, but desert reptiles thrive when given access to UVB for anywhere from two to twelve hours per day. Note: reptiles always require places in the terrarium that provide a complete retreat from view and light exposure. Just as they will retreat under bark or a rock when becoming overheated, so, too, will they retreat from excess UVB.
Vitamin D3 is an important steroid hormone produced in the skin from another molecule, 7-dehydrocholesterol, in the presence of UVB. Its most essential function is to convert the precursor, which has limited chemical activity, into a molecule that easily attaches to the ion calcium. It is D3 that transports calcium into and out of the blood. The most familiar role of calcium is to be deposited to make and maintain bone. Obviously, when calcium levels drop too low, bones begin to break down and are not repaired. The only way calcium is transported across large distances in the body is via D3.
Calcium also has an essential role in the functioning of nerve cells. When one neuron send its impulse to the next neuron in a sequence, it is calcium that causes the chemical neurotransmitters to exit one neuron and jump the synapse to the next. In the absence of calcium around a nerve, that nerve can no longer function; it would be like cutting an electric wire between a socket and a lamp.
The recommended way of providing reptiles with a proper amount of vitamin D3 is by giving the animals foods that carry the vitamin. Unfortunately, there are relatively few vitamin D-rich foods, and some, such as milk, are not appropriate fare for reptiles. Sources include fish oils, fresh ocean fish (cod, tuna, mackerel, sardines, and salmon), eggs, and liver.
D3 supplements should be used sparingly and only under special circumstances, such as when treating for metabolic bone disease, chronic lethargy, or unusually slow responses.
Health Conditions Associated With UVB and D3
Given a proper diet, it is extremely difficult to suffer from vitamin D3 toxicity (hypervitaminosis D3). This is because prolonged exposure to UVB actually begins to break down both previtamin D3 and vitamin D3. Nevertheless, vitamin D3 levels can increase and lead to symptoms to warn that you must do something to reduce levels. Hypervitaminosis D3 leads to higher levels of blood calcium, calcification of soft tissues and impeded of joint movement, malformed bones with external calcium deposits ("bunions"), impaired nerve function, impaired flexibility of the valves of the heart, and destruction of the kidney's nephrons. This condition is very rarely the result of UVB exposure; rather, it comes from the excess administration of vitamin D3 itself. The condition is most easily corrected by withholding further doses of D3.
Lizards may develop lethargy and soft bones, yet have calcium deposits accumulate in muscles and other soft tissues. This is a typical indication of insufficient UVB exposure and not directly related to vitamin D or calcium levels. Such patients need to be exposed to good UVB light for several hours per day. Remission may begin within a few days.
Vitamin D3, calcium, and ultraviolet light in the B-band are essential for the health of the vast majority of vertebrates, including reptiles. Vitamin D is most effective when provided as a foodstuff rather than as a supplement. Deficiencies of vitamin D3 are best treated by providing exposure to quality UVB provided by a special lamp. Excess vitamin D3 is rare because prolonged UVB exposure breaks down available D3. Though vitamin D3 and the mineral calcium are essential to the health of a reptile, primary treatment for either deficient or excess amounts of these substances should be modified use of UVB.