The Importance of Ultraviolet Light and Vitamin D to Reptile Health

Dr. Sprackland is a herpetologist. He got his MA in zoology at San Jose State University and his Ph.D at University College London.

UV Light Facilitates Calcium Production

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.

How Is Vitamin D Absorbed?

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?

Ultraviolet Light

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

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.

Thijmen on December 22, 2018:

i hope you still read this, but is the UVB in terrarium UVB bulbs the same as humans need from the sun? i have a few barely used ones lying around and winter here doesn't really have any sun.

Sacudas Foo on April 29, 2011:

I used to have a small pet turtle and I can relate with the post. It is very important to let the turtle bask under the sun's rays for a few minutes or else his shell will become unhealthy and develop unsightly fungus-like disease. The same goes with our bone. The human skeletal system needs vitamin D, but without the sun our body can't synthesize it, and our bones will not be healthy and sturdy.

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ReptileRevolution from California on March 02, 2011:

Are some reptiles more susceptible to MBD than others?

Monitorman on January 19, 2011:

Hi Robert,

I'm sure you're asking this regularly, but wouldn't you agree that larger lizards (monitors, for example), don't require vitamin-D supplementation due to their meat-heavy diet?

How Important is Natural Sunlight to Tortoises?

My fiancée and I are considering getting a Mediterranean spur thigh tortoise - neither of us have had one before, and we're unsure if it's suited to our current situation - as much as we both love the idea. The tortoise in question is currently 6 months old and is being given away by a friend of my fiancée.

We live in a ground-floor flat with no garden or outside space - however, we do plan to buy our first house (non-negotiably with a garden) within the next 2 years (maximum - we hope less).

We've been told that vitamin D is important for tortoises, especially in early life, and fully plan on getting a UV lamp and a proper "tortoise house" (unsure of the correct term!).

We would rather not get a tortoise at all than selfishly make it fit into our lives at the expense of harming its life or development, so any help would be hugely appreciated!

The Need for UVA Lighting For Reptiles

Reptiles can see much farther into the UVA spectrum than humans… more so than can be provided by "regular" light bulbs. This allows them to perceive patterns and colors beyond what we can see. Depriving them of a large portion of their visible world can cause serious health issues, as well as physiological and behavioral abnormalities.

Leaving a reptile blind to half of their environment causes unnecessary stress, which will generally affect their overall well-being. Lack of UVA light can impair a reptile's ability to recognize friend from foe. The UVA spectrum is used to recognize patterns of their fellow species from other animals, and plays a big role in helping them detect movement. It also helps them recognize their diet foods and generally stimulates the appetite by telling them what's good to eat. UVA also provides the visual cues that controls the basking instincts that they need to regulate their body temperature. UVA generally promotes more activity, foraging, social behaviors and reproductive activities.

The Need for UVB Lighting For Reptiles

UVB light controls the synthesis of vitamin D3 through the skin, which is needed to metabolize and absorb calcium and other minerals. The need for UVB varies by species. Generally speaking, diurnal reptiles (active during the day) need UVB to manufacture vitamin D3. Nocturnal species (primarily active at night) will typically meet their vitamin D3 needs through their food source. Do your research on your chosen reptile.

Without enough vitamin D3, reptiles can suffer from chronic calcium deficiencies leading to bone deformities. Vitamin D3 is also important to proper organ development and regulating the immune system. Reptiles that do not get enough vitamin D3 at a young age can even lose the ability to synthesize D3 through UVB absorption at a later age. Most reptiles do not exhibit signs of these illnesses until they are quite advanced and beyond help, so do your research.

"Full Spectrum" Lighting

I have been to a lot of pet stores that sell reptiles as a side line, and received a lot of bad advice throughout. They simply do not know what they are selling, so you have to do your research. Manufacturers learned long ago that herp enthusiasts were being told to "use full spectrum lighting" so anything that might typically be considered broad spectrum is being marketed as full spectrum. The amount of UVA and UVB output by a fixture can vary greatly. The needs of different reptiles vary greatly. Supplements range from somewhat helpful to actively harmful in overdose situations. But this all goes well beyond the scope of this question.

Researcher Explores Role of Vitamin D in Pet Health

Got vitamin D? Most people know that this compound, a supplement frequently added to milk, plays a role in developing and maintaining healthy bones. But recent findings in humans suggest that vitamin D has a much broader effect on health and immune function.

Dr. Mauria O’Brien, a specialist in emergency and critical care at the University of Illinois Veterinary Teaching Hospital in Urbana, is determined to find out what this means for animals.

“Many studies in humans have found that low vitamin D status is associated with common cancers, autoimmune diseases, cardiovascular disorders and infectious diseases,” says Dr. O’Brien. “It’s possible that finding out more about how our pets use vitamin D and how much they need could help us improve their care and health.”

Vitamin D was first identified in the early 1900s and got its name because it was the fourth “vital amine” that scientists discovered. Officially, vitamins are defined as substances that are not synthesized by the body, yet are necessary for normal function. Later research revealed that vitamin D should probably be called a hormone instead since some species can produce it themselves. But the name stuck and so far, vitamin D is best understood as a regulator of calcium and phosphorus, the major minerals that make up bone.

“A lot is known about how humans, rodents, horses, cattle, pigs and sheep are able to synthesize vitamin D in their skin after exposure to the sun’s ultraviolet light,” says Dr. O’Brien. In fact, vitamin D got its original nickname—“the sunshine cure” —from the marked improvement tuberculosis patients made when they spent more time in the sun.

But Dr. O’Brien found that when she compared the vitamin D values from blood samples of healthy pet dogs and those of healthy dogs who spent all of their time indoors without exposure to sunlight, there was not a difference. And, studies show that cats also do not have the ability to produce adequate amounts of vitamin D in their skin.

“That is one of the aspects of veterinary medicine that makes it so challenging,” says Dr. O’Brien. “Although we know of a vast number of similarities across species, it is never a good idea to assume that any physiological trait is the same without testing to make sure.”

Does this mean that vitamin D isn’t as important for dogs and cats as it is for humans? Not at all.

“In the past 25 years, we’ve learned that nearly every type of cell in the vertebrate body contains receptors for vitamin D, not just the cells associated with skeletal health,” notes Dr. O’Brien.

“The vitamin’s effects on the immune system are especially compelling when it comes to protecting animals from infectious diseases,” she says.

Vitamin D is a link in the complex series of events that occur when the body’s immune system mounts a response to invading pathogens like viruses and bacteria. It essentially stimulates the production of natural antibiotics made by the body.

Clearly, there is a lot more to be discovered about vitamin D and our pets, which is why Dr. O’Brien is leading the way. For now, Dr. O’Brien urges owners to feed their pets a quality commercial diet to ensure their pets are getting enough vitamin D.

Most commercial diets are formulated to contain at least enough vitamin D to protect animals from developing bone abnormalities. Of course, it’s possible that future vitamin D research may suggest the need to increase the standard amount provided in food, but until then, she said “only the pets that are fed inconsistently or that get more table food than pet food are at risk of not getting enough vitamin D.”

She adds that along with feeding a proper diet, making sure a pet maintains a healthy weight and gets plenty of exercise is the best way to protect it from becoming sick.

Shining Some (UV) Light on Rabbit Husbandry

By Mark A. Mitchell DVM, MS, PhD, DECZM (Herpetology)

Rabbits are popular pets, with 3.2 million in U.S. households, according to a 2012 AVMA survey. Veterinarians should be prepared to help educate their clients about best practices for rabbit husbandry.

Rabbits’ lighting needs have not been explored in depth. Rabbits evolved to be crepuscular (active at dawn-dusk) to diurnal (daytime) therefore, they spend a fair amount of their day exposed to natural sunlight, which provides three important spectrums of light:

  • ultraviolet light, associated with the photobiochemical synthesis of vitamin D in some vertebrates
  • visible light, which allows vertebrates to see in a certain range of colors and
  • infrared light, associated with the heat provided by the sun.

The importance of ultraviolet radiation for captive animals is only now being investigated, primarily in reptiles. Because rabbits, much like diurnal reptiles, can synthesize vitamin D when exposed to natural ultraviolet B radiation, we recently studied the impact of artificial ultraviolet B radiation on the vitamin D levels of captive rabbits. We found vitamin D levels significantly higher in rabbits exposed to artificial ultraviolet B radiation than in their cohorts not exposed to ultraviolet B radiation. 1

Given the importance of vitamin D as an essential hormone that regulates many biological functions, a further study was completed by Dr. Megan Watson to evaluate the long-term (6 months) effects of exposing rabbits to ultraviolet B radiation. The findings reinforced the pilot study and showed that rabbits exposed to ultraviolet B radiation maintained significantly higher vitamin D levels over the course of the study. No side effects were found to be associated with regular exposure to ultraviolet B radiation. The research suggests that pet rabbits housed indoors would benefit from exposure to ultraviolet B radiation.

It should be noted that housing indoor rabbits near windows will not provide exposure to ultraviolet B radiation because glass removes these short ultraviolet B wavelengths. However, commercial ultraviolet B light bulbs are available at many pet retailers. Current recommendations are that the lights should be placed within 9 inches of the animal’s cage and changed every 9 months, as the ultraviolet B radiation decays over time in these bulbs.

Studies in chinchillas and guinea pigs found similar results, suggesting that these animals may also benefit from exposure to ultraviolet B radiation. We are planning further research into the role of lighting in the long-term health of pet rodents.

1 Emerson JA, Whittington JK, Allender MC, Mitchell MA. Effects of ultraviolet radiation produced from artificial lights on serum 25-hydroxyvitamin D concentration in captive domestic rabbits (Oryctolagus cuniculi). Am J Vet Res. April 2014, Vol. 75, No. 4 , 380-384.

This article appeared in rvetILLINOIS, Vol. 1, Issue 6.


Vitamin D is an essential hormone that regulates many different functions in vertebrates and can have a protective effect against various disease conditions. Providing exotic pets appropriate access to vitamin D, through the diet, ultraviolet B (UVB) exposure, or a combination of both, is important for veterinarians and exotic pet owners to consider. Although it is commonly thought that most animals derive the vitamin D they need through their diet, there are a number of species that appear to benefit more from UVB exposure. In addition, there has been minimal study to investigate appropriate dietary levels of vitamin D for many of our exotic pets. Although the recommendation of providing UVB lighting has been primarily limited to captive reptiles, research with other species (e.g., birds and small mammals) suggests that these animals may also benefit from this type of lighting. However, the provision of UVB is not without its potential side effects. The purpose of this article is to review the important roles of vitamin D in animals, the different methods animals use to acquire this hormone, the potential clinical signs associated with hypovitaminosis or hypervitaminosis D, the role of artificial UVB lighting in the synthesis of vitamin D, and the potential side effects associated with UVB radiation.

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