Dietary vitamin D or Ultraviolet Light? - A controversial issue.
It has long been known that some species of reptiles which do not normally bask in sunlight- including most snakes and nocturnal or crepuscular lizards - may be kept and bred successfully in captivity with no access to ultraviolet light, provided that adequate levels of vitamin D3 are provided in the diet. These are mainly carnivorous or insectivorous species which might be expected, in nature, to obtain a considerable proportion of their vitamin D3 from their diet. Most of the literature on the husbandry of these species states that UVB is unnecessary.

Diurnal lizards of quite a few species have also been kept and bred successfully by zoos and private keepers supplying them with dietary D3 supplementation but no ultraviolet light.17,39 This would appear to be an abnormal situation for these reptiles since, in the wild, many of these species do bask in sunlight, and would almost certainly utilise ultraviolet light to synthesise much of their vitamin D3. Much skill and considerable experience is needed to ascertain the correct level of supplementation.20 Failure to provide adequate D3 results in metabolic bone disorder, and this condition has been seen frequently in captivity. Oversupplementation causes equally disastrous hypervitaminosis-D. This is presumably because the body has not evolved any efficient way of dealing with dietary excess, since natural foodstuffs are very unlikely to contain toxic levels of vitamin D.5,22 

Certain species, however, do not seem able to utilise vitamin D3 effectively from their diet. This may be because in the wild, their diets are particularly low in vitamin D3 and they are adapted to rely almost entirely upon UVB photobiosynthesis for this vitamin. Vegetarian lizards might be expected to fall into this category, since plants contain predominantly vitamin D2, which is not thought to be utilised by reptiles.8 However, insectivores and even carnivores may fall into this category. Not only green iguanas but also giant day geckos and Komodo dragons fed experimental diets containing vitamin D3 and calcium, but deprived of UVB light, have developed pathological symptoms.1,7,8,20

The exposure of reptiles to UVA provides known physical and psychological benefits.
It is possible that UVB also provides psychological benefits; and reptiles have a natural ability, as we have seen, to use UVB light to create exactly the required amount of vitamin D3. 
Most reptile keepers therefore choose to supply ultraviolet light to all diurnal lizards that are known to bask in sunlight in the wild, rather than deprive them of its benefits and experiment with dietary D3 supplementation.


Providing ultraviolet light to reptiles in captivity.

The best and most balanced source of ultraviolet light is, naturally, the sun. Where reptiles are kept in climates where it is feasible to allow them access to natural, unfiltered sunlight, this is often the ideal option, provided the animals have free access to shade and shelter, as they would in the wild, and the environmental temperatures are suitable for them. Outdoor enclosures are used to great effect in many zoos and private collections.

However, many tropical species need indoor housing, in temperate climates, for much of the year. Over the last decade, increasingly sophisticated light sources have become available to the hobbyist needing to supply captive reptiles with a source of UVB indoors. Today, in the UK there are several brands of fluorescent tubes and compact lamps available in a range of shapes and sizes, and mercury vapour bulbs designed to produce even higher levels of UVB.

The Dangers of Overexposure to Ultraviolet Light.

The benefits of moderate exposure to natural sunlight are undeniable, both for humans and for many species of sun-loving reptiles.22,23 However, it is also well known that in mammals at least, excessive exposure to the sun's ultraviolet rays - or their artificially produced equivalent - can be harmful. The situation in reptiles has not, apparently, been studied in detail, but is almost certain to be similar.

Although, as our test results show, the UV output from many lamps sold for reptile use is only a fraction of that found in natural sunshine, all artificial sources of UV radiation must be treated with respect. Moreover, some of the latest mercury vapour lamps do emit high UV at close range. Manufacturers' recommended minimum distances should always be taken seriously.

Excessive UV radiation has a damaging effect upon living cells.13,45 In mammals both the skin and the eyes may be injured and there are both immediate and long-term effects.

Some skin cells may be killed outright (as in "sunburn"), but surviving cells may suffer damage to their nucleic acids (DNA and RNA) which can predispose to certain types of cancer (photocarcinogenesis) and also suppress the immune system. 

It is believed that reptile skin, being generally much thicker and often more heavily pigmented than mammalian skin, may be more resistant to the detrimental effects of UV light. However, cases of possible "sunburn" have been described; some of these may actually be thermal burns from hot UVB-emitting lamps.33

Excessive exposure to UV lamps is also reported as causing a toxic syndrome and hyperkeratosis (a pathological skin thickening) in side-blotched lizards (Uta stansburiana), lethargy, skin darkening, anorexia and death in iguanas, and egg hatching failure and skin tumours in panther chameleons.1

Damage to the eyes is well documented in mammals. Immediate effects include photokeratoconjunctivitis (snow blindness, "welder's eye") and long-term effects include certain types of cataracts (opacity of the lens). Although there does not appear to be published literature on either of these conditions in reptiles, several reptile keepers have recently described symptoms of photokeratoconjunctivitis in lizards kept in close proximity to new high-UVB output lamps of several types. Typically, the lizard's eyelids swell up and close, and it becomes depressed and lethargic. The condition clears rapidly when the lamp is removed or exchanged for an older one or different type.4
The formation of cataracts has been described in snake eyes following the snake's forced exposure to bright light. However, in some cases the artificial light thought to be responsible was not emitting UVB.10
Reptile eyes do have some differences in their ocular structure and in the composition of the lens and aqueous humour. These differences may afford some protection from UV light.1 

In the absence of more information, we need to ensure as far as possible that our reptiles are not exposed to levels of UVB which would be un-naturally high for a wild animal of that species in its normal habitat. In addition, a reptile should always be able to move out of the effective range of its UVB light. "Extra" UVB above the requirement for D3 production is of no benefit, since as we have seen, the photo-biosynthesis is a self-limiting process; when sufficient is made, additional ultraviolet light merely breaks down any excess formed.