Image courtesy of SOHO/EIT consortium. SOHO is a project of international cooperation between ESA and NASA.

Introduction

Exclusively herbivorous reptiles like lizards and tortoises cannot obtain sufficient amount of vitamin D3 solely from their natural diet. Nevertheless, vitamin D3 is fundamental to ensure normal functioning of many organs. In addition to regulating calcium metabolism, vitamin D3 also acts as a hormone in organ development. Active vitamin D3 also takes part in the functioning of immune system. Furthermore, it controls build up of bone matter and also appears to be important for female fertility. (Jones et. al., 1998).

In plants, the large proportion of D-vitamins consists of vitamin D2 (ergocalciferol) that is not absorbed very well by the intestinal system. Vitamin D3 (cholecalciferol) promotes calcium metabolism much more efficiently, but fresh plants are almost completely devoid of it. Only sun-dried plants, like hay for instance, contain small amounts of vitamin D3 (Raulio, J., pers. comm.). Herbivorous animals must compensate for this deficiency by photosynthesising vitamin D3 by ultraviolet light.

Vitamin D3 is photosynthesised in the skin of terrestrial vertebrates and birds by the action of UVB radiation on 7-dehydrocholesterol (7-DHC). This steroid is most sensitive to radiation in the range of 270-305 nm (fig. 1. MacLaughlin et. al., 1982). This range coincides with the lowest wavelengths of sunlight that can actually penetrate the atmosphere, the lower limit of the active range being 290 nm. While absorbed by a 7-DHC molecule, the UVB photon opens the ring structure of the molecule and converts it to a precursor of vitamin D3 (preD3). Subsequently, this is thermally isomerised slowly, over several days, to cholecalciferol that is the actual vitamin D3.

Fig. 1.

Vitamin D3 is transferred to the liver by the vitamin D binding protein, where it is transformed to calcidiol [25-hydroxycholecalciferol, 25(OH) D3]. Calcidiol is then transferred to the kidneys, which in association with parathyroid hormone, further convert it to calcitriol [1,25-dihydroxycholecalciferol, 1,25(OH)2D3]. A recent study carried out in the University of Tampere (Lou et. al., 2003) suggests that both of these metabolic products have their own significant role in the operation of the organic system: calcidiol acts as a hormone and controls for instance cell division, whereas calcitriol takes part in the calcium / phosphate regulatory mechanism and is thereby the actual active substance while controlling the calcium level of blood serum. It increases absorption of calcium and phosphate through the wall of small intestine and also controls their transfer from bone matter to plasma. Calcitriol also decreases the amount of calcium and phosphate secreted in urine. Since calcitriol receptors have been found in various tissues, it apparently also has several other tasks within organs. For the sake of simplicity, calcidiol and calcitriol are by common consensus called vitamin D, even though a more accurate name for calcidiol would be hormone D.

If excess preD3 is formed in the skin it is further photoisomerized by UVB irradiation to lumisterol and tachysterol. This rapid reaction is photo-reversible: radiation isomerises tachysterol back to preD3, although at a slower rate, and further to lumisterol. Being the least photosensitive product, lumisterol is finally accumulated to plasma. These reactions act as a natural regulation mechanism, preventing excessive synthesis of vitamin D3 under strong UVB irradiation.

The spectral characteristics of light in the UVB/UVA range are an important factor in vitamin D3 photosynthesis. While 7-DHC is sensitive to irradiation up to 315 nm, cutaneous vitamin D3 that has been photosynthesised or obtained nutritionally is destroyed by radiation up to 330 nm (Webb et. al., 1989.) This makes any radiation in the range 315-330 nm highly undesirable for the synthesis of vitamin D3.

The skin temperature also plays a very important role in the synthesis of vitamin D3. This was established in a study with green iguana (Iguana iguana), common frog (Rana temporaria) and human skin samples (Holick et. al., 1995.) In vitro tests showed that a temperature increase from 5 oC to 25 oC accelerated thermal isomerization of vitamin D3 by eight. In a separate study with human and chicken skin at even higher temperatures (40 oC), the tendency remained the same.

To ensure that this complex chain of reactions on reptile’s skin can be completed, sufficiently high irradiation at wavelengths 270-315 nm is required, while higher wavelengths (315-330 nm) should be avoided. The skin temperature must also be high enough. Low UVB irradiation below 315 nm or too low a body temperature of a cold blooded (poikilothermic) animal might create an undesirable situation where new vitamin D3 is no longer produced in the skin and at the time the radiation starts destroying cutaneous vitamin D3.

Under this hypothesis, non-equatorial herbivorous animals should be susceptible to vitamin D3 deficiency. The detrimental effects of photodestruction of vitamin D3, as described above, may however be alleviated by the equilibrium seeking properties of many biological processes (Ball, J., pers. comm.). It is possible, for instance, that the membrane enhancement of the production of vitamin D3 (Holick et. al., 1995) may automatically compensate for the reduced radiation.

It should be noted that the theory and research discussed above relate to human skin. However, the chemistry of the skin of terrestrial vertebrates is similar enough to that of human to justify the theory to be extrapolated to reptiles as well. Human osteoporosis caused by UVB deficiency is well documented in Nordic countries, but this is yet to be demonstrated in wild reptiles.

In conclusion, to ensure sufficient vitamin D3 synthesis, a terrarium must be fitted with efficient artificial lighting with broad-spectrum UVB lamps and temperature must be kept sufficiently high in the basking area. More specifically, the radiation in the region of short wavelengths must reach far enough.

Over recent years, there have been numerous research papers written on the quality of lamps designed for terrarium use. Some studies have included only a few lamps, while some studies cover lamps of which many are no longer available (e.g. Ball, 1995). This makes evaluation of lamps difficult, as there are no comparable results available for current models. For this study, as large as possible selection of lamps available in Finland was obtained. Some lamps that are not locally commonly available were also included, either because they were especially interesting or had received contradictory reviews elsewhere.

Several papers have focused only on the percentage of UVB radiation. As cutaneous vitamin D3 synthesis is only sustained in an extremely narrow band within the crossover of UVB and UVC, the full UVB-range irradiance figure alone does not tell the whole truth about the ability of a lamp to promote vitamin D3 production. More detailed information is required.

For the purpose of this study, the D3 Yield Index was developed to indicate the amount of radiation that can actually participate in the photosynthesis of vitamin D3. A plain index number such as the Yield Index used here shows this in an unambiguous manner. The calculation method for the index was devised in such a way that it can be easily adapted to any reference. Within the framework of this study, the index value is based on radiation energy of the sun in midsummer noon in Finland. Measuring the sunlight with compatible equipment at the equator would enable the index to be adjusted for use as a universal baseline.

The results indicate significant variation in the capability of different lamps to promote the photosynthesis of vitamin D3. They range from half of that of natural sunlight in Finland to virtually nil. On this basis, it is clear that the design of UV lamps for terrarium use must focus more precisely on the spectral characteristics of the UVB range that produces vitamin D3 (i.e., 280-304 nm).