Vitamin supplements
From UroWiki
There is much discussion in the lizard-keeping community about the benefits of using vitamin supplements.
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UVB light source vs. vitamin D3
Every uromastyx owner will agree that the more natural the source, the better. This applies to food (fresh vegetables compared to dried foods from a pet store) as well as to light: natural sunlight is of course the best, while a recommended UVB-emitting bulb such as the Mega Ray does a credible imitation. A vitamin supplement should be considered a last resort for a lizard which isn't getting enough D3 through other sources. There are of course other beneficial effects of full-spectrum lighting, such as fine control over the photoperiod, which supplements can't even approach.
Vitamin D2 vs. vitamin D3
It is a common belief in veterinarian circles that one should not use vitamin D2 supplements with reptiles of this type. Although delicate language is employed (using phrases such as "it is believed" and "until further research is done"), it does seem that a number of researchers are of the opinion that vitamin D2 is all but useless for lizards. Some scholarly works include the following:
D3 (cholecalciferol) is essential to calcium metabolism, and is made in the iguana (and human, for that matter) skin by contact with sufficient UVB wavelengths. Plants contain another type of vitamin D, called D2 (ergocalciferol). D2 is not nearly as efficient, close, in fact to being worthless, at metabolizing calcium, hence the need for D3
Herbivorous species can obtain vitamin D2 from sun cured forage (Ullrey and Bernard 1999) but this is reported to have negligible impact on calcium metabolism in reptiles. It is believed that reptiles utilise vitamin D3 rather than vitamin D2.—Boyer, T. H. (1996) Metabolic bone disease in: Reptile Medicine and surgery, Ed D. R. Mader. W. B. Saunders company, London. pp 385 - 392
Do not assume that the term vitamin D in the label ingredient list is actually D3 because it may be D2 and unusable by reptiles—Susan Donoghue in Mader DR (ed): Reptile medicine and surgery, 2nd edition. Saunders Elsevier, St. Louis, MO: Chapter 18: Nutrition p273
However, there remains some doubt, mostly cast by comments such as this:
Most mammals can use both forms of the vitamins. Whether reptiles are able to use vitamin D2 is not known, and some researchers believe that certain reptiles are not able to use oral vitamin D3 [sic - which seems a bit odd] at all. Therefore, until further research is done, only the vitamin D3 form should be used as a dietary supplement.—Mader DR in Mader DR (ed): Reptile medicine and surgery, 2nd edition. Saunders Elsevier, St. Louis, MO: Chapter 61: Metabolic Bone Diseases p842
Differences between mammals and lizards
The differences between vitamin D2 and D3 metabolism are explained in The case against ergocalciferol (vitamin D2) as a vitamin supplement -- Houghton, L. A., and Vieth, R. (2006) Am. J.Clin. Nutr. 84, 694–697. Although the metabolic pathway is known to be similar, it could well be very different in the fine details. However, the following serves to clarify:
1. The affinities of the specific vitamin D plasma transport proteins for 25-hydroxyergocalciferol and 25-hydroxycholecalciferol were studied in sixty three vertebrate species. 2. Fish, reptile, bird and monotreme plasma proteins bound 25-hydroxyergocalciferol considerably less efficiently than 25-hydroxycholecalciferol. 3. Vitamin D transport proteins from twenty-two placental mammals bound 25-hydroxyergocalciferol and 25-hydroxycholecalciferol with equal efficiency. 4. Proteins from nine mammals bound 25-hydroxycholecalciferol 10-30% more efficiently than 25-hydroxyergocalciferol.
Conclusion
It seems to be that mammals can transport both types of vitamin equally well in plasma, but reptiles transport it "considerably less efficiently" - and humans, who are mammals, don't use vitamin D2 as efficiently as D3. Although relatively unfounded, it might be valid to presume that reptiles will utilise D2 even less efficiently than humans.
