The way in which many vertebrates utilise sunlight in the production of vitamin D3 in the skin, and how this process is also regulated by the light itself, is still the subject of much research. Most of the studies relate to human biology, but researchers such as Holick, Ferguson and Gehrmann in the United States are gathering evidence that the process is very similar in reptile skin.9,14,15,16,24

A simplified animated diagram (Figure 1, below) illustrates the vitamin D pathway.

The pathway begins when a cholesterol, provitamin D, (full name 7-dehydrocholesterol, or 7DHC) is manufactured by cells in the skin. When exposed to UVB at wavelengths between 290 - 315nm, this provitamin D, held within the cell membrane, is converted very rapidly indeed to previtamin D3. The peak production is at 297nm.34

Previtamin D3 is then isomerised (transformed by a re-arrangement of atoms in the molecule) slowly, in warm skin, over several hours, to vitamin D3. Warmth is needed for the reaction to proceed at a normal rate. Reptiles obtain this heat from the sun, as they bask.

The vitamin D3 is released from the skin cell membranes and is taken up by a "vitamin D-binding protein" into the plasma. It is thus carried in the bloodstream from the skin to the liver, where it is hydroxylated to calcediol, 25-hydroxy-vitamin D3. This is the substance which is tested for in blood samples taken to assess the reptile's vitamin D status.1,20,27

Calcediol is then circulated in the bloodstream all around the body. In the kidneys, some is converted to the active hormone calcetriol. This plays a major part in calcium metabolism, governing the levels of calcium in the blood by controlling absorption of calcium from the gut and also from the bones, should dietary levels be inadequate for the body's needs.23

Calcediol has also, in recent years, been found to play a vital part in the normal functioning of other organs. It is taken up by cells throughout the body, and converted intracellularly to calcetriol. This local action has beneficial effects upon the immune system, the cardiovascular system, and in preventing cells in many organs from becoming cancerous by controlling cell division.22,23

There is also new evidence that skin cells in sunlight can actually complete the entire pathway from provitamin D to calcetriol intracellularly, which may increase the skin's resistance to cancer.29,30

Calcediol, in humans, has a half-life of about two weeks in the bloodstream. In some reptiles, this circulating calcediol may act as the body's main store of vitamin D. 3

Vitamin D3 does not remain in high concentrations in the bloodstream. In humans, that which is not hydroxylated to calcediol in the liver is taken up into body fat, where it is apparently stored 23 but we are not aware of any studies ascertaining whether such storage takes place in reptiles, or if it does, how long such a store might last.

Vitamin D3 is a substance that is toxic in large amounts. From the 1920s, vitamin D was added to milk for human consumption to eradicate rickets; however, this was banned in Europe in the 1950s because children were suffering from overdosage.22

In reptiles, too much vitamin D added to the diet leads to hypervitaminosis-D, which causes kidney damage, calcification of the soft tissues, including the major blood vessels, and premature death.42

However, hypervitaminosis-D is not known to occur in basking reptiles (or any other species) obtaining their vitamin D from sunlight, regardless of how long they bask.23 This is because there are inbuilt safety mechanisms preventing overproduction of vitamin D in the skin. Interestingly, these also rely upon ultraviolet light, as can be seen in the animated diagram, Figure 2 (below).

As we saw earlier, when a reptile basks in full sunlight, previtamin D3 is produced very rapidly and accumulates in the skin. Its conversion to vitamin D3 is a much slower, heat dependent process. One might expect huge quantities of preD3 to build up, but this does not happen. This is because preD3 is also sensitive to ultraviolet light up to 325nm; a proportion is converted quite rapidly into two biologically inactive products, lumisterol3 and tachysterol3. These also accumulate in the skin.34

Most of the studies have been conducted on human skin, but the same process is believed to occur in reptiles; lumisterol3 has been isolated from gecko skin samples exposed to sunlight.16

There is also a second line of defence against overproduction of D3. As we saw earlier, vitamin D3, once produced, is carried away in the bloodstream to the liver. However, should excess vitamin D3 build up in the skin - if, for example, more is produced than the binding protein can remove - ultraviolet light breaks this down, as well, into three new substances: two suprasterols and 5,6 trans-vitamin D. This latter product does have some biological activity; the others are believed to be inert.43

What happens to all these inert by-products? Research is ongoing; however, we can speculate that the lumisterol3 and tachysterol3, in particular, might be used as a source of preD3. This is because their production from preD3 is a reversible reaction.

Under ultraviolet light, an equilibrium forms with varying concentrations of the three, depending in part upon the exact wavelengths of the light. The three substances have slightly different action spectra. Lumisterol3 may be converted back to preD3 by light of wavelengths up to 315nm; tachysterol3 responds right up to 335nm, which is in the UVA range.34

Some reptiles may be able to sense whether or not they need vitamin D, and alter the time they spend basking under UVB light accordingly. In one study, panther chameleons (Furcifer pardalis) fed a diet low in vitamin D3 spent more time basking under ultraviolet light than those on a high D3 diet. In addition, they were more attracted to lamps emitting UVB than to equally bright lamps which emitted UVA. Whether they can actually see the UVB is not known, but they do appear to be able to detect it by some means.15

Studies like these show how well reptiles are adapted to make the most of ultraviolet light in their environment.

There's more on the current research into the UV requirements of lizards in our section:
What UV light do reptiles need?

Our own studies and recordings are featured in the sections:
Using the UVB meter and UV light in nature, plus our special investigation into reptile skin and UVB: The Transmission of Ultraviolet Light through Reptile Skin Shed.