Everyone knows that vitamin D3 or cholecalciferol is important for keeping bones healthy and strong. This is because it is the precursor of the active steroid 1,25 dihydroxyvitamin D3 (calcitriol), responsible for calcium metabolism. Three in four Americans are vitamin D deficient and, according to a recent blood test, that includes me. So I've been boning up on vitamin D and it turns out to be essential for skin and hair too.

It was only recently discovered that this vitamin D precursor is in many more tissues than bone, and skin is one of them. Vitamin D metabolism in the epidermis begins with 7- dehydrocholesterol, which produces both cholesterol and previtamin D3. This generates calcitriol when the skin is exposed to UV. Calcitriol is jolly useful with antimicrobial activity (ever noticed how acne clears up in the sun), prevention of several skin deseases, protection from UV damage, and protection of the hair follicle.

In our interview with Dr Melanie Bone, she targeted vit D as the single most important supplement to take. But how much?

Nowadays, the recommended daily intake of D is around 1,000 IUs. A cup of fortified milk will give you nearly half of this amount and a tablespoon of cod liver oil about 1,300 IUs. Ten minutes or so of unprotected exposure to the sun will also do it, but for most of us office bound, sunscreen wearers that is harder to pull off than it might seem. In actual fact, it is pretty difficult unless you live in the tropics. The ten minutes of exposure rule only works when the UV index is greater than three and if you are pale (darker skinned people need longer sun exposure). A UV index of 3+ is a daily occurrence on the equator and possible in the summer in more temperate New York, where there is typically insufficient sunlight to produce vitamin D synthesis in the skin in the winter months. Since, I haven't found any information about a topical vit D cream, I have resorted to taking a daily D supplement.

A small study (on 14 people) conducted by the University of California found that vitamin D supplements helped clear up eczema. It should be noted, though, that the subjects took a whopping 4,000 IUs a day. And most experts agree that the evidence to date suggests that daily intake of 1000-2000 IU vitamin D could reduce the incidence of vitamin D-deficiency-related diseases. A paper published by the University of Tampere in Finland argues that there is an optimal amount of vitamin D (although it doesn't say what that is): too much D leads to premature aging; yet it is important to have the right amount in our bodies as it also regulates fibroblast growth factor-23 (FGF-23), which determines aging and cancer.

New research also points to vitamin D as the potential saviour for rosacea sufferers. Enzymes in the skin of rosacea sufferers cause them to produce antimicrobial peptides in an abnormal form and A study in Belgium of all the research in the last couple of years has made a connection between the regulation of these peptides and vitamin D.

There is a strong association of the sunshine vitamin with hair growth - and nor just because vitamin D helps us process calcium. In order to make use of vitamin D, our skin and hair have vitamin D receptors (VDR). Studies on mice have found that an absence of VDRs prevents them from growing new hair.

An interesting aside regarding our furry friends: the skin of cats and dogs does not synthesize vitamin D. Rather, it is generated from oily secretions into the fur that are obtained by the animal when grooming. Now you know what all that licking is about.