The dermatologist mantra, repeated by countless beauty editors is wear sunblock all day every day, rain or shine. Even if you eschew every other anti-aging product available, then at least wear a high SFP sunscreen. Or so the conventional wisdom goes.

And I have never questioned it; dutifully slathering on my Murad moisturizer with SFP 20 whatever the weather. Until the other day.

This weekend I was in Arizona and talked to Tiffany, an esthetician at the Canyon Ranch spa and health resort. I asked her what she considered to be the most important part of her skincare routine. Given that we were in the middle of a desert and she had a pale and clear complexion, I expected her to say sun protection. On the contrary, she said that the current obsession with sunscreen is not only overrated in terms of anti-aging but is depriving us of vitamin D.

Naturally, I found this extremely intriguing and started to do some research as soon as I could get to a computer. There seems to be a fair amount of evidence to back Tiffany up.

There are two concerns about over-using sunscreen: the first is vitamin D deprivation and the second is the adverse effects on the skin from the chemicals in the products.

There are a number of studies that indicate that people who continuously use a sunscreen will deprive themselves of adequate levels of vitamin D. Note that the levels that scientists now say we need is much higher than were recommended 20 years ago. It was once deemed sufficient to have enough vitamin D to prevent rickets. Now, however, there is growing evidence that vitamin D is important to prevent osteoporosis, colon cancer (the biggest cancer killer after lung cancer), multiple sclerosis, diabetes and autoimmune diseases. Some doctors would like to see the recommended daily level increased to 1,000 IUs.

A Boston University study found that adults - even those taking a multivitamin and drinking milk - were at high risk of vitamin D deficiency if they always wore sun protection and avoided sun exposure.

Other studies show that 65% of Americans don't get enough vitamin D - even if they live in relatively sunny cities such as Atlanta.

If you do live in the sunny south, then about 15 minutes a day of sun exposure (without any protection) is enough to get an adequate vitamin D fix. For those of us in the frozen north, leave the sunscreen off until summer and then only wear it on days when you are likely to be outside all day. And eat plenty of vegetables - anything high in anti-oxidants such as broccoli and leafy greens - will help counteract the effects of sun damage.

I'll give the low-down on chemical sunscreens in a future post.