Have a look at The Vitamin D Solution by Michael Holick. He talks about it . Vitamin D deficiency has been linked to a large number of illnesses, including osteoporosis, MS, arthritis, diabetes, depression and more.
Supplements are not nearly as effective as sunlight. And unfortunately, as far north as the UK is, we are not able to synthesize vitamin D at all during winter (Oct - Mar). It's also harder for people with darker skins to get enough vitamin D.
Michael Holick suggests sensible amounts of time to go sun-screen free, based on your latitude and skin type. I can't find his tables online anywhere but for fair skinned, Northern Europeans, safe exposure in May in the UK would be 25-40 minutes before 11:00.
My mother has had a melanoma removed and I have been very careful about letting my children go out in the sun unprotected. I now realise that I may have set them up for a lifetime of other problems - p4. (paraphrased) a girl having lived in northern latitudes up to age 10 has a 100% increased risk of developing MS no matter where she lives for the rest of her life. Also increased risk of fibromyalgia, chronic fatigue, preeclampsia, unplanned C-section and having a child who will suffer from schizophrenia, not to mention increased risk of internal cancers, hypertension, osteoporosis, arthritis, depression, obesity, type 2 diabetes, dementia ...
I have been tested and am vitamin D deficient but struggle to get out into the sun because I work full time in an office. And it rains at weekends!