cadman:

 

A mate of mine used to pop all sorts of OTC pills in an effort to be "healthy". He was perfectly fit. But anyway they screwed with his liver function so the Doctor told him to stop and the problem went away.

 

Just eat properly and your body will derive all it needs from that.

 

I think if you read the article I posted a link to, the NZ research on vitamin D levels here, and the recent UK research here, then there's plenty of evidence that with what you might call "modern lifestyle" then your body may not derive all the vitamin D it needs from a "proper" diet - whatever that can of worms may mean, combined with low level of sun exposure on the skin.

At particular risk are people with darker skins in colder climates, kids not getting enough sun exposure etc.

You'll also find that supplementation of normal diet with iodine (added to salt), fluoride (added to water, toothpaste, or supplements), folate (for pregnant women), is a very normal accepted practice (despite controversy) , so somewhat destroys the idea that "eating properly" is the perfect solution - as it appears that if that's the case then billions of people do not "eat properly" as they have or are at risk of various deficiency.  Yes it would be great if we could all have perfect diets, but it's very hard to achieve.

The original article based on meta analysis looked at impact of vitamin D levels on severity and symptoms of complications of cold and flu, and found a very positive correlation from supplementation for significant numbers of people (separately) identified as being at risk of deficiency/low level in the NZ population.  

Note that diseases associated with a vitamin D deficiency include rickets, osteoporosis, and certain types of cancer (breast, prostate, colon and ovaries). It has also been associated with increased risks for fractures, heart disease, type 2 diabetes, autoimmune and infectious diseases, asthma and other wheezing disorders, myocardial infarction, hypertension, congestive heart failure, and peripheral vascular disease. (yes I cheated with this paragraph - it's a quote from Wikipedia)

It's generally considered not possible to get enough vitamin D to meet RDA from a "normal" diet - you need to eat a lot of oily fish etc - or get quite a lot of sun exposure - much more I expect than many people would consider "safe".

There's also some recent epidemiological data suggesting that low seasonal vitamin D levels combined with sun exposure when vitamin D serum levels are low may help explain regional variation in melanoma genotypes and phenotypes (ie higher incidence of melanoma deaths in some regions like Southland - despite lower overall UV exposures than in warmer regions).