Researchers who have debunked one of the claimed benefits of vitamin D capsules are sceptical of the links that have been made to a much wider range of medical conditions.
Auckland University physician Professor Ian Reid and colleagues concluded after studying 23 trials that most healthy adults need not take vitamin D supplements for the prevention of the bone-weakening condition osteoporosis.
"This systematic review provides very little evidence of an overall benefit of vitamin D supplementation on bone density," the researchers say in the British journal The Lancet. "Continuing widespread use of vitamin D for osteoporosis prevention in community-dwelling adults without specific risk factors of vitamin D deficiency seems inappropriate."
Most healthy adults in New Zealand got enough vitamin D from the sun, said Professor Reid.
Supplementation might be needed for children at risk of rickets and it might also benefit resthome residents, the frail elderly, veiled women, people with very dark skin and others who couldn't get enough vitamin D from the sun.
"In North America, half of all [older] adults, perfectly fit and healthy people, take vitamin D supplements and what this analysis says is that for almost all of those people, it's a complete waste of time [and money]".
In New Zealand about 12 per cent of people aged over 50 - and about 5 per cent of the whole population - receive state-funded vitamin D supplements but this does not include over-the-counter purchases. In residential care facilities, 74 per cent of elderly residents are given the supplements, to reduce falls.
Use is said to be very high among patients of GPs in wealthier areas. They have become something of a cure-all - or prevent-all - with low blood levels having been linked to multiple sclerosis, winter colds, heart disease, cancer, mental illness and numerous other medical conditions. Professor Reid has found reports of links to 53 conditions.
"When you get 53 different diseases associated with low vitamin D levels and when those diseases are incredibly disparate ... it's pretty hard to see a common biological explanation for those associations except the fact that people don't go outside and lie around in the sun [when they are sick]."
"I think low vitamin D levels are a marker of people being sick, rather than a marker of specific disease conditions."
Another Auckland University researcher, Professor Robert Scragg, is the co-leader of a large trial, one of four in the world, that aims to answer questions about vitamin D.
More than 5000 Aucklanders aged 50 or older have been randomly assigned to receive vitamin D or a placebo in a study whose results are expected in 2016. Rates of cardiovascular disease, respiratory disease and bone fractures will be compared.
Massey University is running studies looking for associations between vitamin D and conditions including respiratory infections, allergies and psoriasis.