Professor Scragg said there was high public interest in vitamin D's potential health benefits.
"GPs are very supportive of it and I know they are prescribing it extensively to patients. Hospital specialists are sceptical. Me, I'm in the middle. My heart says I want it to work. My head says I have to keep an open mind."
Professor Scragg said earlier studies indicated that up to 30 per cent of cardiovascular disease in New Zealanders could be prevented by doubling their vitamin D levels.
Observational studies had shown the risk of many diseases was lowest when levels of vitamin D were around 80 to 100 nanomoles per litre of blood. The most reliable recent survey had found the average level in New Zealand adults was only 50 nanomoles/litre.
The evidence on vitamin D from observational studies was at a tipping point, he said, so cause-and-effect experiments were urgently needed, such as Auckland University's, which would compare rates of cardiovascular disease, infections, bone fractures and other health outcomes among those receiving the vitamin and those on the placebo.
In the $5.5 million trial, financed by the Health Research Council and ACC, patients undergo tests for lung function, blood pressure, arterial stiffness, muscle strength, walking speed and balance.
Professor Scragg said the scepticism about the potential health benefits of increasing levels of vitamin D arose from what had happened with vitamins A, B, C and E. Studies had found high levels of the vitamins were linked to decreased rates of cancer and/or cardiovascular disease. But randomised controlled trials found supplementation with the vitamins either had no effect on the rates of the disorders being studied, or was harmful.
The Auckland trial involves people who were aged 50 to 84 when they joined. The researchers are seeking more in the older end of the age range.
People aged 70-84 who are not taking prescription vitamin D and wish to join the trial can call 0800-843-211.
Q&A: Vitamin D
What is vitamin D?
A fat-soluble substance that helps us to absorb calcium from food.
Where do we get it from?
Sunlight - our skin produces vitamin D when exposed to the sun, but we have to be careful to avoid skin damage from too much sun.
Food - oily fish, eggs, lean meat, dairy products.
Fortified food - milk, margarine and yoghurt with added vitamin D, although the amount available from this source in NZ is considered low.
Nutritional supplements - doctor-prescribed capsules, tablets and drops are subsidised by Pharmac, or can be bought from pharmacies.
Do we have enough vitamin D?
Around 5 per cent of adults are deficient in vitamin D and a further 27 per cent are below the recommended blood level, according to a large survey in 2008/9.
Peak levels are during summer, in line with the amount of ultraviolet light our skin gets, and our lowest levels occur during spring. Maori, Pacific and Asian people have lower vitamin D levels than Pakeha because of darker skin.
What are the possible health effects?
Having too little vitamin D can cause rickets, a bone-building disorder in children. Many observational studies have found associations with a range of disorders including heart disease, type 2 diabetes, bowel cancer, lung infections and the immune-system diseases MS and type 1 diabetes.
Can you get too much vitamin D?
Yes. It can be harmful.
