Risks associated with high vitamin D levels need to be investigated before the vitamin can be added to more Australian and New Zealand foods, warns a Queensland researcher.
Professor John McGrath, a University of Queensland psychiatrist, says several large international studies have linked both low and high levels of vitamin D with increased risk of schizophrenia and death.
"This is a very gloomy prospect," McGrath told a symposium on vitamin D in foods in Melbourne.
"This will put the brakes on how excited we get about telling everyone 'you must take vitamin D'," he says.
But several experts say the proposed amount to be added to foods would not be toxic.
McGrath says a large Copenhagen study of almost 250,000 people attending GP clinics found low levels of vitamin D increased the risk of death two-fold. High vitamin D levels had the same effect.
University of Melbourne health researcher Professor Peter Ebeling said that people surveyed for the study had been referred by doctors and were likely to already be unwell, which could bias the findings.
He says a large United States study of vitamin D and mortality may provide more answers.
The conference, which discussed whether more foods in Australia and New Zealand should be fortified with vitamin D, was told that Canadian products, including milk, had been enriched with the vitamin since the 1940s.
Fortification of milk, soy products and margarine is mandated, University of Saskatchewan professor Susan Whiting told the conference.
Deakin University (Melbourne) researcher Professor Caryl Nowson says about 30 per cent of Australians have low vitamin D levels.
She says it is difficult for people in Australia's southern states and New Zealand to get enough vitamin D from sunlight in the cooler months, and it should be added to more foods.
Currently, Australia mandates only fortification of margarine.
Deakin University's Professor Rob Daly says vitamin D fortification may reduce severe deficiencies. Monash University Associate Professor Andre Renzaho says he has reservations about fortifying foods unless it can be proven to help migrants, who are a high-risk group for being deficient in vitamin D.