AUSTRALIANS are experiencing worsening vitamin D deficiency and the cost of testing has risen a hundredfold since 2000.
But some experts are concerned the testing - which cost Medicare more than $100 million last year - might not be producing better health for those found to be deficient.
Researchers from the University of Sydney and Westmead Breast Cancer Institute said the rise was not leading to tests and treatment for the potential consequences of the deficiency, which can cause brittle bones.
Kellie Bilinski and Steven Boyages also found evidence of overtesting, with nearly 15 per cent of people having four tests a year, and more than 8 per cent having five. One patient had 79 tests in a year, they wrote in the Medical Journal of Australia.
Ms Bilinski, a senior clinical and research dietitian, said many tests were probably unnecessary. If they were limited to three a person a year, based on present costs, $20 million could be saved over 4½ years.
''Our data show that more stringent guidelines should be developed for testing,'' she said. Further research was necessary to establish whether the increased testing was leading to better health results.
In a separate paper, published in the British Medical Journal, Ms Bilinski said it was worrying that increased vitamin D testing had not brought an increase in bone density testing.
There are fewer than 100 bone density tests for every 100,000 people, compared to more than 5000 vitamin D tests.
Rebecca Mason, a professor of endocrine physiology at the Bosch Institute for Biomedical Research at the University of Sydney, said one person having 79 tests was ridiculous as there was generally no need for more than one unless treatment response was being measured. But she said Ms Bilinski's research highlighted the need for more bone density testing, not less vitamin D testing.
''The problem is less that vitamin D testing has gone up and more that bone density testing has not gone up as much as hoped,'' she said.
But bone density tests would not automatically be subsided by Medicare on the basis of vitamin D deficiency, which was implicated in a number of conditions.
''The increases in vitamin D testing have been largely due to the realisation that adequate levels of vitamin D are important, not just for bone and muscle but other health outcomes.'' Professor Mason said she was not overly concerned by recent research linking very high vitamin D supplementation to falls and fractures.
''It is something to keep an eye on but it's not something I would worry about,'' she said. ''In non-urbanised societies people would run around with vitamin D levels that were well above 100, and that would seem to be evolutionarily sensible.''
The most common causes of deficiency are obesity and lack of exposure to sunlight. Guidelines say an adequate level is more than 50 nannomoles per litre of blood.