Scientists have discovered for the first time that vitamin D can be measured in human hair, paving the way for non-invasively testing deficiency of the 'sunshine vitamin'.

Vitamin D deficiency has reached epidemic proportions world-wide, with over one billion people estimated to be affected. Deficiency has been linked with bone health, but it could also be a risk factor for depression, cardiovascular disease, inflammation, diabetes and cancer.

At present, the best way of assessing vitamin D is to measure the concentration of vitamin D in the blood.

However, this can be painful, requires expertise and training along with hygienic conditions and equipment so getting a sample is not always workable.

The study, published in the journal Nutrients, is a major step forward in assessing vitamin D status, researchers said in a statement.

"This study presents the first step towards the development of a novel test for assessing vitamin D status over time," said Lina Zgaga, from Trinity College Dublin in Ireland.

"The idea is that vitamin D is being deposited continuously in the hair as it grows; more might be deposited at times when vitamin D concentration in the blood is high, and less when it's low," Zgaga said.

"Therefore, test based on the hair sample might be able to give doctors a measure of vitamin D status over time -- if hair is long enough, this even might be over a few years," she said.

"Further research is needed to establish the exact relationship between vitamin D concentration in the blood and in hair over time," she added.

"We also need to investigate different factors that might affect vitamin D levels in hair, the most obvious ones being hair colour and thickness, or use of hair products such as hair dye," Zgaga said.

Other applications could also include historical samples from archaeological sites. Hair, along with teeth, are some of the longest lasting surviving biological materials after death.

"It could be possible to for the first time to assess the vitamin D status of historical populations -- Elizabethans, Viking, Celtic, Roman, ancient Chinese, Egyptian," said Eamon Laird, from Trinity College Dublin.

The presence of vitamin D in hair could be interpreted as a personal record of a person's vitamin D status, research said.