London, Feb 24 (IANS): Researchers have found that vitamin D can now be measured by human hair, paving the way for improved diagnosis of deficiency of the sunshine vitamin.

With over a billion people estimated to be affected, vitamin D deficiency -- a risk factor for depression, cardiovascular disease, inflammation, diabetes and cancer -- has reached epidemic proportions worldwide.

While traditional blood analysis captures the levels at a single time point, in contrast, hair, which grow at approximately one centimetre per month, could reflect vitamin D status over several months, capturing the large seasonal differences in the levels.

"The study presents the idea 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," said lead author Lina Zgaga, Associate Professor at Trinity College Dublin.

"Therefore, a 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," Zgaga added.

The findings were published in the journal Nutrients.

However, further research is needed to establish the exact relationship between vitamin D concentration in the blood and in hair over time.

There is also a 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.

The discovery could also have other applications as hair (along with teeth) are some of the longest lasting surviving biological materials after death and thus it could be possible to assess the vitamin D status of historical populations - Elizabethans, Vikings, Celts, Romans, ancient Chinese, Egyptians, the researchers said.