The method measures calcium isotopes, different atoms of the element calcium. These isotopes are present in a person's urine and serve as an indicator of bone strength based on the balance of minerals.
Researchers examined a dozen healthy volunteers over a 30 day period, all of whom were confined to bed rest which mirrors weightlessness in space and can lead to bone loss. Researchers learned that their new test could detect bone loss in as little as 10 days, long before conventional medical scans could produce the same results.
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English - http://www.bbc.co.uk/news/health-18240047
Portuguese - http://veja.abril.com.br/noticia/ciencia/exame-de-urina-pode-levar-a-diagnostico-osseo-mais-preciso-do-que-raio-x