许多年前，科学家们就已经知道人们需要维生素D。维生素D帮助身体吸收钙质，从而促进骨骼和牙齿的生长并使其坚固。研究显示，缺乏维生素D会增大骨折的危险，而加量服用维生素D有助于防止骨折。健康专家们一直相信，每天短时间的日光照射能给人体提供足够的维生素D，因为人

For many years, scientists have known that people need Vitamin D. Vitamin D helps the body use calcium to develop and strengthen bones and teeth. Studies show that lack of the vitamin is linked to an increased risk of broken bones. Extra Vitamin D may help prevent such breaks. Health experts have traditionally believed that short periods of sunlight each day could provide enough Vitamin D. This is because human skin changes sunlight into Vitamin D. The body then can store this substance.