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     U.S. National Institutes of Health (NIH) said Monday that it's to launch a large-scale clinical trial to investigate if a vitamin D supplement helps prevent or delay type 2 diabetes in adults who have prediabetes, and are at high risk for developing type 2.
　　The multiyear study will include about 2,500 people and its goal is to learn if vitamin D, specifically D3 (cholecalciferol), will reduce the risk of developing type 2 diabetes in adults aged 30 or older with prediabetes.
　　People with prediabetes have blood glucose levels that are higher than normal but not high enough to be called diabetes.
　　"Vitamin D use has risen sharply in the U.S. in the last 15 years, since it has been suggested as a remedy for a variety of conditions, including prevention of type 2 diabetes," said Myrlene Staten, Vitamin D and Type 2 Diabetes (D2d) project officer at the National Institute of Diabetes and Digestive and Kidney Diseases ( NIDDK), part of the NIH.
　　"But we need rigorous testing to determine if vitamin D will help prevent diabetes. That's what D2d will do," Staten said in a statement.
　　Past observational studies have suggested that vitamin D could reduce the diabetes risk by 25 percent, but the NIH said it will be not able to make any definitive conclusion until the clinical trial is complete.
　　The trial will directly examine if a daily dose of 4,000 International Units (IUs, 100 mcg) of vitamin D, greater than a typical adult intake of 600 to 800 IUs (15 to 20 mcg) a day, but within limits deemed appropriate for clinical research, helps keep people with prediabetes from getting type 2 diabetes.
　　Half of the participants will receive vitamin D. The other half will receive a placebo, a pill that has no drug effect. Participants will have check-ups for the study twice a year, and will receive regular health care through their own health care providers, the NIH said.
　　The study will be double-blinded, so neither participants nor the study's clinical staff will know who is receiving vitamin D and who is receiving placebo, it said.
　　The study will continue until enough people have developed type 2 diabetes to be able to make a scientifically valid comparison between diabetes development in the two groups, likely about four years, it added.
　　According to data from the 2011 National Diabetes Fact Sheet, an estimated 79 million Americans have prediabetes, and nearly 26 million more have diabetes.
　　相关新闻：美国国家卫生研究院２１日宣布，将启动一项大型临床试验，以检验维生素Ｄ补充剂能否帮助预防或延缓出现糖尿病。
　　此前研究曾表明，服用维生素Ｄ可将糖尿病风险降低２５％左右。美国国家卫生研究院当天发表声明说，过去１５年中，维生素Ｄ在美国的使用量急剧上升，它被建议用作补充剂以改善多种症状，包括预防糖尿病，“但我们需要进行严格的检验才能确定维生素Ｄ是否能够帮助预防糖尿病”。
　　声明表示，这项临床试验将招募大约２５００名３０岁以上的糖尿病前期人群，其中一半参与者服用维生素Ｄ，另一半作为对照组服用没有疗效的安慰剂。维生素Ｄ使用量为每天４０００国际单位（１００微克），远高于典型的每天６００至８００国际单位（１５至２０微克），但仍在安全使用范围内。
　　声明表示，试验参与者每年接受两次检查，估计整个试验需要４年时间，以对两组人群进行科学有效的比较。
　　所谓糖尿病前期，是指患者血糖浓度高于正常值，但又没有达到被称为糖尿病人的程度。糖尿病前期是须予以高度重视的糖尿病预警信号。
　　糖尿病不仅是美国面临的严重问题，在中国也同样超过“警戒级别”。今年９月，中国研究人员在《美国医学会杂志》上报告说，中国成人糖尿病患者已超过１亿，其中七成糖尿病患者不知自己已经患病，糖尿病前期人群达到４．９亿。