维生素D在2型糖尿病中的作用

The Role of Vitamin D in Type 2 Diabetes

　

它能帮助克服胰岛素抵抗吗?

维生素D是一种强大的物质。身体通常会在阳光的刺激下产生自己的维生素D。一个世纪以前，那些得不到足够维生素D的人——通常是那些生活在整个冬天都缺乏阳光的北纬地区的人——有患上佝偻病的危险。今天，尽管许多食物都添加了维生素D，但美国人却发现很难获得足够的维生素D。对于那些在办公室工作的人来说——或者习惯于涂满防晒指数为30的防晒霜来遮挡阳光——阳光并不总是足够的。

近年来，研究人员将低维生素D水平与胰岛素抵抗和糖尿病联系起来。特别是克服胰岛素抵抗，可能是预防2型糖尿病的一种方法。纽约爱因斯坦医学院的助理教授Preeti Kishore, MB, BS说:“现在，有很多间接的证据表明给人们提供维生素D可能有助于胰岛素抵抗。”

不过，尽管旁证对法庭来说可能足够好，但科学家们希望得到更确切的数据，证明一种治疗方法比另一种更好。Kishore对维生素D的研究是由她的导师，美国糖尿病协会的医学博士Meredith Hawkins资助的。她正在仔细观察一种叫做巨噬细胞（Macrophage)的细胞。

巨噬细胞是一种特殊的免疫细胞，可以攻击入侵者，研究人员认为，巨噬细胞还可能有辅助清理工作。当脂肪细胞变得太大时，它们就会死亡，巨噬细胞就会进入并消除坏死组织。基肖尔说，这可能是脂肪组织中巨噬细胞过多的原因，也可能是炎症(巨噬细胞起作用的标志)在超重或肥胖的人群中更为严重的原因。

　

　

对于糖尿病患者来说，最新的研究表明，巨噬细胞的活动有一个额外的缺陷。巨噬细胞产生一种叫做细胞因子（Cytockine)的化学物质，其中一些作为信号传递到身体的其他部位。“它们会损害肝脏和肌肉中的胰岛素作用，”Kishore说。“更高的细胞因子意味着更多的胰岛素抵抗”，这是2型糖尿病的一个关键因素。

基肖尔正在努力寻找一种方法来降低体内巨噬细胞的活性。在注意到巨噬细胞具有维生素D的特殊受体后，Kishore决定观察维生素D缺乏是否会使巨噬细胞更活跃——在此过程中促进胰岛素抵抗和炎症。“当你缺乏维生素D时，巨噬细胞就会处于更加活跃的状态，”Kishore说。“我们相信，当你给人们提供维生素D时，这些炎症反应就会减少。”

　

　

　

这是霍金斯极力鼓励的一个研究领域。“这是一个非常重要的话题——有很多争议，”霍金斯说，他是阿尔伯特·爱因斯坦医学院的教授，也是全球糖尿病计划的负责人。“我们真的没有证据证明维生素D缺乏在糖尿病中的作用。”

Kishore从一小群人开始研究，他们体内的维生素D太少，同时也有胰岛素抵抗(但没有糖尿病)。在对胰岛素抵抗和维生素D缺乏进行了简单的初步筛查后，研究参与者在两只手臂上安装了静脉注射泵，每隔5分钟采集一次血液样本，持续6个小时，以评估胰岛素敏感性。

随后，研究人员让8名受试者服用大剂量的维生素D，这是每天服用复合维生素片的10倍，持续两个月。在研究结束时，他们再次接受了胰岛素抵抗测试。基肖尔说，结果“相当惊人”。“两个月的正常维生素D水平提高了肝脏对胰岛素的敏感度”——衡量肝脏对血液中胰岛素的反应能力——“37%”。最重要的是，当提取和研究脂肪组织的小样本时，Kishore发现，当维生素D水平上升时，巨噬细胞的活性会降低。

霍金斯对她的门徒的结果很热心。“这些研究的数据表明，对于有胰岛素抵抗的人来说，维生素D可以起到很大的作用，”她说。“这是一个廉价、安全、有效的解决方案，可能会产生真正的影响。”

　

　

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The Role of Vitamin D in Type 2 Diabetes

Can it help overcome insulin resistance?

 

Vitamin D is powerful stuff. The body usually produces its own, spurred on by sunlight. A century ago, people who couldn't get enough vitamin D—usually those living in northern latitudes where sunlight is scarce all winter long—risked developing rickets. Today, although many foods are fortified with vitamin D, Americans can find it surprisingly hard to get enough. For people working office jobs—or used to slathering on SPF 30 sunscreen to block the sun's rays—sunlight isn't always enough.

 

In recent years, researchers have linked low vitamin D levels to insulin resistance and diabetes. Overcoming insulin resistance, in particular, could be a way to head off type 2 diabetes before it sets in. "Right now, there's a lot of circumstantial evidence to suggest that giving people vitamin D might help with insulin resistance," says Preeti Kishore, MB, BS, an assistant professor at the Albert Einstein College of Medicine in New York.

 

But while circumstantial evidence might be good enough for a court of law, scientists want more—hard numbers that show one treatment is better than another. Kishore's research into vitamin D is funded by a grant to her mentor, Meredith Hawkins, MD, from the American Diabetes Association (supported by unrestricted funds from Merck). She is taking a close look at a type of cell called a macrophage.

 

Macrophages are specialized immune cells that attack invaders and, researchers now think, may have a secondary function as a cleanup crew. When fat cells get too large, they die, and macrophages move in to eliminate the dead tissue. That, Kishore says, may be why the cells are overrepresented in fatty tissue—and why inflammation, a sign that macrophages are at work, is often more severe in people who are overweight or obese.

 

For people with diabetes, the latest research suggests macrophage activity can have an added drawback. Macrophages at work produce chemicals called cytokines, some of which serve as signals to other parts of the body. "They can impair insulin action in the liver and muscle," Kishore says. "Higher cytokines means more insulin resistance," a key factor in type 2 diabetes.

 

Kishore is working to find a way to turn down the activity of macrophages in the body. After noticing that macrophages have special receptors for vitamin D, Kishore decided to look at whether vitamin D deficiency might be making macrophages more active—contributing to insulin resistance and inflammation in the process. "When you have less vitamin D, the macrophages are in a more active state," Kishore says. "We believe when you give people vitamin D, these inflammatory reactions will be reduced."

 

It was a research field that Hawkins was eager to encourage. "This is an incredibly important topic—there's been a lot of controversy," says Hawkins, a professor at the Albert Einstein College of Medicine and director of the Global Diabetes Initiative. "We really don't have evidence about the role of vitamin D deficiency in diabetes."

Kishore started with a small group of people who had too little vitamin D in their bodies and were also insulin resistant (but did not have diabetes). After a brief initial screening for insulin resistance and vitamin D deficiency, participants in the study were hooked up with IVs in both arms that took blood samples every five minutes for six hours to assess insulin sensitivity.

 

The eight people in the study were then given large doses of vitamin D, 10 times as much as you might get from a daily multivitamin pill, every day for two months. At the end of the study, they were tested again for insulin resistance. The results were "pretty remarkable," Kishore says. "Two months of normal vitamin D levels improved hepatic insulin sensitivity"—the measure of the liver's responsiveness to insulin in the blood—"by 37 percent." On top of that, when small samples of fat tissue were removed and studied, Kishore found that macrophages were less active when vitamin D levels went up.

 

Hawkins is enthusiastic about her protégée's results. "The data from these studies indicate that for people with insulin resistance, vitamin D can make a big difference," she says. "This is a cheap, safe, effective solution that could make a real impact."

http://www.diabetesforecast.org/2011/dec/the-role-of-vitamin-d-in-type-2-diabetes.html