Sunlight, as a source of vitamin D, is free and abundant, at least in most of the world. Supplements are almost as cheap. Yet a growing number of experts think that many people aren't getting enough vitamin D -- particularly those who work and play indoors and slather on sunscreen. And the more experts learn about what D does, the more worrisome a deficiency seems.

It's long been known that D is crucial for strong bones. But new research suggests that it also protects against a wide variety of diseases. A study in the Archives of Internal Medicine last month found that men with low D had a higher risk for heart attacks. Other studies have linked low D with cancer of the breast, ovary, prostate, stomach, bladder, esophagus, kidney and lung. Low levels of D also have been associated with high blood pressure, stroke, diabetes, periodontal disease, rheumatoid arthritis, multiple sclerosis, macular degeneration, mental illness and chronic pain.

'It sounds crazy -- until you realize that vitamin D turns into a steroid hormone that's involved in the maintenance of over 200 human genes,' says John J. Cannell, founder of the nonprofit Vitamin D Council in Atascadero, Calif.

The strongest source, by far, is ultraviolet B rays from the sun, which convert a form of cholesterol into vitamin D in the skin. A person sitting outside in a bathing suit in New York City gets more vitamin D in 20 minutes than from drinking 200 glasses of milk. But UVB rays vary greatly depending on latitude, cloud cover, time of year and time of day. Above 42 degrees north latitude (a line from the northern California border to Boston), the sun's rays don't provide sufficient D from November through February. Researchers looking at latitude and disease rates have found some intriguing patterns. Type 1 diabetes, for example, is much rarer in countries like Cuba than in either New Zealand and Sweden.

Dark-skinned and elderly people don't process vitamin D from the sun as efficiently as younger, fair-skinned people. UVB rays also don't penetrate glass or sunscreen with a factor of 8 or more.

It's difficult to get much D through diet. Few foods contain it naturally -- mainly fatty fish like salmon, mackerel and tuna, as well as liver and egg yolks. Since the 1930s, most milk in the U.S. has been fortified with D to prevent rickets, a bone-softening disease.

It's widely accepted that most people need some supplemental D -- the question is, how much? Current U.S. guidelines, issued in 1997, call for 200 international units from birth through age 50; 400 IUs from 51 through age 70 and 600 IUs from 71 on. But many experts and the American Medical Association are urging the government to revisit those numbers in light of the latest research, and the Institute of Medicine is in discussions to do so.

Some groups aren't waiting. The American Academy of Pediatrics says breast-fed infants should get 400 IUs of supplemental D daily. The National Osteoporosis Foundation urges adults over age 50 to get at least 800 to 1,000 IUs to prevent fractures. Look for D3, which is more potent than D2.

You can learn your vitamin D level with a blood test, which costs about $100. It should measure 25-hydroxyvitamin D, not 1,25-dihydroxyvitamin D. A concentration of less than 20 nanograms per milliliter is considered deficient. Many cancer-prevention benefits have been seen at levels of 30 ng/ml or more. Alan Pocinki, an internist in Washington, D.C., has been testing his patients -- mostly white-collar workers with indoor jobs -- and found about half are below 20 ng/ml.

It's not possible to overdose on vitamin D from the sun; it degrades in the skin at high levels. For supplements, the Institute of Medicine set an upper limit of 2,000 IUs per day, but some experts think that's too low. Vitamin D toxicity can involve kidney stones and heart arrhythmias, but cases are very rare.

Does all this D-iscussion leave you D-izzy? 'Many people are confused by the conflicting messages,' says Len Lichtenfeld, deputy chief medical officer for the American Cancer Society, which suggests getting D from supplements, not the sun. 'We need to know more about what the health benefits are, and what the right level is. We're a country of different ethnicities and locations and sensitivities. We probably can't make a one-size-fits-all recommendation.'

太阳光作为维生素D的一个来源不但完全免费，而且非常充裕，至少对地球上多数地区来说是这样。各种补充添加剂也差不多同样价廉物美。不过，有越来越多的专家认为，许多人体内的维生素D含量不足，特别是那些整天在室内工作娱乐、层层涂抹防晒剂的人更是如此。而且，专家们对维生素D的了解越多，摄取不足的危害似乎也越让人担心。

人们早就知道维生素D对强健骨骼至关重要。但最新研究发现，它还能帮助人体抵御多种疾病的袭扰。Archives of Internal Medicine上个月的一项研究发现，维生素D摄取不足的男性罹患心脏病的风险更高，其他研究还揭示出，维生素D不足与乳腺癌、卵巢癌、前列腺癌、胃癌、膀胱癌、食道癌、肾癌和肺癌有关。此外高血压、中风、糖尿病、牙周疾病、风湿性关节炎、多发性硬化症、黄斑部病变、精神疾病以及慢性疼痛的发生也与人体内维生素D水平过低脱不开干系。

非营利组织维生素D协会(Vitamin D Council)创始人约翰•康奈尔(John J. Cannell)说，这听起来彷佛一派胡言，但如果你了解到维生素D会转化成一种参与维持人体200多种基因的类固醇激素，也许你就不会这么惊讶了。

目前为止，维生素D最有效的来源是阳光中的紫外线B(UVB)，它能将人体皮肤中的一种胆固醇转化成维生素D。如果一个人在纽约户外穿着泳衣晒20分钟日光浴，那么他体内合成的维生素D比喝200杯牛奶吸收的维生素D还要多。不过，UVB强度会随地理纬度、云层覆盖情况、季节和一天具体时间的不同而发生明显变化。在每年11月至次年2月之间，北纬42度（即从加州北部边界至波士顿连线）以北地区的阳光就不足以帮助人们合成足量的维生素D。研究人员对纬度和发病率进行研究后发现了一些颇能引人深思的规律。比如，古巴等国的1型糖尿病患者就比新西兰和瑞典等国少得多。

皮肤黝黑者和老年人借助日光合成维生素D的能力弱于年轻人和肤色较浅的人。而且UVB不能穿透防晒系数在8以上的玻璃或遮阳镜。

人们很难从饮食里摄取大量维生素D。直接含有维生素D的食物很少，只有鲑鱼、鲭鱼、金枪鱼等高脂肪鱼类，以及动物的肝脏和蛋黄。从上世纪30年代开始，美国就在大部分牛奶中添加维生素D，以预防软骨病。

人们普遍接受的一个观点是，大多数人都需要补充一定量的维生素D，问题是，该补充多少？美国1997年颁布的指导意见是，从出生到50岁每天应补充200国际单位(IU)，51-70岁补充400IU，71岁以上补充600IU。但许多专家以及美国医药协会(American Medical Association)都在敦促政府参照最新研究结果对该标准进行修订，Institute of Medicine正在讨论如何着手。

而一些组织已不愿再坐等。美国儿科学会(American Academy of Pediatrics)表示，母乳喂养的婴儿每天需补充400IU的维生素D。全美骨质疏松症基金会(National Osteoporosis Foundation)则建议50岁以上的成年人每天至少补充800-1,000IU以防发生骨折。还有一点，维生素D3比D2的效果更好。

你可以通过测血来了解体内的维生素D水平，检测过程大约需100美元。应测量25-羟基维生素D指标，而不是1,25-二羟基维生素D。如果每毫升血液中前者的含量达不到20毫微克就属于偏低水平。含量在30毫微克/毫升以上时就能产生抗癌能力。华盛顿内科医生艾伦•波辛基(Alan Pocinki)一直在对他的病人进行测试，他们大多是从事室内工作的白领。波辛基发现，大约半数病人的25-羟基维生素D含量低于20毫微克/毫升。

借助日光合成维生素D时不存在过量的问题，因为如果体内含量已较高，新合成的维生素D量会相应减少。对口服补充维生素D的人，Institute of Medicine的建议是不要超过每天2,000IU。但有些专家认为这个上限太低。维生素D可能产生的毒副反应包括肾结石和心律不齐，但非常罕见。

以上这些有关维生素D的讨论是否已经让你头昏脑胀了？美国癌症协会(American Cancer Society)药物副总监兰恩•利奇费尔德(Len Lichtenfeld)说，许多人被各种相互矛盾的信息给搞糊涂了。该协会建议人们通过服用药物补充维生素D，而不是靠晒太阳。利奇费尔德说，我们需要对维生素D对健康的好处以及适宜水平等问题有更多的了解；美国人在人种、地区分布和敏感度方面区别很大，不可能拿出一个一刀切的推荐意见。