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.'

太陽光作為維生素D的一個來源不但完全免費，而且非常充裕，至少對地球上多數地區來說是這樣。各種補充添加劑也差不多同樣價廉物美。不過，有越來越多的專家認為，許多人體內的維生素D含量不足，特別是那些整天在室內工作娛樂、層層涂抹防曬劑的人更是如此。而且，專家們對維生素D的了解越多，攝取不足的危害似乎也越讓人擔心。

人們早就知道維生素D對強健骨骼至關重要。但最新研究發現，它還能幫助人體抵御多種疾病的襲擾。Archives of Internal Medicine上個月的一項研究發現，維生素D攝取不足的男性罹患心臟病的風險更高，其他研究還揭示出，維生素D不足與乳腺癌、卵巢癌、前列腺癌、胃癌、膀胱癌、食道癌、腎癌和肺癌有關。此外高血壓、中風、糖尿病、牙周疾病、風濕性關節炎、多發性硬化癥、黃斑部病變、精神疾病以及慢性疼痛的發生也與人體內維生素D水平過低脫不開干系。

非營利組織維生素D協會(Vitamin D Council)創始人約翰•康奈爾(John J. Cannell)說，這聽起來彷佛一派胡言，但如果你了解到維生素D會轉化成一種參與維持人體200多種基因的類固醇激素，也許你就不會這么驚訝了。

目前為止，維生素D最有效的來源是陽光中的紫外線B(UVB)，它能將人體皮膚中的一種膽固醇轉化成維生素D。如果一個人在紐約戶外穿著泳衣曬20分鐘日光浴，那么他體內合成的維生素D比喝200杯牛奶吸收的維生素D還要多。不過，UVB強度會隨地理緯度、云層覆蓋情況、季節和一天具體時間的不同而發生明顯變化。在每年11月至次年2月之間，北緯42度（即從加州北部邊界至波士頓連線）以北地區的陽光就不足以幫助人們合成足量的維生素D。研究人員對緯度和發病率進行研究后發現了一些頗能引人深思的規律。比如，古巴等國的1型糖尿病患者就比新西蘭和瑞典等國少得多。

皮膚黝黑者和老年人借助日光合成維生素D的能力弱于年輕人和膚色較淺的人。而且UVB不能穿透防曬系數在8以上的玻璃或遮陽鏡。

人們很難從飲食里攝取大量維生素D。直接含有維生素D的食物很少，只有鮭魚、鯖魚、金槍魚等高脂肪魚類，以及動物的肝臟和蛋黃。從上世紀30年代開始，美國就在大部分牛奶中添加維生素D，以預防軟骨病。

人們普遍接受的一個觀點是，大多數人都需要補充一定量的維生素D，問題是，該補充多少？美國1997年頒布的指導意見是，從出生到50歲每天應補充200國際單位(IU)，51-70歲補充400IU，71歲以上補充600IU。但許多專家以及美國醫藥協會(American Medical Association)都在敦促政府參照最新研究結果對該標準進行修訂，Institute of Medicine正在討論如何著手。

而一些組織已不愿再坐等。美國兒科學會(American Academy of Pediatrics)表示，母乳喂養的嬰兒每天需補充400IU的維生素D。全美骨質疏松癥基金會(National Osteoporosis Foundation)則建議50歲以上的成年人每天至少補充800-1,000IU以防發生骨折。還有一點，維生素D3比D2的效果更好。

你可以通過測血來了解體內的維生素D水平，檢測過程大約需100美元。應測量25-羥基維生素D指標，而不是1,25-二羥基維生素D。如果每毫升血液中前者的含量達不到20毫微克就屬于偏低水平。含量在30毫微克/毫升以上時就能產生抗癌能力。華盛頓內科醫生艾倫•波辛基(Alan Pocinki)一直在對他的病人進行測試，他們大多是從事室內工作的白領。波辛基發現，大約半數病人的25-羥基維生素D含量低于20毫微克/毫升。

借助日光合成維生素D時不存在過量的問題，因為如果體內含量已較高，新合成的維生素D量會相應減少。對口服補充維生素D的人，Institute of Medicine的建議是不要超過每天2,000IU。但有些專家認為這個上限太低。維生素D可能產生的毒副反應包括腎結石和心律不齊，但非常罕見。

以上這些有關維生素D的討論是否已經讓你頭昏腦脹了？美國癌癥協會(American Cancer Society)藥物副總監蘭恩•利奇費爾德(Len Lichtenfeld)說，許多人被各種相互矛盾的信息給搞糊涂了。該協會建議人們通過服用藥物補充維生素D，而不是靠曬太陽。利奇費爾德說，我們需要對維生素D對健康的好處以及適宜水平等問題有更多的了解；美國人在人種、地區分布和敏感度方面區別很大，不可能拿出一個一刀切的推薦意見。