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Your mother made you take them. Many doctors agree. Even the Flintstones seem to endorse them. But do you really need a multivitamin?
The answer is ... probably not, although much depends on your age, gender, diet and health. One thing is certain: A one-size-fits-all multivitamin can't precisely meet everyone's needs. People over 50 need extra vitamin B-12, but not as much iron as many multivitamins contain. Children who take adult multivitamins may be getting too much vitamin A. And many women need a separate supplement to get extra calcium, since it's too bulky to fit into a multivitamin.
No wonder the vitamin aisle is so confusing, as manufacturers tailor products for different population segments. One-A-Day brand alone offers 14 versions, from Men's 50+ Advantage to Vitacrave Gummies for kids.
Reading labels can lead to even more confusion. Recommended Dietary Allowances (RDA) for vitamins and minerals, which are set by the independent Institute of Medicine, differ by an individual's age and gender. But for convenience, the Food and Drug Administration requires dietary supplements to list a single Daily Value, usually the highest needed.
For some people, that's too high: The RDA for iron, for example, is 8 milligrams for men and postmenopausal women. But the Daily Value on supplement labels is 18 milligrams, the amount recommended for women of childbearing age. What's more, the Daily Values haven't been updated since 1968, even though the institute's recommendations for some vitamins and minerals have changed.
'In some cases, they are wildly off,' says Paul Coates, director of the National Institute of Health's Office of Dietary Supplements. An FDA spokeswoman said the agency is considering revisions.
There is no standard multivitamin formula. The term applies to any combination of vitamins and minerals in any strength, as long as they are listed on the label.
The label may not even exactly match what's in the bottle. ConsumerLab, a supplement-testing company, reported last week that 10 of 38 multivitamin brands tested contained either more or less of some ingredients than the label indicated.
And price was no predictor of quality. Some multivitamins selling for less than 10 cents a day performed better on the tests than those selling for 50 cents or more.
The Council for Responsible Nutrition, a trade group that represents most multivitamin and other supplement makers, says, 'It concerns us anytime ConsumerLab says we don't meet label claims,' says CRN's chief executive and president. Steven Mister. 'But there is nothing in the report that suggests that consumers are getting levels that will do them harm.'
Are multivitamins even necessary? The dietary supplement industry likens them to nutritional insurance—filling in gaps when people don't eat perfectly balanced meals.
That rationale has helped make multivitamins, introduced in the 1940s, the best-selling dietary supplement, with more than $4.8 billion in sales in 2008, according to Nutrition Business Journal. A third of American adults take them regularly. Use is particularly high among women, children, physicians, the elderly and people with high incomes, low body-mass indexes and healthy eating habits, according to government surveys.
Given those traits and demographics, people most likely to benefit from multivitamins—due to unhealthy habits—also are least likely to take them. Conversely, because people who take multivitamins tend to have other healthy habits, it's difficult to prove what effect multivitamins have, if any, in overall health.
Looking at randomized controlled trials, the gold standard for determining cause-and-effect, a NIH panel in 2007 concluded that 'the present evidence is insufficient to recommend either for or against the use of [multivitamins and minerals] by the American public to prevent chronic disease.' A 2003 U.S. Preventive Services Task Force report found insufficient evidence either for or against taking multivitamins to prevent cancer or cardiovascular disease.
The dietary-supplement trade group says observational studies have shown benefits, including a lower risk of cataracts and colon cancer, and fewer colds.
Such studies, though, can't determine cause and effect, only association. And several large observational trials have come to opposite conclusions.
A study in the American Journal of Epidemiology this year that followed 182,000 people in Hawaii and California for 11 years found no association between multivitamin use and deaths from cancer, cardiovascular disease or any cause.
Compared with over-the-counter and prescription drugs, dietary supplements are lightly regulated. Makers don't need to demonstrate that they are safe or effective. But there are limits to what they can claim. Ads that use terms such as 'support' breast health or mental alertness must note that such statements haven't been evaluated by the FDA and that the product isn't intended to diagnose, treat, cure or prevent any disease.
Still, a scientific consensus has emerged that some groups of people do require more of certain nutrients than they are likely to get from food. Women who might become pregnant should get an additional 400 micrograms a day of folic acid, to reduce the risk of serious neurological issues in a fetus that may occur even before a woman knows she is pregnant.
People over age 50 should get the recommended 2.4 micrograms of vitamin B-12 from supplements or fortified foods because they become less able to absorb it from food as they age. Vegetarians and vegans also need extra B-12.
Infants who are being breastfed should get 400 International Units (IUs) a day of supplement vitamin D until they are weaned. In fact, most Americans need 600 IUs per day, according to a 2010 report from the Institute of Medicine. People who get minimal sun exposure and don't consume much salmon or milk also should supplement their Vitamin D.
On the other hand, some people may get too much of certain nutrients, depending on their age and health, particularly if they use the current Daily Values listed on labels as a guide. For example, 100% of the current Daily Value for vitamin A is 5,000 IUs-which is over the Institute of Medicine's safe upper limit of 2,000 IUs for children and far beyond the RDA for children ages 1 through 3 of just 1,000 IUs. Excess vitamin A can cause headaches, hair loss, visual disturbances and a possibly increased risk of osteoporosis. And smokers should avoid taking extra beta carotene, which has been linked to increased risk of lung cancer.
The consumer-advocacy group Center for Science in the Public Interest has asked the FDA to require warning labels on multivitamins and other supplements for some of these conditions.
Officials at the dietary-supplement trade group say evidence of the need for such warnings is scant, and the upper limit for vitamins is still safe. 'That's not to say that if you cross that threshold, you will have a fatal condition,' says Duffy MacKay, a naturopathic doctor and CRN vice president.
Given all the complexities, it pays to discuss individual needs with a doctor, dietitian or other health professional. Some people may find the à la carte approach works best. Others may like the extra 'insurance' provided by a multivitamin targeted to their age and sex.
Dr. Coates, of the NIH, says based on the best evidence, 'If you are taking multivitamins, there is no reason to stop, and if you are not taking them, there is no reason to start. You are unlikely to harm yourself, whatever you're doing.'
參考譯文：
你媽媽非得讓你吃這些東西。很多醫生也贊同。就連動畫節目《摩登原始人》(Flintstones)似乎也認可它們。但你真的需要服用復合維他命劑嗎？
答案是……或許用不著，不過在很大程度上也要取決于你的年齡、性別、飲食和健康狀況。有一件事是確定的：適合所有人的通用復合維他命劑肯定無法精確地滿足每個人的需要。50歲以上的人需要額外補充維他命B-12，但不少的復合維他命劑中所含的鐵元素對他們來說又過量了。兒童如果服用針對成人的復合維他命，就可能攝入過量維他命A。而很多女性需要單獨補鈣，因為鈣的體積太大，無法合成到復合維他命劑當中。
怪不得林林總總的維他命產品讓人如此困惑，因為生產廠商都會針對不同的人群定制產品。光是One-A-Day這個牌子就有14種不同的維他命產品，從50歲以上男性綜合維他命(Men's 50+ Advantage)到針對兒童的復合維他命咀嚼軟糖(Vitacrave Gummies)等等。
閱讀產品上的標簽可能讓人更犯暈。維他命和礦物質的每日膳食推薦攝取量(Recommended Dietary Allowances)是由獨立的美國醫學研究院(Institute of Medicine)設定的，根據個人的年齡和性別而異。但為了方便起見，美國食品和藥物管理局(Food and Drug Administration)要求膳食補充劑列出一個單一的每日攝入量，通常是所需的最高值。
對于一些人來說這個量就過頭了：比如說男性和絕經后女性的鐵元素每日推薦攝取量是8毫克。但補充劑標簽上標明的每日攝入量為18毫克，這是針對育齡婦女的推薦攝入量。此外，每日攝入量自1968年以來就沒有更新過，雖然美國醫學研究院對于某些種類的維他命和礦物質的推薦攝入量已經有所改變。
美國國家衛生研究院(National Institutes of Health) 下屬的膳食補充劑辦公室(Office of Dietary Supplements)主任保羅•高特斯(Paul Coates)說，在有的情況下二者相差非常大。美國食品和藥物管理局發言人說，該機構正在考慮修訂事宜。
復合維他命劑的配方并沒有通用標準。只要是含有多種維他命和礦物質，無論其含量多少，都可以叫這個名字，只要標簽上列明所含成份即可。
標簽上所列的甚至都未必與瓶子里的內容完全相符。對補充劑進行檢測的公司消費者實驗室(Consumer Lab)上周報告說，其檢測的38個不同品牌的復合維他命劑當中，有10個品牌的產品所含成份的量高于或低于其標簽標示的量。
還有，一分錢也未必就代表著一分貨。在檢測中，售價折合每日成本不到10美分的一些復合維他命劑的表現要好于每日成本在50美分甚至更高的產品。
代表大多數復合維他命劑和其他膳食補充劑生產商的行業組織誠信保健品協會(Council for Responsible Nutrition)說，消費者實驗室每次一說我們的產品與標簽所示不符，我們都會十分關注。但報告中并沒有顯示已經到了會有害于消費者的程度。
復合維他命劑是否確實必要？膳食補充劑行業將它們比作營養保險──當人們的一日三餐沒有達到理想的均衡時，復合維他命劑就會填補其中的攝入不足的部分。
這種理論讓上世紀40年代興起的復合維他命劑成為最暢銷的膳食補充劑。據《營養商業期刊》(Nutrition Business Journal)報導，2008年復合維他命劑的銷售額超過48億美元。三分之一的美國成年人經常服用復合維他命劑。政府調查顯示，女性、兒童、醫生、老年人服用的尤其多，此外還有收入高、體重系數低、注重健康飲食習慣的人群。
考慮到上述特色和人口統計資料，最有可能從復合維他命劑中受益的人群同時也是最不可能服用的，原因是他們不健康的習慣。相反，由于服用復合維他命劑的人往往在其他方面也保持著健康的習慣，因此很難證明復合維他命劑對于總體健康狀況有何種作用、甚至到底有沒有作用。
通過號稱確定因果關系黃金標準的隨機對照實驗，美國國家衛生研究院的一個專家小組在2007年得出結論，認為當前的證據不足以推薦美國公眾為預防慢性疾病而服用或不要服用復合維他命劑和礦物質。2003年美國聯邦預防醫學工作組(Preventive Services Task Force)的一份報告也發現，，對于預防癌癥或心血管疾病，是服用還是不要服用復合維他命劑的問題，尚無足夠的證據做出判斷。
膳食補充劑行業組織宣稱，觀察性研究已經表明服用復合維他命劑對人體有益，包括降低白內障和結腸癌的風險，以及更少患上感冒。
不過這類研究無法證明其中的因果關系，只是表明二者相關聯。而且有幾項大規模的觀察試驗還得出了相反的結論。
《美國流行病學期刊》(American Journal of Epidemiology)今年刊登的一項研究在11年的時間里跟蹤了夏威夷和加利福尼亞州的18.2萬人，結果發現服用復合維他命劑與癌癥、心血管疾病或任何原因引起的死亡之間都不存在相關關系。
相比非處方藥和處方藥，膳食補充劑沒有受到太多監管。生產商并不需要證明它們安全或有效。但生產商宣稱其產品能達到何種效果則受到限制。使用“有助于”乳腺健康或頭腦清醒這類說法的廣告必須注明，這些說法并未經食品和藥物管理局評估，同時其產品也并非用于診斷、治療、治愈或預防任何疾病。
不過，科學界已經有一致的意見，認為一些群體在能從食物中攝入的營養成份之外確實還需要補充特定的營養。可能懷孕的女性應當每日補充400微克的葉酸，以減少胎兒出現嚴重神經系統問題的風險，這種問題甚至有可能在女性知曉自己已經懷孕之前就會出現。
50歲以上的人應當從補充劑或強化食品中攝入每日推薦的2.4微克維他命B-12，因為隨著身體老化，他們從食物中吸收這種成份的能力會減弱。素食主義者也需要額外補充B-12。
母乳喂養的嬰幼兒在斷奶之前應每日補充400國際單位的維他命D。事實上據美國醫學研究院2010年的一份報告，大多數美國人每天都需要600國際單位的維他命D。平時很少曬太陽、也不怎么吃三文魚或者喝牛奶的人也應當補充維他命D。
另一方面，根據人們的年齡和健康狀況，一些人攝入的某些營養成份可能過量，尤其是如果他們以目前維他命產品標簽上所列的每日攝入量作為依據的話。比如說，當前的維他命A每日攝入量全都寫著5000國際單位，這就超過了美國醫學研究院為兒童設定的2000國際單位的安全上限，更是遠遠高于食品和藥物管理局針對1-3歲兒童設定的1000國際單位。過量攝入維他命A可能導致頭痛、脫發、視力障礙，并可能加大骨質疏松癥的風險。抽煙者應當避免攝入過量的β胡蘿卜素，過量攝入已經被證實與肺癌風險升高有關。
消費者權益組織公眾營養健康科學中心(Center for Science in the Public Interest)已經要求食品和藥物管理局強制規定在復合維他命和其他補充劑產品上為上述一些情況設置警示標簽。
膳食補充劑行業組織的官員說，能夠證明有必要標示這種警告的證據不足，而且維他命攝入量的上限仍然是安全的。自然療法醫生、誠信保健品協會的副會長達菲•麥基(Duffy MacKay)說，這并不是說一旦你越過這個界限，就會出現致命的情況。
考慮到所有這些復雜情況，與醫生、營養師或其他健康專業人士討論一下個人需求還是有好處的。一些人或許會覺得缺哪樣就補哪樣的辦法最有效。還有的人則可能喜歡針對其年齡和性別的復合維他命所提供的額外“保險”。
美國國家衛生研究院的高特斯說，照最佳證據來看，如果你正在服用復合維他命，沒有理由停止服用，而如果你沒有服用，也沒有必要開始吃。不管怎樣你都不太可能會傷到自己。
