24Drs好健康報:好健康熱頭條 ─葉酸未必能避免大腸癌
好健康熱頭條 ─葉酸未必能避免大腸癌 Folic Acid May Not Avert Colon Cancer
好健康小單字 –葉酸 (Folic Acid)
好健康熱頭條 ─葉酸未必能避免大腸癌
一般的葉酸補充劑不能預防大腸癌，反而可能會增加成人罹患其他癌症的風險。
主導動物研究的研究者認為，葉酸可以讓人免於大腸癌，這種觀念得到護士健康研究極大的認同，這篇研究發現，攝取最高量葉酸的婦女最不容易罹患大腸癌。
葉酸的效果不僅於此，沒有證據但有跡象顯示補充劑可能也會減低中風和心臟疾病的風險；基於這些原因，許多美國人都開始服用不貴的葉酸補充劑或含有葉酸的綜合維他命。
國家衛生研究所為了找出葉酸是否有預防大腸癌的能力，贊助一項人數超過1千名美國男女的臨床試驗，參與者都切除過直腸息肉，這些息肉若沒有處理會變成癌症。
研究參與者被隨機分配每日服用含有1毫克葉酸的藥丸或是安慰劑，患者也服用低劑量的阿斯匹靈、正常劑量的阿斯匹靈、或是安慰劑。結果呢？北卡羅來納大學教堂山分校醫學系的胃腸肝臟部主任Robert Sandler醫師表示，服用葉酸的人所生息肉就像服用安慰劑的人一樣多。
Sandler醫師向WebMD表示，他們對於葉酸沒有效用感到失望和驚訝，事實上，有一些證據顯示，葉酸會增加癌症風險。
增加癌症風險的跡象並未達到研究人員所謂「統計上顯著差異」，代表可能是機率問題；但是，研究結果令人不安，大部分增加的風險是前列腺癌，服用葉酸的男性中，罹患前列腺癌的機率是7.3%，超過安慰劑組的2.8%。
Sandler醫師表示，這可能是因為葉酸有助於前列腺癌的生長，之前有另一項研究也有類似的結果，但結果並未達統計上的意義，儘管如此，我們現在有兩個實驗顯示，葉酸可能會增加前列腺癌的風險。
Sandler醫師、Dartmouth研究員Bernard F. Cole博士和同事們在6月6日出刊的美國醫學協會期刊（The Journal of the American Medical Association）發表研究結果。
【葉酸的重要】
葉酸是成長中的胎兒急需的營養成分，為了保證孕婦獲得足夠的葉酸，美國和其他已開發國家通常將它添加在麵粉中；Sandler醫師表示，這些少量的葉酸並不重要，所有的研究參與者還是要吃添加大量葉酸的食物，從補充劑中獲取的葉酸含量遠大於從富含葉酸的食品中所得到的。
對嬰兒有益的東西怎麼會對大人不好昵？這篇葉酸研究的評論提出，所有的問題在於時機， Fred Hutchinson癌症研究中心和西雅圖華盛頓大學的評論者Cornelia M. Ulrich博士已經研究葉酸超過十年了。
Ulrich博士向WebMD表示，當我們年輕時，服用葉酸是有益的，因為它能減少基因突變，但隨著我們年紀漸長，大腸中有越來越多的異常物，像是息肉等，葉酸在這個階段可能會增加這些異常物的成長。
Ulrich博士指出，由Cole博士、Sandler醫師和同事們所做的研究，僅注意到研究開始前就經由大腸鏡檢查切除息肉的人，這種人顯然有異常物在大腸中，她表示，葉酸是否可預防大腸正常人的罹患癌症仍是個懸而未決的問題。
但誰的大腸正常？幾乎所有的大腸癌都發生在50歲以上，但是做過大腸鏡篩檢的50歲以上成年人不到三分之一，大腸鏡篩檢仍是了解是否有息肉的主要方法；估計有30%超過60歲的人有息肉，而這些人可能會因為服用葉酸補充劑而有風險。
Ulrich博士表示，只要你年紀大並有息肉，從葉酸似乎就無法有對癌症的好處，反而可能有傷害，有息肉的人依照這個試驗不應該服用葉酸。
Sandler醫師表示，沒有維他命或補充劑被證實可預防癌症；他指出，從他們研究所得到的訊息是，人們應該謹慎服用補充劑來預防疾病，例如癌症等，因為這些療法可能無效，並且可能是有害的。
Sandler醫師指出，每個人從50歲開始就應該定期篩檢大腸癌，雖然大腸鏡可能是了解是否有風險最可靠的方法，但不同人可能喜歡不同的篩檢方法。
Folic Acid May Not Avert Colon Cancer
Popular folic acid supplements fail to protect against colon cancer, but they may increase an adult's risk of other cancers.
Animal studies led researchers to think that folic acid might protect people against colon cancer. That belief got huge support from the Nurses Health Study, which found that women with the highest folic acid intake were least likely to get colon cancer.
And that's not all folic acid is supposed to do. There's evidence -- but no proof -- that the supplement also may cut a person's risk of stroke and heart disease.
For these reasons, many Americans have begun taking inexpensive folic acid supplements or multivitamins that contain folic acid.
To find out whether folic acid has colon-cancer-preventing powers, the National Institutes of Health funded a clinical trial that enrolled more than a thousand U.S. men and women who previously had polyps removed from their colons. Left untreated, these polyps can become cancerous.
Study participants were randomly assigned to take daily pills containing either 1 milligram of folic acid or an inactive placebo. Patients also took low-dose aspirin, regular-dose aspirin, or placebo.
What happened? People who took folic acid got just as many new colon polyps as those who took placebo pills, reports researcher Robert Sandler, MD, chief of the division of gastroenterology and hepatology at the University of North Carolina at Chapel Hill School of Medicine.
"We are disappointed and surprised that it didn't work. In fact, there was some evidence that folic acid increased cancer risk," Sandler tells WebMD.
That evidence of increased cancer risk isn't what researchers call "statistically significant" -- meaning it could be a chance finding. Still, the finding is disturbing. Most of the increased risk came from prostate cancer. Men who took folic acid had a 7.3% chance of getting prostate cancer -- more than the 2.8% risk seen in the placebo group.
"It could be that folate helps prostate cancer to grow," Sandler says. "Another study suggested this previously, but that finding did not reach statistical significance. Nevertheless, now we have two experiments that suggest that folic acid might increase the risk for prostate cancer."
Sandler, Dartmouth researcher Bernard F. Cole, PhD, and colleagues report the findings in the June 6 issue of The Journal of the American Medical Association.
Folic Acid Concern
Folic acid is desperately needed by a developing fetus. In order to ensure that pregnant women get enough folic acid, it is routinely added to flour in the U.S. and in other developed nations.
Sandler says these small amounts of folic acid are not a concern. All of the people in the study continued to eat foods fortified with folic acid. The amount of folic acid people get in supplement pills is far larger than the amount they get from fortified foods.
How could something that is good for babies be bad for grown-ups? It's all a matter of timing, argues an editorial accompanying the folic acid study. Editorial co-author Cornelia M. Ulrich, PhD, of the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle, has studied folic acid for more than a decade.
"What happens is that folate is beneficial when taken when we are young because it decreases genetic mutations," Ulrich tells WebMD. "But as we get older there are more and more abnormalities in the colon -- things like polyps. And at that stage, folic acid probably increases the growth of these abnormalities."
Ulrich notes that the study by Cole, Sandler, and colleagues looked only at people who already had colon polyps removed by colonoscopy before the study began. Such people clearly had abnormalities in the colon. Whether folic acid might prevent cancer in people with normal colons remains an open question, she says.
But who has a normal colon? Nearly all colon cancers occur in people over age 50. Fewer than one in three over-50 adults has had a screening colonoscopy -- still the main way to know for sure whether or not one has polyps.
An estimated 30% of people over age 60 have polyps. And these are the people who might be at risk from taking folic acid supplements.
"Once you are older and have a polyp, there seems no cancer benefit -- and possible harm -- from folic acid," Ulrich says. "People who have had polyps, based on this trial, should not take folic acid."
Sandler says no vitamin or supplement has been proven to prevent cancer.
"The take-home message from our study is people should be cautious about taking supplements to prevent diseases such as cancer because those treatments might be ineffective and could be harmful," he says.
Sandler notes that everyone should get regular screening tests for colon cancer beginning at age 50. Though colonoscopy may be the surest way to know whether you are at risk, different patients may prefer different colon cancer screening tests.
SOURCES: Cole, B.F. Journal of the American Medical Association, June 6, 2007; vol 297: pp 2351-2359. Ulrich, C.M. and Potter, J.D. Journal of the American Medical Association, June 6, 2007; vol 297: pp 2408-2409. WebMD Medical News: "Folic Acid May Lower Stroke Risk." Robert Sandler, MD, chief, division of gastroenterology and hepatology, University of North Carolina at Chapel Hill School of Medicine. Cornelia M. Ulrich, PhD, associate member, Fred Hutchinson Cancer Research Center; associate professor of epidemiology, University of Washington, Seattle.
WebMD Medical News
By Daniel J. DeNoon
Reviewed by Louise Chang, MD
好健康小單字 –葉酸 (Folic Acid)
葉酸又成為維生素B9或維生素M，屬於水溶性維生素B群的一種，因存於綠葉中得此名，為製造紅血球不可或缺的物質，與維生素B12同為造血維生素，若攝取不足，會導致紅血球生成不完全，引起貧血、易疲倦、氣喘、浮腫等症狀，但無過量致毒疑慮。
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