硒、維他命E如何增加前列腺癌風險


  【24drs.com】SELECT(Selenium and Vitamin E Cancer Prevention Trial」研究,試圖確認硒和維他命E這些補充品是否可以預防前列腺癌,備受矚目的新資料顯示,這兩種抗氧化劑確實對男性有風險。
  
  如之前所報告的,硒或維他命E補充品對男性沒有預防效果;事實上,對於某些男性,這些補充品會增加前列腺癌風險。
  
  這篇新研究線上發表於2月22日美國國家癌症研究院期刊,探討服用這些補充品的男性中有哪些人的前列腺癌風險高與其原因。
  
  不過,研究者之一表示,該試驗的公衛資訊依舊是相同的。
  
  第一作者、西雅圖Fred Hutchinson癌症研究中心Alan Kristal博士在聲明中表示,使用這些補充品的男性應停用;硒或維他命E都沒有任何已知的助益,只有風險;許多人認為飲食補充有益、或至少沒有害處,這是錯誤的觀念。
  
  研究世代是SELECT試驗的4,856名男性,這篇大型第3期安慰劑控制試驗共有超過35,000名男性,被隨機指派服用高劑量維他命E (400 IU/天)和/或硒(200 μg/天)補充品。
  
  SELECT試驗始於2001年,預計進行12年,但是在2008年提早結束,當時研究對象平均服用這些補充品5年;研究結果顯示,硒沒有保護效果,維他命E會增加前列腺癌風險。
  
  雖然停止使用這些補充品,但該研究並未終止,繼續追蹤2年後,服用維他命E的男性,其前列腺癌風險在統計上顯著增加了17%,就如同之前的報告。
  
  值得注意的是,服用單一種補充品、或兩種都有服用者,其前列腺癌偵測率都比安慰劑組高(但只有維他命E組的差異達統計上的顯著意義)。
  
  硒是存在於植物如米、小麥、巴西堅果以及海鮮和肉類的非金屬微量元素,在之前的大型皮膚癌預防試驗中,硒和前列腺癌風險降低有關。
  
  根據國家癌症研究院指出,硒是一種可以幫助控制導致癌症之細胞損傷的抗氧化劑。
  
  維他命E廣泛存在於多種食物,特別是蔬菜、蔬菜油、堅果和蛋黃;維他命E和硒一樣被視為抗氧化劑。
  
  在這篇新的案例–世代研究中,1,739名男性在SELECT試驗期間被診斷有前列腺癌,另3,117名男性則無 。
  
  Kristal博士等人發現,在沒有使用補充品的情況下,開始時的硒濃度值和前列腺癌風險無關;不過,他們也發現,開始時的數值低和數值高的男性之間,補充品的影響大有差異;特別是的,硒補充品增加了開始時硒數值高之男性的前列腺癌風險。
  
  在SELECT試驗開始之前,即有證據認為硒補充品對於已經攝取足夠營養的男性並無幫助。
  
  因此,研究者在開始時測量研究對象腳趾甲的硒濃度。他們解釋,計畫檢視這個補充品是否只對那些開始時硒數值低的男性有助益。
  
  相反的,他們發現,開始時硒數值高的男性服用硒補充品會使高等級癌症風險增加達91% ( P= .007);換句話說,硒數值對於這些男性變成是有毒性的。
  
  研究者也報告指出,維他命E增加了男性的前列腺癌風險,但只有那些開始時硒數值低者;特別的是,硒數值低而被隨機指派接受維他命E的男性中,前列腺癌整體風險增加達63% ( P= .02),而高等級癌症風險增加達111% ( P= .01)。
  
  這或許可以解釋為何在2008年SELECT試驗的結果中,只有隨機指派接受維他命E之男性的前列腺癌風險高,而非同時接受維他命E和硒者。
  
  Kristal博士等人寫道,基礎科學有一些證據支持動力學概念;長久以來,維他命E和硒之間即被假設有交互作用,因為它們可以防止脂質過氧化。
  
  他們認為,硒不論來自飲食或補充品,可能可以保護男性來自維他命E的傷害,所以,低濃度的硒對於男性不一定有傷害。
  
  不過,研究者報告指出,這些新結果和探討補充品與癌症的醫學文獻一致,對於健康者並無好處。
  
  他們寫道,文獻認為,補充品的效果取決於使用者的營養狀態,這類補充品對於有足夠營養者,會導致超乎生理需求之曝露,因而無效益或增加癌症風險。
  
  資料來源:http://www.24drs.com/professional/list/content.asp?x_idno=7057&x_classno=0&x_chkdelpoint=Y
  

How Selenium, Vitamin E Increase Prostate Cancer Risk

By Nick Mulcahy
Medscape Medical News

New data from the much publicized Selenium and Vitamin E Cancer Prevention Trial (SELECT), which sought to determine whether these supplements could protect against the development of prostate cancer, confirm that both antioxidants can be risky business for men.

As previously reported, men receive no preventive benefit from either selenium or vitamin E supplements; in fact, for certain men, these supplements actually increased the risk for prostate cancer.

The new study, published online February 22 in the Journal of the National Cancer Institute, explored which men who take these supplements are most at risk for prostate cancer, and why.

However, the ongoing public health message from the trial remains the same, said a trial investigator.

"Men using these supplements should stop, period. Neither selenium nor vitamin E supplementation confer any known [health] benefits — only risks," said lead author Alan Kristal, DrPH, from the Fred Hutchinson Cancer Research Center in Seattle, in a press statement.

"Many people think that dietary supplements are helpful or at the least innocuous. This is not true," he added.

The cohort of 4856 men was culled from SELECT, the larger phase 3 placebo-controlled trial in which more than 35,000 men were randomized to high-dose vitamin E (400 IU/day) and/or selenium (200 μg/day) supplements.

SELECT began in 2001 and was expected to run for 12 years, but it was stopped early, in 2008, after participants had been on the supplements for an average of 5 years. The results demonstrated that there was no protective effect from selenium and suggested that vitamin E increased prostate cancer risk, as reported by Medscape Medical News.

Although the use of the supplements stopped, the study actually continued. After 2 years of follow-up, the men who took vitamin E had a statistically significant 17% increased risk for prostate cancer, as previously reported.

Notably, the rate of prostate cancer detection was higher in the groups that received either supplement alone or a combination of the 2 than in the placebo group (but the difference was significant only in the vitamin E group).

Selenium is a nonmetallic trace element found in plant in foods such as rice, wheat, and Brazil nuts, and in seafood and meat. In a previous large skin cancer prevention trial, it was associated with a reduced risk for prostate cancer. According to the National Cancer Institute, it is an antioxidant that might help control cell damage that can lead to cancer.

Vitamin E is found in a wide range of foods, especially vegetables, vegetable oils, nuts, and egg yolks. Like selenium, vitamin E is considered an antioxidant.

Key: Increased Risk Depends on Baseline Selenium

In this new case–cohort study, 1739 men diagnosed with prostate cancer during SELECT were compared with 3117 men who were not.

Dr. Kristal and colleagues found that baseline selenium status alone, in the absence of supplementation, was not associated with prostate cancer risk.

However, they also found that the effects of the supplements differed substantially between men with low levels at baseline and those with high levels.

Specifically, selenium supplementation increased the risk for prostate cancer in men who already had high selenium levels at baseline.

Before SELECT even began, there was evidence that selenium supplementation would not benefit men who already had an adequate intake of the nutrient.

For this reason, at baseline, the investigators measured the concentration of selenium in the toenails of participants. The plan was to test whether supplementation would benefit only the subset of men with low selenium levels at baseline, they explain.

Instead, they found that men with high selenium levels at baseline who took selenium supplements increased their risk for high-grade cancer by 91% ( P = .007). In other words, the levels of selenium in these men became toxic.

The investigators also report that vitamin E increased prostate cancer risk in men, but only in those with low selenium levels at baseline.

Specifically, in the men with low levels of selenium randomized to receive vitamin E alone, the total risk for prostate cancer increased by 63% ( P = .02) and the risk for high-grade cancer increased by 111% ( P = .01).

This might explain why, in the 2008 SELECT results, only the men randomized to receive vitamin E alone, not those who received both vitamin E and selenium, had an increased risk for prostate cancer.

There is some evidence from basic science to support the idea of a meaningful dynamic. "An interaction between vitamin E and selenium has long been hypothesized because of their activities in preventing lipid peroxidation," Dr. Kristal and colleagues write.

Selenium, whether from dietary sources or supplements, might protect men from the harmful effects of vitamin E, they suggest. So selenium, at low levels, is not necessarily harmful to men.

Nevertheless, these new results are consistent with the medical literature on supplements and cancer, the investigators report. The message is that nothing good is gained in healthy people.

The literature "suggests that effects of supplementation are dependent upon the nutrient status of the target population, such that supplementation of populations with adequate nutrient status, leading to supraphysiological exposure, has either no effect or increases cancer risk," they write.

The study was funded by the National Cancer Institute. The authors have disclosed no relevant financial relationships.

J Natl Cancer Inst. Published online February 22, 2014.

    
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