硒、维他命E如何增加前列腺癌风险


  【24drs.com】SELECT(Selenium and Vitamin E Cancer Prevention Trial」研究,试图确认硒和维他命E这些补充品是否可以预防前列腺癌,备受瞩目的新资料显示,这两种抗氧化剂确实对男性有风险。
  
  如之前所报告的,硒或维他命E补充品对男性没有预防效果;事实上,对于某些男性,这些补充品会增加前列腺癌风险。
  
  这篇新研究在线发表于2月22日美国国家癌症研究院期刊,探讨服用这些补充品的男性中有哪里些人的前列腺癌风险高与其原因。
  
  不过,研究者之一表示,该试验的公卫信息依旧是相同的。
  
  第一作者、西雅图Fred Hutchinson癌症研究中心Alan Kristal博士在声明中表示,使用这些补充品的男性应停用;硒或维他命E都没有任何已知的助益,只有风险;许多人认为饮食补充有益、或至少没有害处,这是错误的观念。
  
  研究世代是SELECT试验的4,856名男性,这篇大型第3期安慰剂控制试验共有超过35,000名男性,被随机指派服用高剂量维他命E (400 IU/天)和/或硒(200 μg/天)补充品。
  
  SELECT试验始于2001年,预计进行12年,但是在2008年提早结束,当时研究对象平均服用这些补充品5年;研究结果显示,硒没有保护效果,维他命E会增加前列腺癌风险。
  
  虽然停止使用这些补充品,但该研究并未终止,继续追踪2年后,服用维他命E的男性,其前列腺癌风险在统计上显著增加了17%,就如同之前的报告。
  
  值得注意的是,服用单一种补充品、或两种都有服用者,其前列腺癌侦测率都比安慰剂组高(但只有维他命E组的差异达统计上的显著意义)。
  
  硒是存在于植物如米、小麦、巴西坚果以及海鲜和肉类的非金属微量元素,在之前的大型皮肤癌预防试验中,硒和前列腺癌风险降低有关。
  
  根据国家癌症研究院指出,硒是一种可以帮助控制导致癌症之细胞损伤的抗氧化剂。
  
  维他命E广泛存在于多种食物,特别是蔬菜、蔬菜油、坚果和蛋黄;维他命E和硒一样被视为抗氧化剂。
  
  在这篇新的案例–世代研究中,1,739名男性在SELECT试验期间被诊断有前列腺癌,另3,117名男性则无 。
  
  Kristal博士等人发现,在没有使用补充品的情况下,开始时的硒浓度值和前列腺癌风险无关;不过,他们也发现,开始时的数值低和数值高的男性之间,补充品的影响大有差异;特别是的,硒补充品增加了开始时硒数值高之男性的前列腺癌风险。
  
  在SELECT试验开始之前,即有证据认为硒补充品对于已经摄取足够营养的男性并无帮助。
  
  因此,研究者在开始时测量研究对象脚趾甲的硒浓度。他们解释,计画检视这个补充品是否只对那些开始时硒数值低的男性有助益。
  
  相反的,他们发现,开始时硒数值高的男性服用硒补充品会使高等级癌症风险增加达91% ( P= .007);换句话说,硒数值对于这些男性变成是有毒性的。
  
  研究者也报告指出,维他命E增加了男性的前列腺癌风险,但只有那些开始时硒数值低者;特别的是,硒数值低而被随机指派接受维他命E的男性中,前列腺癌整体风险增加达63% ( P= .02),而高等级癌症风险增加达111% ( P= .01)。
  
  这或许可以解释为何在2008年SELECT试验的结果中,只有随机指派接受维他命E之男性的前列腺癌风险高,而非同时接受维他命E和硒者。
  
  Kristal博士等人写道,基础科学有一些证据支持动力学概念;长久以来,维他命E和硒之间即被假设有交互作用,因为它们可以防止脂质过氧化。
  
  他们认为,硒不论来自饮食或补充品,可能可以保护男性来自维他命E的伤害,所以,低浓度的硒对于男性不一定有伤害。
  
  不过,研究者报告指出,这些新结果和探讨补充品与癌症的医学文献一致,对于健康者并无好处。
  
  他们写道,文献认为,补充品的效果取决于使用者的营养状态,这类补充品对于有足够营养者,会导致超乎生理需求之曝露,因而无效益或增加癌症风险。
  
  资料来源: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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