加拿大华人论坛 加拿大生活信息关于补充维生素防癌的证据

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这是一个排名不错的SCI流行病学杂志关于维生素防癌功效的综述，结论可能令大家失望，但是也能引人思考。没心情看这么长的，看最后那几句结论。Byers T. Anticancer Vitamins du Jour—The ABCED’s So Far. American Journal of Epidemiology 2010;172:1–3.The editorial surveys the history of anti-cancer vitamins from early reason for hope seen in observational and animal model studies through disappointing and often frightening clinical trials. In alphabetical order, then (citations are omitted and can be found in the full text of the article):Vitamin AAnimal experimental models led us to the notion that cancer risk might be ‘‘materially’’ reduced by supplementation with beta-carotene, a retinol precursor. Although that idea was seductive, we were all disappointed when 2 large randomized controlled trials that began in 1985 in Finland and the United States reported an 18% increased risk of lung cancer caused by high-dose beta-carotene supplementation and a 28% increased lung cancer risk caused by a combination of beta-carotene and retinol. The vitamin A era was over. 此段亮点：高剂量beta胡萝卜素及视黄醇与肺癌风险增加相关。Vitamin BAgain, based on animal experimental evidence and supported by epidemiologic evidence of connections between diets low in B vitamins and increased cancer risk, a large randomized controlled trial was begun in 1985 in central China, where micronutrient deficiency was common and where rates of cancers of the stomach and esophagus were extraordinarily high. Nonetheless, several years of supplementation with a combination of riboflavin (vitamin B2) and niacin (vitamin B3) had no effect on incidence of upper gastrointestinal cancers. Interest in folic acid (vitamin B9) persisted, though, in part because of its striking effect on neural tube birth defects, coupled with speculation about possible benefits of food fortification for diseases such as colorectal cancer that were inversely associated with diets rich in folate-containing foods and supplements. However, a 7-year randomized controlled trial found that high-dose folic acid supplements actually increased risk of colorectal adenomas. The vitamin B era was over.此段亮点1：其中维生素B2及B3的临床试验是在中国进行的，该试验地区人口缺乏微量元素，并且上消化道肿瘤发病率高，按理说应该获益最明显，结果是补充维生素B与发病率没有关系。此段亮点2：长期高剂量叶酸与结直肠腺癌发病率增高相关Vitamin CNext came vitamin C, a popular charge led by none other than Linus Pauling, the brilliant and charismatic 2-time Nobel laureate. Of all the cancers thought to be related to vitamin C deficiency, gastric cancer led the way, and of all the places on Earth where a vitamin C deficiency correction trial might yield benefits for gastric cancer, Linxian, China, would be the best. Indeed, vitamin C was tested in the Linxian trial, but just as for the B vitamins, vitamin C produced no change in gastric cancer rates .Vitamin DOver 2 decades of searching for an anticancer vitamin, we had seemed to skip over vitamin D in its proper alphabetical sequence…the International Agency for Research on Cancer conducted a comprehensive review of the evidence for vitamin D and cancer prevention, concluding that vitamin D may play a protective role in colorectal cancer, but not for prostate cancer, and that the evidence is weak for breast cancer. The conclusion by the International Agency for Research on Cancer about the weakness of the evidence for breast cancer has been a source of controversy among vitamin D protagonists, but subsequent nested cohort studies have found no relation between breast cancer risk and circulating levels of vitamin D. Nonetheless, vitamin D remains the cancer-preventing vitamin du jour.…An outstanding set of papers in this issue of the American Journal of Epidemiology reports on findings about the relation between circulating levels of vitamin D and subsequent cancer risk…These studies found no suggestion of an inverse association between vitamin D levels in the circulation and later incidence of 6 types of cancers (upper gastrointestinal, ovary, endometrial, pancreatic, kidney, and non-Hodgkin lymphoma)… The only association observed in this set of 6 analyses was a troubling one: that risk of pancreatic cancer was doubled for those in the highest quintile of circulating vitamin D levels.…many ongoing randomized controlled trials are now using quite high doses of vitamin D. As we await clearer evidence of benefits from those trials, we will also need to be prepared to be vigilant about their individual and collective power to assess any potential harms.此段亮点：维生素D的益处及危害均未有定论Vitamin EIn 1993, we launched headlong into a love affair with vitamin E fueled by compelling observations that those who chose to take vitamin E supplements were at lower risk of heart disease. Vitamin E supplementation became the rage as several large, randomized controlled trials were mounted. When those results finally came in, the findings were again disappointing: vitamin E supplementation offered no benefit for heart disease, and it slightly increased overall mortality. In the meantime, though, because of a secondary observation that prostate cancer incidence was lower in the vitamin E arm of the same Finnish trial that tested beta-carotene (vitamin E had also been included as a factor), a large factorial trial of vitamin E (and selenium) was carried out for reducing prostate cancer incidence. Disappointment again: there was no effect of either selenium or vitamin E on incidence of prostate cancer. The vitamin E era ended in a whimper.Of course, alternative medicine proponents often dispute this evidence, and it is easy to find claims of efficacy for all of these vitamins in preventing or treating cancer: Vitamin A, Vitamin B, Vitamin C, Vitamin D, Vitamin E.Given the ease with which the evidence can be ignored, cherry-picked, or distorted, the underlying conclusion of this editorial is even more salient and important than the specifics of the failed promise of vitamin supplementation to prevent:It is timely for us to now reflect on the history of the past 25 years of our alphabetical approach to studying single vitamin deficiency states as causal factors for cancer. We have learned some hard lessons along the alphabetical way. We now know that supernutritional levels of vitamins taken as supplements do not emulate the apparent benefits of diets high in foods that contain those vitamins, and we now know that taking vitamins in supernutritional doses can cause serious harm. In short, we have found that the reality of human biology is far more complex than is suggested by our simple ideas.Let me emphasize again these key points:1. Supernutritional levels of vitamins taken as supplements do not emulate the apparent benefits of diets high in foods that contain those vitamins. 2. Taking vitamins in supernutritional doses can cause serious harm. 3. The reality of human biology is far more complex than is suggested by our simple ideas.

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回复: 关于补充维生素防癌的证据维生素ABCDE在临床试验中几乎全军尽墨。不是说营养均衡，健康饮食不重要，只是没那么神，补过量了还有害无益。

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