一項納入13項大規(guī)模隨機對照試驗的Meta分析顯示，服用葉酸補充劑既不會增加也不會降低治療期間的癌癥發(fā)病率。該Meta分析已發(fā)表在《柳葉刀》1月25日在線版上(Lancet 2013 [doi: 10.1016/S0140-6736(12)62001-7])。

從20世紀90年代晚期開始，多個國家開始強制在面粉中添加葉酸以預(yù)防神經(jīng)管缺損，一些人還認為服用葉酸補充劑有助于預(yù)防癌癥。但其他一些國家并未允許對食品進行葉酸強化，也不建議服用葉酸補充劑，部分原因在于擔(dān)心葉酸可能促進癌癥發(fā)生和進展。

納入本項Meta分析的研究均符合下列條件：隨機對照試驗，評估葉酸補充劑的效能，招募至少500名受試者，結(jié)局中包含癌癥發(fā)病率，對受試者進行至少1年的補充劑/安慰劑干預(yù)。最后共納入13項研究，49,621名受試者。挪威公共衛(wèi)生研究所的Stein Emil Vollset博士表示，用葉酸補充劑預(yù)防癌癥的希望和對其誘發(fā)癌癥的擔(dān)憂都被這項Meta分析結(jié)果打消了。

2/3的受試者為男性，基線時平均年齡為64歲。在12項研究中，受試者每日服用葉酸劑量為0.5~5 mg不等，另一項研究則評估了40 mg/d大劑量葉酸的效果。治療時間為1.8~7.4年不等(平均5.2年)。

服用補充劑可使血漿葉酸濃度升高3倍，并使血漿同型半胱氨酸水平降低25%。在檢驗大劑量葉酸的那項試驗中，服用補充劑使得血漿葉酸濃度升高了100倍，但對血漿同型半胱氨酸水平未產(chǎn)生明顯的進一步影響。

葉酸補充劑對總體癌癥發(fā)病率的影響并不顯著。在服用補充劑的受試者中，所有類型癌癥的總發(fā)病率為7.7%，而服用安慰劑者為7.3%，差異不明顯。

延長治療時間或提高補充劑劑量，并不影響總體癌癥發(fā)病率。后者也不隨年齡、性別、基線葉酸水平、基線同型半胱氨酸水平，或者是否住在使用葉酸強化面粉的國家而改變。

盡管在使用大劑量補充劑的那項試驗中血漿葉酸水平增加百倍，但補充劑組(65例癌癥)與安慰劑組(72例癌癥)之間在總體癌癥發(fā)病率方面仍無顯著差異。

葉酸補充劑對特定部位癌癥的發(fā)病率也沒有明顯影響，包括結(jié)直腸癌、肺癌、乳腺癌和前列腺癌。

Some countries fortify flour with folic acid to prevent neural tube defects but others do not, partly because of concerns about possible cancer risks. We aimed to assess any effects on site-specific cancer rates in the randomised trials of folic acid supplementation, at doses higher than those from fortification.

In these meta-analyses, we sought all trials completed before 2011 that compared folic acid versus placebo, had scheduled treatment duration at least 1 year, included at least 500 participants, and recorded data on cancer incidence. We obtained individual participant datasets that included 49 621 participants in all 13 such trials (ten trials of folic acid for prevention of cardiovascular disease [n=46 969] and three trials in patients with colorectal adenoma [n=2652]). All these trials were evenly randomised. The main outcome was incident cancer (ignoring non-melanoma skin cancer) during the scheduled treatment period (among participants who were still free of cancer). We compared those allocated folic acid with those allocated placebo, and used log-rank analyses to calculate the cancer incidence rate ratio (RR).

During a weighted average scheduled treatment duration of 5·2 years, allocation to folic acid quadrupled plasma concentrations of folic acid (57·3 nmol/L for the folic acid groups vs 13·5 nmol/L for the placebo groups), but had no significant effect on overall cancer incidence (1904 cancers in the folic acid groups vs 1809 cancers in the placebo groups, RR 1·06, 95% CI 0·99—1·13, p=0·10). There was no trend towards greater effect with longer treatment. There was no significant heterogeneity between the results of the 13 individual trials (p=0·23), or between the two overall results in the cadiovascular prevention trials and the adenoma trials (p=0·13). Moreover, there was no significant effect of folic acid supplementation on the incidence of cancer of the large intestine, prostate, lung, breast, or any other specific site.

Folic acid supplementation does not substantially increase or decrease incidence of site-specific cancer during the first 5 years of treatment. Fortification of flour and other cereal products involves doses of folic acid that are, on average, an order of magnitude smaller than the doses used in these trials.

British Heart Foundation, Medical Research Council, Cancer Research UK, Food Standards Agency.