[熱點科普] 通俗解讀法國轉基因玉米喂食白鼠實驗

最近媒體熱炒法國轉基因玉米喂食白鼠實驗.這樣的實驗本來應該很有意義,因為轉基因食品的安全性是大家都關心的問題。但是這個實驗的設計卻有很大問題，因而得出的結論不可靠。

實驗的設計有哪些問題呢？

1) 實驗選用的白鼠品種,本身就容易長腫瘤。實驗結果表明不喂食轉基因玉米的白鼠，也長了腫瘤。

2）實驗雖然用了200隻白鼠，但每組隻有10隻。從統計上，這樣的結果沒有代表性。好比拋擲硬幣，我們都知道得到正麵的概率是50%。 但是這個結果要拋擲足夠多的次數之後才能得到。拋1，2，3，4，5次，得到的結果可能與 50% 相差甚遠。下圖引自原始論文。大家可以看到喂食轉基因玉米多（0，11%，22%，33%）的白鼠，反而死得晚（少）！這個結論當然不靠譜，原因是樣本太少。

Fig. 1. Mortality of rats fed GMO treated or not with Roundup, and effects of Roundup alone. Rats were fed with NK603 GM maize (with or without application of Roundup) at three different doses (11, 22, 33% in their diet: thin, medium and bold lines, respectively) compared to the substantially equivalent closest isogenic non-GM maize (control, dotted line). Roundup was administrated in drinking water at 3 increasing doses, same symbols (environmental (A), MRL in agricultural GMOs (B) and half of minimal agricultural levels (C), see Section 2). Lifespan during the experiment for the control group is represented by the vertical bar ± SEM (grey area). In bar histograms, the causes of mortality before the grey area are detailed in comparison to the controls (0). In black are represented the necessary euthanasia because of suffering in accordance with ethical rules (tumors over 25% body weight, more than 25% weight loss, hemorrhagic bleeding, etc.); and in hatched areas, spontaneous mortality.

3）論文中喂食白鼠草甘磷組的容許量，是飲水中人體的容許量。白鼠的體重多少？人的體重多少？對相關毒性的耐受量是什麽關係？ 一般動物毒性試驗的結果外推倒人身上，第一要考慮體重，第二要考慮物種差別。光從體重的角度，老鼠喝一兩白酒和和人喝10斤白酒的結果大致一樣（5兩重的 老鼠和500兩重的人比較）。

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參考文獻：

[1] Claims of GM foods 'link to cancer' disputed by other researchers（http://www.nhs.uk/news/2012/09September/Pages/Claims-of-GM- foods-link-to-cancer-disputed-by-other-researchers.aspx）

[2] Séralini G, Clair E, Mesnage R et al. Long term toxicity of a Roundup herbicide and a Roundup-tolerant genetically modified maize. Food and Chemical Toxicology. Published online September 19 2012

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This study is reported to involve the highest number of rats regularly studied in a GM-diet study. The research also benefits from testing three different dietary concentrations of GM maize over a two-year period, along with GM maize treated with and without Roundup and Roundup alone diluted in water. All the rats in these groups were compared with rats fed only untreated, non-GM feed. The researchers also said that the Roundup concentration in water started at a dose below the range of levels permitted by regulatory authorities.

Animal research such as this is highly valuable for looking at the possible toxic effects. However, claims that GM food may have a similar toxic effect in humans cannot be justified using the results of this study, which many experts argue was poorly conducted.

There are several significant limitations to the research, including the following:

• Although the study did include a large number of rats overall, there were only 10 males and 10 females in each group. All comparisons were made with just one control group of 10 male rats and 10 females, and a larger group of control rats may not have given identical average lifespan and health data. Such a small control group makes it more likely that the results are due to chance.
• Humans are biologically different from rats, and we may not have identical susceptibilities to disease and illness.
• One expert argument was that the rats in this study were a breed already susceptible to tumours, especially if they are given unlimited access to food. This seems plausible as the rats are described to have been virgin albino Sprague-Dawley rats; however, their tumour susceptibility is not discussed in the paper.
• The method of statistical analysis used to assess the results was described by the researchers as being a "robust method for modelling, analysing and interpreting complex chemical and biological data", but is complicated and fairly impenetrable, even to those with training in statistics.
• The rats were fed a regular, concentrated diet of the test substance, and how this dose relates to any human intake is unclear.
• This two-year period roughly equated to a rat’s lifetime. It is difficult to equate this directly to humans. Does it represent lifelong, daily consumption of GM foods treated with herbicides, and at what age adverse effects – if any – may be expected to be seen in humans?

The very unusual way in which the trial was conducted makes it hard to lend much weight to its conclusions. In any case, given the public hostility to GM foods in the UK, it is unlikely that supermarkets are going to start stocking GM foods on the shelf any time soon.

Research and debate into the safe levels of GM foods and herbicides in the diet is likely to continue. 

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.

Links to the headlines

Cancer row over GM foods as French study claims it did THIS to rats... and can cause organ damage and early death in humans. Daily Mail, September 20 2012

GM crop enquiry launched by French government. The Daily Telegraph, September 19 2012

French GM-fed rat study triggers furore. BBC News, September 19 2012

Links to the science

Séralini G, Clair E, Mesnage R et al. Long term toxicity of a Roundup herbicide and a Roundup-tolerant genetically modified maize. Food and Chemical Toxicology. Published online September 19 2012

所有跟帖： 

• 昨晚剛讀了原作。有同感。 樣本太少了。。 -2ndglance- ♂ (108 bytes) () 09/21/2012 postreply 10:14:04