d fats," the authors concluded .
But even before the paper was published, other scientists began pointing out errors, says first author Rajiv Chowdhury, an epidemiologist at the University of Cambridge in the United Kingdom. For instance, the authors took one study on omega-3 fats, one type of unsaturated fats, to show a slightly negative effect while, in fact, it had shown a strong positive effect. The correction means that the meta-analysis now says people who report eating lots of this particular fat have significantly less heart disease; previously, it said there was no significant effect.
Critics also pointed out two important studies on omega-6 fatty acids that the authors had missed. The errors "demonstrate shoddy research and make one wonder whether there are more that haven't been detected," writes Jim Mann, a researcher at the University of Otago, Dunedin, in New Zealand, writes in an e-mail. "If I had been the referee I would have recommended rejection."
Mann and others say the paper has other problems, too. For instance, it does not address what people who reduced their intake of saturated fats consumed instead. A 2009 review  concluded that replacing saturated fats with carbohydrates had no benefit, while replacing them with polyunsaturated fats reduced the risk of heart disease. Several scientists say that should have been mentioned in the new paper.
Chowdhury says the paper's conclusions are valid, however, even after the corrections. Randomized clinical trials are the "hardest" kind of evidence, he says, and they don't show a significant effect of saturated or unsaturated fats. But even one of the paper's authors, Dariush Mozaffarian, of the Harvard School of Public Health, admits that he is not happy with the key conclusion that the evidence does not support a benefit from polyunsaturated fats. "Personally, I think the results suggest that fish and vegetable oils should be encouraged," he says. But the paper was written by a group of authors, he points out. "And science isn't a dictatorship."
Another study author, Emanuele Di Angelantonio of the University of Cambridge, says the main problem is that the paper was "wrongly interpreted by the media." "We are not saying the guidelines are wrong and people can eat as much saturated fat as they want. We are saying that there is no strong support for the guidelines and we need more good trials."
Willett says correcting the paper isn't enough. "It is good that they fixed it for the record, but it has caused massive confusion and the public hasn't heard about the correction." The paper should be withdrawn, he argues.
The controversy should serve as a warning about meta-analyses, Willett adds. Such studies compile the data from many individual studies to get a clearer result. "It looks like a sweeping summary of all the data, so it gets a lot of attention," Willett says. "But these days meta-analyses are often done by people who are not familiar with a field, who don't have the primary data or don't make the effort to get it." And while drug trials are often very similar in design, making it easy to combine their results, nutritional studies vary widely in the way they are set up. "Often the strengths and weaknesses of individual studies get lost," Willett says. "It's dangerous."