evidence of protection judged to be convincing.
The reason for the change was more thorough epidemiology. The earlier studies tended to be retrospective, relying on people to remember dietary details from the distant past. These results were often upended by prospective protocols, in which the health of large populations was followed in real time.
The hypothesis that fatty foods are a direct cause of cancer has also been crumbling, along with the case for eating more fiber. The idea that red meat causes colon cancer is shrouded in ambiguity. Two meta-analyses published in 2011 reached conflicting conclusionsone finding a small effect and the other no clear link at all.
If hamburgers are carcinogenic, the effect appears to be mild. One study suggests that a 50-year-old man eating a hefty amount of red meatabout a third of a pound a dayraises his chance of getting colorectal cancer to 1.71% during the next decade, from 1.28%. Spread over a population of millions, that would have an impact. From the point of view of an individual, it barely seems to matter.
Trying to tweeze feeble effects from a tangle of variables, many of them unknown, inevitably leads to a tug of war of contradictory reports (as the San Diego meeting was winding up, a new paper on high-fat diets and breast cancer suggested there might be a connection after all).
With even the most rigorous studies, it is hard to adjust for what epidemiologists call confounding factors: assiduous eaters of fruits and vegetables probably weigh less, exercise more often and are vigilant about their health in other ways. Some of this can be sorted out with randomised controlled trials, with two large groups of people arbitrarily assigned different diets. But such studies are expensive, and the rules are hard to enforce in the short termand probably impossible over the many years it can take for cancer to develop.
The emphasis at the meeting was on other things: new immunotherapies, the role of chronic inflammation and the endlessly intricate subterfuges of cancer cells. With his focus on nutrition, Willett seemed like the odd man out.
Diet and cancer have turned out to be more complex and challenging than any of us expected, he said, standing thin as a rail at the lectern. There were some reasons for optimism. A study last year suggested that while eating lots of produce had no effect on most breast cancers, vegetables might reduce the occurrence