Warning: This Warning May Not Be Valid

Critics of the government's dietary advice have long questioned the focus on cutting fat–in particular, the recommendation that fat calories be kept below an arbitrary percentage of total calories. Now a controlled intervention study has shown pretty definitively that a low-fat diet does not reduce the risk of breast cancer, colorectal cancer, or cardiovascular disease.

For you Dean Ornish fans out there, I'll concede that the benefits might take longer than eight years to show up, might require more severe restrictions on fat (probably so severe that most people would not be willing to follow them), might relate to different disease risks, or might be so modest that they can't de detected even in a study with nearly 49,000 subjects. But an important lesson nevertheless can be drawn from the increasingly shaky foundation on which advice to cut fat (as opposed to certain kinds of fat, or total calories) rests. The dietary expert Jules Hirsch, physician in chief emeritus at Rockefeller University, told The New York Times this study "should put a stop to this era of thinking that we have all the information we need to change the whole national diet and make everybody healthy." And U.C.-Berkeley statistician David A. Freedman offered this warning: "We, in the scientific community, often give strong advice based on flimsy evidence."

One reason for this tendency is that "public health" doctrine says the government should deliver clear, authoritative-sounding advice, even when it's not justified by the evidence, because otherwise people might not comply. Another reason is that public health calculations give little or no weight to the inconvenience and forgone pleasure entailed by a particular recommendation. From this perspective, if cutting fat might reduce disease risks, and if it seems unlikely that it will raise them, telling everyone to do so makes sense–even if (as with salt limits) only a minority of the population stands to benefit. A more sophisticated (though still collectivist) approach would take the interests of individuals into account because people resent making sacrifices for no purpose and may therefore be less inclined to follow the next recommendation from the experts, even if that one is well-founded.