As we discussed last week, the National Institutes of Health has released the results of its massive $415 million study on diet and women’s health, casting serious doubts on low-fat diet dogmas. Low-fat diets, according to this study, don’t prevent heart disease, colorectal cancer, or breast cancer.

Already, the usual gang of ideologues has pooh-poohed the study, saying that the scientific community has moved on from the low-fat craze. While we agree that it’s important not to put too much stock in any one study, it’s hard to give much credibility to the food cops here, since their response amounts to: “Of course the old food fad is not good science — it’s the new food fad that is good science.”

On Monday The New York Times surveyed the scientific trends surrounding childhood obesity and discovered that dietary interventions — things like banning whole milk or soft drinks — in our schools don’t tend to stand up in the face of the science that those same schools are supposed to be teaching. As the Times points out:

When many [nutrition policies] were subjected to the cold light of rigorous scientific study, they have fallen short. If nothing else, when it comes to fighting obesity, science teaches humility.

Childhood obesity is certainly worth fighting, but not all weapons are created equal. Rockefeller University obesity researcher Dr. Jeffrey Friedman told the Times that measures to fight childhood obesity can be okay, but “the burden should be on those who want to impose them to show they work. To my knowledge, no one has ever done this.” Columbia University obesity researcher Dr. Rudolph Leibel agreed, saying that “those interventions should not be imposed without first showing they have an effect.”

So why do we keep letting feel-good policies like soda bans into our schools, even when the best evidence says they make no difference? Berkeley statistician Dr. David Freedman pointed out to the Times that the public, not to mention scientists, “have this wonderful capacity for ignoring negative evidence.”

Dr. Freedman illustrated his point with a revealing anecdote:

In the 1830’s, Dr. [Pierre-Charles-Alexandre] Louis studied the effect of bloodletting, or bleeding — the standard treatment of the time — on pneumonia.

“The data showed that bleeding didn’t work,” Dr. Freedman said. But, he said, “Dr. Louis rejected this as terrifying and absurd.”

So, he made a recommendation: bleed earlier and bleed harder.