It was repeated so often it was accepted as true: the typical American consumed 95 to 100 pounds of sugar each year. Health experts said that consumption was surely contributing to a nationwide crisis of obesity.

But in a move that has largely gone unnoticed, the Agriculture Department, keeper of the statistics on America’s sweet tooth, has employed new methodology that overnight shaved 20 pounds off its estimate and brought the number down to a precise 76.7 pounds. The decision raises questions about the entire notion of per-capita consumption just as the battles over sugar and sweeteners reach a peak.

“There’s such an implication of precision and accuracy in that decimal point — boy, we’ve got this nailed now,” said Michael F. Jacobson, executive director of the Center for Science in the Public Interest. “But when you take a good look, it’s built on a foundation of sand.”

Jean C. Buzby, the agricultural economist who headed the department’s team responsible for the data that led to the revision in sugar consumption, agreed that it was far from perfect but said it was better than what was used in the past.