The fight to persuade Americans to ditch English units for the metric system in their everyday lives is largely lost. And now even some advocates of grams, meters and the like want to make a tactical retreat from the site of one of their few victories: the grocery aisle.

The nutrition labels that have been mandated by the federal government for nearly 20 years list nutrient quantities in grams. A serving of cereal might be one-quarter cup, but that amount contains, say, 2.5 grams of fat and 4 grams of sugar.