You call that a serving?

A package of Farmer John Classic Polish Style Smoked Sausage sent to us by a California reader shows a link in a bun, ballpark style, with a squirt of mustard. The package's Nutrition Facts label states that the sausage has 170 calories and 16 grams of fat per serving, which the fine print reveals is ... half a link? "No wonder it's so hard to shop and pay attention to diets," the reader lamented.

Befuddled readers often question the logic of serving sizes, and some companies seem to be trying to address their confusion. Ronzoni, for example, cites "about 7" servings per box of spaghetti under its Nutrition Facts but "4 portions" under the box's Perfect Pasta Guide.

At your service

Mandatory labeling of serving sizes began in 1990, when Congress required nutrition information on food labels. The rule let the companies define "serving," which is generally based on data from government surveys in the 1970s and 1980s showing "reference amounts customarily consumed."

Manufacturers could always list calories per container, rather than per serving, on foods that people often consume in one sitting, such as a bag of chips. But that might imply that the full package is an appropriate amount to have at one time, says Brian Wansink, Ph.D., director of the Food and Brand Lab at Cornell University and author of "Mindless Eating."

"The best of all worlds would be packaging that has both the total calories on it and the calories per serving," Wansink says. Some companies do that already.

It doesn't help that serving sizes are often listed by weight. When we asked volunteers to dish out what they estimated to be one serving of peanuts, cheese cubes, and chicken wings based on the standard serving sizes for each (by weight), participants served from 88 percent less to 35 percent more than the listed size.

Moreover, serving size might vary within a single category of foods. Cereals, for example, have one of three labeled servings, depending on their density. When we asked almost 100 kids from 6 to 16 years old to pour out the cereal they'd eat in a sitting, they poured themselves, on average, 50 to 65 percent more than the suggested serving size.

To add to the confusion, calories per serving are usually rounded to the nearest five or 10. A bag of bread we saw advertises "40 calories per slice," for instance, but its Nutrition Facts label states a serving of three slices has 110 calories.