A12-ounce can of diet soda contains two servings, according to the label. One brand of juice, packed in a box with one straw, provides 1.4 servings, says the package. And some pints of frozen yogurt contain 5 1/3 servings while others serve four.

In an effort to bring sense and consistency to food labels, the Food and Drug Administration last month proposed standardized serving sizes for 159 product categories, ranging from caviar to coffee cake. The announcement was made as part of the agency's proposed overhaul of food labels, the centerpiece of which includes mandatory disclosure of nutrition information on all food packages.

That nutrition information -- which includes a listing of calories, fat and sodium -- can fluctuate with the serving size. And that's what concerns FDA.

Currently, serving sizes are left to the manufacturer's discretion, which has lead to discrepancies in nutritional data among different brands, said F. Edward Scarbrough, acting director of the FDA's Office of Nutrition and Food Sciences. "There appears to be a tendency for some manufacturers to manipulate serving sizes to make nutritional claims, such as 'low sodium' or 'low calorie,' " he said.

That practice, he said, was a "big consumer issue" when FDA held public hearings around the country about food labeling proposals.

One troublesome product raised by consumers, Scarbrough said, was 12-ounce diet soft drinks that list the serving size at two, even though the cans appear to be intended for one person. By listing the serving at six ounces, or half a can, soda makers can claim that their products are "very low sodium," according to the FDA's definition of the term. If the serving size were doubled to a whole can, the amount of sodium would be too large to be called "very low." James Finkelstein, spokesman for the National Soft Drink Association, denies that the practice is deceptive.

The FDA is proposing that the serving size for carbonated beverages be set at 12 ounces, as was common practice in the late 1970s.

An FDA study released last year found that almost half of 44 product categories evaluated between 1979 and 1986 had been reduced in size. Aside from diet sodas, servings of some brands of soup, crackers and cake mixes have been reduced, and a serving of bread went from two slices to one. The study concluded that at least some of these adjustments were most likely precipitated by growing consumer concern with overconsumption of calories, salt or fat.

Ironically, this concern is completely different from the one that led some food companies in the 1970s to enlarge their serving sizes. When voluntary nutrition labeling was first instituted in 1973, deficiencies of vitamins, minerals and essential nutrients were a health concern.

FDA's standardized serving sizes are derived from U.S. Department of Agriculture surveys of what people actually eat. The proposed serving sizes are based on 4,000 foods tracked by USDA in a 1977-78 nationwide consumption survey, the most recent and complete data compiled by the federal government.

FDA's Scarbrough said that the serving size section of the agency's labeling overhaul was the most difficult, in part because of the diversity of opinion about what serving sizes should designate. "You can choose serving sizes that are recommended, or you can chose those that are actually eaten," he said, adding that FDA chose what is actually eaten because "it is more in line with truth-in-labeling."

Others believe the specific serving size doesn't really matter since it is so subjective and that as long as it is consistent, consumers can make product comparisons.

Kathy Knuth, spokeswoman for Kraft/General Foods, said that when it comes to serving sizes for salad dressings, "the important thing is that everyone should use the same measure." Currently, a serving of salad dressing is listed on most labels as one tablespoon. Based on actual food consumption patterns, FDA is proposing that the measurement be increased to two tablespoons.

The International Ice Cream Association objects to FDA's proposed six-ounce serving size for ice cream, ice milk, frozen yogurt and sherbet. "We think the proposal is not good because the frozen dessert industry historically has had a four-ounce serving size," said Tom Balmer, director of special programs for the Washington-based trade group. "It's more in line with traditional Dixie cups, and the majority of ice cream scoopers sold in the country deliver a four-ounce serving per scoop."

Eating Right appears on alternate Tuesdays.