In your cache of kitchen gadgets do you have a scale for weighing food? Are your measuring cups and spoons ever-handy for dishing out those suggested serving sizes on the nutrition label? For example, the one that says two tablespoons of salad dressing equals 1 serving?

We know that the amount of food we eat has a direct effect on our health and weight. Obesity has become a critical health issue for Americans. It’s hard not to over-eat these days, though, as huge portions are busting the last notch on our collective belts and we confuse volume with value.

The one thing nutritionists, foodies, and diet gurus seem to agree on is that no matter what we eat, no matter which regimen we’re following, it’s how much that can make the difference between losing those 10 pounds or gaining them. Even here, however, things can be confusing.

”There is a difference between a serving size and a portion,” said Randi Beranbaum, dietician at Tufts University. ”A portion is the amount you put on your plate. A serving is a measured amount for reporting nutrients.”

This matters, she and other professionals say, because typically, we put more on our plate than we need.

Serving sizes are not created equal. Federal agencies, professional associations, and food establishments have somewhat differing guidelines. The Nutrition Facts Label on your breakfast cereal, for example, was developed by the US Food and Drug Administration to help consumers balance important nutrients in a daily, 2,000-calorie diet. The FDA label is based on ”reference amounts,” the volume of food consumed at one sitting as determined by FDA food surveys. Weights and volumes are expressed in standard household measures like tablespoons, cups, and slices, and the labels are designed to let consumers compare information about similar food products.

Within these guidelines, manufacturers still have considerable latitude in deciding what a serving is. The regulations say that ”if a unit weighs more than 50 percent, but less than 200 percent, of the reference amount, the serving size can be one unit.” (Read all the regulations at www.fda.gov. Go to Reference Room, click on Code of Federal Regulations, then type ”nutrition labeling” in Full Text Search.)

For example, look at the label on a can of Coca Cola. The reference amount for carbonated beverages is 8 ounces, according to the regulations, but the can says 12 ounces is one serving. Now, look at the 20-ounce Coca Cola bottle and the label will say it contains 2.5 servings: One serving is now only 8 ounces.

The United States Department of Agriculture goes about this differently. Its Dietary Guidelines for Americans, last revised in 2000, are depicted graphically in the Food Guide Pyramid. The guidelines put ”nutrient standards into food groups based on advice for a healthful diet.” The controversial food pyramid is slated for updating in 2005. John Webster, of the USDA’s Center for Nutrition Policy and Promotion, said, ”The Food Guide Pyramid was designed as a teaching tool, because people usually do not weigh their food and think of foods as items.” In re-assessing the Food Guide Pyramid, Webster said ”it has become evident that consumers don’t know what a serving consists of.”

Meanwhile, restaurants and bakeries get to determine their own serving sizes. We have all been to steak houses where individual, 12- to 16-ounce steaks are on the menu, or to restaurants where plates of piled-high pasta are offered as one serving. Doggie bags are part of the plan, but many people manage to down the entire platter. Depending on who is counting, the customer could have consumed three to six servings of meat or as much as 10 servings of pasta in one sitting in these establishments.

Then there are unpackaged items like bagels and donuts that vary wildly in serving size. A 4-ounce Bruegger’s bagel, often purchased as a single serving, would be two servings according to the FDA, and four servings according to the USDA. Do we really have to do the math for a 670-calorie, 61/2-ounce coffee cake muffin that is considered one serving at Dunkin’ Donuts?

People should learn to ”visualize a real serving,” said Denise Barra, a licensed nutritionist. She referred to the visual from the American Dietetic Association to imagine ”a bar of soap for a serving of meat, a computer mouse for a medium-sized potato, a hockey puck for a bagel, and a tennis ball as a serving of ice cream.” She recommended learning about serving sizes at home because ”we are adapting to super-size portions. … We need to re-learn what it feels like to eat moderate-sized portions.”

”You and I have different caloric and nutritional needs,” she said, ”but a standard serving is a starting point.”

Lexington brothers Andy Olson, 22, who is 6 feet, 3 inches tall and 175 pounds, and Alex Olson, 23, who is 6 feet, 1 inch and 230 pounds, were refilling their breakfast cereal bowls recently. They ate three and four times the serving size stated on the box – three-quarters of a cup, an admittedly small amount for these big guys. Being active, they probably would burn off the 450 calories they consumed.

Breakfast cereals, however, have a wide range of serving sizes, and a fortified cereal like Total can give you too much of a good thing if you don’t pay attention to serving sizes. The idea that if 100 percent is good, then 400 percent is better doesn’t work with some nutrients. Barra cited iron as an example: Men can end up eating several times more iron than their body needs when eating fortified cereals.

”Women have a higher need for iron and get rid of the excess on a monthly basis,” she said. ”Men don’t. If extra iron gets stored, it could eventually lead to health problems.”

While one serving size does not fit all, there is general acceptance of the FDA guidelines. Margo Wootan, director of nutrition policy for the independent watchdog group Center for Science in the Public Interest in Washington, D.C., said, ”Serving sizes on the nutrition label are generally reasonable, and consumers have become familiar with them. However, serving sizes do need some tweaking.” She used the example of ice cream, where the present serving size is an unrealistic one-half cup.

She cautioned that ”consumers should look at the serving size first. Most smaller bags of snacks, like chips or pretzels, are not single servings, and it is unreasonable to think people are sharing these items.”

Recently, Sean Martorano had his hand planted in a bag of microwave popcorn, while Greg Gibbons, his fellow service consultant at Boston Volkswagen in Allston, had hit bottom in a Cracker Jack bag. Both were aware that each bag contained more than three servings and conceded that, if forced, they would share. Still, pulling the Cracker Jack prize from the bottom of his bag, Gibbons said, ”If they intended 31/2 people to be eating this, wouldn’t there be 31/2 prizes?”

These websites may be of further interest:

http://www.cnpp.usda.gov/ (US Department of Agriculture’s Center for Nutrition Policy and Promotion)
http://www.fda.gov/ (US Food and Drug Administration)
http://www.eatright.org/ (American Dietetic Association)
http://www.dunkindonuts.com/ (nutrition information on all their products)
http://www.cspinet.org/ (Center for Science in the Public Interest)