(The Atlanta Journal-Constitution) -- Let's start with a waist-whittling word problem.

If someone who usually gets a small order of fries and a large sweet tea at McDonald's wants to skip one to lighten their lunch, which would cut the most calories?

Answer: It's a wash.

According to nutrition facts posted on the company's website, each contains 230 calories. The point is that it's not easy to guess the amount of calories in menu items.

A study by Healthy Eating Research at the University of Minnesota found that people tend to get calorie counts wrong most of the time. For instance, when a restaurant dish sounded "healthful," such as a chef's salad (930 calories), 90 percent underestimated the calories. That's why many health professionals and now politicians want to see more nutrition labeling on restaurant menus.

Authorized by the health care legislation passed this year, the Food and Drug Administration recently released a draft of guidelines requiring calorie information be posted on menus and menu boards by restaurants and food stores with 20 or more locations and vending machine operators with 20 or more machines.

"Knowledge is power," said Atlanta registered dietitian Marisa Moore. "Having nutrition information available for foods eaten away from home is critical to help make healthy choices."

Moore, a spokeswoman for the American Dietetic Association, said she believes the new federal menu labeling requirement would help. "They will allow you to play detective less often. I've used online nutrition facts for years to help clients make healthy choices away from home. It's helpful to see the 150 calorie difference between the grilled vs. fried chicken sandwich before you ever have a chance to taste it."

Americans consume about a third of their calories from food prepared away from home, so restaurant meals do offer a significant opportunity to provide a teachable moment in nutrition education.

Many major chains and health-focused eateries already voluntarily provide nutrition facts on websites and some right on the menu, but new federal guidelines aim to offer a standardized format. Even with those suggested improvements, it's complicated.

What might be missing: It's not uncommon for restaurants to list nutrition information for sandwiches without including cheese, mayo and special sauces. Entree salads might not include calories in the dressing you add later.

The fudge factor: Information might not be perfect. There's a proposed plus-or-minus 20 percent margin of error. This accounts for the human influence in preparation and variations in product composition, analysis methods and databases.

Your calorie cap: "You need to have a general idea about how many total calories you need to determine how a specific menu choice will fit into your day," Moore said. To get an estimate of your calorie needs, check out www.mypyramid.gov.

Copyright 2010 The Atlanta Journal-Constitution