The numbers appear on all kinds of processed and packaged food in the easy-to-read Nutrition Facts chart. But can you trust these guides on fats, protein and carbohydrates?

Hard as they may be to swallow, the nutrient numbers on food labels, some restaurant menus and recipes printed in cookbooks, newspapers and magazines are not pulled out of a hat. Most of them come from the United States Department of Agriculture (USDA).

"We currently have the largest database in the world, with information on 100 nutrients for over 7,500 foods," says Joanne Holden, research leader at the USDA's Nutrient Data Laboratory (NDL) in Beltsville. For more than a century, the lab and its predecessors have been providing information on the nutrient content of foods consumed in the United States. The lab's main purpose is to manage databases, including the USDA Nutrient Database for Standard Reference, the "gold standard" for nutritionists, the food industry and just about anyone who wants to know about the nutrients in the food they're eating.

Holden said the federal lab gets its facts in four ways: by hiring independent consulting labs to analyze a particular food; from information provided by commodity groups such as the National Cattlemen's Beef Association, National Pork Producers Council or the National Egg Board; by reviewing scientific literature; and occasionally using its own data to derive nutrient values for mixed-dish foods such as chicken pot pie and beef stew.

The government is not the only source of accurate food databases. Many large food companies maintain their own. As a nutrition research scientist for Quaker Oats' snack group division, Sara Murphy oversees a product's development and its nutrient analysis. She also helps maintain a database for each product, which helps the company comply with labeling requirements.

But can you trust the numbers?

Constance Geiger, an assistant professor of foods and nutrition at the University of Utah who has extensively studied food labeling, loosely grades the trustworthiness of nutrition information based on where it appears.

"You can believe with confidence the numbers you see on the Nutrition Facts panel of food labels," Geiger said, because food manufacturers have to comply with strict legal guidelines on how nutrients must be listed.

Geiger also believes that the nutrition information on restaurant menus is trustworthy. "If an item makes a nutrient claim, the FDA does require the restaurant to display the nutrition information" for that particular nutrient. For example, if a menu offered low-fat lasagna, the restaurant would have to list the grams of fat in the lasagna. The most reliable nutrition data comes from major fast-food chains, Geiger says, because they hire independent labs to conduct nutrient analysis; other restaurants likely rely on the Standard Reference database or a consultant using a software program.

Nutritional information provided with recipes in cookbooks, newspapers and national magazines has the potential to be least reliable, Geiger said. "Investigate who provided the numbers," she advised. Were the nutrient values calculated by a registered dietitian with access to current information? Does it list as its source the 12th edition of the USDA's Standard Reference? If not, she said, the numbers could be suspect.