Quick: How many calories are in a tuna sandwich?

If that number doesn't quite roll off your lips, you're not alone. Knowing the nutrients in food -- from calories to grams of healthy fat -- can be challenging. Sure, Nutrition Facts labels provide some key information, but some of it appears in percentages of daily intakes that may not match your own. Plus a lot of food -- think about those leftovers you grab from the fridge, fast food and restaurant fare -- simply doesn't come with much or any nutrition information.

The U.S. Department of Agriculture (USDA) offers a comprehensive nutritional listing of more than 6,000 foods on its Web site www.nal.usda.gov/fnic/cgi-bin/nut_search.pl, but there's no mobility to the database. You have to sit in front of a computer to get the information you need, and the detail can be intimidating. (And let's face it, most people have better things to do than to wade through percentages of omega-3 fatty acids.)

Now, however, there's a fast, fairly easy and mobile way to check the nutrients in a huge array of food and drink. The USDA announced yesterday that it has teamed up with HealtheTech Inc. (makers of BalanceLog, a nutrition-tracking program that works on the millions of handheld computers that run the Palm operating system) to put a consumer-friendly version of its nutrient database in the palm of your hand.

Unveiled yesterday at the American Dietetics Association's annual meeting in Philadelphia, the new database lacks flash and has a clunky name: USDA Nutrient Database for the Palm OS Standard Reference 15. But it downloads to a desktop computer in just about 30 seconds from the USDA Web site www.nal.usda.gov/fnic/foodcomp. The transfer to your handheld will take longer -- about 10 minutes based on our experience, so be patient -- and you will need at least 1.5 megabytes of available memory on your handheld computer. The program is not compatible with Microsoft's Pocket PC operating system.

The program is a slimmed-down version of the comprehensive USDA list. But you probably won't miss the more technical aspects, such as listings of grams of individual amino acids. Instead you can focus on the more relevant basics: calories, carbohydrates, protein and fat (saturated, mono-unsaturated, polyunsaturated), cholesterol and sodium as well as vitamins and minerals for every food in the USDA list. In short, there's more than enough to make you an educated eater when you're on the go.

Experts say it's just the kind of tool that can help hold the line against expanding waistlines. "It's great, because it's putting informaton at one's fingertips that can be used in a very, very positive way," says registered dietitian Leslie Bonci, a spokeswoman for the American Dietetics Association and director of sports nutrition at the University of Pittsburgh.

Studies show that keeping track of food is key for both achieving a healthy weight and long-term maintenance -- the reason that the Lean Plate Club made recording food part of the Everyday Challenge, which was launched in January. For example, a 1998 study of 59 men and women enrolled in a program at the University of Minnesota found that weight loss increased in direct proportion to the consistency with which participants monitored their food intake.

"When people track what they eat, they are much more accountable to themselves," Bonci says. "Research has shown that people who are tracking and monitoring their food are more likely to be successful" at weight loss and at keeping off unwanted pounds.

Of course, just knowing how many calories are in a slice of cake doesn't result in weight loss. You still have to make smart food choices, watch portion sizes and make sure that the calories eaten are less than the calories burned by physical activity -- all tenets of the Lean Plate Club.

The new USDA program doesn't offer any help with recording food and calories. For that, you'll have to either use paper and pencil and do the math or log on to the USDA's Healthy Eating Index Web site (http://147.208.9.133/), which will add things for you and keep up to 20 days of meals that can be tracked against the U.S. Food Pyramid. A similar free service is also available at www.fitday.com, but you must register first. Another option: Fork over $49 to $69 for HealtheTech's BalanceLog program, which works on both desktops and handhelds. The downside: There's no easy way to import USDA figures into BalanceLog.

It's important to underscore that you need to be computer-savvy to use these systems and have the financial means to buy a handheld computer, which can run anywhere from $99 for a reconditioned model to $500 for a new color version. And counting calories in a such a technical way is not for everyone. A paperback calorie counter will also do, along with a small food diary.

For those who like the technical approach, however, there's more good news: Next year, USDA and HealtheTech plan to unveil a similar free service for desktops, according to HealtheTech's chairman and CEO, James Mault. The goal, Mault says, "is to create a software application for handheld formats and desktops that can allow everyone to be able very easily to see nutritional information in everyday foods."

-- Sally Squires

How do you track what you eat? Share your tips -- or ask any nutrition question -- when Sally Squires hosts the Lean Plate Club, our online chat about healthy eating and exercise. You can join live from 1 to 2 p.m. today at www.washingtonpost.com or leave questions ahead of time. To subscribe to the free Lean Plate Club electronic newsletter, log on to www.washingtonpost.com/wp-srv/email/front.htm.