Eating By Numbers: Food labels can stump even the smartest eater.

Remember when surprises in your food used to be fun? A temporary tattoo. A neon-orange whistle. A mini Barbie. These days, though, a surprise inside isn't always a good thing, especially when it comes to trans fats, added sugars, and hidden calories. Fortunately, almost every food product you buy comes with a label designed to take the mystery out of your meals. Unfortunately, it's not always easy to determine what, in fact, that label is telling you — which is where this illustrated guide to nutrition labels comes in. Think of it as your own nutritional decoder ring.

Serving Size

All the information on a nutrition facts panel is based on one thing: serving size. So look for the "servings per container" line, which is already making an appearance on some food labels, though the FDA hasn't decided whether it should be required on all of them. Many items most of us think of as one serving — like a regular 16-ounce bottle of Snapple — are really two servings. If that's the case, double the calories and the rest of the nutritional info. Foods listed in ounces require some quick math and good estimation: If a bag of pretzels holds 12 ounces total and the serving size is 3 ounces, one serving is a quarter of the bag.

Calories

If you're trying to slim down or stay at a healthy weight, this is the most important number on the label. Whether a food is low-carb, low-fat, high-fiber, or gluten-free, you'll gain weight if you take in more calories than you burn. To avoid miscalculating calories, buy food that comes in single servings. Not only is the calorie count a no-brainer, but you'll be less likely to overeat: A study from the University of Illinois at Urbana-Champaign found that people who were given large tubs of popcorn put away 45 percent more of it than those who had medium tubs.

% Daily Value

The FDA recommends eating a specific amount of certain nutrients every day, which is why they're called Daily Values or DVs. They're also known as DRVs (Daily Reference Values) or RDIs (Reference Daily Intakes). Beware: The DVs listed on food labels are based on a 2,000-calorie diet, but women usually take in fewer. As a general guideline, add six percentage points to each DV if you're on a 1,500-calorie diet and two if you're averaging 1,800 calories. So for a 1,500-calorie-a-day woman, a 25 percent DV of saturated fat in a burrito is really a 31 percent DV.

Fats

You need fat — up to 62 grams per day if you're on a 1,600-calorie diet — to synthesize hormones and protect vital organs, among other things. It's the type of fat you eat that makes a difference. Avoid saturated and trans fats, which increase your risk for heart disease by boosting heart-clogging LDL ("bad") cholesterol. Trans fats also lower HDL ("good") cholesterol. Healthy fats (monounsaturated and polyunsaturated) aren't always listed on food labels, so subtract saturated and trans fats from the total fat to find them out, says Alice Lichtenstein, D.Sc., director of the cardiovascular nutrition laboratory at Tufts University in Medford, Massachusetts.

Cholesterol

Too much of this soft, waxy substance can build up in arteries and lead to heart disease, so limit your cholesterol intake to 300 milligrams a day. But also keep this in mind: Only about 25 percent of the cholesterol in your body is absorbed from food. The remaining 75 percent is produced by your liver, and any trans fats you eat (see "Fats") play a major role in this process. The fewer you consume the better.

Sodium

Aim for foods with 5 percent or less of the DV for sodium and avoid those with 20 percent or more to stay within the 2,300-milligram limit recommended by the new dietary guidelines. "Many of us eat restaurant meals loaded with salt, so it's important to choose low-sodium foods when we're grocery shopping," says Dawn Jackson Blatner, R.D., a spokesperson for the American Dietetic Association.

Carbohydrates

Carbs is a big umbrella category for sugars, starches, and sugar alcohols that all share a similar molecular structure and are an excellent source of energy. (Fiber, which you can read more about on page 38, is also listed under carbs.) The right carbs can keep you full and fueled, but simple starches won't. Plus, they lack vitamins and minerals. So look for foods with "whole grain" high on the ingredient list.

Sugars

Our bodies break down fat and protein to create glucose, the fuel we run on. But sugar is already glucose, so our bodies can use it fast — it's a great source of short-term energy. It won't keep you going for long, though, and it doesn't contain any vitamins or minerals. Find foods that combine sugar with healthier ingredients. For example, fructose, the natural sugar found in fruit, is usually accompanied by high-fiber pulp, antioxidant-rich skin, and lots of vitamins and minerals.

Vitamins

Water-soluble vitamins, including C and the Bs, will flush out of your body if you consume more than you need. (So taking huge amounts of them won't do anything but turn your pee orange.) Fat-soluble vitamins A, D, E, and K are stored in our bodies, and consuming too much of these can be toxic, though this is unlikely. Minerals can also cause harm if you go overboard — too much calcium, for instance, can lead to kidney stones and may decrease the absorption of other minerals like iron and zinc. You don't have to worry about OD'ing if you eat a balanced diet, but be careful when taking supplements.

"Not a Significant Source"

Food companies have to list all of the ingredients required by the FDA, but if a product doesn't contain one of those — or has it in only tiny amounts — it gets listed here. (The only exception: If the company makes specific claims — Low in sugar! Zero carbs! — then that category must be clearly defined in the main list.)

Ingredients

The higher the quantity of a particular ingredient in a food, the farther up on the list it will be. "The first three to five ingredients are the most important," Blatner says. If partially hydrogenated oil, high-fructose corn syrup, or enriched flour (all nutritionally lacking) take up the first few spots, you're probably better off without it. Look for whole, natural ingredients, like fruits, vegetables, and whole grains. And avoid foods with ingredient lists containing lots of unrecognizable words — you're probably getting more preservatives and additives and fewer nutrients.