Registered dietitian Jill Weisenberger once had a client who kept a puzzling food journal. The calorie counts were all out of whack. The woman's tuna sandwich had 33 calories. An apple: 144.

Turns out the woman was mistaking a food-calorie book's index for a calorie chart.

It's not too far-fetched, seeing as few common health words are as baffling to us as "calorie." Calories are invisible, yet we try to count them and cut them. Food supplies them, but they're not nutrients.

"They're abstract," said Marion Nestle, a professor of nutrition at New York University, who says calories are "a mess" to explain. "Ordinary mortals cannot count, see, taste, smell or feel a calorie," she said.

As a result, we're bizarrely wedded to a concept we know very little about. Calories are the first thing people look for on the Nutrition Facts panel of food and beverages, according to a survey by the International Food Information Council and Foundation. But just 15 percent of Americans can accurately estimate the number of calories they should be consuming. And less than one-third of us correctly say that "calories in general are most likely to cause weight gain."

"I frequently discourage calorie counting because it can be tedious and ruin the joy of eating," said Weisenberger, a food and nutrition consultant based in Virginia. "But I still want my patients to understand the concept so they can compare different food choices."

And if they don't understand? Focus instead on food choices and portions. "But weight loss won't happen without calorie reduction," she said.

Most of us simply want to know how many calories we need. But to truly grasp how calories can help you reach and maintain a healthy weight, it helps to know the basics.

Q&A

Q Are some calories more fattening than others?

A Not any more than the dollar in your right pocket will buy you more than the dollar in your left pocket. Some foods are more jam-packed with calories than others. You can eat a cup of pasta for about 220 calories, or you can eat about 4 cups of broccoli or you can munch on about one-quarter cup of nuts. They're all about the same calories, but the 4 cups of broccoli will tame your hunger more.

Q How many calories do I need?

A The best way is to look at your weight. If you're gaining weight (and don't want to), you're eating too many calories. You can also get an estimate at mypyramid.gov.

Q How can I cut calories without starving or hating what I eat?

A If you like vegetables, there's no reason to be hungry. They're relatively low in calories and pretty filling because of their fiber and water content. Eat a couple of cups of them at dinner. It's a lot easier to decrease your potatoes and steak when you've got broccoli or zucchini or both to fill you up.

Also try cooking at home. If take-out is your norm, commit to cooking just once or twice each week. And allow indulgences. There's no reason to give up your favorite foods. Eat smaller portions or eat them less often.

By the Numbers

2,000 to 3,000: The range of calories per day that maintains body weight in most people. Bigger or more active people need more. Smaller, more sedentary people need fewer.

3,500: Number of calories in 1 pound of body fat.

A calorie is …

A unit of energy. First described in the 1800s, a calorie is technically the amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius. But in the health and nutrition world, a calorie is the potential energy in food and the amount of energy the body uses, according to the American Dietetic Association's Complete Food and Nutrition Guide.

"We need this reference value in the same way we'd need to know how many pieces of wood to build a certain size house," said San Diego registered dietitian Janice Baker, a certified diabetes educator. "Everyone's body needs different amounts of energy based on height, weight, activity level, age and other factors. A calorie is not good or bad. It just is."

A calorie comes from …

Three nutrients: carbohydrates, fats and proteins. Water, vitamins and minerals are all calorie-free. When we digest food, the nutrients are released, absorbed into the bloodstream and converted to glucose, or blood sugar. This powers the body, allowing us to shiver, blink, remember, breathe or run marathons. The food energy we don't need right away is stored as body fat, regardless of the nutrient it comes from. That means excess carbs are no more fattening than additional calories from any source, including fats and proteins.

Calories can help us lose weight if …

We burn more calories than we take in. "It doesn't matter when you eat them; your body uses the calories from ice cream in the same way, whether you eat at 10 p.m. or 10 a.m. But if you deliberately eat lightly during the day to have a good dinner, then hunger often gets out of control and you overeat," said Susan Roberts, director of the Energy Metabolism Laboratory at Tufts University's Jean Mayer USDA Human Nutrition Research Center on Aging. "Pacing calories is an important component of successful dieting."

Registered dietitian Jill Weisenberger tells her clients that calories are money. "You have a certain amount in your budget, and if you spend too much, you go into debt. If you take more than your calorie allowance, you get fat."

If you want extra money for something special, you might try to earn more or save. "Think of calories the same way: If you want some extra for a special dessert or other treat, earn them by doing extra exercise, or save them from another time," said Weisenberger. "A 500-calorie slice of cheesecake will take an hour or more of really hard exercise. Or you could skip that second piece of buttered toast at breakfast, cut your juice in half and trade in your large sandwich for a smaller one. Or you could combine dietary and exercise changes."