Find out how many calories you need to keep your body fueled and fit.

If you're interested in nutrition or weight loss, you no doubt pay a lot of attention to calories. But do you know what exactly calories are, and how many you really need?

Calories: The Good, the Bad, and the Empty

There is really no such thing as "good" or "bad" calories. "Your body processes each calorie the same," says Kimberly Lummus, MS, RD, Texas Dietetic Association media representative and public relations coordinator for the Austin Dietetic Association in Austin, Texas. But Lummus adds that some foods are far more nutritious than others. "We strive to make our calories the most nutrient-dense that we can, meaning that we are packing in a lot of nutrition for a very small amount of calories. You are optimizing your calorie budget, so to speak."

While calories get a negative rap when it comes to weight control, calories are actually an important source of fuel you cannot live without. "Your body needs calories for energy," says Lummus. Calories are the force behind everything we do, including eating, sleeping, and breathing.

"Calories are how much energy your body gets from the food and beverages that it consumes," says Lummus. Most food sources are composed of some combination of carbohydrates, fats, and proteins, and each of these nutrients contains calories. Yet it's important to stay away from "empty" calories in foods like sweets and soda, warns Lummus.

Calories: Finding Your Magic Number

You must find the right balance of calories every day, depending on your overall goals. "Eating too many calories and not burning enough through physical activities would yield a weight gain, while not eating enough calories [to keep up with your calorie burn] would yield a weight loss," says Lummus.

The number of calories a person needs depends on many individual factors, including age, weight, height, and activity level. When dieticians counsel clients on calorie needs, they take all of these facts into consideration and come up with a suggestion for how many calories are needed to maintain, lose, or gain weight.

In general, men need between 2,000 to 2,400 calories and woman between 1,200 and 1,500 calories per day. Consuming less than 1,200 calories per day can be harmful to your health, notes Lummus, since it may trigger your body to go into starvation mode, causing your body to actually hold onto calories.

Teenagers' caloric needs can vary considerably. For example, teenage boys may require up to 3,000 calories per day, while teenage girls usually need around 2,200 calories each day. "For children, calorie needs are going to change a lot more because they are growing so rapidly," Lummus continues. She says that infants 5 to 12 months of age need around 850 calories daily, 1- to 3-year-olds need roughly 1,300 calories daily, 4- to 6-year-olds need about 1,800 calories daily, and 7- to 10-year-olds require 2,000 calories daily.

"Counting calories is usually not necessary for children," says Lummus. "You just want to make sure that your child is getting all of the requirements from all of the food groups."

Both children and adults should get the bulk of their calories from a variety of healthful foods, including low-fat or fat-free dairy products, fruits and vegetables, whole grains, and lean protein sources — the building blocks of a nutritious diet.