From all you hear, you'd think fat and calories are really bad for you. It's true that some people have too much fat in their diets and eat more calories than they need. But we all need a some fat and calories in our diets to fuel our growth and activities. So what's the truth about fat and calories?

What Are Fat and Calories?

A calorie is a unit of energy that measures how much energy food provides to the body. The body needs calories to work as it should.

Dietary fats are nutrients in food that the body uses to build cell membranes, nerve tissue (like the brain), and hormones. Fat in our diet is a source of calories. When you eat more calories than the body uses, the extra energy is stored as body fat. This is the body's way of thinking ahead: By saving fat for future use, it plans for times when food might be scarce and can use the stored fat as fuel.

Food Labels: Calories

Food labels list calories by the amount in each serving size. Serving sizes differ from one food to the next, so to figure out how many calories you're eating, you'll need to do three things:

1. Look at the serving size.
2. See how many calories there are in 1 serving.
3. Multiply the number of calories by the number of servings you're going to eat.

For example, a bag of cookies may list 3 cookies as a serving size. So if you eat 6 cookies, you are eating 2 servings, not 1. To figure out how many calories those 2 servings contain, you must double the calories in 1 serving.

Low-fat, reduced-fat, light (or lite), and fat-free are common terms you may see on food packages. The U.S. government has strict rules about the use of these phrases: By law, fat-free foods can contain no more than 0.5 grams of fat per serving. Low-fat foods may contain 3 grams of fat or less per serving.

Foods marked reduced fat and light (lite) are a little trickier, and you may need to do some investigating. Light (lite) and reduced-fat foods may still be high in fat. To be labeled light (lite), the food must have 50% less fat or one-third fewer calories per serving than the regular version. Foods labeled reduced-fat must have 25% less fat per serving than the regular version. But if the regular version of a particular food was high in fat to begin with, the reduced-fat version may still be high in fat and may have more added sugar.

4, 4, and . . . 9?

The calories in food come from carbohydrates, proteins, and fats:

• A gram of carbohydrate has 4 calories. 
• A gram of protein has 4 calories. 
• A gram of fat has 9 calories — more than twice as much as the other two.

That's why one food with the same serving size as another may have far more calories. A higher-fat food has many more calories than a food that's low in fat and higher in protein or carbohydrates.

So, the amount of fat in foods can make quite a difference when it comes to total calories in a food.

Not All Fats Are the Same

All types of fat have the same amount of calories, but some fats are better than others. Unsaturated fats are “healthy fats” because they can help lower cholesterol and are good for heart health. They are liquid at room temperature and mostly come from plants.

• Monounsaturated fat is found in olive, peanut, and canola oil; most nuts; and avocados.
• Polyunsaturated fats are found in soybean, corn, and sunflower oils. Fish, walnuts, and flax seeds are high in healthy omega-3 polyunsaturated fats.

Saturated fat and trans fat raise blood cholesterol levels and increase a person's chance of heart disease. Saturated and trans fats are solid at room temperature — like butter, lard, and fat on meat. Saturated fats and trans fats are listed on food labels.

• Saturated fat comes mostly from animal products, but some plant oils, like palm oil and coconut oil, have saturated fat. 
• Trans fats are often found in packaged baked goods, like pastries, cookies, and crackers, and fried foods. Partially hydrogenated vegetable oil is high in trans fats. Hydrogenation is a process that changes liquid oils into a solid form of fat by adding hydrogen. This helps food containing these fats keep for a long time without losing their flavor or going bad.

Fat and Calories in a Healthy Diet

It's a bad idea to try to avoid fat completely. Fats are an important source of energy and they can help you feel full. Fat in your diet is needed to absorb vitamins A, D, E, and K.

Fats should be eaten in moderation. The American Heart Association recommends that people choose healthy unsaturated fats in place of saturated fats and trans fats.

Like fat, you need a certain amount of calories in your diet to fuel your body. Teens come in all sizes and each person's body burns energy (calories) at different rates, so there isn't one perfect number of calories that every teen should eat. You don’t need to count calories to keep a healthy weight. Choose a variety of foods to eat, including vegetables and fruit, whole grains, low-fat dairy, and protein foods. Pay attention to when you feel hungry and stop eating when you feel full.

Your body needs calories to operate — to keep your heart beating, your lungs breathing, and your brain thinking. Your muscles use calories to move. Being active every day keeps your body strong and can help you maintain your weight.

Eating more calories than your body needs can lead to being overweight and other health problems. If you are concerned about your weight, speak to your doctor.