We all know about the common fats that different foods contain. Meat contains animal fat. Most breads and pastries contain vegetable oils, shortening or lard. Deep fried foods are cooked in heated oils. Fats are greasy and slick. Nutritional label from a bottle of olive oil You commonly hear about two kinds of fats: saturated and unsaturated. Saturated fats are normally solid at room temperature, while unsaturated fats are liquid at room temperature. Vegetable oils are the best examples of unsaturated fats, while lard and shortening (along with the animal fat you see in raw meat) are saturated fats.

However, most fats contain a mixture. For example, above you see the label from a bottle of olive oil. It contains both saturated and unsaturated fats, but the saturated fats are dissolved in the unsaturated fats. To separate them, you can put olive oil in the refrigerator. The saturated fats will solidify and the unsaturated fats will remain liquid. You can see that the olive oil bottler even chose to further distinguish the unsaturated fats between polyunsaturated and monounsaturated.

Unsaturated fats are currently thought to be more healthy than saturated fats, and monounsaturated fats (as found in olive oil and peanut oil) are thought to be healthier than polyunsaturated fats. Fats that you eat enter the digestive system and meet with an enzyme called lipase. Lipase breaks the fat into its parts: glycerol and fatty acids. These components are then reassembled into triglycerides for transport in the bloodstream. Muscle cells and fat (adipose) cells absorb the triglycerides either to store them or to burn them as fuel.

You need to eat fat for several reasons: As we will see in the next section, certain vitamins are fat soluble. The only way to get these vitamins is to eat fat. In the same way that there are essential amino acids, there are essential fatty acids (for example, linoleic acid is used to build cell membranes). You must obtain these fatty acids from food you eat because your body has no way to make them. Fat turns out to be a good source of energy. Fat contains twice as many calories per gram as do carbohydrates or proteins. Your body can burn fat as fuel when necessary. Calories A calorie is a measurement of energy.

We tend to associate calories with food, but any sort of energy can be measured in calories. The official definition of a calorie is the amount of energy needed to raise the temperature of a gram of water by 1 degree C. A kilocalorie is 1,000 calories. Just to make life confusing, the "calorie" that you see on packages of food is really a "kilocalorie" in the scientific sense. It makes sense that food contains energy, because most foods burn. For example, if you have ever roasted marshmallows, you probably know that marshmallows burn. What's burning in that case is the sugar in the marshmallow. Fat burns too -- you know that if you have ever seen a grease fire.

Your body "burns" fats, carbohydrates and proteins -- not with flames, but with more controlled chemical reactions that release the energy in different ways. Fats, proteins and carbohydrates have characteristic calorie measurements. One gram of fat contains almost 9 calories (kilocalories) of energy. One gram of any carbohydrate contains 4 calories (kilocalories). One gram of protein contains 4 calories (kilocalories) as well. Knowing these values, you can calculate the number of calories in any food as long as you know how many grams of fat, protein and carbohydrates it contains.

If you were to take any food, dry it out and burn it, the specified number of calories would be released by the flames. If you ingest 3,500 extra calories one day (or over the course of several weeks or months), your body will convert the excess energy to body fat and save it for a rainy day.

To lose 1 pound of fat, therefore, you have to burn off the 3,500 excess calories. You can do that either by exercising or by restricting your calorie intake. The USDA estimates that the average man, 5 feet 10 inches tall and weighing 174 pounds, needs 2,900 calories per day (assuming light to moderate activity). The average woman, 5 feet 4 inches tall and weighing 138 pounds, needs 2,200 calories.