The energy in food is expressed as calories. Fat, protein and carbohydrates all release energy when ingested, and fat, providing nine calories per gram, provides the most energy. Calories are the standard unit in which food products in the United States are labeled, however, the scientific unit of energy is actually a kilocalorie, providing 4.184 kilojoules of energy.
Energy From Fat
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Fat is the most concentrated source of energy providing nine calories per gram. This is more than twice the calories provided by proteins and carbohydrates. The calorie density of fat makes a high-fat meal very filling, such as a meal containing butter, whole-fat dairy products, red meat and fried foods. Fats serve several important functions in your body. They are incorporated into your cell membranes and create a flexible protective barrier to the cell walls that allow only appropriate molecules to enter the cell. Fats insulate your vital organs and help to control basic functions of metabolism. Additionally, many compounds, such as fat-soluble vitamins, are stored in fatty tissue.
Energy From Protein
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Dietary protein provides four calories per gram. Proteins play many vital roles in your body. They are the building blocks of muscles, bones, skin and blood as well as being instrumental in the function of enzymes, hormones and vitamins. Meat, fish, poultry, eggs, nuts and diary are great sources of protein that provided many needed vitamins including B vitamins.
Energy From Carbohydrate
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Carbohydrates, like protein, provide four calories per gram. Carbohydrates are broken down into glucose by the digestive system, which is the universal energy source for the cells of your body. Fiber is a type of carbohydrate that is not broken down into glucose; it passes through your body undigested. The 2005 Dietary Reference Intakes recommend 14 grams of fiber for every 1,000 calories consumed.
It is important to consume a balanced diet with fat, protein and carbohydrates. The calories you require depend on many factors such as age, sex, height, weight and activity level. The USDA has named five food groups, which contain either fat, carbohydrates, protein or a combination of the three, that all have an important place in a healthy diet. These food groups are vegetables, fruits, grains, protein foods and dairy foods.
- National Institutes of Health: Inside Life Science - What do fats do in the body?
- Unites States Department of Agriculture: Proteins – why is it important to make lean or low fat choices from the protein foods group?
- Institute of Medicine of the National Academies: Dietary Reference Intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids
- United States Department of Agriculture: Choose My Plate - Choose a food group
Melanie Di Stante is a registered dietitian and a NCBDE-certified diabetes educator with more than 10 years of experience. She received a bachelor's degree in dietetics from the University of Connecticut and a master's degree in human nutrition from the City University of New York. Di Stante has been writing professionally for more than 10 years, contributing to local newspapers and "Today's Dietitian."