Calories come from macronutrients found in the foods you eat. When you eat food, it enters your digestive system. The food is broken down in your stomach and small intestine and them absorbed into your body. After the food you eat has been absorbed, your body can then use the calories in the food for energy. The process of turning the food you eat into energy is called energy metabolism.
Macronutrients
Calories tell you how much energy your body can use from the food. The three macronutrients found in food that contain calories are carbohydrate, protein and fat. Carbohydrate and protein both contain four calories per gram, while dietary fat contains nine calories per gram. Pound for pound, dietary fat contains more calories and energy than carbohydrate or protein.
Carbohydrate
Carbohydrates include sugar, starch and fiber. Sugar and starch can be broken down into smaller components and absorbed by your body for energy while fiber typically does not provide a significant source of calories. Carbohydrate digestion starts in the mouth and stomach where enzymes start to break down sugar and starch. They are primarily digested and absorbed in the small intestine.
Sugars are easily broken down to glucose and fructose by enzymes found in your small intestine. They are then absorbed into your blood stream. Complex carbohydrates, such as whole grains, are first broken down into smaller pieces by enzymes from your intestines and pancreas. Once they are broken down into smaller sugars, then they are digested into glucose and fructose and absorbed into your bloodstream.
Dietary Fat
When your body breaks down the fat in your diet, your small intestine uses bile acid from your liver and the fat from the food you eat to form a micelle. The micelle is absorbed into your lymphatic system and then taken to your liver where it is processed into lipoprotein particles. These lipoprotein particles are then sent to organs and tissues for energy use or storage.
Protein
Protein is found in many food sources such as meat, dairy and legumes. Protein in your food is broken down to smaller particles by enzymes in your stomach. Enzymes from your pancreas as well as your small intestine help digest the protein into small components called amino acids. These amino acids are the building blocks for protein and are absorbed in your small intestine, where they can be used for building muscle or other tissues. Your body usually relies on carbohydrate or fat as a source of calories for energy.
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