They must be identified on the label as a dietary supplement. Some may have a mild reaction while the reaction in others may be severe and require medical attention. The energy available in food is measured in calories , a unit of heat. In chemistry and physics, a calorie is defined as the amount of heat needed to raise the temperature of one gram of water by one Celsius degree. A food calorie is actually a kilocalorie - the amount of heat needed to raise the temperature of 1000 grams of water by one Celsius degree.
Digestion occurs in the gastrointestinal tract, also known as the alimentary canal .
While individual metabolism rates are different, food generally takes from 18 to 20 hours to pass through the alimentary canal. While the teeth physically break down food into small particles, saliva begins to mix with the food. Saliva is a mixture of water, mucus, and the digestive enzyme called salivary amylase. The mucus in saliva softens and lubricates food and helps hold the food together. The salivary amylase begins the chemical digestion of carbohydrates by breaking down starch into the disaccharide maltose.