When you bite off a piece of bread and chew, you begin digestion, but you could never absorb nutrients if the process stopped there. As you continue chewing, your mouthful becomes sweet if you're eating carbohydrates because enzymes in your saliva begin to break complex carbs down into simple sugars. Later, in the stomach and intestines, proteins and fats are also broken down by enzymes. Without enzymes, digestion could never be completed, and you would die of malnutrition, even if you munched and chomped all day.
Digestion and Absorption
Digestion and absorption are two very different processes that work together to ensure your gut is able to extract all the nutrients your body needs, according to Purdue University. Digestion is where nutrients in food are broken down into their component parts. Chemical digestion refers to the work performed by enzymes throughout your digestive tract, which break the bonds that hold molecules together so that proteins, carbohydrates and fats are split into single molecules. Only these smaller molecules can pass through the lining of your small intestine and be absorbed into your body.
In your Mouth
The process of chewing food is called mechanical digestion. While it's not a chemical process, chomping your food into small pieces is the first step in chemical digestion because enzymes can only work successfully on smaller pieces of food. Glands in your mouth secrete saliva, which moistens the food and makes it stick together into lumps that can easily be swallowed. Saliva also contains the enzyme amylase. When chewing carbohydrate-rich foods, salivary amylase breaks down the carbs into smaller molecules of sugar.
In Your Stomach
The most important enzyme excreted by the glands lining your stomach wall is called pepsin, which breaks proteins down into soluble peptides. Pepsin needs acidic conditions to work properly, so hydrochloric acid is also excreted into the stomach. Churning by strong muscles that line your stomach helps mix the enzyme into your food. The acidic conditions eventually destroy the salivary amylase, but by the time your food, now a corrosive mass called chyme, leaves your stomach, both carbohydrate and protein digestion are well advanced.
The Small Intestine
A significant amount of digestion occurs in the small intestine. Bile from the gallbladder neutralizes chyme, making it alkaline. Bile salts emulsify fats and oils into droplets, allowing enzymes to begin the chemical digestion of fats. Pancreatic juice containing a mixture of enzymes also enters the small intestine. These enzymes include proteases to break down proteins, lipases that digest fats and more amylase to finish splitting carbohydrates. Fats are broken down into fatty acids and glycerol, proteins into peptides and amino acids, and carbohydrates become simple sugars such as glucose and fructose.
The Large Intestine
In the past, it was believed that everything of nutritional value had been digested by the time it reached the large intestine. Only salt, water and some vitamins were believed to be absorbed there. However, research continues to show a more complex role. Even though the large intestine does not produce enzymes, bacteria that thrive there continue the digestive process by fermenting carbohydrates that weren't digested in the small intestine. This process produces a small amount of energy, as well as vitamin K, according to Colorado State University.