The primary role of the human digestive system is to break down and absorb nutrients from food (see References 1). There are three major groups of dietary nutrients, collectively called the macronutrients: carbohydrates, proteins and fats. A typical Western diet is composed of 50 to 60% carbohydrates, 10 to 20% protein, and 30 to 40% fat (see References 2). Macronutrients are large molecules that are made of smaller building blocks, called micronutrients. Macronutrients are too large to be directly absorbed by the gastrointestinal tract, so they must be broken down into their micronutrient constituents to be absorbed. Digestive enzymes play the vital role of breaking down macronutrients into micronutrients (see References 3).
Background
An enzyme is a protein in the body that allows biochemical reactions, like the metabolism of nutrients, to occur. There are three main digestive enzymes: amylases digest carbohydrates, proteases digest proteins and lipases digest fats. Digestive enzymes work by a method called hydrolysis; they split macronutrients into micronutrients through the addition of a water molecule (see References 3).
Amylases
Amylases hydrolyzes starches. Carbohydrates are stored as starch in plants and are the largest source of calories in the human diet. They are broken down into simple sugars like glucose, galactose and fructose. Amylases are produced and secreted by the salivary gland and the pancreas. Salivary amylase initiates carbohydrate digestion in the mouth and is degraded in the stomach. Pancreatic amylase facilitates carbohydrate digestion in the small bowel, where the majority of carbohydrate hydrolysis occurs. Starches are broken down into sugar molecules and absorbed by the small intestine. Some starches cannot be hydrolyzed by digestive enzymes; these are commonly referred to as fiber and pass through the gastrointestinal tract intact and undigested (see References 1, 2 and 3).
Proteases
Proteases hydrolyze proteins. Proteins are vital to nearly all functions of the body, including cell growth, tissue repair, immunity, and energy. They are broken down into amino acids (see References 3). Proteases are manufactured and secreted by the stomach and small intestine. Initial protein digestion begins in the stomach by proteases called pepsins I and II. The pepsins are manufactured and secreted by the stomach cells. Eighty percent of protein digestion occurs in the small intestine, where the pancreas secretes additional proteases, mainly trypsin and chymotrypsin (see References 1). The majority of amino acid absorption occurs in the small intestine.
Lipases
Lipases hydrolyze fats. Fats play versatile roles in the body, including providing structure to cells, supplying a concentrated source of energy, and facilitating the absorption of vitamins A, D, E and K (see References 3). Fats are broken down into fatty acids and glycerol. Fat digestion begins in the stomach, where approximately 10% of total lipid degradation occurs. It continues in the small intestine where pancreatic lipases are secreted. Fats are primarily absorbed in the small intestine, but some are also absorbed in the colon (see References 1, 2 and 3).
Other Enzymes
There are many other types of digestive enzymes to aid in the hydrolysis of dietary nutrients. These include cholesterol esterases (cholesterol), nucelotidases (nucleic acids), ribonucleases and deoxyribonucleases (ribonucleic acids and deoxyribonucleic acids), lactase (the sugar lactose) and maltase (the sugar maltose). Water, minerals, simple sugars and most vitamins are usually absorbed in their basic form (see References 3). Carbohydrates, proteins, and lipids need to be converted to their building blocks by digestive enzymes before they can be absorbed (see References 3).
References
- Digestive Diseases and Sciences; "Human Pancreatic Digestive Enzymes"; D,C, Whitcomb, M.E. Lowe; 2007.
- The Science and Practice of Nutrition Support; "Nutrient Digestio, Abosrption, and Excretion"; M.H. DeLegge, C. Ridley; 2000.
- Krause's Food, Nutrition, and Diet Therapy (11th Ed.); L. K. Mahan and S. Escott-Stump; 2004.
