How are Nutrients Incorporated Into the Body?
Food Must be Digested Before Nutrients can be Absorbed into Tissues
Oct 30, 2009 Adrienne Larocque
Nutrients can be divided into two main types. Macronutrients supply the body with energy and represent the greatest nutritional need (in the amount of hundreds of grams) on a daily basis. Carbohydrates, fats and proteins are sources of energy. Micronutrients are needed in much smaller amounts (milligrams or micrograms), but are equally important for good health. These include vitamins and minerals.
Nutrients normally enter the body when a person eats food or drinks liquids through the process called ingestion. There are several processes that determine where nutrients ultimately end up. These are:
- ingestion
- digestion (mechanical and chemical)
- absorption by the bloodstream
- metabolism
- incorporation into tissues
- egestion
- excretion
Food must be broken down mechanically and chemically during digestion. Chewing is the main mechanism for making food into smaller pieces; smaller particles have higher surface area. Higher surface area increases the rate of chemical reactions that transform large, complex molecules into simpler ones that can be absorbed across the layer of cells that line the intestines (relatively little absorption actually occurs in the stomach). These chemical reactions are catalyzed by specialized proteins known as enzymes.
Digestion and Absorption of Carbohydrates
To be absorbed, carbohydrate structures must be disassembled into their constituent monosaccharide or simple sugar structures (mainly glucose as well as fructose and galactose). Humans manufacture the enzymes necessary to break down sugars and starches into monosaccharides. However, we do not have the enzymes for digestion of fiber. As a result, fiber is indigestible and passes out of the body through a process called egestion.
Starches are polysaccharides consisting of a combination of amylose and amylopectin. Amylase, the enzyme that breaks down these starches, is present in small amounts in saliva and large amounts in pancreatic secretions released into the small intestine. Sucrose, lactose and maltose are disaccharides (sugars) that require sucrase, lactase and maltase, respectively, to be hydrolyzed into monosaccharides for transport from the across the intestinal lining and into the blood.
Digestion and Absorption of Fats
Most fats in the diet are known as triglycerides or triacylglycerols. They consist of three fatty acid chains, each linked by an ester bond to one of three OH groups on a glycerol molecule. Digestion of triglycerides is slow, and mainly occurs in the top of the small intestine. The first step requires the presence of bile salts (produced in the liver but stored in the gallbladder) to emulsify fats. Emulsification is necessary to break up fats into small droplets (i.e., increasing surface area) to facilitate the action of lipase.
Lipase is a water-soluble enzyme produced in the pancreas that hydrolyzes lipids. Lipase eventually separates triglycerides into constituent glycerol and fatty acid molecules which can then diffuse into intestinal cells. Inside the cells, triglycerides are reformed and combined with protein and cholesterol into chylomicrons. These lipoprotein particles pass into the lymphatic system which eventually carries them to the bloodstream.
Digestion and Absorption of Proteins
Enzymatic digestion of proteins begins in the stomach where proteases are secreted. Protein molecules consist of long chains of amino acids and must be broken down into individual amino acids or short peptide chains before absorption can occur. Once broken into amino acids or short peptides, they are absorbed into the bloodstream and transported to where they are needed.
Digestion and Absorption of Vitamins and Minerals
The majority of mineral absorption occurs in the small intestine where metals are present as dissolved ions in watery intestinal juices. Water-soluble vitamins are absorbed directly into the bloodstream. Fat-soluble vitamins such as A, D, E and K, can only be digested, absorbed and transported if fats are present.
Numerous transport mechanisms operate, depending on the specific nutrient as well as the amount available to be absorbed. Also, the presence of absence of other nutrients can affect absorption. For example, the presence of animal protein enhances zinc absorption. Vitamin C improves uptake of iron. Transport of calcium and phosphorus across epithelial cells is greatly enhanced by the presence of Vitamin D.
How Nutrients are Used by the Body
Nutrients are necessary for metabolic reactions. Catabolic reactions involve breaking down molecules like glucose and fatty acids into usable forms of energy like adenosine triphosphate (ATP). In contrast, anabolic reactions involve the construction of more complex molecules from simpler ones. These reactions result in new cellular material, ranging in size and complexity from proteins and cells to tissues and organs.
In addition to supplying energy, fat and protein are necessary for the growth, maintenance and repair of cells and tissues. Micronutrients are necessary for both structure and function. For example, the mineralization of bone requires the use of calcium, magnesium, phosphorus and Vitamin D. Waste products of metabolic reactions and nutrients that are not stored in internal tissues are excreted or removed from the body through organs such as kidneys, lungs, skin and hair.
Sources of Information
To learn about this subject in detail, consult the excellent site Pathophysiology of the Digestive System
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