The food you eat contains the nutrients that serve as building blocks, and provide energy and nourishment throughout your body. In food, nutrients are contained in large molecules that are chemically and physically bound together. Digestion is the process of breaking down these tightly bound molecules into individual nutrients that can be taken into your body and used to support its functions. Simply defined, digestion is cutting things down to a size in which they can be absorbed into your body.
Digestion occurs in the gastrointestinal tract-the 20 to 30 foot long tube extending from your mouth to your anus. Whatever you eat flows through this system, but until it is absorbed through the intestinal tract, the nutrients in food are physically outside of your body. This is because the gastrointestinal tract functions like an internal skin and provides a barrier between whatever you ingest from the outside (external) world and your internal bloodstream and cells. Part of the digestion process, then, is the selective transport of nutrients through the cell wall that lines your intestinal tract. Once transported across the intestinal barrier to the inside of your body, these nutrients can enter your bloodstream and circulate to all of your tissues to maintain organ function, support your need for energy, and provide for growth and repair of new cells and tissues.
While digestion can be simply defined, its mechanics are quite complex. This is because your food contains so many different sizes, shapes, and types of individual molecules, all tightly entwined, and because each of these types of molecules is chemically distinct. Digestion uses both mechanical processes, such as chewing and grinding, which help separate the different types of molecules, as well as chemical processes, in the form of enzymes that can cut the bonds within the molecules, to release small nutrients into your system. An analogy is two or more necklace chains of different types twisted, knotted, and interlocked together. Digestion would be the process of untwisting and separating the chains, usually requiring cutting them in a couple of places, and then pulling them apart and further cutting each of them into many smaller pieces, so they can become building blocks for other necklace chains. Food is a very complex mixture of different types of very large molecules-the proteins and some carbohydrates; mid-range sized molecules-such as fats; and a wide variety of smaller molecules including vitamins, minerals, small carbohydrates like sugars, and other phytonutrients, which are protective substances found in plants (phyto = plant). Most foods you eat are a mixture of all of these different molecules, and since you need a variety of types of nutrients, your body must be able to digest these varied types of molecules in food.
The size, as well as the type of molecule, makes a difference in how a food is digested, the nutrients that are derived from it, and where these nutrients are taken up by your body. Each type of molecule has its own challenge with respect to digestion. Proteins are extremely important because they constitute the majority of the structural tissue in your body, such as bone and connective tissues that provide the shape and form to which your cells attach. Proteins are involved in just about every function in the body as well since enzymes are proteins, and enzymes are the molecules in the body that do much of the work, like building new tissue or removing damaged tissue. Proteins are also message carriers in your body, transporting hormones from one place to another, and transporting signals across your cell membranes to your DNA.
Your body is constantly making new proteins to replenish what's lost from tissue damage or to provide for growth. Enzymes are continually being produced a new to replace older, less functional enzymes. Therefore, to maintain optimal health, your body needs a continuous supply of the nutrients to support protein production.
Proteins are made up of smaller molecules called amino acids that are strung together by chemical bonds like beads on a chain. To become an active, functional protein, this string of amino acids folds in on itself forming a twisted and entwined, three-dimensional structure. An individual protein molecule can be as small as 200 to as large as 5,000 amino acids strung together. In order to make the protein your body needs, it must obtain the protein building blocks, the amino acids, from the proteins in food. Although vegetables and grains do provide some protein, you get most of your protein from nuts, legumes, eggs, fish, meats, and dairy products. When you eat these protein-containing foods, your body must take the large protein chains in them and cut them down to either individual amino acids or dipeptides (two amino acids, di=two, peptide=amino acid) before you can absorb them. Once absorbed, the amino acids are transported through your bloodstream to the tissues that need them, such as muscles. Then, your body uses these amino acids to reconstruct its own proteins in the forms you need to support your tissue's growth and repair.
Your body produces enzymes called proteases to help break down the proteins in food to the amino acids. Proteases cut proteins between specific amino acids to produce the smaller peptide chains. Before the proteases can act on the protein, the protein must first be untwisted, a process called denaturation, which results in a long single-chain protein. Proteins are denatured in the stomach, with the help of the stomach acid (hydrochloric acid), the mixing action of the stomach, and the protease pepsin.
After denaturation in the stomach, the long single-chain protein is transported to the proximal small intestine, the duodenum, which contains several types of proteases. These proteases act on the protein chain, cutting it further until only dipeptides and single amino acids are present. The amino acids and dipeptides are absorbed in the small intestine, primarily in the middle section, the jejunum. A healthy adult is estimated to need around 40 to 65 grams of protein per day. If this is not provided in the food you eat, your body will begin to break down muscle and other tissues to obtain the amino acids it needs. Inadequate intake and digestion of amino acids from protein can lead to stunting, poor muscle formation, thin and fragile hair, skin lesions, a poorly functioning immune system, and many other symptoms.
In plant and animal foods, the amino acids you need are mainly provided in the form of large protein molecules that require all aspects of protein digestion-denaturation in the stomach and protease action in the intestines-before absorption. Free amino acids, which require no processing by the body before absorption, may also be present but are generally not found in large amounts.
In processed foods, protein is sometimes provided as hydrolyzed protein, which means it has been chemically cut into smaller chains from two to 200 amino acids called peptides. These peptide fragments may be easier for your body to digest; that is, they may not need to be denatured in the stomach, but are still too large for direct absorption and must be digested in the intestine. Some specially produced foods for hospital or healthcare use are made of elemental amino acids; these products provide the amino acids themselves and require no digestion before absorption. Fats, also called lipids, are required for many important functions in your body. Fats are a main component of the membranes of all the cells in your body: without fats, your cells would have no covering or boundary. By providing the membrane around all your cells, fats are vital for insulating your body from the outside world. Fats also can be used to provide energy and are involved in supporting the immune system, brain health, and cardiovascular function.
There are many different types of fats, but only a few are essential, which means your body cannot create them internally, so you must take them in through your diet. These essential fats include an omega-6 fatty acid (linoleic acid), and an omega-3 fatty acid (linolenic acid), and are found in the highest amount in nuts, seeds, and fish. Meat contains high levels of fats that are not considered essential, called the saturated fatty acids, and it also contains cholesterol, which is also not essential and is digested in the same way as fats. High amounts of the non-essential saturated fats, and too little of the essential fats can result in problems with the immune system, hardened arteries, and scaling skin, among other symptoms.
As well as being a necessary part of your diet, during digestion, fats also act as carriers of the fat-soluble vitamins (A, D, E, and K) and the carotenoids, thus enabling their absorption. (Carotenoids, such as beta-carotene, are a group of highly colored fat-soluble compounds in plants with a wide range of health protective effects.) Without fats in your diet, you would also not be able to absorb these important vitamins, and would show deficiency symptoms such as problems with blood clotting (vitamin K), weak bones (vitamin D), or vision disturbances (vitamin A). Fats are present in food primarily as three fat molecules attached to a backbone molecule called glycerol, but your body can't absorb this molecule directly. Like protein, your body must first break down this larger molecule into smaller ones. For example, after you eat a piece of salmon, which contains essential fats, your body must first remove, or strip-off the fat molecules from the glycerol backbone to which they are attached. This process is called hydrolysis, and the types of enzymes that hydrolyze fats from glycerol are called lipases. Lipases are secreted under the tongue, in the stomach, and from the pancreas; therefore, fat hydrolysis begins the minute fats enter your mouth and continues in your stomach, where the majority of fat hydrolysis occurs.
After hydrolysis, the absorption of fats is complicated by the fact that, like any oil, they are insoluble in water, and therefore the body has a system in place to provide a solubilized fat aggregate. The body uses bile acids, which act as detergents, to make fat globules, or aggregates. After aggregation with bile, the fat aggregates, also called miscelles are transported to the small intestine, where they can be taken up directly by the intestinal cells and absorbed into the body.
Absorption of the fat from the miscelles begins in the first part of the small intestine, the duodenum, with the majority of absorption occurring in the mid-section of the intestine, the jejunum. The bile acids generally stay behind in the intestinal tract, acting more as a shuttle. Carbohydrates are a varied combination of both very small and very large molecules and comprise about 40 to 45 percent of the energy supply for your body. You get most of your carbohydrates from cereals, fruits and vegetables. Small carbohydrates, like table sugar (sucrose) or glucose, provide a sweet taste to foods. Larger carbohydrates, like starches or fiber, provide substance to foods. Examples of these larger carbohydrates include gums, gels, or pastes, like you get with bread or cookie dough. When cooked, these foods have a structure, like a slice of bread or a cracker, but are mainly composed of different types of carbohydrates. Only the individual small sugar molecules, called monosaccharides (mono=one; saccharide=sugar), can be absorbed directly. Glucose and fructose are examples of monosaccharides. Since carbohydrates exist in food not only as monosaccharides, but also as many combinations of these monosaccharides linked together, your body has to cut these carbohydrates down to their individual monosaccharide units.
Many of the simple sugars that give food its sweet taste are found as two small sugars bonded together. For example, when you eat a bowl of cereal, your body must digest the sucrose (table sugar), which is made of two small sugars, to its monosaccharides. To do this, it uses an enzyme called sucrase, which cuts sucrose to produce glucose and fructose, a process called hydrolysis. The milk on the cereal gets its sweet taste from the carbohydrate called lactose, which is cut (hydrolyzed) into monosaccharides by lactase, to produce galactose and glucose. The majority of carbohydrate hydrolysis occurs in the small intestine; that is, these carbohydrates are mainly transported to the small intestine before they are cut into the monosaccharides glucose, galactose, and fructose. After hydrolysis, these individual monosaccharides are then absorbed directly in the duodenum and jejunum.
Cereals are also high in fiber and provide your body with this important nutrient. Fiber is made of very large carbohydrates containing types of chemical structures that aren't broken down, or digested, by your body. Fiber travels through your gastrointestinal tract intact and ends up in the large intestine, where it provides nutrition for the intestinal bacteria that ferment it. Fiber is called soluble or insoluble, depending on its ability to take up water and to be fermented in the large intestine. Plants store their energy by stringing together many glucose molecules into a long complex of several hundred to several thousand glucose molecules. Plant foods that have stored energy, for example seeds that must provide energy for the young plant when it starts growing, are high in starch. When the young plant starts growing, the starch is broken down to form glucose for energy. Starch is found in food as amylose starch, which is a straight chain starch, and amylopectin starch, which is a branched chain starch.
When you eat foods with starch, like corn or potatoes, your body digests this very large carbohydrate in much the same way as it digests protein. Your body uses a number of enzymes to cut down a large, linear starch chain into the small individual units that are linked together, the glucose molecules, which can then be absorbed in the intestines. The enzymes that breakdown starches are called amylases. Amylases are very important because starch is prevalent in our diet and a main source from which we derive glucose, the primary sugar molecule the body uses for energy. Amylases actually cut starch down to two-sugar units, maltose and isomaltose, and then other enzymes, called maltase and isomaltase, hydrolyze these two sugars into the individual monosaccharide glucose.
Amylases are produced in the mouth and, therefore, when you eat starch it is immediately acted upon, beginning the process of starch breakdown. This is one of the reasons why thoroughly chewing rather than gulping your food is so important. Since the smaller sugars that come from amylase action on starch are sweeter tasting, if you hold a cracker in your mouth and swish saliva around it, you may notice the appearance of a sweeter taste.
One special kind of starch is found in some foods, such as raw, green bananas. It is called resistant starch, and gets its name because it is resistant to digestion. Therefore, resistant starch is more like a fiber, traveling through the intestinal tract undigested until it reaches the large intestine where, like fiber, it may be fermented by the bacteria in the colon. Vitamins and minerals are quite varied in structure and amount in the foods you eat. They can be found in food in a free form, chemically bound to a larger molecule, or tightly encased inside a food aggregate. In most cases, they are liberated during eating by the mechanical process of grinding. They may also be liberated during the breakdown of the large molecules like proteins and starch, in which they may be encased.
Since your body requires specific amounts of these key nutrients, most vitamins and some minerals have active transports in place for absorption and are taken into the body in very specific ways. These active transports act as shuttles, picking up the vitamin or mineral and taking it through the intestinal cell wall into the body, where it may be directly released or transferred to another transport molecule. Since vitamins and minerals are small and are usually found in much lower levels than amino acids, carbohydrate, and fats, these active transports must select and pull these important molecules out of the food and take them into your body. Active transports require energy to function properly.
Calcium and iron are examples of minerals that are taken into the body by active transport. Most of the water-soluble vitamins have an active transport in place as well, and these active transports are primarily found in the middle section of the small intestine, the jejunum. Some minerals, like iron and calcium, are absorbed in the first part of the small intestine as well as the jejunum. The fat-soluble vitamins (vitamins A, D, K, and E), as discussed above, are absorbed with fat miscelles, and therefore require fat to be present for their full absorption.
Magnesium is a mineral of tremendous importance for bone health, energy production, and overall healthy functioning throughout the body since it activates more than 300 cellular enzymes. Like calcium, magnesium must be constantly supplied to maintain optimal function. Magnesium doesn't have an active transport, but depends entirely on dietary intake and a healthy intestinal lining for its absorption, and can be absorbed throughout the entire small intestine and even in the colon. Low intakes of magnesium, or loss of ability of the intestinal tract to absorb magnesium due to intestinal inflammation or disease, can result in a variety of problems such as muscle twitching or tremors, weakness, irritability and restlessness, depression, and weak bones. Magnesium is found at highest levels in whole foods such as grains but is often removed during processing. Whole grain bread and cereals will have a much higher amount of magnesium than white bread, which is made from refined flour.
Vitamin B12 is also absorbed differently from the other vitamins and minerals. First, it is most commonly found attached to proteins, and therefore requires protein breakdown to be liberated. Then, it requires a protein made in the stomach, called intrinsic factor, for its absorption, but is not absorbed until the vitamin B12-intrinsic factor complex reaches the final part of the small intestine, the ileum. Optimal digestion of vitamin B12 is dependent on your ability to make a healthy amount of stomach acid, since protein breakdown requires stomach acid and research has shown that intrinsic factor is also not secreted in adequate levels when stomach acid is low.