All About Digestion, Absorption and Metabolism


Sections
  1. Basic information
  2. Digestion
  3. Absorption
  4. Metabolism

It used to be that when you looked up a spice, you'd find out how to use it in cooking. Now much of the online information is about how to use spices medicinally. Nothing wrong with that! But before we can understand how individual spices act in the body, it's essential to know how food in general is used by the body. That's because spices and herbs adhere to the same basic fundamentals even if each has its own specific health benefits as well.

Basic Information

One of the biggest problems with Digestion, Metabolism and Absorption is that these three terms are often confused with each other. They are distinct and separate processes, but many internet authors use the terms interchangeably or mix up their purposes. The fact that they may take place at the same time at different points in the body can make the picture even more confusing.

Digestive Pathways Digestion is the series of steps that break down foodstuffs into their component parts. Foods are composed of starches (carbohydrates), proteins, fats, and a long list of vitamins, minerals and other things, all stuck together with various mechanical, electrical and chemical bonds. Digestion breaks down those bonds, separating the constituents into individual components (or at least smaller combinations of things).

Food has to be broken down for several reasons. First, most foods contain different compounds that can't all be absorbed through the same pathways. So they have to be divided up into smaller units of similar composition. Second, the molecules in our foods are too large to be absorbed without being separated into smaller pieces. And finally, different processes are needed to turn different kinds of nutrients into energy for our bodies. For example, a spice may contain fats, starches and proteins. They can't all be used in the body by the same processes. So they must be broken down into individual compounds which are then combined with other like things. Digestion takes care of all these needs.

Absorption is the process of moving nutrients from the intestines into the bloodstream. Most nutrients are absorbed through the lining of the small intestine, but water-soluble nutrients are absorbed differently from fats and fat-soluble compounds.

Metabolism converts nutrients into forms that can be used (or stored in the body), and also into forms that are removed (excreted) from the body. The technical word for the latter is xenotoxins, or just toxins. The use of 'toxin' does not mean that these things are poisons, only that they are not accumulated within the body. The desirable ones have a part to play, but are removed when that role is accomplished. In fact, many of the vitamins and minerals that are important components of our foods are also xenotoxins that are metabolized and removed within a specified time frame for each one.

The sequence of absorption and metabolism is not the same for all foods, nor for medications. Some are metabolized only in the liver, a few in the kidneys, and some things are not metabolized at all. For some, metabolism begins in the small intestine before the food or drug ever reaches the bloodstream. Metabolism is frequently a multi-step process, with one phase taking place in the small intestine and the final step in the liver and other tissues. Regardless of the specifics for any particular food or drug, the end result is to prepare it for use and possible storage in the body, and in the case of xenotoxins, to facilitate its elimination when it has completed its purpose.

DIGESTION

Most people think of digestion as something that happens only in the gut, but it actually begins in the mouth with a change in your saliva. When you nose smells food, and even when your brain knows a regular meal time is approaching, serous glands in your mouth and on the surface of your tongue secrete alpha-amylase, an enzyme that begins the process of breaking down fats and starches. Amylase digests starches into maltose and dextrin, two simpler starches. The serous glands also secrete lingual lipase, which begins the digestion of medium and long-chain triglycerides (types of fat).

If your parents told you to chew your food properly (do parents say that any more?), the reason is that it gives your digestive system a head start on breaking down the food before it ever gets to your stomach.

Once in the stomach, the food is subjected to acids, various gastric lipases and mechanical churning to continue breaking it down. The upper part of the stomach (the fundus) mixes the food with gastric lipases, pepsin and gastric acids. These continue the process of breaking down fats and proteins that began in the mouth. This is not digestion in the sense of making nutrients directly available for absorption, but it does break down larger complex molecules (especially the proteins) into smaller units that the small intestine will digest further.

The lower part of the stomach (the antrum) contracts rhythmically to mechanically crush and homogenize the contents, and churn them into the half-digested mixture called "chyme." Because the sections of the stomach are not physically separate from each other, the contents are exposed repeatedly to gastric juices and enzymes as stomach contractions move the food around.

Chyme passes into the small intestine at a rate closely controlled by the nervous system. This is essential, to keep the alkaline environment of the small intestine from being overloaded with the acidic contents of the stomach. Secretin, a peptide produced by the duodenum (the first part of the small intestine), stimulates the pancreas to release fluids rich in sodium bicarbonate. This raises the pH of the chyme to the optimum pH for pancreatic enzymes and bile to complete the task of digesting the food.

At the end of these processes, the compounds that started out as fats, proteins and carbohydrates are reduced to monoglycerides, amino acids and monosaccharides. Please note that this is a very simplified explanation. It isn't meant to be a treatise on any of the topics, just an overview of the differences between digestion, absorption and metabolism.

ABSORPTION

Up to this point, although water-soluble and fat-soluble nutrients are acted upon differently, they are still all together in the digestive tract. Now, however, the pathways for water-soluble and fat-soluble nutrients diverge.

The small intestine is where the bulk of nutrient absorption takes place. Like the stomach, it has distinct sections. The one closest to the stomach is called the duodenum and the one closest to the large intestine is the ileum. In between are all the winding loops of the jejunum. This image gives an idea of how the small intestine is situated in the abdominal cavity.

The jejunum is lined with a layer of special cells called 'enterocytes,' present on the surface of columnar forms called villi. These have an unusual feature. The outside of each enterocyte is covered with additional tiny projections called microvilli, so tiny that they can be seen individually only with an electron microscope. With an optical microscope, they look like a fuzzy layer on the outside of the enterocytes. If you come across a reference to the 'brush border,' of the intestine, it is these microvilli that it refers to, and the term 'brush border' came about because of the fuzzy appearance. Their presence hugely increases the surface area available for nutrient absorption.

Each of the villi has a capillary through it, along with a tube (called a lacteal) that connects with the lymph system. The monosaccharides and other water-soluble products of digestion pass through the enterocyte membranes into the villi and into the capillary inside. From there, they go to the hepatic portal vein and to the rest of the bloodstream.

The long-chain fats (all but a few oils like coconut oil and palm oil) are repackaged within the enterocytes into packets called chylomicrons. These move from the enterocytes into the lacteals, where they enter the lymph system and ultimately the bloodstream.

Short- and medium-chain fats, however, go straight into the bloodstream via the portal vein, along with the monosaccharides and peptides. The red and blue colors of the capillary sections in Figure 2 above indicate that each villus has an incoming blood supply from the arteries and an outgoing blood vessel leading to a vein (in this case, the portal vein).

The term "portal vein" gets tossed around a lot without much explanation of what it actually is. Unlike the other veins in the body, it does not go to the heart. It travels only from the small intestine to the liver, a distance of about 3 inches in the average adult. Direct absorption into the portal vein has been cited as one reason to prefer coconut oil over long-chain oils such as olive oil and most others. Some have even promoted the use of MCT oils (extracts of medium chain triglycerides from coconut oil). Yet this has mixed benefits. Anything that is carried along with short and medium-chain fatty acids undergoes hepatic metabolism before reaching the primary bloodstream circulation, and these metabolic changes can have a significant effect on pharmaceuticals. On the other hand, olive and other long-chain oils take longer to enter bloodstream circulation, as they must detour through the lymph system. But they do not undergo hepatic metabolism immediately, so fat-soluble molecules packaged with them will reach the bloodstream mostly unchanged.

Unrefined coconut oil is approximately 50% medium and short-chain fatty acids with the balance being long-chain fatty acids. It seems reasonable to assume that about half of any fat-soluble nutrients would be carried by those lipids directly into circulation via the portal vein, and about half would be packaged with the long-chain fatty acids and arrive later via the lymph system. This is a very general assumption, though, and would vary widely depending on individual physiology and with the mix of foods in any given meal. It isn’t meant to stipulate any specific percentages: it just provides an overall picture of the two different routes via which fat-soluble nutrients may enter the bloodstream.

METABOLISM

'Metabolism' is used in a very fuzzy way by many people to indicate the rate at which foods are used in the body and waste is excreted. People say 'I have a slow metabolism,' for example (sometimes as a way to justify their inability to reach a desired weight loss goal). But metabolism actually means the conversion of compounds into forms that are either used up, or stored, or excreted. In many cases, all three metabolic steps may take place for a given compound. In addition, some pharmaceuticals (called "pro-drugs") require at least one metabolic step to achieve the active form of the drug.

Metabolic pathways vary widely, depending on the kind of nutrient (carbohydrates, fats and proteins).

Carbohydrates, now in the form of monosaccharides, are used in many ways. In plants and some bacteria, they're rearranged into new polysaccharides to form different kinds of tissues such as cell walls.

One of the monosaccharides, glucose, is transformed in the liver into glycogen, where some is stored. More glycogen is stored in skeletal muscle tissue, to be used in our daily activities. Some glucose continues to circulate in the bloodstream, and is taken up by cells in the body where it participates in the ATP cycle that provides all our body's energy. Where obesity is present, excess glucose is converted into triglycerides and is stored in fat cells. A type of reaction called glucolysis transforms glucose into pyruvate, which is subsequently used in other kinds of reactions.

The digestion of proteins results in amino acids. Their metabolic pathways create new proteins and new amino acids, depending on what is needed in various parts of the body. They also participate in all the enzymatic reactions in the body (enzymes are proteins). They are not stored in our cells, as both fats and carbohydrates are, but must be present in sufficient quantity in our food in order for us to be healthy. Amino acids can also be used by the body to create glucose and lipids. Because of this, they can provide energy in times of food deprivation, via a sequence of metabolic pathways. They are so essential to our health that if the diet is deficient in protein, the body will steal it from skeletal muscle in order to provide energy to keep the liver working properly and to sustain the rapidly dividing cells that line the intestines.

Fats enter the bloodstream still packaged into chylomicrons. Once in the bloodstream, they are again broken down by an enzyme called lipoprotein lipase into triglycerides. The triglycerides are further broken down into fatty acids, glycerol and cholesterol. The fatty acids and glycerol enter cells to participate in the generation of energy, and the cholesterol circulates again to the liver.

Different kinds of fats are used (or stored) throughout the body. They take part in the creation of steroids and hormones, they become an integral part of cell membranes, and they make up a significant part of the neural tissue in the brain. Fat storage provides an energy reservoir for periods of food shortage in the form of triacylgycerols. Finally, cholesterol bonds with two other compounds in the liver (taurine and glycine) to make bile, which is stored in the gallbladder and ejected into the intestines to participate in the digestion of one's next meal.

So that is a very simplified picture of how digestion, absorption and metabolism manage the foods we eat (and the spices we flavor them with). Some spices have a potent effect on one or all of those processes, while others have a more subtle effect, or may be present primarily for their flavor. But they all follow the same physiological pathways, so spice up your food and enjoy it!