Digestive Wellness: Strengthen the Immune System and Prevent Disease Through Healthy Digestion, Fourth Edition (10 page)

Figure 2.2
The GI mucosa.
(The McGraw-Hill Companies, Inc./Al Telser photographer)

Figure 2.3
Lamina propria and epithelial tissue.
(The McGraw-Hill Companies, Inc.)

The lamina propria connects the digestive system to the lymphatic system for digestion of fats and to the blood for absorption of nutrients (
Figure 2.3
). The lamina propria is where lymphatic nodules, lymphocytes, plasma cells, and macrophages form the first line of defense against infections. These are lymphatic nodes (like the ones that swell in your throat when you have a bad cold) that run throughout the digestive system but are most prominent in the tonsils, small intestine, appendix, and large intestine. The lymphatic system circulates fluids throughout your body, drains excess fluids from the fluid between cells (interstitial fluid), initiates immune response against infection and allergy, transports the fat-soluble vitamins A, D, E, and K, and brings digested fats into the bloodstream, among other things.

The lamina propria is also where cytokines, such as IL-6, IL-10, TNF-alfa, and others, are produced. Some cytokines are inflammatory, while others are healing.

The final layer of the GALT is the muscularis mucosae, which is a thin layer of smooth muscle that runs from the stomach through the small intestine. (See
Chapter 9
.)

The small intestine is hardly small. If this coiled-up garden hose were stretched out, it would average 15 to 20 feet long. If spread flat, it would cover a surface the size of a tennis court. Here food is completely digested and absorbed. Nutrients are absorbed through hundreds of small, fingerlike folds called villi in the epithelium, which are covered, in turn, by millions of microvilli. (Think of them as small loops on a towel that then have smaller threads projecting from them.) The villi and microvilli are only one cell layer thick, but they perform multiple functions of producing digestive enzymes, absorbing nutrients, and blocking absorption of substances that aren’t useful to the body.

The intestinal wall has a paradoxical function: it allows nutrients to pass into the bloodstream while blocking the absorption of foreign substances found in chemicals, bacterial products, and other large molecules found in food. Some foods we eat and medications we use irritate the intestinal wall, and it can lose the ability to discern between nutrients and foreign substances. When this occurs, there is a problem of increased intestinal permeability, commonly known as “leaky gut syndrome.” This syndrome contributes to skin problems, food sensitivities, osteoarthritis, migraine headaches, and chronic fatigue syndrome. (See
Chapter 4
.)

The small intestine has three parts: the duodenum, the jejunum, and the ileum. The duodenum is the first 12 inches of the small intestine, the jejunum composes about 40 percent of the digestive system (about 11 feet), and the ileum composes the last segment (about 8 feet). The jejunum and ileum are connected by the ileocecal valve. Each nutrient is absorbed at specific parts of the small intestine. The duodenum has an acidic environment that facilitates absorption of some minerals, including chlorine, sulfur, calcium, copper, iron, thiamin, manganese, and zinc. We also begin the process of absorbing fat-soluble vitamins (A, D, E), fats, and some water-soluble vitamins (B
₁
, B
₂
, B
₆
, C, and folic acid). People with low hydrochloric acid levels may become deficient in one or more of these nutrients because they need acid for absorption. In the jejunum, we continue absorption of nutrients plus sugars, proteins, and amino acids. In the ileum, we finish the job of digesting many nutrients and add absorption of cholesterol, B
₁₂
, and bile salts. And finally, in the colon, we absorb potassium, water, salt, vitamin K, and short-chain fatty acids. If you look at the nutrient absorption chart in
Figure 2.4
you can see specifically where each nutrient is absorbed along the digestive tract.

The pancreas has two main roles: to aid in the digestion of food and to produce insulin and glucagon, which regulate blood sugar levels, thereby maintaining both digestive and global function.

When food passes from the stomach to the duodenum, cholecystokinin is secreted and enhanced by secretin. This stimulates the pancreas to secrete bicarbonate-rich alkaline fluid, essentially baking soda, which neutralizes the acidity of the chyme. The hydrochloric acid has already finished its work and a more neutral pH is where the rest of the digestive system functions best. The pancreas also manufactures and secretes specific digestive enzymes. Pancreatic amylase digests starches and sugars. The protein-splitting enzymes are called trypsin, chymotrypsin, carboxypeptidase, and elastase (also called pancreatic elastase). Pancreatic lipase and colipase break fats into fatty acids and glycerol. Ribonuclease and deoxyribonuclease digest old RNA and DNA. Once our food has been fully digested, nutrients can be absorbed into the bloodstream and used by the cells. Low secretion of pancreatic enzymes can lead to nutritional deficiencies. For example, vitamin B
₁₂
requires protein-splitting enzymes to separate it from its carrier molecule, so poor pancreatic function can lead directly to vitamin B
₁₂
deficiencies.

The second role of the pancreas is the production of hormones, including insulin, glucagon, somatostatin, and pancreatic polypeptide in the pancreatic islets (also called islets of Langerhans). Insulin is secreted when blood sugar levels rise; glucagon is secreted when blood sugar levels are low. Common problems in the pancreas are diabetes, which is a systemic disease, and pancreatitis, that is, inflammation of the pancreas.

Figure 2.4
Nutrient absorption chart.

The liver is the most complex of the body’s organs. It performs more than 500 functions and is critical to most of our metabolism. I once heard the dean of a medical school say, “I’d rather run all of the operations of General Motors for a day than be my own liver.” Your four-and-a-half-pound liver manufactures 13,000 chemicals and has 2,000 enzyme systems, plus thousands of synergists that help with body functions. It regulates the metabolism of carbohydrates, fats, and proteins; it manufactures bile to emulsify fats for digestion; it makes and breaks down many hormones, including cholesterol, testosterone, and estrogens; it regulates blood sugar levels; it processes all food, nutrients, alcohol, drugs, and other materials that enter the bloodstream and lets them pass, breaks them down, or stores them. It is a storage house for many nutrients: glycogen, fats, vitamin B
₁₂
, vitamins A, D, E, and K, and zinc, iron, copper, and magnesium. Your liver can store five to seven years of vitamin B
₁₂
, four years of vitamin A, and up to four months of vitamin D. Proteins synthesized in your liver transport vitamin A, iron, zinc, and copper into your bloodstream. Practically all vitamins and minerals we take in need to be enzymatically processed by the liver before we can use them. Several vitamins are converted into their active forms: carotene to vitamin A, folic acid to 5-methyltetrahydrofolic acid, and vitamin D to its active form 25-hydroxycholecalciferol. Your liver also produces proteins and lipoproteins that allow your blood to clot. The liver can lose as much as 70 percent of its capability and not show diagnosable liver disease. It can also regenerate itself after being injured.

The liver breaks down toxins ingested with our foods and those that are produced by bacterial metabolism. With these chemicals and enzymes, it “humanizes” nutrients so that the cells can use them. If the liver becomes too congested to enzymatically process these nutrients, we do not get the benefit from them.

Bile, manufactured by the liver and stored by the gallbladder, buffers the intestinal contents due to its high concentration of bicarbonates. It also emulsifies fats. Bile is a soaplike substance made of bile salts, cholesterol, and lecithin. It makes fats more water-soluble, increasing their surface area so that the enzymes can split them for the cells to use. It’s essential for absorption of fats, the fat-soluble vitamins A, D, E, and K, and some minerals. Bile also secretes immunoglobulins that protect our intestinal mucosa. Drugs and other toxins are eliminated from the liver through bile. The brown color of stool comes from the yellow color of bilirubin in bile.

The liver is also part of our immune system. The Kupffer cells filter bacteria and debris from the blood. The liver also stores environmental toxins like radioactive substances, pesticides, herbicides, food preservatives, and dyes. The liver will detoxify what it can, but if it can’t break down a particular substance, it stores it there and in tissues throughout the body.

The gallbladder is a pear-shaped organ that lies just below the liver. The gallbladder’s function is to store and concentrate bile, which is produced by the liver. When you eat a food that contains fat, cholecystokinin is released from the duodenum, which stimulates the gallbladder and liver to release bile into the common duct that connects the liver, gallbladder, and pancreas to the duodenum. Bile emulsifies the fats, cholesterol, and fat-soluble vitamins you’ve eaten by breaking them into tiny globules. These create a greater surface area for the fat-splitting enzymes (lipase) to act on during digestion.

The most common problem of the gallbladder is gallstones. When bile becomes too concentrated, stones may form, which can cause pain, nausea, and discomfort. Another common issue is bile reflux, where bile backs up into the stomach. Gallbladder disease is directly related to diet.

The appendix is a small, fingerlike sac that extends off the beginning of the colon. Until recently, the function of the appendix was a mystery. Now we know it contains a great deal of lymphatic tissue and is important for fetal and early childhood development. Hormones produced in the appendix beginning about the 11th week of pregnancy help regulate fetal metabolism. The appendix contains a lot of lymphatic tissue and is especially important in immune health in the first decades of life. In the developmental years, the appendix produces secretory IgA and helps with the maturation of B-lymphocytes (a type of white blood cell). These functions help to support local immune function.

When all nutrients have been absorbed, water, bacteria, and fiber pass through the ileocecal valve to the large intestine and colon. The ileocecal valve is located by your right hip bone and separates the contents of the small and large intestines.

The colon is short, only three to five feet long. Its job is to absorb water and remaining nutrients from the chyme and form stool. Two and a half gallons of water pass through the colon each day, two-thirds of which come from body fluids. The
efficient colon pulls 80 percent of the water out of the chyme, which is absorbed into the bloodstream.

The large intestine has three main parts: the ascending colon (up the right side of the body), the transverse colon (straight across the belly under the ribs), and the descending colon (down the left side of the body) to the rectum, where feces exit the body. Stool begins to form in the transverse colon. If the chyme passes through the colon too quickly, water is not absorbed, causing diarrhea. Stool that sits too long in the colon becomes dry and hard to pass, leading to constipation. About two-thirds of stool is composed of water and undigested fiber and food products. The other third is composed of living and dead bacteria.

The large intestine contains the majority of commensal and probiotic bacteria by far. In the colon, bifidobacteria ferment fibers that become short-chain fatty acids: butyric, propionic, acetic, and valerate. Butyric acid is the main fuel of the colonic cells. Low butyric acid levels or an inability of the colon bacteria to properly metabolize butyric acid has been associated with ulcerative colitis, colon cancer, active colitis, and inflammatory bowel disease.