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cells. Until food is absorbed, it is essentially outside the body—in a tube going through it. If the gut is inflamed, as with gluten intolerance or celiac disease, there can be malabsorption, which is typified by an inability to gain weight, lack of growth in children, anemia, and diarrhea.
    Assimilation is the process by which fuel and nutrients enter the cells. Technically this isn’t part of the digestive system, but it is the ultimate goal of the digestive process.
    Elimination is the last step. In digestion we excrete wastes by having bowel movements (defecation). These wastes are comprised of indigestible food components, waste products, bacteria, cells from the mucosal lining that are being sloughed off, and food that has not been absorbed.
Mouth
    The main function of the mouth is chewing and liquefying food. The salivary glands, located under the tongue, produce saliva, which softens food, begins dissolving soluble components, and helps keep the mouth and teeth clean. Saliva contains amylase, an enzyme for splitting carbohydrates, and lipase, a fat-digesting enzyme. Only a small percentage of starches are digested by the amylase in your mouth, but they continue to work for about another hour until the stomach acid inactivates the enzymes. On the other hand, the lipases become activated once reaching the stomach, beginning the process of fat digestion. Saliva also has clotting factors, helps to buffer acids, allows us to swallow, and protects our teeth, oral mucosa, and esophagus. Saliva also reabsorbs nitrates from our foods, primarily green leafy vegetables and beets. These nitrates are converted into nitrites by bacteria on our tongues, concentrating this a thousand times higher than those found in plasma. When this nitrite-rich saliva gets swallowed into acidic gastric juics, it converts into nitric oxide (NO), reducing inflammation throughout the body.
    Chewing also stimulates the parotid glands, behind the ears in the jaw, to release hormones that stimulate the thymus to produce T cells, which are the core of the protective immune system.
    Healthy teeth and gums are critical for proper digestion. Many people eat so fast, they barely chew their food at all and then wash it down with liquids. That means the stomach receives chunks of food instead of mush. This undermines the function of the teeth, which is to increase the surface area of the food. These people often complain of indigestion or gas. In
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John Robbins describes three men who survived in a concentration camp during World War II by chewing their food very well, while their compatriots perished. Simply by chewing food thoroughly we can enhance digestion and eliminate some problems of indigestion.
    The most common problems that occur in the mouth are sores on the lips or tongue—usually canker or cold sores (herpes)—and tooth and gum problems.
Esophagus
    The esophagus is the tube that passes from the mouth to the stomach. Here peristalsis begins to push the food along the digestive tract. Well-chewed food passes through the esophagus in about six seconds, but dry food can get stuck and take minutes to pass. At the bottom of the esophagus is a little door called the cardiac or esophageal sphincter. It separates the esophagus from the stomach, keeping stomach acid and food from coming back up. It remains closed most of the time, opening when a peristaltic wave, triggered by swallowing, relaxes the sphincter. The most commonesophageal problems are heartburn (also called gastroesophageal reflux disease, or GERD), hiatal hernia, Barrett’s esophagus, and eosinophilic esophagitis (EE).
Stomach: The Body’s Blender
    The stomach is the body’s blender. It chops, dices, and liquefies as it changes food into a soupy liquid called chyme, which is the beginning of the process of protein digestion. The stomach is located under the rib cage, just under the heart.
    After reaching your stomach, your food may stay in the top part of your stomach for