10.9: The Colon

The undigested material moves out of the small intestine and into the colon. The colon is only about 5 feet long and its width averages about 2 1/2 inches. Its lining is far simpler than that of the small intestine—it has no villi and few of the folds that characterize the small intestine. Indeed, after all the furious activity of the small intestine, the colon seems like a quiet pool after churning rapids. But just as was seen in peeking into Tom’s stomach, the colon can react to emotion. This was shown vividly in an experiment in which a healthy medical student’s colon was being monitored as it was implied that he had a tumor there (even pretending to take a biopsy)—and then told that this wasn’t true (see Fig. 10-5).⁶

Figure 10-5: Stress and the Colon.

Life in the Colon

Just as life is more prolific in the pool of a mountain stream than it is in the tumbling rush of the rapids, so is there more life in the colon— microbial life. Microbes alter bile pigments to make the stool brown, and alter certain residual amino acids to give stool its characteristic odor. They also synthesize some vitamins, such as vitamin K, which are then absorbed.

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Vitamin K deficiency is uncommon because of bacterial production of vitamin K, but can occur when the colon bacteria are markedly cut back, e.g., after taking antibiotics for a long time. A vitamin K injection is routinely given to newborns in the U.S. as a preventative measure, since the colon is generally free of bacteria at birth.

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Microbes need nutrients to sustain themselves, and they use all they can get from whatever comes their way. They produce gaseous by-products when they “eat” substances such as certain indigestible polysaccharides in beans.

Some of this gas is absorbed in the colon and discharged in the breath. The rest is expelled through the anus. The expulsion of gas (flatus) through the anus is called flatulence—useful information for those seeking an alternative word.

Gut Microbiome

Our body harbors trillions of microbes (bacteria, viruses, fungi, etc.) that are collectively called the human microbiome. We each carry a unique mix of the microbes such that they can even be used to identify us. Most of these microbes reside in our gastrointestinal tract (gut microbiome), and mostly in our colon.

Gut microbes play a major role in health and disease, affecting not only gut diseases like irritable bowel syndrome, but our susceptibility to conditions like asthma, diabetes, and obesity.7 What we eat is a major determinant of the particular mix of microbes in our gut. As an analogy, people who like spicy food flock to restaurants that serve spicy food. Our diet not only affects our health directly, but indirectly by the metabolic effects of the microbes residing in our gut.⁷

Although imbalances in gut microbes have been characterized and linked to various health issues, proof of cause-and-effect is often lacking. This is an area of intense research. Most of the marketing of various substances to restore “microbial balance” and health has been ahead of the science. Probiotics contain microbes said to be helpful to one’s health. Prebiotics are substances said to promote growth of beneficial microbes.

The primitive state of this field is illustrated by the use of fecal transplants to restore a healthy mix of microbes in the colon.8,9 In an early case, a woman was hospitalized with a severe kidney infection, which was successfully treated with antibiotics. The antibiotic treatment, however, also caused the emergence of an antibiotic-resistant microbe (C. dificile) in her colon that threatened her life. In desperation, a slurry of stool from the patient’s son was delivered in a tube threaded through her nose to repopulate her colon with normal microbes to overcome the infection. This case was followed up by a placebo-controlled trial that showed the efficacy of fecal transplants to treat this condition.

Using fecal transplants in controlled trials is diffcult for many reasons, including the safety of donor stools. Needless to say, the home use of fecal transplants aided by YouTube videos is risky.

Much progress has been made in identifying gut microbes, as in the Human Microbiome Project hmpdacc.org but there’s still a ways to go in establishing cause-and-effect,10 and then being able to safely target specific changes to our microbiome. Data is being collected on the interaction of our diet and our genes and our gut microbiome, and may lead to individualized dietary recommendations based on a person’s genes and gut microbiome.¹¹

Lactose Intolerance

The reduced ability to digest lactose (lactose intolerance) is common among adults. We’re born with high levels of lactase (the lactose-digesting enzyme) in our small intestine, which supports digestion of the ample lactose in milk, our first food. But as the intestine matures, the amount of lactase normally declines to low levels, and the ability to digest lactose declines accordingly.

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Recall that lactose is a double sugar (glucose+ galactose), and only single sugars can be absorbed. Lactase splits lactose into glucose and galactose.

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When lactose is consumed in amounts larger than can be fully digested, the undigested lactose proceeds to the colon, where bacteria make a meal of it. The person hosting this meal may suffer the consequences, in the form of diarrhea, gas, and abdominal cramps.

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Many who think they are lactose intolerant might not be. A double-blind study of people who said they were severely lactose-intolerant found that the gastrointestinal symptoms of those given regular milk didn’t differ from those given milk with “predigested” lactose.¹²

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Describing adult populations as lactase deficient is misleading because it implies an abnormality. In fact, about 75% of adults, worldwide, have low levels of lactase. The high level of lactase found in some populations (such as Northern Europeans and the pastoral Fulani tribe of Nigeria) is an anomaly, probably due to a selective mutation thousands of years ago that conferred a nutritional advantage to members of dairying cultures. Low levels of lactase in adults are found in about 90% of Asians and about 75% of African Americans and Native Americans. In contrast, less than 20% of Caucasian adults of Northern European origin have low levels of lactase.

Milk is the only food that naturally has lactose. But since milk is made into and added to other foods, lactose is found in a variety of food products. In practical terms, adults with low levels of lactase are concerned with the amount of milk and milk products they can comfortably consume. This is best determined by a person’s own eating experience. Some adults with low lactase levels, for example, can’t tolerate gulping down a glass of milk on an empty stomach but find no discomfort in drinking a glass of milk leisurely throughout a meal.

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Some people have sucrose intolerance, resulting from low levels of sucrase enzyme. The condition is analogous to lactose intolerance.

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Yogurt and “acidophilus milk” are better tolerated because the added bacteria (Lactobacillus acidophilus) have “predigested” some of the lactose. Cheese and ice cream are also better tolerated because they’re high in fat and contain relatively little lactose. The high fat content of these foods slows the passage of the lactose from the stomach to the intestine. (Also, the high fat content ordinarily limits how much we eat.)