6.6: Colon

The colon is the main site of bacterial action (lots of bacteria live here). It’s about 4 feet long, about 2½ inches wide, and—unlike the small intestine—isn’t lined with villi. Water is easily absorbed here, turning the liquid from the small intestine into semisolid stool. (This “easy absorption” makes it possible to administer some drugs as a suppository via the rectum.)

Bacteria make up about 30% of the dry weight of stool. They alter bile pigments to make the stool brown and alter certain residual amino acids to give stool its characteristic odor. The bacteria also make some vitamins, such as K, which are then absorbed. Vitamin K deficiency is uncommon because of its ample production in the colon, but a deficiency can occur if you take antibiotics for a long time. A vitamin K injection is given to newborns in this country as a preventative measure, because there are no bacteria in the colon at birth.

A healthy colon contains a healthy mix of trillions of microbes. Characterizing them and their health effects is an active area of research, but is only in its early stages—far from what ads, labels, and sound bites would have you believe.

The primitive state of knowledge is illustrated by the use of fecal transplants to repopulate the colon with a normal mix of bacteria. Its more recent use began in 2006 when a young physician had a patient with a life-threatening intestinal infection of an antibiotic-resistant bacteria [C. difficile] that emerged from antibiotic treatment of a urinary infection. In desperation, a slurry of stool from the patient’s son was delivered in a tube threaded through her nose to repopulate her colon. It saved her life. This was followed up by a placebo-controlled trial that showed the efficacy of fecal transplants to treat such conditions.

There’s hope of altering colon bacteria to treat a wide range of conditions, from Irritable Bowel Syndrome to obesity to cancer, but there is sparse evidence at this time as to how and if it works. Using fecal transplants in controlled trials is difficult for many reasons, including the safety of donor stools. The home use of fecal transplants aided by YouTube videos is risky.

In hopes of replacing fecal transplants, capsules of purified microbes have been developed, but it’s hard to know which ones and in what amounts to include. The Human Microbiome Project was launched in 2008 to create a database of the estimated 10,000+ microbial species that colonize our bodies, and to study their health effects.

Gas

Colon bacteria need nutrients to sustain themselves (they grow by multiplying), and they use whatever they can from what comes their way. They break down substances like undigested lactose and certain indigestible carbohydrates in beans, and can make gas as a byproduct. For this reason, beans cause gas, as does undigested lactose.

Some of the gas is absorbed into the blood from the colon and expelled in the breath (increased breath hydrogen after a dose of lactose is a positive test for lactose intolerance). 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). Some jokes are based on the fact that one of the gases is methane, which is combustible.

As a side-note to the earlier mention of starch-blocker pills, people who ingested starch blockers along with their pasta and rice should have been glad that the pills didn’t work. Bacteria in the colon would have had a feast on all that starch coming their way. The gaseous result would have been “explosive!”

Diarrhea

Many diseases can cause diarrhea. In developing countries, a common cause is drinking water contaminated by disease-causing bacteria (usually from sewage). Severe diarrhea causes massive water loss, which can cause severe dehydration—a major cause of infant death in developing countries. The infants with severe diarrhea can’t get a saline solution intravenously (to get water and electrolytes in fast enough), as they could in developed countries. Lacking advanced medical care, many infants have been rescued from fatal dehydration by their mothers giving them cooled, boiled (sterilized) water mixed with a bit of sugar and salt (to speed water absorption).

Some diarrheas aren’t related to disease. Emotional upset or lactose intolerance can cause it. The diarrhea of lactose intolerance occurs because the lactose pulls water into the colon. Ingestion of a lot of sugar alcohol such as sorbitol and mannitol can similarly cause diarrhea. Sorbitol and mannitol are commonly used as sweeteners in sugarless gum and dietetic foods, and are absorbed slowly. Thus, some may remain unabsorbed and proceed to the colon and produce diarrhea (and gas), as in lactose intolerance.

Constipation

Constipation is the difficult or infrequent passage of stool. Many people complain of it because of the mistaken belief that defecation (bowel movements) must occur daily to be healthy and normal. In fact, it’s quite normal to be irregular. Twice a day or three times a week is well within normal.

The volume of stool depends mainly on the amount of fiber you eat. Fibers like cellulose help prevent constipation by holding water, making the stool bulkier and softer and thus easier to pass. Eating or drinking can cause an urge to defecate (gets the intestines moving); infants usually defecate after a meal. Adults often suppress or ignore this urge. Those worried about constipation might be wise to heed “nature’s call,” because it’s when defecation is easier.

Don’t be unduly concerned with bowel movements, but consult your physician if there’s a big change in the usual pattern of bowel movements or in the stool’s appearance. A change to a very light color might be from fat malabsorption due to insufficient bile production; a change to a black color might be due to blood from a bleeding ulcer.

Diverticulosis

Diverticulosis is an outpouching of the colon wall, much like a hernia. An estimated 30 to 40% of people over age 50 in the U.S. have one or more of these outpouchings (called diverticula),* although many of them don’t suffer any ill effects. If an outpouching fills with bacteria and stool or becomes ulcerated, it can be painful. If an ulcer erodes a blood vessel, severe bleeding can occur. If an ulcer erodes completely through the colon wall, severe problems can occur from bacteria leaking into the normally germ-free abdominal cavity.

With a low-fiber diet, colon muscles can go into spasm when squeezing against an insufficiently bulky stool. This increases the pressure inside the colon, and can eventually cause outpouchings at weak points (think of the outpouchings between your fingers when you squeeze a partially filled balloon). Fiber such as cellulose helps prevent diverticulosis by providing the bulk that helps the colon move things along with less squeezing.

Imagine trying to push mashed potatoes through a resilient rubber tube by squeezing the tube. If the tube is filled with mashed potatoes, you don’t have to squeeze very hard to move the potatoes through. If the tube is only half-filled, you have to use more pressure to push against the lesser bulk. Over time, this excess pressure inside the tube may cause outpouchings of any weak spots.

^{*When diverticula become inflamed, it’s called diverticulitis; -itis means inflammation of, as in dermatitis (inflamed skin/ derma), appendicitis (inflamed appendix), gastritis (inflamed stomach).}

Colon Cancer

Colon cancer is the second leading cause of death from cancer in the United States (lung cancer is first), so there’s a lot of interest in the possibility that diet may affect risk. As seen by the array of high-fiber cereals in the supermarket, fiber’s possible role is well advertised. Fiber might lower risk, but this hasn’t been proven.

In looking at colon cancer throughout the world, high rates have been linked with diets low in fiber and high in fat, refined sugar, and animal protein. (These dietary factors tend to be linked, e.g., diets high in animal protein tend to be high in fat and sugar and low in fiber.) Other studies, e.g., comparing vegetarians with non-vegetarians, also show a link between more fiber and less colon cancer. But these studies show only links rather than causal relationships.

Studies in lab animals have been inconsistent. Some show protective effects, some show no effect, and some even show more colon cancer with more fiber. The probable reason for these variable results is that the diets differ in the kind and amount of fiber. Oat bran, for example, isn’t the same as wheat bran. Even differences in how finely the fiber is ground can change the outcome.

As with most diseases, genetic susceptibility varies. For most colon cancers, the cause isn’t known, though there are several suspects. A roundabout way of examining some possible causes is to begin by comparing the colon with the small intestine, where cancer is rare. This gives insight into both the logic and the difficulties of determining the relationship of diet to colon cancer.

A key event in cancer is a change that enables a cell to multiply out of control. In the small intestine, cells multiply only in the crypts at the base of the villi. As can be seen from Figure 6.2, the presence of the many villi helps protect these crypt cells from outside contact with carcinogens (cancer-causing agents). If cancer were to occur in any cell on the villus (i.e., a cell that has migrated up out of the crypt) this usually wouldn’t matter because that cell would be shed and gone in a matter of hours or days.

In contrast, the colon has crypts but no villi, putting the crypt cells (i.e., the dividing cells) in more direct contact with the contents of the colon. The colon partially protects itself by secreting mucus. Some fibers stimulate this secretion, and this could be one way in which fiber could help prevent colon cancer.

As a general rule, the length of exposure to a carcinogen has a bearing on whether cancer is induced in a cell (e.g., more smoking = more lung cancer). Transit through the colon is much slower than in the small intestine. Fiber helps move substances faster through the colon, which shortens the exposure to any carcinogens. But if the length of exposure were crucial, we’d expect that people with a history of constipation would be more likely to get colon cancer. This doesn’t seem to be the case.

It’s also generally true that the higher the concentration of carcinogens, the greater the risk of cancer (e.g., smoking unfiltered vs. filtered cigarettes). Low-fiber diets are typically high in fat, animal protein, and refined sugar, so it is logical to consider whether a high concentration of fat, animal protein, and refined sugars is a direct cause of colon cancer.

It’s highly unlikely that these are a direct cause. These dietary substances aren’t carcinogens themselves, and none of the refined sugar and only small amounts of the fat and protein even reach the colon.

Substances that are ingested but not absorbed might be expected to play a role because they’re more concentrated in the colon. But if the ingested substances are carcinogens, they should cause some stomach cancer as well, even though they would be at lower concentrations in the stomach.

In other words, we might expect that populations with a lot of colon cancer also have a lot of stomach cancer if ingested carcinogens were responsible. This isn’t the case, e.g., Japan has low rates of colon cancer but high rates of stomach cancer, whereas it’s the opposite in the U.S.

Where might carcinogens in the colon come from if they aren’t ingested? The most likely suspects are breakdown products of some kind, such as components of bile, bacterial breakdown products, etc. Trying to identify a culprit is like looking for the proverbial needle in a haystack.

Each person’s colon has a unique blend of thousands, of substances, including a unique blend of bacteria that subsist on whatever’s available. Just as restaurants serving different foods have different clientele, so it is with the colon and its bacteria. Different bacteria can make different substances. Some bacteria, for example, can change bile acids into carcinogens.

If bile or its breakdown products are a major culprit, eating a lot of fat and protein could play an indirect role because both stimulate bile secretion. A prospective study of more than 88,000 nurses found that colon cancer was 2½ times higher among those who ate beef, pork, or lamb as a main dish every day, compared to those who ate meat less than once a month. Most cuts of beef, pork, and lamb are high in fat and protein, so this finding is consistent with the hypothesis that eating lots of fat and protein increases the risk. But keep in mind that the comparison groups probably have other lifestyle differences—dietary and otherwise—that may not have been taken into account.

Clearly, the possible effect of diet on colon cancer isn’t simple or straightforward. But based on current evidence, we’re advised to eat a diet with less animal fat, more fiber, and only moderate amounts of protein as a possible way to lower risk of colon cancer (i.e., eat more vegetables, fruits, and whole grains, and less animal fat). Also, don’t forget that a nutritious diet is needed for healthy tissue, and healthy tissue is less susceptible to disease. The dietary recommendations discussed in Chapter 4 are the basis of what’s currently considered to be the best diet.

With so little known about prevention, screening is important. The colon (and rectum) can be visually inspected with a fiber optic camera passed through the anus (colonoscopy). Polyps (abnormal growths) can be seen and removed. Most polyps don’t result in colon cancer, but colon cancer typically starts as a polyp. Abnormal tissue in the colon or rectum can also be biopsied.