12.4: Assessing Risk

Causes of most cancers are hard to pinpoint. Environmental and genetic factors interact throughout a lifetime. As noted earlier, scientists often start by looking for links, e.g., cancer is less common among people who are physically active. If cause-and-effect relationships are plausible, scientists explore the links in laboratory experiments and clinical trials.

Direct Contact—Breathing, Eating, Drinking, Chewing

We’d expect eating or breathing carcinogens to have the most effect on the mouth, throat, esophagus, stomach, and lungs. There’s overwhelming evidence that smoking and tobacco are the main cause of cancer at all these sites except the stomach. We might expect that dietary carcinogens would cause stomach cancer. If so, counting stomach cancers might be a way to estimate the amount and potency of carcinogens in our diet.

Stomach cancer and its relationships to diet and to H. pylori were discussed in Chapter 4. Stomach cancer has fallen steadily in this country for decades (Table 12-1); we now have one of the lowest rates in the world (Fig 12-4).

Smoking includes not only tobacco, but marijuana (see Table 8-1). Most of us know that smoking causes lung cancer, but many don’t know that smokeless tobacco (chewing tobacco, snuff/dip) also causes cancer. Users not only get more cancer in the mouth and throat, but become addicted to the nicotine that enters the bloodstream through the tissues in the mouth.

There are many carcinogens in smoke and tobacco. For simplicity, they are grouped here into tar (the solids in smoke) and nitrosamines. Smoking is estimated to cause 90% of lung cancers in men, 80% of lung cancers in women, and 30% of all cancer deaths. Breathing smoke around smokers (secondhand smoke) can also cause lung cancer. Relative to smoking and tobacco, carcinogens in food don’t cause much cancer by direct contact. However, there are food substances that aren’t carcinogens themselves (e.g., alcohol) but can, by direct contact, make tissue more susceptible to cancer.

• Tar: What carcinogens in tar and soot did to the nose and scrotum of chimney sweeps is similar to what happens in the respiratory system of smokers. The description he smokes like a chimney is fitting. It matters, of course, what people smoke, how often, how long, if the cigarettes are filtered, etc. Smoking tobacco vs. marijuana is hard to evaluate. Most marijuana smokers smoke fewer cigarettes, but breathe in the unfiltered smoke more deeply, in larger amounts, and hold it in longer. They also smoke the marijuana cigarette to a shorter length (less filtering by the cigarette itself). Smoke has carcinogens; so does food prepared by smoking (e.g., smoked fish). Food cooked over a grill can also be “smoked” by smoke from dripping fat. Eating a lot of smoked food may raise the risk of cancer in the esophagus and stomach. The advice is to eat smoked food only in moderate amounts.
• Nitrosamines form when nitrite (NO₂) combines with amines, which are natural components of food and tobacco. Nitrosamines are potent carcinogens in lab animals, are found in cigarette smoke, smokeless tobacco, snuff, and are thought to raise the risk of cancer in the tissues that they contact. Nitrosamines in food are linked to a higher risk of colon cancer.

Most of the nitrosamines in our diet come directly or indirectly from nitrite-cured meat—bacon, frankfurters, ham, bologna, pepperoni, pastrami, sausages, corned beef, etc. (Before malting methods were modified, our main dietary source of nitrosamines was beer.)

Nitrite is added to preserve meat and prevent botulism.It also gives meat a distinctive flavor that people like, and combines with the meat’s myoglobin (Chap. 9) to form nitrosomyoglobin, which gives meat a distinctive red/pink color (otherwise, it can look gray). In response to consumer-desire for “natural” (vs. “chemical”), celery-juice powder is sometimes substituted for nitrite (weiners with no added nitrite or nitrate). Celery is rich in nitrite, so adding the powder provides the nitrite in nitrite-cured meat, so it’s still purposely added nitrite.

Nitrosamines can form in food during cooking or in the acid conditions of the stomach. Vitamin C (ascorbate) is sometimes added to nitrite-cured meats to lessen nitrosamine formation. We can also do this by eating vitamin-C-rich foods at the same time as nitrite-cured meats—orange juice with breakfast bacon, bacon/lettuce/tomato sandwich, corned beef and cabbage, sausage and sauerkraut, etc. (orange juice, tomatoes, and cabbage all have vitamin C).

Nitrite occurs naturally in saliva, but at a much lower concentration than nitrite-cured meat. The advice is to eat vitamin-C-rich fruits and vegetables, and only moderate amounts of nitrite-cured meats.

• Alcohol, is estimated to cause 6% of all cancers in the U.S.; how alcohol does this can be different for specific types of cancer. Animal studies indicate that alcohol is capable of raising the risk of cancer, in general, by increasing the activation of potential carcinogens (e.g., it raises the activity of some liver enzymes that can make some substances potentially carcinogenic) and by suppressing DNA repair (e.g., alcohol can lower the activity of an enzyme that repairs DNA damage).

All levels of alcohol intake are associated with increased cancer risk (i.e., there’s no “safe level”). Risk increases with successively higher alcohol intake. Scientific evidence for alcohol as a cause of cancer is strongest for cancers of the mouth, throat, esophagus, liver, colon, rectum, and breast. Reducing alcohol intake is an important and underutilized means of cancer prevention.

• Salt: High salt intake along with eating lots of salt-cured, smoke-cured, and pickled food is linked with a higher risk of stomach cancer. It’s hard to sort out specific effects of salt, since these foods may also contain nitrosamines. The advice is to eat salty and salt-cured foods only in moderation.

Colon Cancer

Possible interactions between diet and colon cancer were discussed in Chapter 6: Dietary fat stimulates bile secretion, and some breakdown products of bile can act as carcinogens. Other substances in the diet, like calcium, can combine with some of these carcinogenic breakdown products and make them harmless. Different diets, encouraging growth of different bacteria in the colon, can change—for better or worse—conditions for developing colon cancer. The villains can be sly.

Colon cancer usually starts as a polyp, which can be seen and removed during a colonoscopy. Colonoscopy screening is recommended, starting at age 45, and younger for those with a family history of colon cancer.