12.7: Prevention

Continual news reports of substances that can cause cancer leave us with the fear that we’re inescapably surrounded by carcinogens. In fact, we are, and nature is a major contributor. Sunshine can cause skin cancer;† naturally occurring radiation in the air can cause lung cancer; and there are natural substances in food that can cause cancer.

It’s no surprise that nature has also given us good defenses, e.g., melanin in skin, DNA-repair mechanisms, a powerful immune system. After all, if our image of cavemen is correct, we evolved in a hostile environment with little clothing to protect us from the sun, few ways of preserving food, and only an open fire for cooking.

For all the worry about environmental causes of cancer, most are related to personal choice. (Pollution isn’t a major cause in this country, as it is in some countries.) For example, we can lessen our exposure to tobacco and alcohol, avoid sunburn, eat a heathy diet, exercise regularly, and maintain a healthy weight.

Clearly, if you have one risk factor, it’s all the more important to avoid other risk factors. If you smoke plus breathe in asbestos fibers or radon, you raise the risk of lung cancer even more. What if you add to that, smoking, heavy alcohol consumption, and a poor diet?

Also, we may choose to take some risks because they are exceeded by benefits. Many women are willing to trade small increases in risk of breast cancer for the benefits of having a first child after age 30, having fewer children, menopausal hormone therapy to prevent hot flashes, etc. Also, such women typically have more education and higher socioeconomic status—both very closely linked to better health—and menopausal hormone therapy lowers the risk of osteoporosis.

Geographical clusters of cancers are alarming and should be investigated, but keep in mind that they may have little to do with “environmental toxins.” Cigarette smoking in 2019, for example, ranged from 25% of adults in West Virginia to 8% in Utah.

Smoking is the single most common cause of cancer in this country, causing about 30% of all cancer deaths, and is a major risk factor for heart disease, emphysema, and chronic bronchitis.

Lung cancer was unusual in the 1800s. The few who smoked cigarettes had to roll their own, and the smoke was too harsh to inhale comfortably. This was changed by the discovery of the fluecuring of tobacco (making the smoke mild and pleasant to inhale) and the machine-production of cigarettes. Cigarettes were heavily marketed; smoking increased dramatically; and an epidemic of lung cancer followed. (The lag time between smoking and lung cancer is about 25 years.)

Smoking in the U.S. peaked in the mid-1960s. (The surgeon general’s report on the link between smoking and lung cancer came out in 1964). As seen in Table 12-1, the age-adjusted death rate/ 100,000 from cancer of the lung and bronchus in 1930 was 4 for men, 3 for women. In 1990, it was 91 for men, 37 for women. Smoking has declined in the U.S., but lung cancer remains the leading cause of death from cancer in both men and women in the U.S.

Tobacco companies spend billions a year in advertising to recruit new smokers and make smoking more socially acceptable. Most smoking begins in adolescence. Cigarette smoking has fallen in this age group, but the use of E-cigarettes has risen.

Tobacco companies also promote their products in areas of the world where malnutrition, infectious disease, and inadequate health care are prevalent. In some developing countries, tobacco use is now higher than it ever was in the U.S.

^{†A hypothesis about the increase in skin cancer is that we’re exposed to more ultraviolet radiation because of a decreasing ozone layer between us and the sun.}

Diet and Cancer

Follow the dietary advice for good health that is discussed in Chapter 4. Healthy bodies are more resistant to damage and infection. The emphasis in cancer prevention is to eat a diet rich in plant foods. Such a diet tends to be low in fat and rich in fiber, vitamins, and minerals.

Many plant foods also have non-nutrient substances that may be protective. Cabbage-family (cruciferous)* vegetables have substances shown in animal studies to protect against cancer. In human studies, eating more of these vegetables is linked to a lower risk of colon and rectal cancer. Substances in soybeans are being studied for possible protection against breast and prostate cancer.

Diets rich in plant foods also tend to be rich in carotene and vitamins C and E—all can act as antioxidants to protect against cell damage. The need for C and E is covered in the Recommended Dietary Allowance (RDA). There’s no RDA for carotene; we’re simply advised to eat a daily serving of a carotene-rich fruit or vegetable.

^{*Cruciferous comes from the Latin crux, meaning cross (as in crucifix); some vegetables in this plant family have flowers that form the pattern of a cross. Cruciferous vegetables include cabbage, broccoli, cauliflower, turnips, brussels sprouts, radishes, kale, and kohlrabi.}

Carotenes/Carotenoids

Carotenoids are a group of several hundred bright yellow, orange, and red pigments made in plants. They’re in fruits, vegetables, flowers, and foliage, and give fall leaves their beautiful colors. About 20 of these carotenoids can be converted to vitamin A in the body. For simplicity, the various carotenoids that have vitamin A value will be referred to collectively as carotene or carotenes.* In food composition tables, carotenes are given vitamin A values, even if the food doesn’t have any actual vitamin A. Eating carotene-containing foods can meet our vitamin A needs by the conversion of carotene to vitamin A.

Carotenes are found in yellow-orange fruits and vegetables (they were first isolated from carrots in 1931, giving carotene its name), and also in dark-green vegetables (the green color of chlorophyll masks the yellow-orange color of carotenes). Carotenes are made only by plants but are found in animal tissue when animals eat carotene-rich plants. Poultry and egg producers routinely add carotenes (e.g., from alfalfa or corn) to poultry feed to make the egg yolk and chicken skin more attractive to the consumer. Carotene is also added to margarine (otherwise, it would be white).

Like vitamin A, carotene is fat-soluble. But unlike vitamin A, it isn’t toxic when eaten in large amounts. This is because carotene itself is not toxic, and the body stores excesses rather than converting it to vitamin A. The skin of people who eat a lot of carotene (e.g., drink carrot juice) looks yellow-orange. As a fat-soluble substance, the carotene colors the fat under the skin.†

Some foods rich in carotene, given as Retinol Equivalents (RE)** in ½ cup cooked food, are: canned pumpkin (2700), carrots (1915), sweet potatoes (1285), spinach (735), kale (480), broccoli (110). Other foods in various portions: ½ cantaloupe (860), 1 mango (805), 1 hot chili pepper (485), 3 apricots (275), 1 cup romaine lettuce (105), 1 avocado (105), 1 nectarine (100), 5 prunes (95), 1 tangerine (75), and 10 sprigs of parsley (50).

Carotene-rich food seems to protect against cancer. To say it another way, diets low in carotene-rich food seem to raise the risk of cancer. The advice to eat a daily serving of a fruit or vegetable rich in carotene doesn’t mean you must eat it each and every day. If you “pig out” on pumpkin pie and sweet potato on Thanksgiving, you may store enough carotene for months (it’s fat-soluble and stored in the body).

Scientists measure nutrients precisely, but we need to think only in terms of portions of foods. Keep in mind that food has more than single nutrients. Broccoli, for example, is a cruciferous vegetable that’s rich in vitamin C, fiber, carotene, and B-vitamins.

Since the body can convert carotenes to vitamin A, it’s hard to separate the effects of carotenes from those of vitamin A in human studies.†† But carotenes have an advantage, since vitamin A can act only as vitamin A, whereas carotene can also act as an antioxidant (some carotenoids without vitamin A value can also act as antioxidants).

Vitamin A helps maintain epithelial tissue (tissue that covers the internal and external surfaces of the body, e.g., skin and tissue lining the respiratory and digestive tracts). Thus, it’s no surprise that diets low in carotene/vitamin A have been linked to higher risk of cancer—especially lung and esophageal cancer.

Studies showing protective effects of specific nutrients shouldn’t be interpreted to mean that we need supplements. Part of the reason we self prescribe supplements so readily is because we hear, for example, that a study shows that people with more carotene in their blood (i.e., who eat more foods rich in carotene) have less cancer. We forget that a link isn’t proof of cause-and-effect.

Carotene can be a marker for a diet rich in fruits and vegetables and a healthier lifestyle. Furthermore, those who eat more fruits and vegetables may smoke less, exercise more, live in cleaner air, etc. Also, many people resort to supplements because they aren’t sure what carotene is, what foods have it, or whether it’s helpful only in pills.

In 1747, when James Lind showed that lemons and limes protected against scurvy (Chap. 1), we didn’t know what it was in lemons and limes that did this. Finding that it was vitamin C didn’t mean that we needed to take supplements. Rather, it allowed us to choose to eat any of a variety of foods having vitamin C (kiwi, green peppers, strawberries, etc.). When we want more of a particular nutrient, a sensible thing to do is to eat more of the foods that are rich in that nutrient.

If it were found that x amount of carotene protects against cancer, chances are that diets in line with dietary advice given in Chapter 4 provide enough. If not, the experts will revise the recommendations. The advantage to sticking to food as a source of carotene is that such food is also rich in other healthful substances. In fact, most double-blind studies of carotene pills found that the pills didn’t lower the risk of cancer (and in some studies increased the risk). Unlike the vitamin-C-deficiency-disease scurvy, the diet-cancer relationship of other food is very complex.

^{*Some carotenes provide more vitamin A than others. (Vitamin A, itself, isn’t made by plants; it’s found exclusively in animal products.) Beta-carotene is one of the most common carotenes in food. Its chemical structure is nearly identical to 2 molecules of vitamin A linked together.
†Some carotenoids are sold as suntan pills. These aren’t usually carotene; they’re selected for their brown-orange color (tan) rather than for vitamin A value. Tanning pills containing the carotenoid canthaxantin are illegal, although they are sometimes sold via mail-order or in health-food stores. Canthaxantin can cause problems by accumulating in the liver.
**Retinol (vitamin A) Equivalent (RE) is a measure of carotene’s potential conversion to vitamin A in the body. Thus, RE values given for plant foods give an estimate of carotene. The RDA for women is 700 RE; it is 900 RE for men (Appendix A-4).
††Cats can’t convert carotene to vitamin A, so are useful in studying specific effects of carotene. This also means that cats must get their vitamin A from meat or milk (vitamin A occurs naturally only in animal foods) or in fortified cat food. Vegetarian cats wouldn’t make it in the wild!}

Carcinogens

As will be discussed in Chapter 16, there are many carcinogens that are normal components of food, many of which could technically be called natural pesticides. After all, a plant’s chemical resistance to pests must have been crucial to a plant’s survival in evolution (plants don’t have fangs or claws and can’t run).

Also, note that we usually don’t eat the seeds (the plant’s next generation). If we do swallow the seeds, they’re often undigested and exit surrounded by “fertilizer” (human manure). Seeds often are bitter, and can contain toxins.*

Many natural carcinogens have been identified, especially in plants. They’re so prevalent in food, we really can’t avoid them. There’s little doubt that the benefits of eating fruits, vegetables, and grains far exceed the risks from the natural carcinogens in them. Also, any risk from residual pesticides sprayed on food crops may be relatively trivial.

A bigger concern is that people may buy fewer fruits and vegetables because of fear of pesticides and higher prices. Unwarranted restrictions on effective and inexpensive pesticides raise the price of the very fruits and vegetables that are thought to reduce the risk of cancer. While it’s important not to downplay the presence of natural and man-made carcinogens in our food, the danger shouldn’t be exaggerated, either.

^{*Apricot pits have cyanide (the poison used in the Jonestown mass-suicide in Guyana). Laetrile is an apricot-pit extract touted as a cancer cure. Promoters call it vitamin B17, but there’s no such vitamin. Due to public pressure, scientists did a controlled (and costly) study of laetrile in cancer patients; it didn’t help.}