Cancer

What is it?

Cancer is a group of over 100 diseases in which abnormal cells grow and spread in an uncontrolled way. It is one of the leading causes of death in developed countries, with lung cancer responsible for more deaths than any other type of cancer. Many factors play a role in the development of cancer, and these may vary between the different types of disease. They include nutritional, environmental, genetic, social, emotional, psychological and spiritual factors. It is only in the last 20 years that diet has been accepted as playing a vital part in the prevention of cancer.

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Symptoms of cancer

As cancer occurs in many forms, the symptoms of one type of cancer may be quite different to those of another. However, there are certain warning signs that often occur. These include:

Change in bowel or bladder habits
A sore that does not heal
Unusual bleeding or discharge
Thickening or lump in the breast or any other part of the body
Indigestion or difficulty swallowing
Obvious change in a wart or mole on the skin
Nagging cough or hoarseness.

However, these symptoms are not always warning signs of cancer as they can also be caused by less serious conditions. A person who has any of these symptoms should see a doctor who can determine what the problem is. There are several tests which can help to determine if cancer is the cause of a particular medical problem. These include a biopsy, in which a sample of tissue is removed and checked under a microscope for cancer cells.

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Diagnosis of cancer

The Cancer process

Normally, cells divide to produce more cells only when the body needs them. If cells divide when new ones are not needed, they form a mass of excess tissue, called a tumor. Tumors can be benign (not cancer) or malignant (cancer). The cells in malignant tumors can invade and destroy nearby tissues and organs. Cancer cells can also break away from a malignant tumor and travel through the body to form new tumors in other places. The spread of cancer is called metastasis.

Cancer develops in two stages, an initiation stage and a promotion stage. During the initiation stage, the genetic code of a healthy cell is altered by a substance known as a carcinogen. This is followed by the promotion stage in which the abnormal cell is encouraged to multiply. The initiation stage happens frequently and quickly while the promotion stage is more lengthy. In some cases it may take as long as 30 years for cancer to become apparent.

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Causes of cancer

Many factors contribute to the development of cancer. Some of the most common include smoking, chemical pollutants, excess sun exposure, radiation, some pharmaceutical drugs, alcohol, viruses and psychological influences. Nutritional factors include food chemicals, obesity, diets high in saturated and polyunsaturated fats and protein, and nutrient deficiencies.

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Treatment of cancer

Cancer is treated with surgery, radiation, chemotherapy, hormones, or biological therapy. Treatment may involve just one method or a combination of methods. The choice of therapies depends on many factors including the type and location of the cancer, whether the disease has spread, a patient's age and general health.

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Preventing cancer

While it is impossible to avoid all cancer-causing substances, it is possible to lower the risk of developing the disease by avoiding risks where feasible, and increasing consumption of nutrients that act to prevent development of cancer. It is important to avoid exposure to the risk factors mentioned above. Other lifestyle factors, including effective relaxation and stress management techniques can play a large part in reducing the risk of developing cancer. Regular checkups and self-examinations can also be useful in preventing death from cancer as they may reveal the disease at an early stage, when treatment is likely to be effective.

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Cancer and diet

Between 20 and 60 per cent of deaths from cancer may be diet-related, making diet second only to tobacco as the most influential factor in the development of cancer. Cancers particularly influenced by diet include those in the colon, prostate, ovary, uterus, breast, skin, vulva, kidneys, cervix, stomach, esophagus, mouth and liver. The body has many mechanisms to thwart the progress of cancer; such as detoxification of carcinogens, preventing and correcting damage to DNA, immune stimulation, and sealing off an abnormal cell growth. All these mechanisms rely on good nutrition.

A good diet can prevent cancer and a bad one can increase the risk, but it is not always clear exactly what is good and what is bad. It is known that nutrients play a role in contributing to or preventing cancer, but the exact relationship between dietary ingredients and cancer is elusive. There are many different types of cancers and some of these can take years to develop, thus making it difficult to pinpoint cause and effect. Food contains many chemicals, known and unknown, and the effects of many of these have not yet been investigated.

The dietary guidelines from the American Cancer Society are:

Choose most of the foods you eat from plant sources.
Limit your intake of high fat foods, particularly from animal sources.
Be physically active: achieve and maintain a healthy weight.
Limit consumption of alcoholic beverages, if you drink at all.

Foods to avoid

Food contains many substances which can cause cell mutations or promote cancer. High intakes of saturated fat, sugar, alcohol, artificial sweeteners and food additives may cause cancer. Alcohol may promote the growth of abnormal cells, and food additives known as nitrites which are found in processed meats can be converted in the body to carcinogenic nitrosamines. Frying, smoking, barbecuing and broiling fatty meat and fish may produce cancer-causing chemicals, and these foods should be avoided as much as possible.

Foods to include

There are many substances in food which prevent the development or progression of cancer. No specific diet is guaranteed to prevent cancer but a diet which is low in fat and high in fruit and vegetables will help reduce the risks. Fruit and vegetables are rich in vitamins, minerals, fiber and other cancer-preventing compounds. A person whose diet is high in fruit and vegetables also tends to eat less fatty and high calorie foods. Studies from many different countries consistently show that diets high in fruit and vegetables reduce the risk of cancer.

Most experts recommend eating at least two fruits and three vegetables, especially dark orange and green ones, every day. Eating a variety of foods is also very important as no single food provides all the nutrients a person needs, and different nutrients protect against different types of cancer. Cruciferous vegetables such as broccoli, cabbage and cauliflower may have particularly beneficial effects as they contain high levels of vitamins and minerals and other phytochemicals.

Fiber

Fruit and vegetables are also high in fiber. A high fiber diet may reduce the risk of several cancers, including colon and rectal cancer, by binding to potentially toxic bile acids, moving food more quickly through the intestines and exerting beneficial effects on gut bacteria. Fiber has also been shown to protect against other cancers such as those of the breast and prostate.

Fat

Maintaining a healthy weight is very important as obesity increases the risk of developing cancer, and many studies have shown that low fat diets protect against cancer. High fat diets are associated with an increased risk of many types of cancers. This is particularly true for diets high in saturated fats such as those from animal sources. High levels of certain polyunsaturated fats also appear to increase the risk of some cancers while monounsaturated fats do not. (See page 7 for more information).

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Vitamins, minerals and cancer

Dietary antioxidants

There is a lot of evidence to suggest that insufficient amounts of antioxidants can increase the risk of cancer. In groups of people who have low levels of these nutrients, the cancer rates are higher. Vitamin C is the body's most powerful water soluble antioxidant while vitamin E and carotenes are lipid soluble. Antioxidants neutralize metabolic products, including free radicals; prevent carcinogens from attacking DNA and cell membranes; inhibit chromosome aberrations; restrain replication of transformed cells; suppress actions of cancer promoters; and induce regression of pre-cancerous lesions. Antioxidants can also boost immunity and counter the immune-suppressive effects of oxidized cholesterol and other substances, and may also slow or halt the growth of tumors by enhancing communication between cells, and stimulating the activity of the immune system.

Vitamin A

Many studies suggest that high blood levels of vitamin A can help prevent certain forms of cancer, particularly cancers of epithelial tissues, such as the lung, mouth, stomach, colon, cervix and uterus. Vitamin A plays an important role in the growth and differentiation of cells, strengthening the immune system and suppressing cell transformation into cancerous cells. Vitamin A deficiency- related changes resemble cancer in these cells.

Breast cancer

Breast tissue may be particularly sensitive to the tumor-suppressive action of vitamin A. In a study published in 1997, researchers at Harvard School of Public Health compared the concentrations of various forms of vitamin A in the breast fat tissue from 46 cancer patients and 63 women with benign breast lumps. They found an increased risk of disease in those with low levels of vitamin A.¹

Results of a 1997 study suggest that the development of lung cancer may be due to a decreased ability of cells to respond to vitamin A-related compounds known as retinoids. When researchers at the University of Texas looked at the lung tissue from 79 patients with lung cancer and 17 without lung cancer they found that all the healthy cells carried receptors that bound retinoids. However, only 42 to 76 per cent of the cancerous cells had this ability. Of the six different types of retinoid receptors, three were found at lower levels in cancer cells.² This study raises the possibility that increasing dietary intake of vitamin A or taking supplements can be used to reduce the risk of lung cancer.

Carotenes

Many population studies have suggested that diets high in carotenes can protect against several types of cancer including those of the cervix, ovaries, uterus, mouth, gastrointestinal tract, lung, prostate and breast. Other types of studies have shown that cancer victims often have lower carotenoid levels than healthy individuals.

In a study published in 1991, researchers investigated the links between beta carotene and cancer in New Zealand families. The study involved 389 people diagnosed with cancer and 391 hospital patients without cancer. They also assessed the family members of the study participants to compensate for the fact that changes in beta carotene levels may have occurred after the cancer developed. Low levels of beta carotene were found in people with a number of cancers, including those of the lung, stomach, esophagus, small intestine, cervix, and uterus. Low levels of beta carotene were also found in the relatives of these cancer patients. The strongest findings were those for lung cancer. In this study patients with cancers of the breast, colon, prostate, and skin did not have lower levels of beta carotene and neither did their families. The results of this study suggest that the cancer sites associated with low levels of beta carotene are, in general, sites for which smoking is a strong risk factor.³

Lung cancer

Several population studies have investigated the links between lower levels of carotenes and lung cancer. In a study published in 1998, researchers at Johns Hopkins University measured nutrient levels in blood samples from 258 patients with lung cancer and compared these with those in samples from 515 people free of cancer. Blood concentrations of cryptoxanthin, beta carotene, lutein and zeaxanthin were significantly lower among the cancer patients. Small differences were noticed for alpha carotene and lycopene.⁴

In a 1994 study, researchers compared the diets of 413 non-smokers suffering from lung cancer with those of 413 people without cancer. The results showed that high dietary intake of fruit and vegetables and beta carotene was linked to a decreased risk of lung cancer in both men and women.⁵

Breast cancer

In a study published in 1998, researchers in Missouri examined blood levels of various nutrients in women who developed breast cancer after donating blood to a bank over a ten year period. They then compared these levels to women who were free of cancer. They found lower levels of the carotenes beta cryptoxanthin, lycopene, lutein and zeaxanthin in patients who developed breast cancer.⁶ In a study published in 1996, Italian researchers investigated the relationship between selected nutrients and breast cancer risk in 2569 women with the disease and 2588 women with no history of cancer. The results showed significantly less risk in women with high beta carotene intakes.⁷

In a 1994 study published in the British Journal of Cancer, West Australian researchers investigated the effect of increased intake of beta carotene on survival in breast cancer patients. Over a six-year period, only one death occurred in the group with the highest consumption of beta carotene, while there were eight and 12 deaths in the intermediate and lowest groups of consumption respectively.⁸

In a study published in the Journal of the National Cancer Institute in 1996, researchers examined the links between dietary intake of carotenes(including nonfood supplements), and premenopausal breast cancer risk. The study involved 297 premenopausal women 40 years of age or older who were diagnosed with breast cancer from November 1986 to April 1991. These were compared with 311 women without cancer. The results showed a reduction in risk associated with high intake of several nutrients including beta carotene, lutein and zeaxanthin.⁹

Prostate cancer

High beta carotene intakes may improve survival in those with prostate cancer, according to results from the Chicago Western Electric Study published in 1996. The study involved 1899 middle-aged men who were followed for a total of 30 years. During that time, 132 men developed prostate cancer and survival was found to be less likely in those with low beta carotene intakes.¹⁰

Lycopene-rich tomatoes seem to be linked with a lower risk of prostate cancer. In a study published in 1995, researchers at Harvard Medical School assessed the links between diet during a one-year period, and prostate cancer in almost 48 000 men in the Health Professionals Follow-up Study. They found that men who ate more foods such as tomatoes, pizza and tomato sauce which are high in lycopene were less likely to be at risk of prostate cancer.¹¹

Colon cancer

In a 1997 study done in Italy, researchers assessed carotenoid levels in four healthy patients, seven patients with pre-cancerous lesions and seven patients with colon cancer. They found significantly lower carotenoid levels in the cancer patients.¹²

Cervical cancer

Population studies suggest that low carotenoid levels, including beta carotene may increase the risk of cervical cancer. In a 1993 study, researchers examined the relationship between cervical cancer and carotenoid levels in 15 161 women who donated blood in 1974. Over the next 16 years, 50 women developed cancer. The blood samples of these women were compared with those from 50 women free of cancer and the results showed that the levels of total carotenes, alpha carotene, beta carotene, cryptoxanthin, and lycopene were significantly lower among cancer cases than they were among controls.¹³ Laboratory studies show that beta carotene can slow the growth of cervical cancer cells.¹⁴ Increasing intake of beta carotene may help to overcome this tissue-specific deficiency.¹⁵

Vitamin C

People with high vitamin C intakes seem to have a reduced risk of almost all forms of cancer. The protective effect seems to be strongest for cancers of the esophagus, larynx, mouth and pancreas. Vitamin C also seems to provide some protection against cancers of the cervix, liver, stomach, rectum, breast and lungs.¹⁶ However, in many of these studies it is not possible to tell whether the protective effect is due to vitamin C, vitamin E, or carotene, to a combined effect of these nutrients, or even due to additional substances found in food. Results from the Western Electric Study published in 1995 suggest a link between low vitamin C levels and death from cancer. The researchers obtained information on diet and other factors from 1556 employed, middle-aged men. During the follow-up period, 231 men died from cancer. The results showed that those with the highest vitamin C and beta carotene intakes were 40 per cent less likely to die of cancer than those with the lowest intakes.¹⁷

Prostate cancer

Vitamin C seems to improve survival in those with prostate cancer. Researchers involved in the Western Electric Study examined the links between dietary beta carotene and vitamin C and the risk of prostate cancer in 1899 middle-aged men over a 30-year period. During this time, prostate cancer developed in 132 men. The results showed that associations between vitamin C intake and risk of prostate cancer differed depending on whether the cancer was diagnosed during the first 19 years of follow-up or the next 11 years of follow-up. Overall, higher intakes of vitamin C and beta carotene were linked to improved survival.¹⁸

Lung cancer

Vitamin C may also help to protect against lung cancer. Researchers involved in a 1997 study obtained dietary information from 561 men from the Dutch town of Zutphen, in 1960, 1965, and 1970. During the period from 1971 to 1990, 54 new cases of lung cancer were identified and analysis of the diets of the men showed an increased risk of lung cancer in those with lower fruit and vegetable and vitamin C intakes.¹⁹

Stomach cancer

Results from a study published in 1995 suggest that low vitamin C intake is linked to an increased risk of stomach cancer. In the 1960s, researchers collected detailed dietary information and in 1987, they assessed average food intakes. They then examined the links between this information and death from stomach cancer. The results showed that the average intake of vitamin C was strongly related to the risk of stomach cancer. However, vitamin C intake was not related to the risk of lung and colorectal cancer in this study.²⁰ Other studies have shown similar results.²¹

Colon cancer

In a study published in 1992, researchers investigated the links between fruits and vegetables and vitamin C intake in 11 580 residents of a retirement community who entered the study free from cancer. During the period from 1981 to 1989 a total of 1335 cases of cancer were diagnosed. The results showed a decreased risk of colon cancer in women with higher vitamin C intakes. Supplemental use of vitamins A and C also showed a protective effect on colon cancer risk in women.²²

In a 1994 study, Italian researchers investigated the relationship between estimated intake of certain nutrients, including vitamin C, and the risk of disease in 828 patients with colon cancer, 498 with rectal cancer and 2024 people without cancer. Those in the highest intake group for vitamin C had a 60 per cent lower risk of cancer than those in the low intake group.²³

Breast cancer

The results of a 1994 study suggest that women with high vitamin C intakes have a lower chance of dying of breast cancer than those with the low intakes.²⁴ The study involved 678 women who were diagnosed with the disease from January 1982 through June 1992. However, results from the Nurses Health Study did not show a protective effect against the disease.²⁵

Vitamin E

Several population studies show that low levels of vitamin E are linked to the development of certain cancers, including those of the mouth, liver, lung, colon, rectum, cervix and breast.

Cancers of the gastrointestinal tract

Results from the Iowa Women's Health Study show that high intakes of antioxidants including vitamin E are linked to lower risks of colon, oral, pharyngeal, esophageal and gastric cancers.²⁶ As part of the study, the results of which were published in 1993, researchers analyzed the links between vitamin E and colon cancer in 35 215 women aged 55 to 69 years without a history of cancer. During the follow-up period, there were 212 cases of colon cancer. The results showed that low vitamin E intake increased the risk of colon cancer, with those in the high intake group having 30 per cent of the risk of those in the low intake group. The protective factor was stronger in the younger women.²⁷

Breast cancer

Some studies suggest that low vitamin E levels increase the risk of breast cancer. In a study published in 1992, researchers investigated the relationship between blood levels of various nutrients including vitamin E, and the risks of breast cancer and proliferative benign breast disease (BBD) in postmenopausal women. Women who had a high intake of vitamin E from food sources only, had around 60 per cent less risk of breast cancer compared to those in the low intake group.²⁸ However, not all studies have shown protective effects.²⁹

Cervical cancer

In a 1990 study, Utah University researchers investigating the relationship between cervical cancer and dietary intake of antioxidant vitamins and selenium in 266 women with cervical cancer and 408 women without the disorder found that women with high vitamin E intakes had a 40 per cent lower risk of cervical cancer.³⁰ Blood levels of vitamin E have also been found to be low in women with cervical cancer.³¹

Lung cancer

High vitamin E intakes may decrease the risk of lung cancer. In 1974 and 1975, researchers at Johns Hopkins School of Hygiene and Public Health, Baltimore, collected blood samples from 25 802 volunteers. They assessed vitamin E levels in samples from 436 cancer cases and 765 matched control subjects. The results showed that high vitamin E levels protected against lung cancer.³²

Selenium

Epidemiological studies suggest that the risk of cancer is reduced in areas where the soil is high in selenium. Blood samples taken from large groups of people also show that they are more likely to develop cancer if they have low blood levels of selenium and the antioxidant selenium-containing enzyme, glutathione peroxidase. Low serum, dietary and soil selenium levels are particularly associated with lung, gastrointestinal tract and prostate cancers. Selenium may be most effective when combined with vitamin E.

Colorectal cancer

In a 1997 study of the relationship between selenium and colon cancer, researchers at the University of North Carolina determined selenium levels in patients referred for colonoscopy. The results showed that those with the lowest selenium levels had almost four times the risk of colon cancer when compared to those with the highest levels.³³

In a German study published in 1998, researchers investigated the selenium and glutathione peroxidase levels in 106 colorectal cancer patients and compared these to those in people without cancer. When average selenium levels in the cancer patients were compared with those in the control group, no significant differences were found. However, a significant reduction of serum glutathione peroxidase activity was seen in cancer patients. Those patients with low selenium levels had lower survival times and rates than the patients with higher selenium levels. The lowest selenium level was found for patients with advanced tumor disease. It is unclear from the results of this study whether low selenium levels are a cause or effect of cancer.³⁴

Lung cancer

In a study published in 1993, Dutch researchers examined the links between long-term selenium status and lung cancer among 120 852 Dutch men and women aged 55-69 years. The results showed that the lung cancer risk in those with the highest intake of selenium was half that of those in the lowest intake group. The protective effect of selenium was concentrated in subjects with a relatively low dietary intake of beta carotene or vitamin C.³⁵

Antioxidant supplements

The supplements most widely used to protect against cancer are the antioxidants. There have been several studies showing that high doses of antioxidants are beneficial, particularly in those who are deficient. A five-year study of almost 30 000 adults in Linxian, China found a 13 per cent reduction in cancer death rates in those given vitamin E, selenium and beta carotene supplements.³⁶ Several studies suggest that the most beneficial effects are seen when antioxidants are given in combination rather than alone. Antioxidants protect each other from damage and interact in many body functions.

Vitamin A

Lung cancer

Vitamin A supplements have been used to prevent cancer recurrence in smokers who had undergone surgery for lung cancer. In a 1993 study, researchers gave daily doses of 90 000 mcg RE (300 000 IU) to 307 patients took for one year. After a follow-up period of 46 months, the number of patients with either recurrence or new tumors was 56 (37 per cent) in the vitamin A group and 75 (48 per cent) in the control group. Eighteen patients in the treated group developed a second primary tumor, and 29 patients in the control group developed 33 second primary tumors.³⁷

Other studies support the use of vitamin A as a cancer preventive in those at high risk of disease. In a 1998 study done in Western Australia, 1024 blue asbestos workers known to be at high risk of diseases such as mesothelioma and lung cancer, were enrolled in a cancer prevention program using vitamin A. Half the subjects given 30 mg per day of beta carotene and the other half 7500 mcg RE (25,000 IU). The workers were followed up for a five-year period. Four cases of lung cancer and three cases of mesothelioma were observed in those in the vitamin A group, and six cases of lung cancer and 12 cases of mesothelioma in the beta carotene group. In the retinol group, there was also a significantly lower rate of death from all causes.³⁸ When the researchers compared these results with those workers who had not taken part in the study, they found that those taking part in the study had significantly lower death rates than non-participants.³⁹

Leukoplakia

Vitamin A has also been shown to exert protective effects against leukoplakia, a pre-cancerous change in mucous membranes. It often occurs in the mouth and throat and is related to smoking. In a study done in 1997, researchers tested the effects of the retinyl palmitate form of vitamin A on leukoplakia of the larynx. The treatment period was five weeks and the doses used ranged from 90 000 mcg RE per day (300 000 IU) to 270 000 mcg (1 500 000 IU) per day. Complete remission was observed in 15 out of 20 patients and partial response was seen in the remaining five patients.⁴⁰

Carotenes

Beta carotene supplements have been used in cancer and cardiovascular disease prevention trials including the Finnish Alpha Tocopherol Beta Carotene Cancer (ATBC) Prevention Study, the US Carotene and Retinol Efficacy Trial (CARET) and the US Physicians Health Study. In 1996, these studies reported results which received wide publicity. Results from the ATBC study showed an 18 per cent increase in lung cancer deaths in men who took daily supplements of 20 mg of beta carotene.⁴¹

The CARET study, which involved 18 000 smokers and people who had been exposed to asbestos was stopped 21 months early due to a 28 per cent increased risk of lung cancer, a 26 per cent increase in the risk of death from cardiovascular disease and a 17 per cent increase in overall deaths in the group receiving the supplements.⁴² Results from the 12-year Physicians Health Study suggest that beta carotene supplements have no effect on the risk of cancer.⁴³ Further analyses of these results support the suggestion that beta carotene is susceptible to oxidative damage by alcohol and the gases in cigarette smoke.⁴⁴ Other antioxidants, such as vitamins C and E may help to exert protective effects against this damage. In smokers, dietary vitamin C supplementation should accompany beta carotene supplementation.⁴⁵ (See page 55 for more information.)

Vitamin C

Vitamin C supplements may have a part to play in cancer prevention.

Stomach cancer

Vitamin C supplements may be useful in helping to prevent stomach cancer. In a 1996 study, researchers gave 32 patients 500 mg of vitamin C twice daily for two weeks. Levels in gastric juices and gut tissues were increased, raising the possibility of increased protection against free radicals.⁴⁶

A 1997 Japanese study suggests that vitamin C may inhibit the growth of Helicobacter pylori, a stomach bacterium that increases the risk of ulcers and stomach cancer. Vitamin C-rich diets have been found to decrease the risk of stomach cancer. This has been attributed to the antioxidant ability of vitamin C. However, vitamin E, which is also an antioxidant, does not inhibit the growth of Helicobacter pylori. This suggests that vitamin C may exert its protective effects through a biochemical mechanism. This research suggests the possibility of a safe, side effect-free alternative to antibiotics for the treatment of ulcers.⁴⁷

Colon cancer

Vitamin C supplements may also help to prevent colon cancer. In a 1992 study, twenty patients with colorectal cancer were given vitamins A, C, and E for six months and 21 patients with adenomas received placebo. The results showed that supplementation with vitamins A, C, and E was effective in reducing pre-cancerous abnormalities.⁴⁸ Vitamin C supplements may also be beneficial in the treatment of prostate cancer.⁴⁹

Vitamin C and cancer treatment

Controversy surrounds the use of vitamin C in the treatment of cancer. The Nobel Prize winner, Linus Pauling and his colleagues have used vitamin C to improve survival times in cancer patients but these results have not been repeated in other studies. Vitamin C may also benefit cancer patients who are undergoing radiation treatment by enabling them to withstand greater doses of radiation with fewer side effects.⁵⁰

Vitamin E

Vitamin E supplements, especially when combined with selenium, have shown beneficial effects in the prevention of certain types of cancer, including breast cancer. Analysis of results from a 1996 US National Institute on Aging study showed a 22 per cent decrease in the risk of death from cancer in those taking vitamin E supplements.⁵¹

Prostate cancer

According to more results from the ATBC study published in 1998 in the Journal of the National Cancer Institute, vitamin E reduces the risk of prostate cancer among smokers. Researchers studied the effects of 50 mg (75 IU) in Finnish men and the results showed a 32 per cent decrease in the incidence of prostate cancer and a 41 per cent decrease in prostate cancer deaths among the men taking vitamin E, compared with those who took no vitamin E.⁵²

Selenium

Recent large scale studies in Linxian, China found reduced risk of cancer when selenium supplements were given to those living in selenium-deficient areas.⁵³ Other studies have shown that selenium supplements protect against some types of cancer such as rectal, ovarian, colon, lung and cervical cancers. However, there are also studies, including the Harvard Nurses Health Study which do not show a protective role for selenium against cancers at any major site. Laboratory studies have shown that selenium can slow tumor cell growth.

Results of a 1996 study showed that selenium supplements were associated with a 50 per cent reduction in deaths from cancer. Researchers at the Arizona Cancer Center set out to test the effectiveness of selenium supplements on the prevention of skin cancer in over 1300 patients. Participants received a placebo or 200 mcg selenium per day over a period of 4.5 years and a total follow-up of 6.4 years. While the results did not show any reduction in skin cancer risk, the selenium group had a 37 per cent reduction in cancer incidence and a 50 per cent reduction in cancer mortality. The effects appeared strongest for prostate (63 per cent lower risk), colorectal (58 per cent lower risk) and lung (53 per cent lower risk) cancers.⁵⁴

Suggested doses of supplements

The doses of antioxidants used in cancer prevention trials are, in many cases, higher than those which could be obtained from the diet. Many nutrition experts recommend taking high daily doses of supplements to help prevent cancer. This may be particularly important in anyone who has a family history of disease or is often exposed to risk factors.

Suggested doses are:

Beta carotene: 10 to 30 mg
Vitamin E: 200 to 800 IU (134 to 536 mg alpha TE)
Vitamin C: 1000-2000 mg
Selenium: 200 mcg

Vitamin D

Vitamin D is involved in normal cell growth and maturation, and so may play a part in cancer prevention. Laboratory experiments show that vitamin D can inhibit the growth of human prostate cancer⁵⁵ and breast cancer cells.⁵⁶ Lung cancer and pancreatic cancer⁵⁷ cells may also be susceptible to the effects of vitamin D. Sunlight also seems to be protective against several types of cancer including ovarian,⁵⁸ breast and prostate cancers; and this effect may be mediated by vitamin D levels. Synthetic vitamin D-type compounds are being investigated for their potential as anticancer drugs.

Colorectal cancer

In a 1996 study, researchers conducted a population-based case-control study to examine the relationship between vitamin D intake and disease among 352 people with colon cancer, 217 people with rectal cancer and 512 healthy people in Stockholm, Sweden. The researchers used questionnaires to assess the vitamin D intake for the preceding five years. The results showed that those with the highest vitamin D intakes were around half as likely to get cancers of the colon or rectum than those with the lowest intakes.⁵⁹

Results from the Harvard Nurses Health Study published in 1996 suggest a link between vitamin D and colorectal cancer. The study involved 89 448 female nurses and covered the time period from 1980 to 1992 during which 501 cases of colorectal cancer were documented. The results showed a link between intake of total vitamin D and risk of colorectal cancer.⁶⁰

Prostate cancer

In a study published in 1996, researchers in a Boston hospital collected blood plasma samples from 14 916 participants in the Physicians' Health Study and measured vitamin D levels. Their analysis included 232 cases diagnosed up to 1992 and 414 age-matched control participants. Their results showed a slightly reduced risk of prostate cancer in those with high vitamin D levels.⁶¹

Genes affecting the way a man's body utilizes vitamin D could affect his risk of prostate cancer. A 1996 National Institute of Environmental Health Sciences study found that men with a particular type of vitamin D receptor gene are less likely than others to develop the type of prostate cancer that requires surgery. Researchers looked at the receptor genes in 108 cancer patients and 170 men without cancer. The results showed that 22 per cent of cancer patients had a particular gene while only eight per cent of the cancer-free men did. These findings support the theory that vitamin D plays an important role in prostate cancer development.⁶²

Folic acid

Those with diets low in folic acid may have a higher risk of cancer than those who eat large amounts, particularly cancers of the cervix, lung and colon. Folic acid is vital for the maintenance of the genetic code and regulation of cell division in both healthy and tumor tissues. Folic acid deficiency leads to changes similar to those seen in cancer and may affect the repair of DNA and increase chromosome fragility. It may also diminish the ability of the immune system to fight cancer cells and viruses. Deficiency has been shown to affect a gene involved in suppressing tumor formation.⁶³

Colorectal cancer

Results from the Alpha-Tocopherol Beta carotene Study published in 1996 suggest a relationship between folate status and colorectal cancer. The study involved male smokers aged from 50 to 69. The researchers measured folate levels in 144 cases of colorectal cancer and 276 healthy people. Those with higher dietary folate intakes had a reduced risk of colon cancer. Men with a high alcohol, low folate, low protein diet were at higher risk for colon cancer than men who consumed a low alcohol, high folate, high protein diet.⁶⁴

Cervical dysplasia

Low blood levels of folic acid may increase the risk of cervical dysplasia (pre-cancerous changes in the cells lining the cervix), possibly by enhancing the effect of other risk factors. Researchers from the University of Alabama investigated the links between folate deficiency and cervical dysplasia in 294 women with the disorder and 170 healthy women. They also assessed the impact of factors such as smoking, oral contraceptive use, human papillomavirus (HPV) infection, and number of sexual partners. The results showed that at low folate levels the risk of dysplasia caused by HPV infection was increased.⁶⁵

Vitamin K

Vitamin K injections are often given to babies after birth to reduce the risk of internal bleeding. In the early 1990s, researchers reported a possible increase in the risk of childhood cancers in those who were given these injections. However, the results of studies are inconclusive. This link was examined in four studies published in the British Medical Journal in 1998. The results of two of the four studies suggest that there is no association between vitamin K injections and cancer; one could not exclude the possibility; and the fourth suggested a possible increase in the risk of leukemia. (See page 177 for more information.)

Calcium

High intakes of calcium-containing foods are linked to a lower risk of developing colon cancer, although the latest research suggests that the protective effect is not very marked. Calcium may exert its protective effects by binding to cancer-causing fats and bile acids in the intestine and normalizing the growth of cells in the intestinal wall. Calcium may also normalize the growth of cells in the intestinal wall, thus protecting against cancerous changes. Low calcium intake may also increase the risk of breast and cervical cancers.

Results from the Health Professionals Study, involving almost 48 000 men aged from 40 to 75 also suggest that a higher intake of calcium from foods and supplements slightly reduces the risk of colon cancer.⁷¹ Data from the Nurses Health Study which involved over 89 000 nurses also showed a small reduced risk.⁷² Results from the Iowa Women's Health Study published in 1998 suggest that calcium can decrease the risk of rectal cancer. Researchers analyzed information from 34 702 postmenopausal women who responded to a mailed survey in 1986. After nine years of follow-up, 144 rectal cancer cases were identified. The results showed that high total calcium intake reduced the risk of rectal cancer.⁷³ Other results from this study show a reduced risk of colon cancer in women with high intakes of calcium and vitamin D.

Copper

Copper may act to prevent cancer. Animal studies have shown that copper has a protective role and this may be due to its antioxidant properties as part of copper-zinc superoxide dismutase.

Fluoride

Some evidence suggests that water fluoridation may be linked to some types of cancer, although this is controversial. A study published in 1996 reported on the relationship between fluoride concentration in drinking water and deaths from uterine cancer in Okinawa, Japan. Fluoride was added to the water supplies in the region in the period from 1945 to 1972. The results showed significant links between the time of water fluoridation and deaths from uterine cancer.⁷⁴

Iodine

Hypothyroidism and iodine deficiency are associated with a higher incidence of breast cancer.

Iron

In some population studies, high iron levels have been associated with an increased risk of throat and gastrointestinal cancers while others have not shown links.⁷⁵ Results from a study assessing the links between body iron stores and cancer in 3287 men and 5269 women participating in the first National Health and Nutrition Examination Survey found an increased risk with high iron levels.⁷⁶ Some experts believe that the findings of increased risk are due to causes such as defects in iron metabolism, rather than diet alone. Some studies have shown that iron can inhibit tumor development while others have shown that it might enhance it. Iron may increase the risk of cancer through its effect on free radical formation.

Manganese

A form of the antioxidant enzyme, superoxide dismutase, contains manganese. Proper function of this enzyme helps protect against free radical damage which can cause cancer.

Molybdenum

Population studies show that people living in areas where the soil is molybdenum- deficient have been found to have an increased risk of stomach and esophageal cancers.³ This may be because molybdenum-deficient plants are unable to metabolize carcinogenic compounds known as nitrosamines, which are present in high levels in food.

Zinc

Zinc supplements have been used to improve taste perception in people taking medications which reduce taste sensation, and in cancer patients undergoing radiation therapy.⁷⁷ This can be valuable in helping to maintain normal weight and nutrient intake during treatment.

Vanadium

Some evidence suggests that vanadium may limit the initiation and frequency of tumors in animals. Its role in humans is unclear.

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Supplements and cancer

Folic Acid

Folic acid supplements can help to reduce the risk of cancerous changes in several areas such as the cervix, lung and gastrointestinal tract. In a 1997 study, researchers at the Cleveland Clinic investigated the links between folate supplements and cancerous changes in 98 patients with ulcerative colitis. Patients taking the supplements had a 30 per cent lower risk of developing cancerous changes in the bowel. The lower the folate levels the more advanced the degree of cancerous changes in the cells.⁶⁶ In a 1997 Italian study researchers also studied the effects of folate supplements on pre-cancerous cell changes in ulcerative colitis. The results showed that folate reduced these changes.⁶⁷ Folic acid may also help to prevent the pre-cancerous changes in lung tissue caused by smoking.⁶⁸

Folic acid supplementation may protect abnormal cells from becoming cancerous and may reverse cervical dysplasia in some cases. A 1996 study done at the University of Alabama suggests that supplements may be useful in preventing the initial changes but do not appear to affect the progress of established disease.⁶⁹ Some researchers have found a higher risk of abnormalities in cervical tissue in women using oral contraceptives and suggest that folic acid supplements are beneficial in preventing cervical dysplasia in these women.⁷⁰

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Nutrients and cancer

Essential fatty acids

The levels and types of fat in the diet seem to influence cancer risk, and disease progression. High intakes of omega-6 polyunsaturated fatty acids seem to increase the risk of cancers while high intakes of omega-3 fatty acids may provide protection. Animal studies have demonstrated that polyunsaturated omega-6 fatty acids stimulate carcinogenesis and tumor growth and metastasis, whereas long-chain omega-3 fatty acids inhibit these processes. Reducing total fat intake and increasing the ratio of omega-3 to omega-6 fatty acids in the diet may be particularly useful for groups at a relatively high risk for breast or prostate cancer, and may also be useful after surgery to help prevent disease recurrence.⁷⁸

Dietary intake of essential fatty acids may play a role in prostate cancer cell proliferation. Epidemiological studies have demonstrated that men whose dietary intake is high in omega-6 fatty acids have a higher incidence of clinical prostate cancer.⁷⁹ Diets high in omega-3 fatty acids may have protective effects. Other research suggests that omega-3 fatty acids inhibit breast cancer and that the degree of this inhibition depends on background levels of omega-6 fatty acids. Results from the European Community Multicenter Study on Antioxidants, Myocardial Infarction, and Cancer (EURAMIC) study published in 1998, suggest that an increase in the ratio of omega-3 fatty acids to total omega-6 fatty acids in fat tissue decreases the risk of breast cancer. In this study, total levels of omega-3 or omega-6 fat were not consistently associated with breast cancer.⁸⁰

Population and laboratory studies suggest that omega-3 fatty acids may help to prevent and inhibit colon cancer. In a study published in 1995, death rates for colorectal cancer in 24 European countries were correlated with current fish and fish oil consumption, and with consumption ten and 23 years previously. In men, there was a reduced risk of death from colorectal cancer and current intake of fish, a weaker link with fish consumption ten years earlier, and none with consumption 23 years earlier. The researchers concluded that fish consumption is associated with protection against the later stages of colorectal cancer, but not with the early initiation stages.⁸¹

Omega-3 fatty acid supplements

Omega-3 fatty acids may be beneficial in preventing as treating cancer as they seem to exert tumor-suppressive effects.⁸² In a study published in 1997, Norwegian researchers studied the relationship between incidence of lung cancer and intake of dietary fats, high fat foods, fish, and fish products in 25 956 men and 25 496 women aged from 16 to 56. During the follow-up period, 153 cases of lung cancer were identified. The results showed that those who took cod liver oil supplements had around half the risk of those who did not.⁸³

Essential fatty acids may also boost immune function which may help in cancer prevention and treatment. Studies on the immune T cells in cancer patients taking fish oil capsules suggest that omega-3 fatty acids bring about beneficial changes. In a Greek study published in 1998, researchers investigated the effect of dietary omega-3 polyunsaturated fatty acids and vitamin E on the immune status and survival in both well-nourished and malnourished cancer patients. The study involved 60 patients with solid tumors who were randomized to receive dietary supplementation with either fish oil (18 g of omega-3 fatty acids) or placebo daily. The authors measured various indicators of immune function. The results showed that omega-3 fatty acids had a significant immune- enhancing effect and seemed to prolong the survival of malnourished patients.⁸⁴

Gamma-linolenic acid

Gamma-linolenic acid has been shown to be effective in killing cancer cells and is well-established as a topical treatment for some types of cancer, including bladder cancer.⁸⁵ It has also been shown to kill various other types of cancer cells.⁸⁶

There are many other compounds under investigation for their anticancer potential, including soybeans, tea and garlic. Bioflavonoids, colored pigments from fruit and vegetables, may also have anticancer properties. Cruciferous vegetables such as broccoli, cauliflower and cabbage also contain anticancer compounds.

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