Cancer, the second leading cause of death in the United States,
is a group of conditions of uncontrolled growth of cells originating
from almost any tissue in the body.
Cancer may arise in any organ in the body, but tumours of the lung,
colon and rectum, breast, skin, and prostate occur most frequently, and
are variably associated with dietary factors.
The relationship between diet and cancer has been a topic of
considerable controversy in research as well as in public policy. It
has been estimated that as many as 35% of all cancer deaths in the
United States are attributable to diet. In 1984, the American Cancer
Society published a set of dietary guidelines that recommended reducing
fat intake to lower the risk of some types of cancer. This
recommendation has been supported by the National Cancer Institute and
the Surgeon General's Report on Nutrition and Health.
The National Cancer Institute initiated a low-fat dietary
intervention program (The Women's Health Trial), to study the effects
of a low-fat diet on the incidence of breast cancer in women at
elevated risk for the disease.
Women in the intervention group reduced their total fat intake to
approximately 20% of total calories over a 12 month period, mainly by
decreasing their fat intake from milk products, red meats, and
fats/oils. In addition the overall quality of the diets improved.
The intervention produced a number of beneficial dietary changes
beyond its primary objective to reduce total fat intake. These dietary
changes are consistent with the dietary guidelines for reducing cancer
risk, developed by the American Cancer Society and the National Cancer
Institute, and included: eating more complex carbohydrates from fruits
and vegetables, eating more foods high in vitamin A and C, weight loss,
drinking less alcohol, and eating less salt-cured, smoked, and
nitrate-cured foods (bacon, baked ham, pork sausages and hot dogs).
Current dietary guidelines recommend that individuals reduce their
fat intake, not only for the prevention of cancer but also to reduce
the risks of other chronic diseases such as coronary heart disease,
hypertension, obesity, and diabetes. In this study the participants
successfully decreased their fat intake to 20% of total calories while
maintaining nutritional adequacy in their diets and without major
changes in the types of foods they were consuming. [Journal of the American Dietetic Association]
It appears that a high vegetable, low-fat, low-calorie diet protects
against rectal cancer. Risk decreases with an increased intake of
carotenoids, vitamin C, and dietary fibre fromvegetables. [Freudenheim, et al]
Records from insurance companies suggested that overweight people were
at higher risk for cancer than normal or underweight people.
Tumours of the upper aerodigestive tract (laryngeal, pharyngeal,
oral, and oesophageal cancers) are alcohol related, and there is
increasing evidence linking alcohol and breast cancer in women. For
oral and oesophageal cancers, one cohort study documented a quadrupled
mortality rate for persons who drink six or more alcoholic beverages a
day, compared with nondrinkers.
Table 2.16
National Cancer Institute Dietary Guidelines.
Several mechanisms have been proposed to account for observed
associations between diet, digestive processes, and cancer. These
include:
- Carcinogens in food that are present naturally, that are
inadvertent contaminants, or that form as products of cooking or
preservation. - Diet-induced metabolic activation or deactivation of
carcinogens. For example, formation of oxygen radicals and lipid
perioxidation products can be retarded or blocked by normal enzymatic
processes or by the selenium or beta-carotene present in foods. - Biological formation of carcinogens, as with conversion of bile
acids to tumour-promoting chemicals by normal intestinal bacteria. The
bacteria that accomplish this conversion may be affected by diet. - Enhancement (eg. by fats) or inhibition (eg. by vitamin A) of promotion.
- Nutrient imbalance may impair immunity and thus may influence
early rejection of malignant cells or the ability of cells to repair
damaged DNA.
Role of Vitamin A and Carotenoids in Cancer
A large body of evidence suggests that foods high in Vitamin A and carotenoids
are protective against a variety of cancers. The strongest evidence for
the role of vitamin A in the prevention of human cancer comes from
epidemiologic studies that correlate consumption of
carotenoid-containing vegetables or foods with a high vitamin A index
to protection against cancer of the lung.
An important issue, related to these studies is whether the
protective effects attributed to vitamin A activity, are truly
attributable to vitamin A, or whether they are due to some other factor
that may be present in the foods. For example, a study of lung cancer
among New Jersey white males showed a protective effect for fruits and
vegetables that was greatest for dark yellow-orange and green
vegetables, but no statistically significant effect for retinol,
carotenoids, or vitamin A activity. [Ziegler et al]
Because retinoids are required for normal cell differentiation,
their deficiency leads to improper differentiation of stem cells in epithelial tissue. In animals, retinoids may inhibit initiation and promotion
stages of carcinogeneses. Retinoids may also have a role in reversing
cancerous changes.
Antioxidant chemicals are thought to protect against certain
promoters of carcinogeneses. Foods containing vitamin A have been shown
to protect against the formation of oxygen radicals and lipid
peroxidation, and beta-carotene is a very efficient neutralizer of
oxygen radicals.
Large amounts of retinoids in the blood or tissues, can be toxic and
may cause birth defects and adverse effects on the skin, liver, and
neurologic tissue. Excessive intake of preformed vitamin A or retinoid
supplements should be avoided, especially by pregnant women. However,
increased intake of carotenoids from food alone is unlikely to have any
adverse effects, other than skin discolouration at very large intakes.
Role of Other Dietary Constituents in Cancer
Foods contain both nutritive and nonnutritive components. Most of
the later are present naturally, but some are added during formulation,
processing, and cooking. Studies have shown that some specific
nonnutritive substances can promote tumour development in animals. For
example, aflatoxin, a potent carcinogen derived from mold on grains,
legumes, or nuts is a naturally occurring toxin in these foods.
Experimental and epidemiological data suggest an association between
alcohol consumption and human cancer that is strongest for certain head
and neck cancers. Alcohol intake and smoking act synergistically to
increase the risk for cancer of the mouth, larynx, and oesophagus.
Although alcohol has an effect independent of smoking in increasing
cancer risk, it remains uncertain whether the responsible agent is
alcohol itself or any of the more than 400 other chemicals identified
in alcoholic beverages. The nutrient deficiencies produced in
alcoholics could be associated with impaired immune function,
permitting increased carcinogenesis. A slightly greater risk for breast
cancer in women has been associated with an average of one drink per
day in a cohort study of 89,538 American women. [Willett et al]
Vitamin C functions as a chemical-reducing agent and antioxidant.
Human studies have shown a protective association between foods that
contain vitamin C and cancers of the oesophagus, stomach, and cervix.
Vitamin C blocks the formation of carcinogenic nitrosamines from
nitrates and prevents oxidation of certain other chemicals to active
carcinogenic forms.
An association between protein consumption, especially animal
protein, and the incidence of certain cancers has been observed in
several human epidemiologic studies. Smoked and charred foods acquire
polycyclic aromatic hydrocarbons, some of which are known to be
carcinogenic in animals. These and other potential carcinogenic agents
may be formed within foods during cooking in amounts that may be
related to temperature and duration of cooking at very high
temperatures.
International epidemiologic evidence suggests that populations
consuming diets high in salt-cured, salt-pickled, and smoked foods have
a higher incidence of stomach and oesophageal cancers. oesophageal and
stomach cancers are also associated with poor nutrition. For decades
sodium nitrate has been added to cured meats at levels of about 200
parts per million to prevent botulism. Nitrate can react with secondary
amines to form carcinogenic nitrosamines. Bacteria in the mouth or
intestine, however, reduce nitrate to nitrite in appreciable amounts.
Clearly, a nutritious diet providing adequate amounts of all
nutrients and proper calorie content to achieve desirable weight is
important for general health and for vigourous defence mechanisms
against cancer as well as other diseases.
Nutrition Programs and Services
Food Labels:Evidence related to the role of dietary
factors in cancer suggests that food manufacturers should include on
package labelsinformation about nutritional content of the food,
especially for fat and carbohydrate components (including fiber).
Food Services:Evidence related to the role of dietary
factors in cancer suggests that the public might benefit from increased
availability of foods high in fibre and low in fat.
Food Products: Evidence related to the role of dietary
factors in cancer suggests that foods low or reduced in calories and
fat and high in fibre should be made increasingly available by food
manufacturers. [The Surgeon General]
- Journal of the American Dietetic Association: Research, Page 802 - 809, June 1990.
- Freudenheim,
J.L., Graham, S., Marshall, J.R., Haughey, B.P., and Wilkinson, G., A
case study of diet and rectal cancer in western New York , American
Journal of Epidemiology, 131:612, 1990. - Ziegler,
R.G., Mason, T.J., Stemhagen, A., Hoover, R., Schoenberg, J.B.,
Gridley, G., Virgo, P.W., & Fraumeni, J.F., Carotenoid intake,
vegetables, and the risk of lung cancer among white men in New Jersey ,
American Journal of Epidemiology 123:1080, 1986. - Willett, W.C., and MacMahon, B., Diet and Cancer; an overview , New England Journal of Medicine 310:697, 1984.
- The Surgeon General's Report on Nutrition & Health, 1988.
