Vegetarian Diets and Cancer Prevention
Satnam Sekhon, BHE, RDN
Nutrition Consultant, British Columbia Cancer Agency, Vancouver, BC,
Canada

http://www.vegetariannutrition.net/a...Prevention.php

Although vegetarianism is relatively new in North America, it has
existed in Asia for centuries, especially in followers of religions
such as Hinduism and Buddhism. In the 19th century, North Americans
chose to become vegetarian primarily for spiritual and moral reasons.
As the scientific research linking nutrition and health started to
become apparent, and with the discovery of vitamins in fruits and
vegetables in the early 20th century, the public developed the
perception that vegetarian diets were healthy (1). Most research on
vegetarianism has been done in the past 30 years, mostly in developed
countries looking at the association between vegetarianism and
nutritional deficiencies.

There are many reasons other than religion for choosing a vegetarian
diet. They include health benefits, the belief that it improves
athletic performance, and political, social, ecological and ethical
reasons (2). Many adolescents adopt a vegetarian lifestyle because of
concerns about animal welfare, as well as concerns about their own
weight (3-5). Just as there are many reasons for becoming vegetarian,
so are there many degrees to which animal foods are avoided. In other
words, not all vegetarians eat the same way (6). The new terms being
used for semi-vegetarianism are new wave, quasi, partial and
transitional. Such individuals do not strictly follow lacto-ovo or
vegan diets but largely exclude meat and animal products. This may or
may not include poultry, fish or seafood. The danger is that, with
inadequate knowledge, they may not include more plant-based proteins,
such as beans and lentils and nutrientrich greens at the same time
that they exclude animal products (7,8). Some may still have a diet
that is high in saturated fat from large amounts of high-fat dairy
products and eggs. Donovan and Gibson (9) noted that lacto-ovo and
semi-vegetarian adolescent females had energy intakes less than
two-thirds of Canadian recommendations and concluded that they were
more at risk for nutrient inadequacies than those following omnivorous
diets.

A special exposure group used in diet and cancer prevention studies
has been Seventh-Day Adventists (SDA). The SDA Church is a Protestant
denomination comprising over 10 million members worldwide. This group
practices a temperate and healthy lifestyle, abstaining from alcohol,
tobacco, and other harmful drugs. Many are lacto-ovo-vegetarians,
believing that flesh food and shellfish are better omitted from the
ideal diet.

It is the opinion of this author that increasing numbers of cancer
survivors are also adopting semi-vegetarian diets and healthy
lifestyles post-treatment, as seen in counseling sessions, but there
is little research to confirm this.

Over the last decade, we have seen the results of long-term studies
comparing cancer rates among vegetarian groups and the general public
in various countries. An observational cohort study of approximately
11,000 British subjects was conducted by Key et al (10) to investigate
the association of dietary habits with mortality in a cohort of
vegetarians and other health-conscious people. Diet was assessed using
a short questionnaire that asked participants if they smoked and were
vegetarian (this was not defined further) and to record their usual
frequency on six dietary factors. The investigators noted that 62% of
the participants considered themselves vegetarian. The cancer
standardized mortality ratio (SMR) is the observed number of incidents
of cancer that have occurred after the start of a study in comparison
with the expected number of incidents, which is usually acquired from
age-specific rates of the general population. Key et al (10) concluded
the SMR for all malignant neoplasm was 0.50 for men and 0.76 for
women, indicating a significantly lower incidence of cancer in the
vegetarian groups.

The vegetarian population demonstrated an all cancer mortality rate
50% that of the general population for men and 76% for women. The SMRs
were reduced for cancer of the stomach, large intestine, rectum,
pancreas, lung, and bladder, but slightly increased for the prostate.
Daily consumption of fresh fruit was associated with a reduced
mortality from all cancers combined. Limitations of this study (10)
are that the dietary habits did not differentiate by type of
vegetarianism as that may have changed during the 17-year follow- up.
Also, the questionnaire did not include all food groups and other
health-related behaviors, such as exercise and past smoking habits.

Cancer incidence was monitored by Mills et al (11) in a population of
approximately 34,000 non-Hispanic white Seventh-Day Adventists in
California between 1976 and 1982 and compared with a similar group of
whites in Connecticut. Relative risks were calculated using data
obtained from a detailed lifestyle questionnaire. The SMR for all
cancers was 0.73 for men and 0.92 for women, indicating a
significantly lower risk of developing cancers at most of the major
cancer sites (buccal cavity, stomach, lung, pancreas, rectum, bladder,
and large intestine). Surprisingly, the SMRs for prostate and
gynecological cancer were 1.25 and 1.6, respectively. The authors
attribute this to Seventh-Day Adventists possibly taking advantage of
preventive medical measures and higher use of hormone replacement
therapy in this population.

Frentzel-Beyme and Chang-Claude (12) assessed mortality and morbidity
risks as related to nutritional status of moderate (eat fish or meat
occasionally) and strict (avoid fish and meat completely) vegetarians
in a total cohort of 1904 self-identified vegetarians in Germany. The
questionnaire collected information on dietary habits including
alcohol consumption, smoking habits, physical activity, previous
medical history, and socioeconomic characteristics. The followup
period was 11 years and the SMR for all cancers was 0.48 for men and
0.74 for women. The SMR was lower for cancer of the intestinal system,
stomach, and colon. Cancer mortality was most strongly associated with
duration of vegetarian status (20 years or more of a vegetarian
lifestyle decreased cancer mortality by more than 50%) and moderate
vegetarians appeared to be at lower risk for cancer. In the author?s
opinion, the influence of other factors such as health-conscious
behavior and a healthy lifestyle seem to indicate stronger effects
than nutrition itself and this may partly explain the generally better
health of moderate vegetarians.

In another British study (13), Thorogood et al investigated the health
consequences of a vegetarian diet in approximately 6000
non-meat-eaters and 5000 meat-eating controls. Each participant
completed a questionnaire concerning diet, lifestyle factors, medical
history, and body mass index. Approximately 95% of the non-meat-eaters
were lactoovo vegetarians (who may have eaten meat or fish once a
week) or strict vegans. Recruitment of participants took place through
advertisements or word of mouth and the participants in turn recruited
friends or relatives as the controls. After 12 years of follow-up,
non-meat-eaters had significantly lower SMRs for cancer compared to
meat eaters (0.50 and 0.80, respectively). A 40% reduction in cancer
mortality was observed due to diet, and it did not appreciably change
when adjusted for smoking, body mass index, and social class. A
limitation of this study is a potential bias of the participants. The
investigators did not differentiate between different types of
vegetarians but simply grouped the participants as non-meateaters
(although this was not the objective of the study). They concluded
that the protective effect of diet was large, but the data do not
provide justification for encouraging meat-eaters to change to a
vegetarian diet.

A review article looking at the association between vegetarianism,
dietary fiber, and gastrointestinal (GI) disease (14) concluded that
vegetarians have a lower incidence of GI cancer. The authors state the
benefits of a vegetarian life-style may be conferred on
non-vegetarians by eating a carefully planned non-vegetarian diet
consisting of increased fruit, vegetables, and fiber.

Conclusion
There are many reasons that it is difficult to demonstrate a direct
causal relationship between vegetarian diets and reduced cancer
incidence based on current research. These include long latency
periods for cancers, the presence of other lifestyle factors (smoking,
exercise) and known risk factors (fat in red meat, cooking methods),
as well as the possible interrelationship among these factors. Also,
the definition of ?vegetarian diets? in the literature is
inconsistent. Nevertheless, a comprehensive review by Steinmetz and
Potter (15) concluded that the scientific evidence regarding a
protective role for vegetable and fruit consumption in cancer
prevention is generally consistent. Active research indicates that it
is not only the vitamins, minerals, or fiber that make plant foods
beneficial to health, but the phytochemicals found in these food as
well (16,17). As a result, it is difficult to conclude whether it is
the decrease in meat and/or fat, the increase in fruit and vegetables,
or other lifestyle factors that provide the beneficial effect in
vegetarians; most likely they all play a role (18).
It is reasonable to conclude that vegetarians have a reduced incidence
of cancer in comparison to the general population, but it is not know
to what extent vegetarian diets can play a role. A well-planned
vegetarian diet can be a healthy choice and may reduce the risk of
certain types of cancer, but it is important to include a wide array
of foods for maximum benefit. A healthy diet including small amounts
of lean meats and animal products and a high consumption of
plant-based foods can also be a healthy alternative. Dietitians should
help clients to choose the diet that works best with their lifestyle
and promote other positive lifestyle factors, such as incorporation of
regular physical activity and maintaining an appropriate weight.