Vegetarian Diets and Cancer Prevention
Satnam Sekhon, BHE, RDN
Nutrition Consultant, British Columbia Cancer Agency, Vancouver, BC, Canada
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.
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.
The following is a list of various resources for the professional as well as the client who may want to incorporate more plant-based foods into his or her diet.
- Food, Nutrition and the Prevention of Cancer: A Global Perspective. American Institute for Cancer Research; 1997. ISBN 1-899533-05-2.
- Becoming Vegetarian: A Complete Guide to Adopting a Healthy Vegetarian Diet, by V Melina, B Davis and V Harrison. Book Publishing Company, 1995. ISBN 1-57067-013-7
- The Dietitian's Guide to Vegetarian Diets: Issues and Applications by Mark and Virginia Messina. Aspen Publishers, Inc, 1996. ISBN 0-8342-0635-8.
- Almost Vegetarian by Diana Shaw. 1994. ISBN 0-51788-20-6.
- The Occasional Vegetarian by Karen Lee. Warner Books Inc, 1995. ISBN 0-446-51792-5.
- Meatless Meals for Working People: Quick and Easy Vegetarian Recipes, by D. Wasserman and C Stahler. The Vegetarian Resource Group, 1998. ISBN 0-931411-06-8.
- Lean and Luscious and Meatless, by B. Hinman & M Snyder. Prima Publishing, 1991. ISBN 1-55958-110-7.
- Jane Brody's Good Food Book, by Jane E Brody. WW Norton & Company, 1995. ISBN 0-393-02210-2.
- Meatless Dishes in Twenty Minutes, by Karen A Levin. Contemporary Books Inc, 1993. ISBN 0-8092-3810-1.
- Moosewood Restaurant Cooks at Home, by the Moosewood Collective. Simon Schuster/Fireside, 1994. ISBN 0-671-81954-5.
- Simple, Lowfat & Vegetarian, by Suzanne Havala, Vegetarian Resource Group, 1994. ISBN 0-931411-09-2.
- New Laurel's Kitchen, by L Robertson, C Flinders, B Ruppenthal. Ten Speed Press, 1986. ISBN 0-89815-167-8.
- Full of Beans, by V Currie and K Spicer. Mighton House, 1993. ISBN 0-965688-1-9.
- The Amazing Legume: Cooking with Lentils, Dry Beans and Dry Peas, by Alice Jenner. Centax Books, Canada, 1994. ISBN 0-9691901-0-7.
- American Institute for Cancer Research, http://www.aicr.org
- Vegetarian Resource Group, http://www.vrg.org
- U.S. Soyfoods Directory, http://www.soyfoods.com Position of The American Dietetic Association: Vegetarian Diets. J Am Diet Assoc 1997;97:1317-1321.
- Vegetarian Nutrition and Healthletter, School of Public Health, Loma Linda University, Loma Linda, CA 92350.
- Whorton JC. Historical development of vegetarianism. Am J Clin Nutr 1994;59(suppl): 1103S-9S.
- Johnston, PK. Vegetarians among us: implications for health professionals. Top Clin Nutr 1995;10(2):1-6.
- Ryan YM. Meat avoidance and body weight concerns: nutritional implications for teenage girls. Proc Nutr Soc 1997;56:519-24.
- Santos MLS and Booth DA. Influences on meat avoidance among British students. Appetite 1996;23:197-205.
- Neumark-Sztainer D, Story M, Resnick MD, Blum RW. Adolescent vegetarians: behavioral profile of a school-based population in Minnesota. Arch Pediatr Adolesc Med 1997;151:833-8.
- Clifford M. Nutrition counseling of the vegetarian. Top Clin Nutr 1995; 10(2):44-7.
- Szabo LB. The health risks of newwave vegetarianism. Can Med Assoc J 1997;156(10):1454-5.
- Ball M. Vegetarian, vegan or meat eater: the pros and the cons. Aust Fam Phys 1997;26(11):1269-74.
- Donovan UM and Gibson RS. Dietary intakes of adolescent females consuming vegetarian, semi-vegetarian,and omnivorous diets. J Adoles Health 1996;18:292-300.
- Key TJA, Thorogood M, Appleby PN, Burr ML. Dietary habits and mortality in 11000 vegetarians and health conscious people: results of a 17-year follow up. BMJ 1996;313:775-9.
- Mills PK, Beeson WL, Phillips RL, Fraser GE. Cancer incidence among California Seventh Day Adventists, 1976-1982. Am Clin Nutr 1994;59(suppl):1136S-42S.
- Frentzel-Beyne R and Chang-Claude J. Vegetarian diets and colon cancer: the German experience. Am J Clin Nutr 1994;59(suppl):1143S-52S.
- Thorogood M, Mann J, Appleby P, McPherson K. Risk of death from cancer and ischemic heart disease in meat and non-meat eaters. BMJ 1994;308:1667-70.
- Nair P and Mayberry JF. Vegetarianism, dietary fibre and gastro-intestinal disease. Dig Dis 1994;12: 177-85.
- Steinmetz KA and Potter JD. Vegetables, fruit, and cancer prevention: a review. J Am Diet Assoc 1996;96:1027-39.
- Craig WJ. Phytochemicals: guardians of our health. J Am Diet Assoc 1997;97(suppl 2):S199-S204.
- Dwyer J. Is there a need to change the American diet? In AICR, ed. Dietary Phytochemicals in Cancer Prevention and Treatment. New York; Plenum Press, 1996:189-98.
- Dwyer J. Vegetarian eating patterns: science, values, and food choices – where do we go from here? Am J Clin Nutr 1994;59(suppl): 1255S-62S.
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Isoflavone supplements (derived from soy beans) were found to have a favorable effect on cognitive function, especially verbal memory, in postmenopausal women.