DEFINITIONS
A vegetarian is a person who does not eat meat, fish, or fowl or products containing these foods. The eating patterns of vegetarians may vary considerably.
The lacto-ovo-vegetarian eating pattern is based on grains, vegetables, fruits, legumes, seeds, dairy products, and eggs but excludes meat, fish, and fowl. The lacto-vegetarian excludes eggs as well as meat, fish, and fowl. The vegan, or total vegetarian, eating pattern is similar to the lacto-vegetarian pattern, with the additional exclusion of dairy and other animal products. Even within these patterns, considerable variation may exist in the extent to which animal products are avoided, from person to person. People choosing macrobiotic diets are frequently identified as following a vegetarian diet. The macrobiotic diet is based largely on grains, legumes, and vegetables. Fruits, nuts, and seeds are used to a lesser extent.
Common reasons for choosing a vegetarian diet include health considerations, concern for the environment, and animal welfare factors. Vegetarians also cite economic reasons, ethical considerations, world hunger issues, and religious beliefs as their reasons for following their chosen eating pattern.
Perceptions and practices of self-defined current vegetarian, former vegetarian, and nonvegetarian women; 2002
HEALTH IMPLICATIONS OF VEGETARIANISM
Vegetarian diets offer a number of advantages, including lower levels of saturated fat, cholesterol, and animal protein and higher levels of carbohydrates, fiber, magnesium, boron, folate, antioxidants such as vitamins C and E, carotenoids, and phytochemicals. Some vegans may have intakes for vitamin B-12, vitamin D, calcium, zinc, and occasionally riboflavin that are lower than recommended.
Nutrient intakes and eating behavior scores of vegetarian and nonvegetarian women; 1995
BIOCHEMICAL CONSIDERATION
Proteins.
Plant protein can meet requirements when a variety of plant foods is consumed and energy needs are met. Research indicates that an assortment of plant foods eaten over the course of a day can provide all essential amino acids and ensure adequate nitrogen retention and use in healthy adults, thus complementary proteins do not need to be consumed at the same meal. Estimates of protein requirements of vegans vary, depending to some degree on diet choices.
A recent meta-analysis of nitrogen balance studies found no significant difference in protein needs due to the source of dietary protein.
Another study, based primarily on the lower digestibility of plant proteins, other groups have suggested that protein requirements of vegans may be increased by 30% to 35% for infants up to the age of 2 years, 20% to 30% for 2- to 6-year-old children, and 15% to 20% for those 6 years and older, in comparison with those of non-vegetarians. However, the quality of plant proteins varies. Based on the protein digestibility corrected amino acid score (PDCAAS), which is the standard method for determining protein quality, isolated soy protein can meet protein needs as effectively as animal protein, whereas wheat protein eaten alone, for example, may be 50% less usable than animal protein.
Cereals tend to be low in lysine, an essential amino acid. This may be relevant when evaluating diets of individuals who do not consume animal protein sources and are relatively low in protein.
Although some vegan women have protein intakes that are marginal, typical protein intakes of lacto-ovo-vegetarians and of vegans appear to meet and exceed requirements. Athletes can also meet their protein needs on plant-based diets.
In conclusion, nutrition care professionals should be aware that protein needs might be higher than the RDA in vegetarians whose dietary protein sources are mainly those that are less well digested, such as some cereals and legumes. Plus, dietary adjustments such as the use of more beans and soy products in place of other protein sources that are lower in lysine or an increase in dietary protein from all sources can ensure an adequate intake of lysine.
Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults; 2003
Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids; 2002
Plant proteins in relation to human protein and amino acid nutrition; 1994
Protein Quality Evaluation; 1991
Iron.
Plant foods contain only nonheme iron, which is more sensitive than heme iron to both inhibitors and enhancers of iron absorption. Inhibitors of iron absorption include phytate (or inositol hexakisphosphate, called phytate in its salt form); calcium; teas, including some herb teas; coffee; cocoa; some spices; and fiber. Because iron intake increases as phytate intake increases, effects on iron status are somewhat less than might be expected. However, vitamin C and other organic acids found in fruits and vegetables can enhance iron absorption and can help to reduce effects of phytate. Plus, some food preparation techniques such as soaking and sprouting beans, grains, and seeds can hydrolyze phytate and may improve iron absorption. Other fermentation processes, such as those used to make soy foods like miso and tempeh, may also make iron more available, although not all research supports this.
Whereas many studies of iron absorption have been short term, there is evidence that adaptation to low intakes takes place over the longer term and involves both increased absorption and decreased losses.
In conclusion, studies shows that iron absorption would be significantly reduced if a diet were to be high in inhibitors and low in enhancers. Recommended iron intakes for vegetarians are 1.8 times those of nonvegetarians because of lower bioavailability of iron from a vegetarian diet.
Studies typically show iron intake by vegans to be higher than that of lacto-ovo-vegetarians and of nonvegetarians, and most studies show iron intake by lacto-ovo-vegetarians to be higher than that of nonvegetarians. For this reasons, incidence of iron deficiency anemia among vegetarians is similar to that of nonvegetarians. Although vegetarian adults have lower iron stores than nonvegetarians, their serum ferritin levels are usually within the normal range.
Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc; 2001
Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages; 1999
Inositol phosphates with different numbers of phosphate groups influence iron absorption in humans; 1999
Dietary intake and iron status of Australian vegetarian women; 1999
Nonheme-iron absorption, fecal ferritin excretion, and blood indexes of iron status in women consuming controlled lactoovovegetarian diets for 8 wk; 1999
The Dietitian’s Guide to Vegetarian Diets: Issues and Applications; 1996
Effect of traditional Oriental soy products on iron absorption; 1990
Chelating properties of dietary fiber and phytate: The role for mineral availability; 1990
The effects of organic acids, phytates, and polyphenols on the absorption of iron from vegetables; 1983
Zinc.
Because phytate binds zinc (as well as iron), and animal protein is believed to enhance zinc absorption, total zinc bioavailability appears to be lower on vegetarian diets. Also, some vegetarians have diets that are significantly below recommended intakes for zinc. Although overt zinc deficiency has not been seen in Western vegetarians, the effects of marginal intakes are poorly understood. Zinc requirements for vegetarians whose diets are high in phytate may exceed the RDA.
Compensatory mechanisms may help vegetarians adapt to lower intakes of zinc.
Also, the same mechanism used to reduce phytate introit for iron are valid to avoid zinc reduction.
Moving toward a plant-based diet: Are iron and zinc at risk; 2002
Zinc intake and status in Australian vegetarians; 2000
Zinc absorption, mineral balance, and blood lipids in women consuming controlled lactoovovegetarian and omnivorous diets for 8 weeks; 1998
Content and bioavailability of trace elements in vegetarian diets; 1994
Calcium.
Calcium is present in many plant foods and fortified foods. Low-oxalate greens (bok choy, broccoli, Chinese/Napa cabbage, collards, kale, okra, turnip greens) provide calcium with high bioavailability (49% to 61%), in comparison with calcium-set tofu, fortified fruit juices, and cow’s milk (bioavailability in the range of 31% to 32%) and with fortified soymilk, sesame seeds, almonds, and red and white beans (bioavailability of 21% to 24%). Figs and soy foods such as cooked soybeans, soy nuts, and tempeh provide additional calcium. Factors that enhance calcium absorption include also adequate vitamin D and protein.
Oxalates present in some foods can greatly reduce calcium absorption, so vegetables that are very high in these compounds, such as spinach, beet greens, and Swiss chard, are not good sources of usable calcium despite their high calcium content. Phytate may also inhibit calcium absorption. Also, diets high in sulfur-containing amino acids (eggs, meat, fish, poultry, dairy products, nuts and many grains) may increase losses of calcium from bone, as well as excessive sodium intake may also promote calcium losses.
Calcium intakes of lacto-vegetarians are comparable with or higher than those of non-vegetarians, whereas intakes of vegans tend to be lower than both groups and often below recommended intakes. Some studies show that the ratio of dietary calcium to protein is more predictive of bone health than calcium intake alone. Typically, this ratio is high in lacto-ovo-vegetarian diets and favors bone health, whereas vegans have a calcium to protein ratio that is similar to or lower than that of non-vegetarians. All vegetarians should meet the recommended intakes for calcium, established for their age group by the Institute of Medicine. This can be accomplished, in nonpregnant, nonlactating adults, by consuming at least 8 servings per day of foods that provide 10% to 15% of the Adequate Intake (AI) for calcium, as indicated in the Vegetarian Food Guide Pyramid and Vegetarian Food Guide Rainb other stages of the life cycle are available.
A new food guide for North American vegetarians; 2003
Bioavailability of the calcium in fortified soy imitation milk, with some observations on method; 2000
Choices for achieving adequate dietary calcium with a vegetarian diet; 1999
Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D and Fluoride; 1997
Meat consumption and its associations with other diet and health factors in young adults; 1991
Vitamin D.
Vitamin D status depends on sunlight exposure and intake of vitamin D fortified foods or supplements. Note that those with dark skin require longer exposure, and also sun exposure may be inadequate for those living in Canada and at northern latitudes in the United States, especially in winter months, for those in smoggy regions, and for those whose sun exposure is limited. Furthermore, infants, children, and older adults synthesize vitamin D less efficiently.
Vitamin D3 (cholecalciferol) is of animal origin, whereas vitamin D2 (ergocalciferol) is a form acceptable to vegans. Vitamin D2 may be less bioavailable than vitamin D3, which could raise the requirements of vegetarians who depend on D2 supplements to meet vitamin D needs. Low vitamin D levels and reduced bone mass have been observed in some vegan populations at northern latitudes who did not use supplements or fortified foods, particularly children following macrobiotic diets and adult Asian vegetarians. If sun exposure and intake of fortified foods are insufficient, vitamin D supplements are recommended.
Evidence that vitamin D-3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D-2; 1998
Vitamin D and bone health; 1996
Bone density and cortical thickness in nutritional vitamin D deficiency: Effect of secondary hyperparathyroidism; 1988
Riboflavin.
Some studies have shown vegans to have lower intakes of riboflavin , compared with nonvegetarians; however, clinical riboflavin deficiency has not been observed.
Nutrient intakes and eating behavior scores of vegetarian and nonvegetarian women; 1995
Vitamin B-12.
Sources of vitamin B-12 that are not derived from animals include B-12 fortified foods (such as some brands of soymilk, breakfast cereals, and nutritional yeast) or supplements. Unless fortified, no plant food contains significant amounts of active vitamin B-12. Foods such as sea vegetables and spirulina (uni-cellular blue-green alga in the shape of a spiral coil) may contain vitamin B-12 analogs; neither these nor fermented soy products can be counted on as reliable sources of active vitamin B-12. Lacto-ovo-vegetarians can get adequate vitamin B-12 from dairy foods and eggs if these foods are consumed regularly instead. Vegetarian diets are typically high in folic acid, which can mask the hematological symptoms of vitamin B-12 deficiency. Studies indicate that some vegans and other vegetarians do not regularly consume reliable sources of vitamin B-12 and that this is reflected in less than adequate vitamin B-12 status. Absorption is most efficient when small amounts of vitamin B-12 are consumed at frequent intervals.
A regular source of vitamin B-12 is crucial for pregnant and lactating women and for breastfed infants if the mother’s diet is not supplemented. Infants born to vegan mothers whose diets lack reliable sources of this vitamin are at especially high risk of deficiency. Maternal vitamin B-12 intake and absorption during pregnancy appear to have a more important influence on vitamin B-12 status of the infant than do maternal vitamin B-12 stores.
In conclusion, it is essential that all vegetarians use a supplement, fortified food, dairy products, or eggs to meet recommended intakes of vitamin B-12.
Total homocysteine, vitamin B12, and total antioxidant status in vegetarians; 2001
Metabolic vitamin B12 status on a mostly raw vegan diet with follow-up using tablets, nutritional yeast, or probiotic supplements; 2000
Relative weight, weight loss efforts and nutrient intakes among health-conscious vegetarian, past vegetarian and nonvegetarian women ages 18 to 50; 2000
Observations on transfer of vitamin B12 from mother to fetus and newborn; 1958
Vegetarianism and vitamin B-12 (cobalamin) deficiency
Effect of Vitamin B12 Treatment on Haptocorrin
Severe vitamin B12 deficiency in infants breastfed by vegans
Vitamin A / Beta Carotene.
Because preformed vitamin A is found only in animal foods, vegans get all of their vitamin A from conversion of dietary carotenoids, particularly beta carotene. Research suggests that absorption of beta carotene from plant foods is less efficient than previously believed. This suggests that vegans intake of vitamin A is about half of what previous studies have suggested, and intake by lacto-ovo-vegetarians may be 25% lower than previously shown. Despite this, vegetarians have been reported to have higher serum carotenoid levels than nonvegetarians. Vitamin A requirements can be met with the inclusion of three servings per day of deeply yellow or orange vegetables, leafy green vegetables, or fruits that are rich in beta carotene. Cooking increases beta carotene absorption, as does the addition of small amounts of fat to meals.
Bioavailability of lutein from vegetables is five times higher than that of beta carotene; 1999
Estimation of carotenoid accessibility from carrots determined by an in vitro digestion method; 2002
The Dietitian’s Guide to Vegetarian Diets: Issues And Applications; 1996
N-3 Fatty Acids.
Whereas vegetarian diets are generally rich in n-6 fatty acids (especifically linoleic acid), these diets can be low in n-3 fatty acids , resulting in an imbalance that can inhibit production of the physiologically active long chain n-3 fatty acids, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Diets that do not include fish, eggs, or generous amounts of sea vegetables generally lack direct sources of EPA and DHA. Recently, vegan sources of DHA derived from microalgae have become available as supplements in nongelatin capsules. Algae sources of DHA have been shown to positively affect blood levels of DHA and of EPA through retroconversion. Most studies show vegetarians, and particularly vegans, to have lower blood levels of EPA and DHA than nonvegetarians. It is recommended that vegetarians include good sources of linolenic acid in their diet. Those with increased requirements (eg, pregnant and lactating women or those with diseases associated with poor essential fatty acid status) or those at risk for poor conversion (eg, people with diabetes) may benefit from direct sources of long-chain n-3 fatty acids, such as DHA-rich microalgae.
Supplementation with an algae source of docosahexaenoic acid increases (n-3) fatty acid status and alters selected risk factors for heart disease in vegetarian subjects; 1996
Fatty acid composition of erythrocyte, platelet, and serum lipids in strict vegans. Lipids; 1995
Achieving optimal essential fatty acid status in vegetarians: Current knowledge and practical implications.
Iodine.
Some studies suggest that vegans who do not consume iodized salt may be at risk for iodine deficiency; this appears to be particularly true for those living in iodine-poor areas. Bread can be a source of iodine because some dough stabilizers contain iodine. Concern has been raised about vegetarian diets that include foods, such as soybeans, cruciferous vegetables, and sweet potatoes, that contain natural goitrogens (anti-thyroid drugs). However, these foods have not been associated with thyroid insufficiency in healthy people provided iodine intake is adequate. The adult RDA for iodine is easily met by one-half teaspoon of iodized salt daily. Some vegetarians may have very high intakes of iodine because of consumption of sea vegetables.
The Oxford Vegetarian Study: An overview; 1999
Increased risk of iodine deficiency with vegetarian nutrition; 1999
VEGETARIANISM THROUGHTOUT THE LIFE CYCLE
Well-planned vegan, lacto-vegetarian, and lacto-ovo-vegetarian diets are appropriate for all stages of the life cycle, including pregnancy and lactation. Appropriately planned vegan, lactovegetarian, and lacto-ovo-vegetarian diets satisfy nutrient needs of infants, children, and adolescents and promote normal growth. Vegetarian diets in childhood and adolescence can aid in the establishment of lifelong healthy eating patterns and can offer some important nutritional advantages. Vegetarian children and adolescents have lower intakes of cholesterol, saturated fat, and total fat and higher intakes of fruits, vegetables, and fiber than nonvegetarians. Vegetarian children have also been reported to be leaner and to have lower serum cholesterol levels.
VEGETARIANISM IN DISEASE
Obesity.
Vegetarian eating patterns have been associated with lower body mass index (BMI). In a study of 4,000 men and women in England comparing the relationship between meat consumption and obesity among meat eaters, fish eaters, lacto-ovo-vegetarians, and vegans, mean BMI was highest in the meat eaters and lowest in the vegans. BMI was lowest in those lacto-ovo-vegetarians and vegans who had adhered to their diet for 5 years or longer. Factors that may help to explain the lower BMI among vegetarians include differences in macronutrient content (lower protein, fat, and animal fat intake), higher fiber consumption, decreased alcohol intake, and greater consumption of vegetables.
Prevalence of obesity is low in people who do not eat meat; 1996
Cardiovascular Disease.
An analysis of five prospective studies involving more than 76,000 subjects showed that death from ischemic heart disease was 31% lower among vegetarian men compared with nonvegetarian men and 20% lower among vegetarian women compared with nonvegetarian women. Death rates were also lower for vegetarian men and women compared with semivegetarians, those who ate fish only or ate meat less than once per week. The lower rates of heart disease among vegetarians are explained in part by their lower blood cholesterol levels. A review of 9 studies found that, in comparison to nonvegetarians, lactoovo- vegetarians and vegans had mean blood cholesterol levels that were 14% and 35% lower, respectively. Although the lower average BMI of vegetarians may help to explain this, even when vegetarian subjects were heavier than nonvegetarian subjects, the vegetarians had markedly lower plasma lipoprotein values. Average triglyceride levels tend to be similar in vegetarians and nonvegetarians.
Some, but not all, studies have shown lower high-density lipoprotein (HDL) levels in vegetarian subjects. Lower HDL levels may be due to the type or amount of dietary fat or to lower alcohol intake. This may help to explain the smaller differences in heart disease rates between vegetarian and nonvegetarian women because HDL may be a more important risk factor than LDL levels for women. A number of factors in vegetarian diets may affect cholesterol levels. Although studies show that most vegetarians do not typically consume low-fat diets, saturated fat intake is considerably lower among vegetarians than nonvegetarians, and vegans have a lower ratio of saturated to unsaturated fat in their diets. Vegetarians also consume less cholesterol than nonvegetarians, although the range of intake varies considerably across studies. Vegan diets are free of cholesterol. Vegetarians consume between 50% and 100% more fiber than nonvegetarians, and vegans have higher intakes than lacto-ovo-vegetarians. Soluble fiber may lower risk for cardiovascular disease by reducing blood cholesterol levels.
Other factors in vegetarian diets may impact cardiovascular disease risk independent of effects on cholesterol levels. Vegetarians have higher intakes of the vitamin antioxidants vitamins C and E, which may reduce oxidation of LDL cholesterol. Isoflavones, which are phytoestrogens found in soy foods, may also have antioxidant properties as well as enhancing endothelial function and arterial compliance. Although there is limited information available about intake of specific phytochemicals among population groups, vegetarians appear to consume more phytochemicals than nonvegetarians because a greater percentage of their energy comes from plant foods.
Not all aspects of vegetarian diets are associated with reduced risk for heart disease. Some but not all studies have found higher serum homocysteine levels in vegetarians compared to nonvegetarians. Homocysteine is believed to be an independent risk factor for heart disease. Inadequate intake of vitamin B-12 may be the explanation. Vitamin B-12 injections lowered homocysteine levels in vegetarians, many of whom had low B-12 levels and high serum homocysteine. In addition, low intakes of n-3 fatty acids and a high ratio of n-6 to n-3 fatty acids in the diet may raise risk of heart disease among some vegetarians. There are only limited data on the role of vegetarian diets as intervention for heart disease. Vegetarian diets used in these studies have usually been very low in fat. Vegetarian diets can be planned to conform to standard recommendations for the treatment of hypercholesterolemia.
Vascular dilatory functions of ovo-lactovegetarians compared with omnivores. Atherosclerosis; 2001
Isoflavone phytoestrogens consumed in soy decrease F(2)-isoprostane concentrations and increase resistance of low-density lipoprotein to oxidation in humans; 2000
Cardiovascular risk factors in vegetarians. Normalization of hyperhomocysteinemia with vitamin B(12) and reduction of platelet aggregation with n-3 fatty acids; 2000
Mortality in vegetarians and nonvegetarians: Detailed findings from a collaborative analysis of 5 prospective studies; 1999
Consensus Panel Statement: Guide to Preventive Cardiology for Women; 1999
Cholesterol-lowering effects of dietary fiber: A meta-analysis; 1999
Common carotid intima-media thickness and arterial stiffness: Indicators of cardiovascular risk in high-risk patients. The SMART Study (Secondary Manifestations of Arterial disease); 1999
The effect of diet on plasma homocysteine concentrations in healthy male subjects;1999
The Dietitian’s Guide to Vegetarian Diets: Issues and Applications; 1996
Diet and serum lipids in vegan vegetarians: A model for risk reduction;1991
Plasma lipids and lipoproteins in vegetarians and controls; 1975
Hypertension.
Many studies show that vegetarians have both lower systolic and diastolic pressures with differences between vegetarians and nonvegetarians generally falling between 5 and 10 mm Hg. In the Hypertension Detection and Follow-Up Program, blood pressure reduction of just 4 mm Hg caused marked reduction in mortality from all causes. In addition to having lower blood pressures in general, vegetarians have markedly lower rates of hypertension than meat eaters. Even semivegetarians are 50% more likely to have hypertension than vegetarians. Even when body weights were similar between subjects, vegetarians had lower blood pressures. Placing nonvegetarian subjects on a vegetarian diet led to reduced blood pressure in normotensive and hypertensive subjects. A number of studies have controlled for various factors that might help to explain the lower blood pressures of vegetarians and the hypotensive effects of changing to a vegetarian diet. The lower blood pressures do not appear to be due to lower BMI, exercise habits, absence of meat, milk protein, fat content of diet, fiber or differences in potassium, magnesium, or calcium intakes. Because sodium intake of vegetarians is comparable or only modestly lower than that of nonvegetarians, sodium does not explain the differences either. Suggested explanations include a difference in blood glucose-insulin response because of a lower glycemic index of vegetarian diets or a collective effect of beneficial compounds from plant foods.
The Dietitian’s Guide to Vegetarian Diets: Issues And Applications; 1996
A randomized controlled trial of the effect on blood pressure of dietary nonmeat protein versus meat protein in normotensive omnivores; 1988
Low blood pressure in vegetarians: Effects of specific foods and nutrients; 1988
Effect of dietary fats and carbohydrate on blood pressure of mildly hypertensive patients; 1987
A randomized controlled trial of the effect of dietary fiber on blood pressure; 1987
Nutrient intake, blood pressure, serum and urinary prostaglandins and serum thromboxane B2 in a controlled trial with a lacto-ovo-vegetarian diet; 1986
Blood pressure in vegetarians and non-vegetarians: A cross-sectional analysis; 1985
Low blood pressure in vegetarians: The possible roles of potassium; 1983
The relationship of blood pressure to diet and lifestyle in two religious populations; 1983
Blood pressure lowering effect of a vegetarian diet: Controlled trial in normotensive subjects; 1983
The role of the sympathetic nervous system and catecholamines in the regulation of energy metabolism; 1983
Blood pressure and diet in normotensive volunteers: Absence of an effect of dietary fiber, protein, or fat.1981
Five-year findings of the hypertension detection and follow-up program.Reduction in mortality of person with high blood pressure, including mild hypertension; 1979
Diabetes.
Vegetarian diets can meet guidelines for the treatment of diabetes, and some research suggests that diets that are more plant-based reduce risk for type 2 diabetes. Among the possible explanations for a protective effect of vegetarian diet are the lower BMI of vegetarians and higher fiber intake, both of which improve insulin sensitivity. However risk for diabetes was still 80% higher in nonvegetarian men after adjustment for weight. In men, meat consumption was directly associated with increased risk of diabetes.
Among women, risk increased only when meat consumption exceeded five servings per week.
Evidence-based nutrition principles and recommendations for the treatment and prevention of diabetes and related complications; 2002
Does a vegetarian diet reduce the occurrence of diabetes?; 1985
Cancer.
Vegetarians have an overall lower cancer rate compared with the general population, but it is not clear to what extent this is due to diet. When nondietary cancer risk factors are controlled for, differences in overall cancer rates between vegetarians and nonvegetarians are greatly reduced, although marked differences remain in rates of certain cancers. An analysis from the Adventist Health Study that controlled for age, sex, and smoking found no differences between vegetarians and nonvegetarians for lung, breast, uterine, or stomach cancer but did find that nonvegetarians had a 54% increased risk for prostate cancer and an 88% increased risk for colorectal cancer. Other research has shown lower rates of colon cell proliferation in vegetarians compared with nonvegetarians and lower levels of serum insulin-like growth factor-I, thought to be involved in the etiology of several cancers in vegans compared with both nonvegetarians and lacto-ovo-vegetarians. Both red and white meat have been independently linked to increased risk for colon cancer.
Research suggests that a number of factors in vegetarian diets may impact cancer risk. Vegetarian diets come closer to matching the dietary guidelines issued by the National Cancer Institute than do nonvegetarian diets, particularly in regard to fat and fiber intakes. High fiber intake is believed to protect against colon cancer, although not all research supports this. The environment of the colon of vegetarians is strikingly different from that of nonvegetarians. Vegetarians have a lower concentration of potentially carcinogenic bile acid and fewer intestinal bacteria that convert the primary bile acids into carcinogenic secondary bile acids. More frequent elimination and the levels of certain enzymes in the colon enhance elimination of potential colon carcinogens. Most research shows that vegetarians have lower levels of fecal mutagens. Vegetarians do not consume heme iron, which has been shown to lead to the formation of highly cytotoxic factors in the colon increasing colon cancer risk. Finally, vegetarians most likely have higher intakes of phytochemicals, many of which have anticancer activity.
Hormones and diet: Low insulin-like growth factor-I but normal bioavailable androgens in vegan men; 2000
Lack of effect of a high-fiber cereal supplement on the recurrence of colorectal adenomas. Phoenix Colon Cancer Prevention Physicians’ Network; 2000
Associations between diet and cancer, ischemic heart disease, and all-cause mortality in non-Hispanic white California Seventh-day Adventists; 1999
Red meat and colon cancer: The cytotoxic and hyperproliferative effects of dietary heme; 1999
Bile acids and pH values in total feces and in fecal water from habitually omnivorous and vegetarian subjects; 1993
Dietary intake of fiber and decreased risk of cancers of the colon and rectum: evidence from the combined analysis of 13 case-control studies; 1992
National Cancer Institute dietary guidelines: rational; 1988
Seventh-day Adventist vegetarians have a quiescent proliferative activity in colonic mucosa; 1985
Bowel function measurements of individuals with different eating pattern; 1986
Diet and DNA-modifying activity in human fecal extracts; 1981
Fecal microbial flora in Seventh Day Adventist populations and control Subjects; 1977
Osteoporosis.
Osteoporosis is a complex disease affected by a variety of lifestyle, dietary, and genetic factors. Although some data indicate that osteoporosis is less common in developing countries with a mostly plant-based diet, these studies have relied on hip fracture data, which has been found to be unreliable for comparing bone health across cultures. There is little evidence to suggest that bone mineral density differs between western nonvegetarians and lacto-ovo-vegetarians.
A number of studies have shown that high protein intake, from animal foods in particular, causes increased excretion of calcium and raises calcium needs. The effect is believed to be due to the increased acid load from metabolism of sulfur-containing amino acids (SAA). However, grains are also high in these amino acids, and some research shows that SAA intake was similar between nonvegetarians and vegetarians. Despite this, there is some evidence that postmenopausal women with diets high in animal protein and low in plant protein had a high rate of bone loss and a greatly increased risk of hip fracture.
Although excessive protein intake may compromise bone health, there is evidence that low protein intakes could raise risk for poorer bone health. Although there are very little reliable data on bone health of vegans, some studies suggest that bone density is lower among vegans compared with nonvegetarians. Vegan women, like other women, may have low calcium intakes despite the availability of nondairy sources of well-absorbed calcium. Some vegan women may also have protein intakes that are marginal, and vitamin D status has shown to be compromised in some vegans. The lower serum estrogen levels of vegetarians may be a risk factor for osteoporosis. In contrast, short-term clinical studies suggest that soy protein rich in isoflavones decreases spinal bone loss in postmenopausal women. Higher intake of potassium and vitamin K among vegetarians may also help to protect bone health. However, the data suggest that a vegetarian diet does not necessarily protect against osteoporosis despite lower animal protein content.
A high ratio of dietary animal to vegetable protein increases the rate of bone loss and the risk of fracture in postmenopausal women; 2001
A threshold for low-protein diet-induced elevations in parathyroid hormone; 2000
Dietary protein intake and urinary excretion of calcium: A cross-sectional study in a healthy Japanese population; 1998
Protein intake and urinary excretion of protein-derived metabolites in aging female vegetarians and nonvegetarians; 1991
Vegetarian lifestyle and bone mineral density; 1988
Renal Disease.
High intake of dietary protein may worsen existing kidney disease or increase risk for those who are susceptible to this disease because protein intake is associated with a higher glomerular filtration rate (GFR). The GFR of healthy vegetarians is lower than that of nonvegetarians and even lower in vegans. The type of protein consumed may also have an effect, with plant foods having a more beneficial effect on GFR than animal protein. GFR was 16% higher in healthy subjects after eating a meal containing animal protein compared with a meal with soy protein. Because the pathology of renal disease is similar to that of atherosclerosis, the lower serum cholesterol levels and reduced cholesterol oxidation resulting from a vegetarian diet may be beneficial for those with kidney disease.
Renal, metabolic, and hormonal responses to proteins of different origin in normotensive, nonproteinuric type 1 diabetic patients; 1995
Renal, metabolic and hormonal responses to ingestion of animal and vegetable proteins; 1990
Dietary composition and renal function in healthy subject; 1987
Renal functional reserve in humans. Effect of protein intake on glomerular filtration rate; 1983
Dementia.
Although rates of dementia differ markedly throughout the world, differences in diagnostic criteria make cross-cultural comparisons difficult. In the United States, among SDA, those who ate meat were more than twice as likely to develop dementia. Those who had eaten meat for many years were more than three times as likely to develop signs of dementia. Diets high in antioxidants have been found to protect cognitive function. The lower blood pressure of vegetarians may also be protective. There is also some evidence that lower blood cholesterol protects against dementia. Higher homocysteine levels are linked to increased risk of dementia, and this may present one risk factor for vegetarians who do not get adequate vitamin B-12.
Serum folate and the severity of atrophy of the neocortex in Alzheimer’s disease: Findings from the Nun Study; 2000
Decreased prevalence of Alzheimer’s Disease associated with 3-hydrozy-3-methylglutaryl coenzyme A reductase inhibitors; 2000
Nutrients, age and cognitive function; 1998
The incidence of dementia and intake of animal products: Preliminary findings from the Adventist Health Study; 1993
Diverticular disease.
Gear and colleagues found that both male and female vegetarians aged 45 to 59 years were 50% as likely to have diverticulitis with nonvegetarians. Although fiber is believed to be the most important reason for this difference, other factors may have an effect as well. High-fat diets, independent of fiber intake, have been associated with increased risk of diverticulitis. Meat intake may also increase risk. Older research suggests that meat consumption may promote growth of bacteria that produce a toxic metabolite that weakens the wall of the colon.
A prospective study of diet and the risk of symptomatic diverticular disease in men; 1994
Diet and diverticulosis; 1985
Symptomless diverticular disease and intake of dietary fibre; 1979
Gallstones.
In a study of 800 women aged 40 to 69 years, nonvegetarians were more than twice as likely as vegetarians to suffer from gallstones. The relationship held even after controlling for the three known risk factors for gallstones: obesity, gender, and aging.
Effect of vegetarianism on development of gall stones in women; 1985
CLINICAL CASES
Dietary vitamin B12 deficiency in an adolescent white boy; 2002
Nutritional vitamin B12 deficiency in a breast-fed infant of a vegan-diet mother; 1986
REFERENCES
Some parts of this article are based on the 2003 reports of the American Dietetic Association and Dietitian of Canada
This is their Statement about their report:
“It is the position of the American Dietetic Association and
Dietitians of Canada that appropriately planned vegetarian
diets are healthful, nutritionally adequate, and provide
health benefits in the prevention and treatment of certain
diseases.”
Valerio Stefanone
Davide Tinti
