Cutting Edge Vegetarian Nutrition
Cutting Edge Vegetarian Nutrition
Brenda Davis, RD/ 2005
A vegetarian is a person who does not consume animal flesh foods, including meat, poultry and fish. While vegetarians are generally divided into two categories – lacto-ovo vegetarians (those who consume milk and eggs) and vegans (those who eliminate all animal products), much diversity exists within each category. There are “junk food” vegetarians” who rely heavily on fast foods and processed foods, “organic vegetarians” who eat only organic whole foods, “raw vegetarians” who eat only 100% raw plant foods, and endless other variations. In addition, there are plenty of self-described “vegetarians” who eat fish, chicken, or even meat, on occasion or in moderation. According to the latest roper polls, an estimated 4-10% of Americans call themselves vegetarian, while just under 3% fit the definition of vegetarian according to food intake data. A couple of decades ago, it was estimated that about 95% of vegetarians were lacto-ovo, and about 5% were vegan. Today, estimates suggest that at least a third to a half of all vegetarians are vegan, with one large roper poll showing upwards of 64% being vegan. Considering the variability of dietary practices among vegetarians, individual assessment is important to providing constructive nutritional guidance.
According to the official position of the American Dietetic Association and Dietitians of Canada (2003), appropriately planned vegetarian diets are healthful, nutritionally adequate, and provide health benefits in the prevention and treatment of certain diseases. The position paper adds: “Well-planned vegan and vegetarian diets are appropriate for all stages of the life-cycle including during pregnancy, lactation, infancy, childhood, and adolescence. Vegetarian diets offer a number of health benefits including lower levels of saturated fat, cholesterol, and animal protein and higher levels of carbohydrates, fiber, magnesium, potassium, folate, antioxidants such as vitamins C and E, and phytochemicals. Vegetarians have been reported to have lower body mass indices than nonvegetarians, as well as lower rates of death from ischemic heart disease; lower blood cholesterol levels; lower blood pressure; and lower rates of hypertension, type 2 diabetes, and prostate and colon cancer.”
While there is no question that vegetarian and vegan diets can provide excellent nutrition, simply becoming vegetarian or vegan does not guarantee healthful choices. The key nutrients of concern to vegetarians and vegans are protein, trace minerals (primarily iron, zinc and iodine), essential fatty acids, vitamin B12, calcium and vitamin D. Vegetarian and vegan diets can easily meet the current recommendations for all of these nutrients when appropriately planned. In fact, many these nutrients are currently being added to foods commonly consumed by vegetarians (e.g. calcium, vitamins B12 and D to soymilk, and iron, zinc and vitamin B12 to meat substitutes).
Protein
Sufficient protein can be obtained from plants alone when a variety of plant foods are consumed and energy needs are met. Complementary proteins do not need to be consumed at the same meal, as an assortment of plant foods eaten over the course of a day can provide all of the indispensable amino acids required. Most people require approximately 0.8 g protein per kg body weight, assuming adequate energy intakes. Protein needs are higher during times of growth (pregnancy, lactation, infancy, childhood and adolescence), for some athletes (especially competitive athletes), and for people recovering from certain illnesses. Protein needs many be increased by 10-20% (slightly higher for infants and children) when diets consist primarily or exclusively of whole plant foods, which have reduced digestibility. This increase is unnecessary for those who regularly include processed plant foods such as tofu and soymilk, or dairy products and eggs in their diets. This amount of protein is not difficult to get in any vegetarian or vegan diet containing a variety of plant foods. Even elite athletes can meet all of their protein needs without animal protein.
Trace Minerals
There is some evidence that trace mineral absorption can be compromised in phytate and fiber-rich plant-based diets. The trace minerals of greatest concern are iron, zinc and iodine.
Iron
Plant foods contain only nonheme iron, which is more sensitive than heme iron to both inhibiters and enhancers of iron absorption. The principle inhibitors of iron absorption include phytate, calcium, tannins from tea and fiber. Vitamin C and other organic acids from fruits and vegetables enhance iron absorption. Studies show that iron absorption is about 10% from a plant-based diet and about 18% from an omnivorous diet. Consequently, the recommended intake for iron has been set 1.8 times higher for vegetarians than nonvegetarians. Some food preparation techniques, such as soaking, sprouting, yeast leavening and fermenting can reduce the phytate content of plant foods, which may in turn improve iron absorption. There is evidence that adaptation to low iron intakes takes place over time. Most studies show iron intake by vegetarians to be higher than that of omnivores. Incidence of iron deficiency anemia among vegetarians is similar to that of nonvegetarians, although vegetarians tend to have lower iron stores. To meet recommended intakes, good sources of iron such as legumes, seeds, nuts, dried fruits and iron-fortified foods should be included in the daily diet. Iron enhancers consumed at the same time as iron-rich foods can help to maximize absorption. Vegetarians should be cautioned against sprinkling foods with wheat bran, which is highly concentrated in phytates that significantly impair mineral absorption.
Zinc
Zinc bioavailability appears to be significantly lower in vegetarian diets. Zinc concentrations also tend to be lower in plant foods than animal foods, thus average intakes of zinc in some vegetarians are below recommended intakes. Phytates and calcium are the primary inhibitors of zinc absorption, thus vegetarians with high phytate intakes may require zinc intakes beyond the RDA. Some evidence suggests that vegetarians may adapt to lower zinc intakes over time. Some food preparation techniques, such as soaking, sprouting, yeast leavening and fermenting may reduce binding of zinc by phytate and increase zinc bioavailability. To meet recommended intakes for zinc, plenty of zinc-rich foods such as legumes, nuts, seeds and whole grains should be included in the daily diet. Zinc-fortified foods can also help boost zinc intakes.
Iodine
Several studies suggest that vegans who avoid iodized salt may be at risk for iodine deficiency. Vegetarian diets that are rich in foods containing natural goitrogens, such as soybeans, cruciferous vegetables, sweet potatoes, millet, and raw flaxseed may reduce iodine uptake. However, these foods only appear problematic when iodine intake is inadequate. The adult RDA for iodine is 150 mcg, and can easily be met with one-half teaspoon of iodized salt daily. In the United States, about half of the general population uses iodized salt, while in Canada, all table salt is fortified with iodine. Sea salt and salty seasonings such as soy sauce and tamari are not generally iodized. Seaweeds are also rich iodine sources, and are an excellent choice for those wishing to limit sodium consumption. Only one-tenth of a teaspoon of kelp powder per day is necessary to provide the RDA of 150 mcg iodine. The upper limit for iodine is 1100 mcg, which can be exceeded with large intakes of seaweed. The iodine content of plant foods depends on the iodine content of the soil, and varies greatly throughout the world. Dairy products contain iodine because iodine-containing disinfectants are used to clean milking machines and holding tanks.
Essential Fatty Acids
While vegetarian diets are generally lower in total fat, saturated fat and cholesterol than are nonvegetarian diets, they generally offer no advantages over nonvegetarian eating patterns where essential fatty acid balance and intake are concerned. Indeed, several reports suggest vegetarians may actually be at a disadvantage. Vegetarian, especially vegan diets, are relatively low in alpha-linolenic acid (ALA – 18:3 n-3) compared with linoleic acid (LA – 18:2 n-6), and provide little, if any, of the more physiologically active highly unsaturated omega-3 fatty acids, eicosapentaenoic acid (EPA – 20:5 n-3) and docosahexaenoic acid (DHA – 22:6 n-3). Clinical studies suggest that tissue levels of long-chain omega-3 fatty acids are depressed in vegetarians, and particularly in vegans. Conversion of ALA by the body to the more longer-chain metabolites is inefficient - approximately 5-10% for EPA and <2-5% for DHA. Thus, total omega-3 requirements may be higher for vegetarians than for nonvegetarians, as vegetarians must rely largely on conversion of ALA to EPA and DHA. Vegetarians are well advised to include at least 1.5% alpha-linolenic acid in the diet. Intakes of 3-5 grams are sufficient for most people. Intakes of omega-6 fatty acids should be adequate, but not excessive (not more than 8% of calories). Vegetarians, especially those at risk for poor EFA conversion, and those with increased EFA needs (such as pregnant and lactating women), may benefit from direct sources of EPA and DHA. Lacto-ovo vegetarians who consume eggs can select the DHA-rich variety providing about 60-150 mg DHA per egg. Microalgae is the most promising source of long-chain omega-3 fatty acids for people who do not consume fish, and is available in supplement form. Supplements presently on the market provide DHA at levels of approximately 100-300 mg, and several brands are sold in nongelatin caps.
Vitamin B12
Vitamin B12 is produced mainly by bacteria, and plant foods are not reliable sources of this nutrient. Studies indicate that vegetarians, and particularly vegans, have reduced B12 status relative to nonvegetarians. This is especially true in those individuals that do not include a reliable source of vitamin B12 in the diet. It is essential that all vegans, and lacto-ovo vegetarians over 50 years of age, use a supplement or fortified foods to meet recommended intakes for vitamin B12. Lacto-ovo vegetarians under the age of 50 years can also use dairy products or eggs to meet recommended intakes. Animal products are not considered reliable sources of vitamin B12 for those over 50 years as their ability to cleave B12 from the protein it is bound to can be significantly reduced. Lack of vitamin B12 in the diet causes megaloblastic anemia, nerve damage, gastrointestinal disturbances, and elevated homocysteine. Elevated homocysteine can potentially eliminate any cardioprotective effects of a vegetarian diet. To get sufficient vitamin B12 from foods or supplements, vegans require at least 3 mcg in fortified foods in two or more meals, 10 mcg of supplemental B12 daily or 2000 mcg B12 weekly. Seaweed, fermented foods and organic vegetables are not reliable sources of vitamin B12. Although seaweeds contain some true B12, they also contain variable amounts of noncobalamin corrinoids or B12 analogs, which can attach to B12 receptor cites, crowding out true B12 without performing its necessary functions.
Calcium
Calcium requirements can be met on both lacto-ovo and vegan diets. Calcium intakes of lacto-vegetarians are similar to or higher than that of nonvegetarians, while vegan intakes are generally lower, and often below recommended intakes. Calcium is present in many plant foods and fortified foods. Low-oxalate greens (broccoli, kale, collards, okra and Chinese greens) provide highly bioavailable calcium (49 to 61%), calcium-set tofu, fortified fruit juices, and cow’s milk provide calcium with good bioavailability (31 to 32%), and fortified soy milk, sesame seeds, almonds, and most legumes provide calcium with moderate bioavailability (21to 27%). Other calcium-rich plant foods include figs, certain seaweeds, and several calcium-fortified foods such as soy yogurt and some cereals. Adequate intakes (AI) of calcium are estimated to be1000-1300 mg of calcium per day (adolescents 9-18 years – 1300 mg; adults 19-50 years – 1000 mg; adults 50+ years – 1200 mg). For most people, the easiest way to insure sufficient calcium on a completely plant-based diet is to use a combination of calcium-fortified foods and calcium-rich plant foods. Selecting a variety of calcium-rich foods throughout the day also helps to enhance absorption. 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. Phytates may also inhibit calcium absorption. However, some foods with relatively high contents of both phytate and oxalate, like soyfoods, still provide well-absorbed calcium. Diets rich in sulfur-containing amino acids may increase losses of calcium from bone, although this appears to be most significant when calcium intakes are low. Excessive sodium intake is also an important contributor to urinary calcium losses, while caffeine and phosphoric acid have lesser negative impacts.
Vitamin D
Vitamin D can be obtained from warm summer sun or fortified foods. While small amounts of vitamin D occur naturally in some plant foods, this is rarely sufficient to meet daily requirements. Sun exposure to the face, hands, and forearms for 10 to 15 minutes per day is sufficient during the warmer months for light-skinned people. Those with dark skin require considerably greater exposure. For those living in Canada and at northern latitudes in the United States, there are several months of the year where vitamin D production from sun exposure will be insufficient to meet vitamin D requirements.
Foods that are fortified with vitamin D include cow’s milk, some brands of soymilk and other fortified non-dairy beverages, as well as some breakfast cereals and margarines. Vitamin D3 (cholecalciferol) is of animal origin whereas vitamin D2 (ergocalciferol) is of plant origin. Vitamin D2 is only about 60% as bioavailable as vitamin D3, thus if this is the primary source for vegans, requirements may be increased.
Selected References
American Dietetic Association and Dietitians of Canada. Position of the American Dietetic Association and Dietitians of Canada: Vegetarian Diets. J Am Diet Assoc. 2003;103:748-765.
Appleby PN, Thorogood M, Mann JI, Key TJ. The Oxford Vegetarian Study: an overview. Am J Clin Nutr. 1999;70:525S-531S.
Davis B, Kris-Etherton P. Achieving optimal essential fatty acid status in vegetarians: Current knowledge and practical implications. Am J Clin Nutr. 2003;78(3 Suppl):640S-646S.
Davis B, Melina V. Becoming Vegan. The Book Publishing Company, Summertown TN, 2000.
Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D and Flouride. Washington, DC: National Academy Press; 1997.
Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Institute of Medicine, 2002.
Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, D.C.: National Academy Press, 2000.
Joint FAO/WHO Expert Consultation. Protein Quality Evaluation. FAO Food and Nutrition Paper 51. Rome, 1991.
Messina V, Mangels, R and Messina V. The Dietitian's Guide to Vegetarian Diets: Issues and Applications. Second Edition. Sudbury MA. Jones and Bartlett Publishers.
Messina V, Mangels R. Considerations in planning vegan diets: children. J Am Diet Assoc. 2001;101:661-9.
Sanders TA. The nutritional adequacy of plant-based diets. Proc Nutr Soc. 1999;58:265-9.
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