February 2015 Issue
Plan Healthful Vegan Diets
By Virginia Messina, MPH, RD
Today's Dietitian
Vol. 17 No. 2 P. 40
Learn how to better counsel vegan clients and patients so they can meet nutrient needs and avoid shortfalls.
People choose to become vegans for a variety of reasons. They may have concerns about the treatment of farmed animals; they may want to improve their health; or they may have a desire for a more environmentally sustainable lifestyle. The number of vegans as a percentage of the population is relatively small—about 2% of US adults1—but there's evidence that those numbers are growing. In fact, most dietetics professionals can expect to work with vegan clients and patients periodically.
Vegan diets exclude all meat, fish, poultry, dairy products, and eggs. Some vegans, particularly those who are vegan for ethical reasons, may exclude foods that involve animal byproducts in processing even if those products don't end up in the food. For example, most commonly available cane sugar is processed using bone char and may not be acceptable to some vegans. It's important to consider these individual perspectives and practices in counseling vegan clients and patients.
Evidence on the health benefits of vegan diets comes from two large prospective epidemiologic studies: The Adventist Health Study-2 in North America, and the EPIC-Oxford study in England. Findings show that vegans typically have lower BMIs,2 a decreased risk of type 2 diabetes,2 reduced blood cholesterol levels,3 and a probable reduction in hypertension.4 Rates of heart disease are lower among vegan men but not women,5 and vegans may have a lower risk of certain cancers.6,7
The factors that may be responsible for these health benefits include a higher intake of fiber, fruits, vegetables, nuts, and legumes, and lower intakes of saturated fat and cholesterol.8
At the same time, research suggests that calcium intakes often are low in vegans.9 Those who don't supplement also may have low or marginal vitamin B12,10 vitamin D,11 iodine,12 and omega-3 fatty acid status.13 Because of antinutrients in plant foods, there also are challenges in meeting needs for iron and zinc. As a result, RDs must take all of these nutrients into consideration when helping clients plan a vegan diet.
The good news is that nutrient shortfalls are easily avoided on vegan diets as long as clients pay attention to food choices.
Calcium From Plants
Several decades ago, the acid-ash hypothesis of osteoporosis suggested that calcium needs might be lower in diets that exclude or limit animal proteins. According to this theory, modern Western diets, which are rich in animal protein, result in higher endogenous acid production, which promotes higher rates of bone turnover.
More recent research has challenged the acid-ash hypothesis of osteoporosis and newer theories suggest that protein is protective for bone health and not detrimental.14-16 Unfortunately, many popular vegan websites and books continue to suggest that vegans have lower calcium requirements due to the absence of animal protein from their diets. This may be one reason why many vegans don't meet the recommended dietary allowance (RDA) for calcium.9 Some evidence suggests that these lower intakes are associated with higher fracture risk.16,17 As a result, it's important for dietitians to assist vegan clients in identifying calcium-rich plant foods that can help them meet current RDAs for calcium.
Bioavailability of calcium from plant foods varies over a wide range. Absorption of calcium from the cruciferous vegetables collards, kale, mustard greens, turnip greens, bok choy, and broccoli, can be as high as 60%, which is twice the absorption rate of calcium from cow's milk.18 Theoretically, vegans could meet the biological requirement for calcium with a lower total calcium intake if most of it comes from these highly bioavailable sources. For example, 1 cup of cow's milk contains nearly twice as much calcium as 1 cup of bok choy, but the amount of calcium absorbed from both is similar. It's fair to assume, though, that most vegans get calcium from a variety of foods with varying levels of absorption and therefore, should aim to meet the RDA for calcium.
Most plant milks made from almonds, soybeans, hempseeds, cashews, and rice are fortified with calcium in amounts similar to the levels in cow's milk, and absorption rates also are similar. Calcium-set tofu, made with calcium sulfate, also can be an excellent calcium source.
Other foods such as white beans, soybeans, chickpeas, sesame tahini, navel oranges, and figs contribute moderate amounts of calcium. High-oxalate vegetables such as spinach and chard have poor bioavailability and make only negligible contributions to calcium intake.
Meeting Protein Needs
Humans require nine essential amino acids and adequate nitrogen for synthesis of nonessential amino acids. While grains, legumes, vegetables, nuts, and seeds provide all essential amino acids, their amino acid pattern is a poorer match to human needs compared with animal proteins.
A longstanding recommendation was to consume plant foods in complementary combinations during meals to supply an optimal amino acid pattern. It's now recognized that complementary proteins don't need to be consumed at the same meal. Instead, a variety of plant foods consumed over the course of a day will provide adequate essential amino acids.19 Furthermore, the body maintains a pool of essential amino acids that are available for protein synthesis. These come from a number of sources, including sloughed intestinal cells and digestive enzymes.20
However, dietitians should be mindful of a couple of important issues regarding vegan protein needs. First, vegan diets that don't include legumes may fall short of meeting needs for the essential amino acid lysine even if total protein needs are met. Grains and beans typically are low in this amino acid, and consuming enough of these foods to meet the protein RDA wouldn't ensure that the RDA for lysine is met. Therefore, vegans should consume at least three daily servings of legumes, which are rich in lysine. This food group includes dried beans; soyfoods, such as soymilk, tofu, tempeh, and vegetarian meats; and peanuts or peanut butter.
A second issue is that protein needs may be approximately 10% higher for vegans because of lower digestibility of plant proteins. This translates to 0.9 g of protein per kg of body weight for vegans compared with 0.8 g of protein per kg of body weight for omnivores.21 This amount of protein is easily supplied when vegans consume a variety of plant foods, meet calorie needs, and include legumes in their meals.
Enhancing Iron and Zinc Absorption
Legumes, whole and enriched grains, and some vegetables and dried fruits are especially rich in iron. Vegan diets often are higher in iron than omnivore diets and are frequently higher in iron than lacto-ovo vegetarian diets since dairy foods are a poor source of this nutrient.9
The issue in vegan diets is iron bioavailability, since the nonheme iron in plant foods is often bound to phytates, resulting in poor absorption. Other dietary factors, especially vitamin C, counter the effects of phytate. A meal including just 50 mg of vitamin C (the amount in 1/2 cup of cooked broccoli) can significantly affect absorption. Importantly, the vitamin C must be present at the same time as the iron.22 Therefore, vegans should give some attention to including good sources of vitamin C in as many meals and snacks as possible.
The best sources of zinc in vegan diets are legumes, nuts, and seeds. Vegan diets often are lower in zinc compared with diets that include animal foods, however, and zinc from plant foods has lower bioavailability. Vitamin C doesn't affect zinc absorption, but some food preparation practices can improve both zinc and iron absorption. Both minerals are better absorbed from leavened bread through the fermenting action of either yeast or sourdough. Encouraging vegans to replace some of the grains in their diet with whole grain bread can be an important way to improve mineral status. Toasting nuts and seeds also can improve zinc absorption as can sprouting legumes and grains.23
Dietary Fat
Vegan diets typically are somewhat lower in total fat than diets containing animal products and are significantly lower in saturated fat.9 Even with their reduced fat intake, vegans have no trouble meeting needs for the essential omega-6 fatty acid linoleic acid, which is widely available in the food supply. In contrast, the essential omega-3 fatty acid alpha-linolenic acid (ALA) is found in significant amounts in only a handful of plant foods. The best sources are ground flaxseed (the fat is poorly absorbed from whole flaxseed), and walnuts. Walnut, canola, and flaxseed oils also provide ALA. One or more of these foods should be included in vegan diets daily.
Like all people who don't eat fish, vegans have low blood levels of the long chain omega-3 fatty acids DHA and EPA.13 Although these fats can be synthesized from the essential fat ALA, the process is inefficient in many people. It's unclear whether the lower levels in vegans are harmful, but it may be wise for them to follow recommendations of major health organizations to supplement with these fats.
Vegan supplements of DHA and EPA derived from microalgae are available.
Fat-rich plant foods such as nuts, seeds, and healthful vegetable oils, including olive, walnut, soy, and canola, also can help vegans meet nutrient needs, provide phytochemicals for reducing chronic disease risk, and improve absorption of fat soluble nutrients and phytochemicals. Nuts and seeds are particularly important for providing zinc.
Essential Supplements
All vegans need to take a vitamin B12 supplement or consume foods fortified with this nutrient. There are no known plant foods that are natural sources of vitamin B12. Some foods that have been credited with providing vitamin B12, most notably fermented soyfoods, seaweeds, spirulina, and sourdough bread, appear to provide only B12 analogues that have no actual vitamin activity.24 But misinformation on popular vegan websites and in books may encourage vegans to depend on these foods to meet their requirements. Not surprisingly, research consistently shows suboptimal B12 status among vegans who don't supplement.10
Vegans should choose supplements containing cyanocobalamin, which is the form of the vitamin that has been well studied. Limited research suggests that methylcobalamin can be effective in maintaining a healthy B12 status, but only at high doses.25
Because vitamin B12 absorption is inversely related to dosage, recommended intake depends on how often the vitamin is consumed. The RDA of 2.4 mcg assumes consumption of small amounts of vitamin B12 from food throughout the day. Those who consume fortified foods, including breakfast cereals, plant milks, and vegetarian meats can opt for two servings per day of foods fortified with at least 1.5 mcg of B12 per serving. For vegans who get all of their B12 from a single daily supplement, 25 to 100 mcg is recommended to compensate for much lower absorption. Most B12 supplements contain much higher doses than this, however, and some vegans may prefer to take a supplement providing 1,000 mcg twice per week. Vitamin D supplements or fortified foods are recommended for anyone who lacks adequate sun exposure regardless of the type of diet followed. Ethical vegans may opt for supplements of vitamin D2, ergocalciferol, which is derived from yeast or mushrooms exposed to ultraviolet rays. The more common vitamin D3, cholecalciferol, is derived from animals. Both forms of the vitamin are absorbed and raise blood levels of 25 hydroxyvitamin D, although vitamin D3 is somewhat more effective than D2.26 Because levels of vitamin D2 may decline more quickly than those of vitamin D3, it's important for vegans to take daily supplements rather than larger doses less frequently.
The major sources of iodine in North American diets are iodized salt and dairy products. Plant foods also provide iodine, but the amounts are variable. Although little is known about the iodine status of vegans, one study of Boston-area vegans showed suboptimal levels of urinary iodine. Iodine status also may be poor among vegans in iodine-poor areas in northern Europe.27 Seaweeds can be excellent sources of iodine but amounts also are variable in these foods.28 One study of pregnant women in Japan found that thyroid health of their newborns was affected by the mothers' excessive iodine intake from seaweed.29 In contrast, vegans using kelp supplements were found to have low iodine status.27 Because of these variable amounts in both land and sea plants, vegans may benefit from small amounts of supplemental iodine from either iodized salt or supplements.
Mapping Out a Plan
Because vegan diets are outside of the mainstream, most clients will need some guidance for planning healthful diets that meet nutrient needs. With a little attention paid to plant foods that are sources of calcium, protein, iron, zinc, essential fats, vitamins B12 and D, and iodine, vegan diets can provide all essential nutrients and may offer some health benefits as well.
The science of planning healthful vegan diets easily can be translated into six simple guidelines.
1. Include good sources of well-absorbed calcium in meals. Food guides for vegans show that calcium-rich foods can be found in all vegan food groups.30 Foods that provide approximately 10% of the adult RDA for calcium include 1/2 cup calcium-fortified plant milk, yogurt, or juice; 1/2 cup calcium-set firm tofu, tempeh, or soybeans; 1 cup cooked collards, kale, bok choy, or mustard greens; 1 cup okra or white beans; 2 cups broccoli or cabbage; one-half of a calcium-fortified energy bar; and two navel oranges.
2. Incorporate at least three servings per day of legumes in meals. Although, with the exception of peanut butter, legumes are relatively uncommon in American diets, there are many popular options for vegans. These include hummus wraps, peanut butter sandwiches, peanut sauces on vegetables, bean burritos, veggie burgers and other vegetarian meats, cereal with soymilk, and scrambled tofu.
A serving of legumes includes 1/2 cup cooked dried beans, peas, or lentils; 1/2 cup tofu, tempeh, or edamame; 1 cup soymilk; 1/4 cup peanuts or soynuts; and 2 T peanut butter.
3. Emphasize eating good sources of vitamin C with as many meals and snacks as possible to ensure iron absorption. The best sources of vitamin C are citrus fruits, strawberries, green leafy vegetables, peppers, cauliflower, cabbage, and tomatoes. Examples of meals that pair these foods with iron-rich legumes and grains include oatmeal with sliced strawberries or orange juice, lentil soup with spinach and tomatoes, and tofu stir-fries with broccoli, onions, and peppers.
4. Use food preparation practices that reduce the phytate content of foods to enhance iron and zinc absorption. Whole grain breads made with yeast or sourdough, toasted nuts and seeds, and sprouted grains and legumes are sources of well-absorbed iron and zinc.
5. Recommend sources of healthful fats in the diet. Include a daily serving of a food rich in ALA, the omega-3 fatty acid. Any of the following will meet the daily ALA requirement for a vegan adult male (women can reduce the serving size by about one-third): 4 tsp canola or walnut oil, 1 tsp flaxseed oil, 1 T hempseed oil, eight walnut halves, and 4 tsp ground flaxseed.
Vegans should include small additional servings of nuts and seeds in meals to increase zinc intake. A serving is 1/4 cup of nuts or 2 T of seeds.
6. Choose appropriate supplements. To meet vitamin B12 (cyanocobalamin) requirements, clients can consume two servings per day of foods fortified with at least 1.5 mcg of vitamin B12 per serving, a daily supplement providing 25 to 100 mcg of vitamin B12, or a supplement providing 1,000 mcg twice per week.
To meet their vitamin D needs, suggest clients take 600 IUs daily if sun exposure is inadequate. For iodine requirements, patients should take 1/4 tsp of iodized salt daily or 90 mcg supplemental iodine three times per week.
To meet DHA and EPA needs, they should supplement with 200 mg to 300 mg of DHA and EPA combined two to three times per week.
— Virginia Messina, MPH, RD, is a writer and speaker on vegetarian and vegan diets for the public and health professionals. She's coauthor of Vegan for Life, Vegan for Her, and Never Too Late to Go Vegan, as well as a textbook on vegetarian diets for health professionals. She's also a former coauthor of the Academy of Nutrition and Dietetics Position on Vegetarian Diets.
References
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30. The plant plate. The Vegan RD website. http://www.theveganrd.com/food-guide-for-vegans