Critics of vegetarianism claim that people who choose this diet will have to "balance incomplete proteins" to synthesize complete protein in the body. Proteins that come from fruits, vegetables, legumes, and grain products are "incomplete" in the sense that they do not provide all nine essential amino acids (protein building blocks) in a single food; thus, according to critics, a vegetarian (and especially a vegan) must be careful to consume appropriate quantities of complementary proteins each day to ensure that he or she is getting enough dietary protein.
Contrary to this popular belief, inadequate protein intake is rarely a concern for vegetarians. Better understanding of protein nutrition shows that, for adults, it is not necessary to consume special combinations of foods to meet requirements for the right balance of amino acids. Many plant foods, such as grains and legumes, provide protein. Soy foods and dairy products are particularly rich in high-quality protein. Protein needs are easily met when vegetarians consume a variety of plant foods and eat enough food to meet calorie needs. In fact, most meat-eaters consume far more protein than they actually need.
Critics also point out that there is no way to get vitamin B12 via food except through animal products. Tempeh and other fermented soy products do not contain consistent enough quantities of B12 to meet the body's requirements. Vegetarians must therefore eat B12-fortified foods or take vitamin supplements on a daily basis to prevent a deficiency.
Some critics fault vegetarian, and particularly vegan, diets for not providing sufficient calcium. In truth, most people, not just vegetarians, eat less than optimal amounts of calcium. Dairy products are a good source of dietary calcium, but they are not the only source. Green leafy vegetables and tofu are also good sources, and are usually eaten in large quantities by vegetarians. Vegans--who eat no animal products--are the most likely to suffer from calcium deficiency. Many Americans--vegetarians and meat eaters alike--could benefit from taking a calcium supplement.
Vegetarians eat less iron than non-vegetarians, and the iron they eat is somewhat less absorbable. As a result, vegetarians are more likely to have reduced iron stores. However, iron deficiency is not usually caused by a lack of iron in the diet alone; an underlying cause--such as iron loss in menstrual blood--often exists. Although iron is found in a variety of different foods, its availability to the body (bioavailability) varies significantly. This is determined by whether it is found in the form of heme and non-heme iron. Heme iron is found only in meat, fish, and poultry. It is absorbed much more readily than non-heme iron found primarily in fruits, vegetables, dried beans, nuts, and grain products, such as bread and iron-fortified breakfast cereals. Including a source of vitamin C at each meal improves absorption of vegetarian-source iron.
Strict vegans may become iodine deficient if they do not consume enough iodine, readily available in iodized salt.
Very few foods are good sources of vitamin D, which is why milk is fortified with it. Vegetarians who do not consume milk should be sure to get 20 to 30 minutes of sun exposure every day since this promotes the synthesis of vitamin D in the body. People who live in cloudy, smoggy, or northern areas, and people who do not leave their residences, need foods fortified with vitamin D. These fortified foods include breakfast cereals and some brands of soy milk and rice milk. Since people with dark skin make vitamin D less efficiently, they may also need supplemental sources of vitamin D or increased sun exposure.