Protein. Research shows that lacto-ovo vegetarians generally get the recommended daily amount of protein, which is easily obtained from dairy products and eggs. (Women need about 0.4 grams of protein per pound of body weight per day. Because the protein in vegetables is somewhat different from animal protein, vegans may need 0.45 grams of protein per pound of body weight per day.) There are many plant sources that can help vegans meet their protein needs, including peas, beans, lentils, chickpeas, seeds, nuts, soy products, and whole grains (for example, wheat, oats, barley, and brown rice). Vegetarians used to be told that they had to combine "complementary" plant proteins (rice with beans, for example) at every meal to get all the amino acids contained in meat protein. Now, health experts say that such rigid planning is unnecessary. According to the American Dietetic Association, eating a wide variety of protein sources every day is sufficient.
Vitamin B12. Vitamin B12 is found only in animal products, but those products include dairy foods and eggs, so most vegetarians get all they need. If you avoid animal products altogether, you should eat foods fortified with vitamin B12 (certain soy and rice beverages and breakfast cereals) or take a vitamin B12 supplement to avoid a deficiency, which can cause neurological problems and pernicious anemia.
Iron. Studies show that in Western countries, vegetarians tend to get the same amount of iron as meat eaters. But the iron in meat (especially red meat) is more readily absorbed than the kind found in plant foods, known as non-heme iron. The absorption of non-heme iron is enhanced by vitamin C and other acids found in fruits and vegetables, but it may be inhibited by the phytic acid in whole grains, beans, lentils, seeds, and nuts.
Zinc. Phytic acid in whole grains, seeds, beans, and legumes also reduces zinc absorption, but vegetarians in Western countries do not appear to be zinc-deficient.
Omega-3 fatty acids. Diets that include no fish or eggs are low in EPA and DHA. Our bodies can convert ALA in plant foods to EPA and DHA, but not very efficiently. Vegans can get DHA from algae supplements, which increase blood levels of DHA as well as EPA (by a process called retroversion). DHA-fortified breakfast bars and soy milk are also available. Official dietary guidelines recommend 1.10 grams per day of ALA for women, but vegetarians who consume little or no EPA and DHA should probably get more than that. Good ALA sources include flaxseed, walnuts, canola oil, and soy.
https://www.health.harvard.edu/staying-healthy/becoming-a-vegetarian
