There is much more that could be written on this topic, and it will likely be updated in the future. While it is possible to obtain all necessary nutrients from a vegan diet, few people actually do so, and supplementation is often advisable. Vitamin B12, for example, is found only in animal products, making supplementation essential for most vegans. Although it is technically possible to obtain B12 from healthy bowel flora, few individuals have intestinal function robust enough to supply adequate amounts.
Research appearing in Clinical Chemistry (2001;47:1094–1101) found that vegans had higher homocysteine levels and poorer antioxidant status when compared to non-vegetarians.
Vegans also tend to be deficient in vitamin A, zinc, and calcium. The omega-3 fatty acid DHA (docosahexaenoic acid) is particularly difficult to obtain without supplementation. DHA is most readily available from animal sources, especially fish, making it challenging to achieve adequate intake on a strict vegan diet.
Vegan mothers who breast-feed should be especially mindful of vitamin B12 intake. Vitamin B12 is essential for brain development, and deficiency can result in growth retardation and brain atrophy in newborns. This is supported by research appearing in the European Journal of Pediatrics (1991;150(3):205–208) and the Journal of Obstetrics, Gynecology and Reproductive Biology (2005;34(6):610–612). Similarly, inadequate DHA intake during pregnancy and lactation may adversely affect the development of the child’s nervous system.
Another challenge for vegans is obtaining complete protein. A complete protein contains all essential amino acids. Animal products generally provide all essential amino acids, while most plant proteins are incomplete. It is possible to obtain all essential amino acids on a vegan diet, but it requires careful planning and food combining.
