With all due repect to Dr. Heaney in the above letter, his claims critical of meatless diets are not factually correct. For writings by doctors who are knowlegeable about these issues, I would recommend the work of Neal Barnard, M.D., Dean Ornish, M.D., John McDougall, M.D., Michael Klaper, M.D., and others.
As was mentioned in the comment above, even the very mainstream American Dietetic Association, the main professional association for registered dieticians in the United States, acknowledges the safety and many health benefits of vegetarian/vegan diets. They state:
"It is the position of the American Dietetic Association that appropriately planned vegetarian diets, including total vegetarian or vegan diets, are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases. Well-planned vegetarian diets are appropriate for individuals during all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence, and for athletes.... The results of an evidence-based review showed that a vegetarian diet is associated with a lower risk of death from ischemic heart disease. Vegetarians also appear to have lower low-density lipoprotein cholesterol levels, lower blood pressure, and lower rates of hypertension and type 2 diabetes than nonvegetarians. Furthermore, vegetarians tend to have a lower body mass index and lower overall cancer rates. Features of a vegetarian diet that may reduce risk of chronic disease include lower intakes of saturated fat and cholesterol and higher intakes of fruits, vegetables, whole grains, nuts, soy products, fiber, and phytochemicals."
For the full ADA statement, see the PDF link at http://www.eatright.org/about/content.aspx?id=8357