Vegetarian diets can cause medical problems in both children and adults if adequate protein, vitamins and minerals are not eaten. Children may have to eat a more than they can handle to get adequate protein etc.
I had a telephone call the other day from a sixty-year-old woman who asked my thoughts about why she had had a fracture of her leg and a hip replacement. Now she has a new crack in the bone of her leg. I knew she was a vegetarian and in asking her about her diet, I found she was eating little protein and calcium. Even with the three problems with her bones, her doctor had not asked about her diet or done a test for osteopenia or osteoporosis, calcium, or Vitamin D. She was not drinking milk or taking any calcium supplement. I must admit that I was not surprised the doctors were providing such inadequate care. Even surgeons should know that diet and calcium intake can make a difference. The woman is not overweight and gets exercise when she is not bedridden with her hip and leg problems. The orthopedic surgeon she was seeing was technically good, but certainly wasn't looking at the whole patient.
I realize few medical schools teach anything about nutrition. I have been told by medical students and house staff that I am the only doctor they have ever heard ask about diet. Even gastroenterologists, in my expereince, don't ask about food intake, which always amazes me.
Children on a vegetarian diet can get into real trouble with malnutrition. Dr. Henry Legere in his book, Raising Healthy Eaters, notes that a child must eat anywhere from three to seven times as much non-meat protein to get the amount of protein in a single serving of meat or cheese. Two cups of beans would equal two slices of cheese. Amazing! Also, a child has to eat much larger quantities of everything to get what is needed for growth and development.