From Dr. Fuhrman’s book Eat to Live:
Could restricting my intake of animal products or eating a strict vegetarian diet cause me to develop vitamin deficiencies?
A strict vegetarian diet is deficient in meeting the vitamin B12 needs of some individuals. If you choose to follow a complete vegetarian (vegan) diet, it is imperative that you consume a multivitamin or other source of B12, such as fortified soymilk. My vegetarian menu plans and dietary suggestions are otherwise rich in calcium and contain sufficient iron from green vegetables and beans. They contain adequate protein and are extremely nutrient-dense.
Vitamin D, often called the sunshine vitamin, is another common deficiency I find when I check the blood levels of my patients. Most of us work indoors and avoid the sun or wear sunscreen, which lowers our vitamin D exposure. Some of us don’t absorb it as well and just require more. So, given all the data that is available today and my personal experience with patients, I advise more people to consume an appropriate multi.
My observations suggest that vegetarians would be foolish not to play it safe, either by taking a B12 supplement or a multi or by consuming foods that have been fortified with vitamin B12. Another option for those who loathe taking vitamins is to have their blood checked periodically. Checking your B12 level alone is not sufficient. Methylmalonic acid (MM) must be checked to accurately gauge if the level of B12 in your body is enough for you.