Excerpts taken from Peter Singer's updated edition of Animal Liberation.
ON PROTEIN:
"The protein intake of the average American exceeds the generous level recommended by the National Academy of Sciences by 45 percent. Other estimates say that most Americans consume between two and four times as much meat as the body can use. Excess protein cannot be stored." p 181
Some is converted into carbohydrates for storage, the main by-product of this process is ammonia which is filtered out of the body through the kidneys (urine) and sweat.
"...most of the plant foods we eat--not just nuts, peas, and beans, but even wheat, rice, and potatoes--contain enough protein in themselves to provide our bodies with the protein we need." p 182
ON B12 for VEGANS:
"Only vegans, who take no animal products at all, need to be especially careful about their diet. There appears to be one, and only one, necessary nutrient that is not normally available from plant sources, and this is vitamin B12, which is present in eggs and milk, but not in a readily assimilable form in plant foods. It can, however, be obtained from seaweeds such as kelp, from a soy sauce made by the traditional Japanese fermentation method, or from tempeh, a fermented soybean product eaten in parts of Asia, and often now available in health food stores in the West. It is also possible that it is produced by microorganisms in our own intestines. Studies of vegans who have not taken any apparent source of B12 for many years have shown their blood levels of this vitamin still to be within the normal range. Nevertheless to make sure of avoiding a deficiency, it is simple and inexpensive to take vitamin tablets containing B12." p 182