Adapted from Dr. Fuhrman's book Disease Proof Your Child:
Animals eat their macronutrients; they don't fabricate them from the air. All protein, all fat, and all carbs are made from soil and water with energy from the sun via photosynthesis. Animals then get all the fat, protein, and carbohydrates for energy from plants. Amino acids are the building blocks of protein. All animals, directly or indirectly, receive protein (amino acids) from plants. The lion eats the antelope; the antelope got the protein it supplied to the lion from the grass. Green vegetables (the soil) supplied the nitrogenous compounds to make the protein for the antelope and ultimately the lion.
In North America, about 70 percent of dietary protein comes from animal foods. Worldwide, plants provide 84 percent of calories. In the 1950s human protein requirements studies were first conducted that demonstrated that adults require twenty to thirty-five grams of protein per day.1 Today, the average American consumes 100 to 120 grams of protein per day, mostly in the form of animal products. People who eat a completely vegetarian diet (vegan) have been found to consume sixty to eighty grams of protein a day, well above the minimum requirement.2 Vitamin B12, not protein, is the missing nutrient in a vegan diet.
In modern times, the plant foods we eat are well washed and contain little bacteria, bugs, or dirt, which would have supplied B12 in a more natural environment such as the jungle or forest. To assure optimal levels of B12 in our diet, we require some form of B12 supplementation when eating a diet with little or no animal products.
UPDATE: Animal protein can be part of an extremely healthy diet! Dr. Fuhrman explains this in detail in an earlier post.
1. Jenkins DJ, Kendall CW, Popovich, et al. Effects of a very-high-fiber vegetable fruit and nut diet on serum lipids and colonic function. Metabolism 2001:50(4);494-503.
2. Rose W. The amino acid requirements of adult man. Nutritional Abstracts and Reviews 1957;27:631.