Theoretically, you could get adequate protein from a strictly vegetarian diet, but practically it would be extremely difficult. For one thing, it would require the eating of huge amounts of food. Most vegetarian diets make excellent use of eggs, milk, and cheese, and to that extent are only partly vegetarian.
Animal proteins are stressed in your reducing diet for another reason. Recent knowledge indicates that Vitamin A is not always well absorbed in the form in which it occurs in green plants. Such plants, normally rated as excellent sources of the vitamin, contain it in yellow pigments of which the chief is carotene.
This does not become the vitamin until it is altered in your liver. It has been found that some persons absorb as little as 5% of available carotene; hence, though their food contains plenty of Vitamin A units, they derive no benefit from it.
Animal foods, however—eggs, butter, liver, milk, cheese —contain the true vitamin. The animal has done all the work of converting the carotene into Vitamin A, saving you the trouble.
That this is no trifling virtue is indicated by the belief of many authorities that Vitamin A is likely to be deficient in many reducing diets. There is a natural tendency to cut down on milk, butter, and cream—relatively rich in Vitamin A—because these contain considerable amounts of fat.
There is real danger in eliminating all dairy products from a self-chosen reducing diet. One man did just that and got his case reported in medical records. His skin became dry and rough; his hair grew brittle, lost its luster, and also lost its anchorage, starting to fall out. His dry skin tormented him with its itchiness. When he finally went to a doctor, he was promptly placed on a rational diet and his symptoms cleared up with Vitamin A concentrates.