The body's needs for vitamin A can be entirely obtained from plant foods.
Vitamin A is principally found in animal products. Plants do contain beta-carotene, a substance that the body can convert into vitamin A. The impression given by some vegetarian sources is that beta-carotene is just as good as vitamin A. This is not true.
Firstly, the conversion from carotene to vitamin A can only take place in the presence of bile salts. This means that fat must be eaten with the carotenes to stimulate bile secretion. Additionally, infants and people with hypothyroidism, gall bladder problems or diabetes either cannot make the conversion or do so very poorly. Lastly, the body's conversion from carotene to vitamin A is not very efficient: it takes 46 units of carotene to make one unit of vitamin A. What this means is that the sweet potato (containing about 25,000 units of beta-carotene) you just ate will only convert into about 4,000 units of vitamin A (assuming you ate it with fat and do not have a thyroid or gall bladder problem) .
Relying on plant sources for vitamin A, then, is not a very wise idea. This is why good-old-fashioned butter is a virtual must in any diet. Butter from pasture-fed cows is rich in vitamin A and will provide the intestines with the fatty material needed to convert vegetable carotenes into active vitamin A. Vitamin A is all-important in our diets, for it enables the body to use proteins and minerals (17).