Vitamin A in vegetables is in the form of beta-carotene which is converted
to 2 retinol (vitamin A) molecules in the body. Animal derived retinol is
more potent (eg from liver, kidneys). There are many other carotenes in
plant foods, they are well absorbed after eating, and a regular supply is
best. Getting a variety of carotenoids from a variety of plant foods is a
good idea. Iron from spinach si said to be poorly absorbed because there is
so much oxalic acid. Poor Popeye!

Beta carotene levels from food are associated with lower rates of certain
cancers (eg lung cancer in smokers) but supplements paradoxically cause an
increase in cancer, possibly because they inhibit the digestion of other
carotenes from the diet! Similarly vitamin E supplements inhibit the
absorption of other tocopherols which is why vitamin E trials are so
lacklustre but dietary vitamin E is so good (because you get the other
tocopherols).

Vitamin C supplements are useless because they can generate free radicals in
combination with iron. Vitamin C is a an antioxidant but not all free
radicals are generated by oxidants, iron is a reductant and the worst of all
free radical generators. Get your vitamin C from your diet and it won't
cause problems. Paleo diets have more vitamins than any other diet so
supplements are generally not needed, except perhaps some fish oil and some
cod liver oil capsules when you're not getting enough fish or liver.

Ben Balzer