Antioxidants bioavailability and supplementation

It is known that people who eat adequate amounts of fruits and vegetables high in antioxidants have a lower incidence of cardiovascular disease, certain cancers, and cataracts.
These two food groups are rich in antioxidants, however the phytochemicals' repartition in the food and bioavailability can differ greatly.

Repartition in the food
Compared to an apple with its skin, a peeled apple has lost 25 % of its content of phenolic acid and 30 % of its flavonoids.

Whole walnuts contain 740 mg per 100 g of ellagic acid whereas bleached walnuts only contain 14 mg per 100 g.

Antioxidants are not present evenly in the food. It seems that the skin is always richer in these.

Bioavailability
It is not because a food is rich in a type of phytochemical that we will benefit from it.

In one study, broccoli and peas induced a larger beta-carotene response in plasma than whole leafy and chopped spinach despite a 10 times lower beta-carotene content in the former vegetables. (Karin H. van het Hof et al. J. Nutr. 130:503-506, 2000).

In the same article we learn that the absorption of lutein from mixed vegetables is greater than the one of spinach (respectively 67 % and 45 %). The bioavailability of lutein appears to be lower from green leafy vegetables than from other vegetables. The bioavailability of beta-carotene and lycopene is enhanced by homogenization (juices, paste) and heat treatment.

According to another study, 100 grams of unpeeled fresh apple (about two-thirds of a medium-sized apple) provides the total antioxidant activity of 1,500 milligrams of vitamin C.
(Nature 405:903)

The bioavailability of phytochemicals depends on the type of phytochemical, the quantity, the interactions with others nutrients, the matrix...

Supplementation
Does the isolated phytochemicals provide the same phytochemical activity as the whole food containing the phytochemical?

Even though tremendous studies are being done on these phytochemicals we still do not have enough information about them. All these phytochemicals works in synergy and it seems that it is better to consume the complex mixture of phytochemicals present in whole food rather that the purified compounds found in pills.

Just as an example, an 8 years study showed that a supplementation of beta-carotene had increased by 20 % the risk of developing a lung cancer compared to those you did not consume supplements.