If you favor those milk chocolate bon-bons over the dark variety,
you might be better off switching. Eating dark chocolate — but
not milk chocolate — raises levels of antioxidants in the blood,
which could help protect against heart disease.

Researchers found that the rise in antioxidant levels is much
lower when milk chocolate is eaten or when dark chocolate is
eaten along with milk. Although the exact reason is unclear, it
may be that milk interferes with the absorption of antioxidants
from chocolate.

The findings come from a study of 12 healthy volunteers who, on
different days, ate 100 grams (about 3 ounces) of dark chocolate
alone, 100 grams of dark chocolate with milk, or 200 grams of
milk chocolate.

Blood analysis one hour after dark chocolate was eaten showed a
significant rise in antioxidant levels, including levels of
epicatechin, a well-known dietary flavonoid.

In contrast, not much change in antioxidant levels was seen after
eating of milk chocolate or dark chocolate with milk. Moreover,
epicatechin absorption into the bloodstream was much lower than
when dark chocolate was eaten alone.

The researchers say their findings highlight the possibility that
other items in the diet could reduce the antioxidant activity of
flavonoids in the body.

Source: Nature, August 28, 2003