Acorns provide nutrients and minerals, but they contain tannins, which can cause pain, kidney failure, and even death in horses. Cattle also cannot metabolize (or process) the tannins in them.

Other large animals, like bears, deer, and pigs, can “de-toxify” their acorns with no problem. So can smaller mammals, like mice, rats, and squirrels. Pigeons, jays, and some species of ducks and woodpeckers eat them without ill effect. Moth and weevil larvae can eat them from the inside out as they develop.

Some animals, such as certain types of squirrels, collect acorns and store them in caches. If they are stored near groundwater, that groundwater can enter and soak the oak-tree nuts, and then leach the tannins out. Other animals “buffer” their digestive tracts with other foods that neutralize the tannins.

Ancients humans, including Iberians and lower-class Greeks, relied on acorns as a food source. The Japanese ate them in times of famine. Some Native American tribes stored enough acorns to last two years, in case there was a shortage of them. Some people still eat acorns in a simple soup. During the US Civil War, Confederates used acorns to make a drink resembling coffee, when the Union cut off access to coffee beans. Some Koreans use acorns to make a jelly called dotorimuk and noodles called dotori guksu.

White-oak acorns are suitable for people to eat after a simple roasting.

More Info: en.wikipedia.org