When corn becomes a staple food—like corn tortillas in Mexican cuisine—it must undergo a process called nixtamalization to make the nutrients available for digestion. e_bone says it's a method developed in Mesoamerica thousands of years ago. But when corn was introduced to Europe, they didn't use nixtamalization, and vitamin deficiencies spread.
The process consists of boiling and steeping corn in an alkaline bath, often made with slaked lime (cal) or ashes, says lapositivista. Niacin in the corn then becomes available for human bodies, and the grains absorb nutrients from the cal itself. "Eventually, the outer coating of the corn becomes gelatinous, the cal/water mixture is drained and the corn is rinsed, removing the outer coating," lapositivista says. The corn can then be used whole in pozole or ground for other uses.
Does that mean that fresh corn on the cob, which obviously hasn't undergone nixtamalization, isn't nutritious? Not quite, jumpingmonk says. Corn on the cob is a once-in-a-while food that's eaten as a vegetable side dish, rather than as a staple food. The fiber and nutrients in fresh corn are still valuable, as long as you're getting niacin and other nutrients from different foods.
Discuss: Corn on the Cob vs. Nixtamalization