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When maize became the staple of poor peasants in 18th- and 19th-century northern Italy, southern France and Spain, pellagra became epidemic, killing and disabling many who lived on polenta. The cause was that Europeans adopted the grain but not the Mesoamerican practice of nixtamalization, cooking maize with lime or ash, which releases niacin and prevents the disease.
Maize spread across the Old World mainly through peasants rather than through state or elite promotion. It yielded heavily on marginal land, needed little labor or equipment, and was often left out of tithe and tribute systems built around established cereals. That let the poor grow it for their own subsistence, largely unseen by those who taxed them, until its effects on population and labor became too large to ignore.
Following a cheap, high-calorie staple shows who profits from making survival cheap. In Warman's account, maize fed enslaved people in the Atlantic trade and on African provisioning grounds, and it fed peasants pushed onto frontier and marginal lands. This lowered the cost of keeping workers alive while the gains from their labor went to others. A crop's productivity can therefore strengthen systems of extraction rather than free those who grow it.
After centuries of spreading as a peasant subsistence plant, maize was turned into a standardized industrial input through hybrid seed, fertilizer, machinery and the US Corn Belt model. Mexican peasant producers, who had created and maintained maize's diversity, were then made to compete with subsidized agribusiness grain. The crop's homeland became dependent on imports of its own domesticate.