The Archer Daniels Midland wet mill on the outskirts of Decatur, Ill., rises like an industrial behemoth from the frozen, harvested cornfields of Central Illinois. Steam billowed in the 20-degree cold last week, as workers turned raw corn into sweet, ubiquitous high-fructose corn syrup. Three miles away, a Primient mill, which sprawls across 400 acres divided by North 22nd Street, was doing the same.

To Robert F. Kennedy Jr., President-elect Donald J. Trump’s nominee for secretary of health and human services, this bedraggled city — set deep in Trump country — is the belly of the agribusiness beast, churning out products that he says poison America, rendering its children obese and its citizens chronically ill.

To the workers here, those mills — the largest in the world — are their livelihoods.

“It’d have a huge impact,” a 37-year-old electrician who would identify himself by only his first name, Tyler, said of Mr. Kennedy’s declaration of war on corn syrup and corn oil. He was grabbing lunch at Debbie’s Diner in the shadow of the mills. “That shuts down Central Illinois, if A.D.M. shuts down.”