“Eat well—it’s good for your brain” sounds like sensible advice. But scientifically, it hides a difficult question: What, exactly, connects food with thinking? Diet may influence the brain. But the relationship can also run in other directions. Young people with stronger cognitive skills may make different food choices. Family income, sleep, puberty, body composition, and many other factors may affect both diet and cognition. Even when diet and thinking are related, identifying the brain structures involved is far from simple. In a recent study, my colleagues and I examined these relationships in 3,308 adolescents from the Adolescent Brain Cognitive Development (ABCD) Study. We combined information about their diets, performance on cognitive tests, and structural MRI scans. Our central question was not whether one particular “brain food” makes adolescents smarter. It was whether broader patterns of eating are associated with cognition—and whether part of that association is also reflected in the structure of the developing brain. Looking Beyond Individual Foods People do not consume nutrients in isolation. They eat combinations of fruits, grains, dairy products, legumes, …