It wouldn't be as airtight of an experiment to show causality, especially since again no evidence points to vitamin deficits being a direct mechanism. You'd probably have to track prenatal health, and you'd have to measure known genetic ADHD markers to account for ADHD being highly genetic when running statistical analyses.

But good luck getting the cash to fund it if there isn't already compelling evidence that vitamin deficit is directly linked to ADHD. Investors usually don't like "taking risks for the sake of exploration" unfortunately