So one time this culture discovered that a readily-available plant had a really tasty component, and they began harvesting, and then cultivating and later, processing and refining and making all kinds of analogs and variations on the tasty stuff. Everyone loved it, and it sold like gangbusters, and the refineries and plantations and products proliferated; money everywhere. Soon the white powder was in everything. Every household had a bag of it, but that was just for extra, since even things in which the taste of it wasn't desired usually contained at least a little. Many people just ate it by the spoonful anyway, or carried it with them in easily-gnoshable packaging — besides being tasty, it was energizing, especially in the heavily-refined and concentrated forms. If you speculated about how many people *needed* it to get through a regular day…but that was silly; nobody speculated. Everyone ate it. All the children. *Especially* the children.
Who knew how early on the side-effects started, the rates of diseases that were once rare spiking, the birth-defects, the overdoses and organ failures. When nobody's watching, a lot can happen, and nobody questioned the world's favorite food, especially not after a couple generations of inculcation. Only the furthest-flung and least-communicative cultures rarely or never ate it, and their comparative longevity and health was chalked up to a million other factors.
But generations take their toll, numbers can swerve sharply when pressures are applied to linked and dynamic systems, and by the time the culture sees it, their invisible group addiction to the machine-refined white powder from a harmless plant, will they be able to avoid dying out?