Open food stamp markets that accept only food stamps, that only provide healthy food options. Generic labeled foods would be the cheapest, while still providing proper nutrition. As a more free market alternative, provide incentives to privately owned markets to stock these generic food stamp-only products along with their usual inventory.
Provide subsidies to farmers who reserve a portion of their milk, fresh meats, fresh vegetables, etc., for these markets or participating private markets, to be sold below market value. This would be far less expensive than funding medical care for diabetes, heart disease, and other obesity-related conditions.