Diet guidelines from shopping habits

Supermarkets must be awash with great data on peoples shopping habits from their store cards. I hypothesize that it would be possible to accurately determine someones diet from their supermarket shopping habits (although I realise that people still buy a lot of their snack food, takeaways, etc outside of supermarkets). If such a hypothesis proved to be true, the data would allow identification of the most dietary "at risk" people, who could be referred for medical help. Alternatively, the supermarket could take it upon themselves to make suggestions to help people, although I doubt this would go down well with customers.