I've thought about something similar but get hung up on data entry. How do you make it not miserable to get food purchase data into the app, given that food doesn't have an RFID tag in it, and not everything (produce) has a barcode? I like the receipt idea - partner with grocery stores to print a qr-code with a machine readable list of items and (maybe) expiration dates. Having to do any kind of manual scan on each item I bring home from the store feels like too much behavior change to become part of my everyday routine. Maybe there's an opportunity to do it for institutional kitchens where inventory is required by default?

It would be fun to build the fitbit and basis integrations. Also, Randall Munroe offered a different approach: http://xkcd.com/1109/ :)