the problem isn't one that can be solved with prompts. If I gave a panel of food and nutrition experts (human or machine) a bunch of pictures of food, they still wouldn't be able to tell if, e.g. a slice of cake was made with whole milk or skim.

The "pic of packaged food --> LLM --> nutrition DB call" pipeline is workable, but many users of these apps are using them for fresh prepared foods, which is just an unworkable problem without either an understanding of the preparation process or a bomb calorimeter.