Not really. What you do is create a temporary account for each session, unless one exists already. If the user chooses to create an account later, it is not really created, but the temporary account is promoted to a permanent one, and the details like username are filled in.

Again, check out stackoverflow.com. They do this.

Unlike stackoverflow.com the food recommendations generated by Usable Health are dependent on each of the answers in the sign up process. For example someone who weighs a 100 pounds will receive different recommendations from someone who weighs 200 pounds so unfortunately there is no way of getting around the complete sign up / nutritional profile creation process.

I see! Well, then I agree that my suggestion might not be a good fit. (I was fooled by the fact that I could try it without entering any details)