There is an insightful interview over at wii.com in which Nintendo President Satoru Iwata talks with legendary game designer Shigeru Miyamoto about Wii Fit.

It seems that early in the development Miyamoto and his team considered the DS as a portible diary for recording food intake.

"We even thought about making it so that you could take your DS out with you and store data on what you ate. With health as part of the Wii catalogue, we also saw the subject of food ahead of u"

When talking about how they wanted Wii Fit to do things that the many fitness DVD's out there couldn't do Miyamoto commented

"Many DVDs related to exercise are already on the market, so when we started thinking about what couldn’t be done with DVD, we hit on the fact that we could check whether or not you were actually exercising. So we continued the experiments while using radio exercise music, and by measuring balance we found out we were able to do a lot of things quite accurately."

"if you’re in the middle of exercising by standing on one leg, and lose your balance and put your foot down, they’ll tell you, “Did I see you put your foot down during the exercise?” You couldn't do that on DVD."

You can read more over at wii.com