Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and "Governing America."

Princeton, New Jersey (CNN) -- New Jersey Gov. Chris Christie made headlines last week when one of his aides admitted that he had surgery to lose weight. Christie said that the surgery had nothing to do with politics and everything to do with his health and his family. Christie said: "It's not a career issue for me. It is a long-term health issue for me and that's the basis on which I made this decision. It's not about anything other than that."

It is impossible to know whether we should take Christie at face value. Given that there has been ongoing speculation about his presidential aspirations for 2016, often coupled with discussions of his struggle with weight, it is certainly not unreasonable to wonder whether these are related.

Why do Americans care about the weight of a candidate and why is appearance an issue in presidential elections? There is very little chance that the issue will go away. Certainly, old-fashioned bias has something to do with this concern....