I thought I knew what you were saying, and then I wasn't sure.

Are you saying, "Well, sure, fat people diet, so it's obvious that people who diet are more likely to be fat?"

If that's what you're saying, I don't think it's universally true. If you mean, "People *think* they're fat (even if they're) not, and then they diet, and that leads to them getting fatter, not thinner," then I think that's true.

Care to expand/clarify?