Nobody disputes the obvious fact that it is possible to make anyone lose weight - starvation is one classic method that has been around for a long time. But that isn't the problem. We know how to make a person thinner, but we don't know how to make people thinner, because people like to eat fatty foods, they like to be sedentary, they dislike dieting and often don't have the willpower for it. Having an effective diet is only part of a solution if people are resistant to it.

Saying calorie restriction is a way to make people thinner is like saying quitting cigarettes is a way to lower cancer rates: not true enough.

Yes: diet and exercise. That's exactly how you make people thinner. The problem -- there are competing truths here -- is that diet and exercise isn't the easy solution it seems to be. How do you implement calorie restriction and exercise? This is what Paul is talking about when he says we don't know. (But I think his choice of words suggests that the "diet and exercise" message is debunked or something.)

Re: "There aren't any health differences to speak of for people between BMIs of about 20 and 35"

Uh, reality check!

There not any doubt that health is affected over this range (imagine here the BMI of a single individual varying over this range) but the problem -- to mediate again -- is that BMI is not a reliable measure of unhealthy fat, so you can't accurately compare the health of two different people by their BMIs, or really have a good gauge of your own health from your current BMI. Paul Campos is exploiting this confusion. It would be incorrect (and intellectually dishonest) to connect this message about BMI with the message that it's just as healthy to be 30 pounds overweight.

Some discussion of the connection between health risks and BMI here and the limitations of BMI as a measure here.

I haven't read the book, so I don't know if Paul Campos actually asserts that being fat is healthy, and unrelated to diet and exercise, or if he is deliberately misleading in order to whip up interest in his book.

Yet there's at least one true message: for people who have a healthy weight but a high BMI (for their height, thanks Psychohistorian), they can consider themselves healthy because they're not overweight.