Though diet book authors claim they have a once-size-fits-all solution for weight gain, be it giving up sugar or salt or wheat, the fact is that we have the luxury of being able to eat in excess for a very low cost. 

But 'do everything in moderation' is also ineffective for losing or maintaining weight, because the more people like a food, the more forgiving their definitions of moderation are, 

A paper in Appetite describes the relative meanings attached to moderation based on perceptions, based on surveys.

"We asked people to tell us what they think moderation is, in terms of quantity," said Michelle vanDellen, an assistant professor in psychology at
University of Georgia. "For instance, the research team asked participants to define how many cookies would be moderation, how many would be indulgence and how many would be considered what you should eat. People do think of moderation as less than overeating, so it does suggest less consumption. But they do think of it as more than what they should eat. So moderation is more forgiving of their current desires. ... The more you like a food, the more of it you think you can eat in moderation."

This adds to conflicting notions about dieting. While thin people are immediately told they should seek psychological counseling to offset the body image issues brought on my American culture, fat people are also demonized because they are a greater burden on the health system. 

And with so many fad diets on the New York Times bestselling list, all of them noting that other diets don't work, people have gotten the impression that no diets work. 

Just eat in moderation, the saying now goes. Which is fine, assuming you know what moderation is for your persona metabolism, which is predicated on various biological and lifestyle factors.