I am one of the people who thinks that "calories out > calories in" is most important. I've tried (in the spirit of Feynman) to poke holes in this, so here's the stuff that would persuade me I'm wrong.

1) satiety. Some foods make you feel full, so even though they're "fat" you eat less. (This doesn't counter CO>CI but explains why people fail at it).

2) abysmal quality of research on weight loss. Lots of the research is poor quality and we really don't know what works. Better quality research is always a good thing.

3) gut flora and genetics. Genetics plays a role in everything so I'm sure that it is easier for some people to be fat than others because genes. We know that some gut flora helps some people stay slim. Nwe know that some other gut flora helps some people become fat. A double whammy would be unfortunate.

4) evil food industry. Seeing a bag of sugar (in whatever form, but especially candy) sold as "LOW FAT FOOD!!!" is infuriating. I tend to ignore the HFS arguments but I agree that the vast amounts of sugar eaten today is very harmful. Making food "hyper paletable" and making it seem like it is much quicker to get that food than prepare your own healthy food has done a lot of harm. (See popcorn chicken cersus a roast chicken).

5) judgementalism. CO>CI may be true but it is simplistic and it is often given with judgemental messages. As HN middlebrow dismissal shows contrarianism is human nature, thus saying "CO>CI! Develop some willpower, go for a jog!" Will devolve into discussion about a bunch of stuff. People like me need to find a better way to deliver that message.