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"But actually they weren't aware that they were not eating less."
This is why I advocate the method of using a Beeminder weight goal (or some equivalent), weigh yourself every day, and don't eat for the rest of the day when you are above the center line. When you are below it, you can eat whatever you want for the rest of the day.
This doesn't take very much willpower because there is a very bright line, you don't have to carefully control what or how much you eat, it's either you eat today or you don't.
That has some issues. First, changes in water retention jitter your daily weight by a pound or two. Second, you assume good tolerance for intermittent fasting. If you weight yourself in the morning, decide you're not going to eat for the whole day, and then suffer a major sugar crash in the afternoon, that will be problematic.