I think the reason some people might believe this claim is because we’re taught in school that the moon’s gravity causes the tides. I think the reasoning goes, “well if the moon’s gravity can affect the tides, surely it can affect smaller things too”
In theory it has some kind of affect but not anything anyone would notice. Like I’m not even sure it would show up on a scale and anyone trying to measure it with something sensitive enough to do so would have a hell of a time separating the change from normal things like eating, going to the bathroom, sweating, or just breathing.
If we say for a moment that it does have an affect on people the next question would be what effect might there be. Again I think a major issue would be measuring results when there would be so many other things at play. Also what are we measuring? What might the proposed effect be?
This idea mostly sounds like pseudo science or a half assed attempt at supporting some new age idea.