Clinical perspective, offered as context rather than as advice. The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either. Those three things can all be true at once, and most arguments here are two people holding different parts of that.

That is the short version; the long version is somebody else's post.

BethLabQueen said:

If you are going to change something, change one thing and give it long enough to express itself.

Coming at BethLabQueen’s question from a different direction. Worth answering the question that was asked rather than the one behind it. The narrow version usually has an answer; the broad version usually does not, and answering the broad one is how a thread stops being useful.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

One concrete data point for the thread. The boring version of this is the one that works, and the boring version is: measure a baseline, change one variable, wait, measure again under the same conditions. Nobody wants that answer and it is still the answer.

One thing that is still open after BethLabQueen’s answer:

What did you change at the same time, and can you separate the two now?

Moderator note: good thread. Keeping it here rather than moving it, because the question is general enough to be useful.