I am confused about weight loss. Specifically, it is past time that I update somewhat away from the (practical usefulness of?) the "calories in, calories out" model.

Some background:

For the last fourteen months, I've been logging everything I eat and tracking calorie intake. I lost about 15 pounds in the first 9 weeks at almost exactly the expected rate. After that, I did bulk-cut cycles (running 250cal surplus for a few weeks, then a 750cal deficit for a few weeks, with transition weeks at maintenance between phases), but have failed to make any further progress to speak of in body composition, whether in muscle mass gained or fat lost. I seem to have hit a wall at 170 pounds and about 20% bodyfat.

The standard response is that self-reported calorie intake is notoriously inaccurate, a problem to which I do not imagine myself immune. But I don't think that's enough to explain the results: when bulking, my weight goes up at the expected rate, and when at maintenance my weight does in fact hold constant. So I appear to be well-calibrated for those. But when running a deficit, I lose only a fraction of the expected amount, and I find it hard to believe that I mysteriously become that much less accurate while cutting, to a degree that almost entirely cancels out the planned deficit. Those are the times when I'm being most scrupulous!

Also, my physical performance during workouts unmistakably tanks during a cut, which is exactly the expected result of a large calorie deficit, but should not happen if I'm nearly at maintenance.

So at this point, I am much less sure that calorie balance is the sole determinant of weight loss. At best, it seems to be "technically true, but useless": a year of specifically focusing on calorie balance has fallen far short, both of theoretical predictions and of what I would consider a satisfactory rate of progress.

I wish I knew what alternative hypothesis to update towards.