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I found two contradicting views on what happens with water retention in a caloric deficit:

  1. Water goes down because in the caloric deficit we use up the glycogen which binds water
  2. Water goes up because in the caloric deficit the cortisol level increases

Both points can be made "even more true" with heavy exercising, i.e. we use more glycogen in muscles so less water stays in the body, we compound the cortisol increase so more water stays in the body.

Which is true? I suspect both are correct but everything depends on other factors/context which I don't know yet. If so, then what are these scenarios in which these claims can only make sense?

Background:

I saw people use 1) to explain why the weight loss can begin with a significant weight drop, i.e. the scale exaggerates the actual fat loss. This also explains the yo-yo effect when someone abruptly interrupts a fad diet and experiences a massive weight gain (it's mostly regained water).

On the other hand, I also saw articles that use 2) to explain why the weight loss may appear stale, without any weight drop or even with an increase, while there is a "hidden" ongoing fat loss. This also explains why a refeed can result in a weight drop, i.e. cortisol level decreases after caloric increase so water drops.

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Everything you've stated is correct. The missing variables are timing, duration, and intensity of the effect.

When eating in a caloric deficit, glycogen will deplete a little. Remaining in a deficit will ensure that this depletion never really fully goes away. So there will be this immediate drop in weight which will not go away until you start eating at least maintenance calories again. This "flush" can be pretty dramatic depending on how severe the deficit and the size of the person. The biggest drop will be at the beginning of the diet and will happen again in smaller increments as calories are lowered.

Cortisol fluctuates up and down over time in response to various stresses. Though when we say "high cortisol" we mean above the normal range which is a result of constant stress for many days or weeks depending on the severity of the stresses.

Normal range: example of cortisol in normal range

Elevated range: example of high cortisol in response to chronic stress

With elevated cortisol, not only will you hold on to water, but you'll feel other effects like an inability to sleep despite being exhausted.

After a few weeks of dieting, you may find you'll hold on to water. In my personal experience, I become "softer" because there's extra water underneath the skin. The weight on the scale doesn't really go up but it stops going down which means that the weight of the water is equal to the weight that I'm losing.

Cortisol can be lowered back to normal ranges by removing the stresses1. This usually involves taking a break from working out for a couple days, ensuring sleep is adequate, and eating slightly more for a few days. It's common for people to suddenly "whoosh" or "flush" and their weight will drop afterwards. Then it's ok to go back to normal routine until the cycle repeats. If this doesn't happen, then it may not have been water retention and it means your maintenance calories dropped. Unfortunately at that point you have to drop calories.

After the goal weight is achieved or when taking a break, calories will go back up. Glycogen will be replenished. The sudden drop in weight at the beginning of the diet will come back. This is where people will suddenly panic and drop calories again thinking they just lost all their hard work. Though this isn't really fat gain as long as the caloric intake is within the maintenance calorie range.

1You may Google "how to lower cortisol" and come across cortisol blockers which are marketed to lower cortisol back to normal levels. There's no solid evidence that these work. It's best to just be patient, and let your body handle it the way it was designed to.

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