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@brouhaha

[You likely know all this better than I do. I’m not trying to mansplain it to you, but explaining for the benefit of other readers.]

I am not upset with you, but if i were to get upset I’d be far more likely to be upset of the use of the sexist term “mansplain” than I would from the actual desire to explain. That said, your good :)

Did you urinate more than usual?

Nope, less than usual in fact. I think all day I peed once, maybe twice at most.

If it’s not water loss from sweating

I actually was quite cold all day, just turned the heat on a moment ago.. if there was any sweat it certainly wasnt noticable.

the main way you lose body weight is by fat broken down into ketones (lipolysis, ketosis)

Actually this part is partly true (there is glycogenesis and glycolysis cycles too). But yea the bulk of fat winds up as ketones for energy.

excreted by urination.

This is the part where your mostly wrong. ketones, once broken down for energy, result in the byproducts that are mostly water and carbon dioxide. While you are correct that the part you break down into water is exreted by your kidneys (or sweat) you are incorrect in your assertion that this is the route the majority of your weight is lost.

When breaking down ketones or sugar the resulting water is only 16% of the weight loss by mass. 84% of your weight loss is actually expelled in the form of CO2. When you loose weight you literally loose the overwhelming amount of your weight through your lungs.

The other alternative is muscle tissue being broken down (ketoacidosis), which is very bad, but shouldn’t normally happen.

While ketoacidosis is bad it is not the normal route a healthy person undergoes to convert protein into energy. Ketoacidosis is an unhealthy state, usually unique to diabetics, where you have both high glucose, and high ketone count leading to Ph imbalance and potentially death.

Typically proteins (dietary and muscle) are converted through protein metabolism, which starts with protein catabolism. Derriving energy from protein is actually quite normal if you consume more protein than your body needs.