I was at a dinner talking with docs about mineral balance and in particular salt. A few thought the "body's control systems are sophisticated". I countered with no, they are pretty good but there is evidence you can really screw them up, especially with food intake.

So I have more questions than answers, but water is held to offset salt or sodium to keep from getting toxic/too high. When I reduced my salt intake, I reduced inflammation in my joints, fingers and feet. It was as if that all shrunk a bit. I have been avoiding high sodium intake since then and feel great.

When I used to drink coffee, the kidneys were in hyperdrive so it had the diuretic effect, but that also purged other valuable minerals. Not a good thing.

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Training Tweets: https://twitter.com/Jagersport_com
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Get some basic labs. Check for Electrolytes, Creatinine and albumin. Urinalysis.

This could be nothing or it could be major.

Without more information it is impossible to know.

I spent 2 weeks in the hospital and 6 months on chemo last year with Goodpastures syndrome.
https://en.wikipedia.org/wiki/Goodpasture_syndrome

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The above poster has eschewed traditional employment and is currently undertaking the ill-conceived task of launching his own hardgoods company. Statements are not made on behalf of nor reflective of anything in any manner... unless they're good, then they count.
http://www.AGNCYINNOVATION.com

Observations and thoughts:
1) Training in hot weather + soda (and other salty food) - can cause radical swings in body weight. I used to do 20 mile runs in 100+ weather. I would finish at 163. I might weigh as much as 185 a day later. (Normal being 167).
2) High training stress might cause low hormone levels, which might cause fat gain. (I think this has happened to me a couple times).
3) Salty food causes water retention.
4) Some food is heavy.
5) Scales have margin of error and don't work properly.

Bottom line- during the summer time, I often can't figure out what is going on with my weight. Doctors have no f#ing idea.

Right now my scale consistently says I have gained 5 lbs. This since I started training more and eating better.
Is it water retention because of too much salt and heat?
Fat build up because of hormone imbalance, associated with lots of training stress?
Is it weird muscle build up?

No idea. But I am not going to pay a doctor to tell me I need to watch more TV.

I've been having something similar to you, only the swelling never subsides.

My weight can fluctuate 2.5 to 3 kgs from day to day. I have Tanita scales and often when I am lowest my body fat % is highest, suggesting less water.

Salty food can cause water retention for me also.