I've kept close track of my weight after my last ultra, and it reflects a similar discussion over at Kevin Sayer's website. I unfortunately don't have any data for my weight 24 hours after running.
Days after the ultra / weight
Day 1: 123
Day 2: 124.5
Day 3: 122
Day 4: 117
Day 5: 119
A big increase around 48-60 hours after the event, followed by what looks like an overcompensating plummet to far below my normal weight. What gives?
Well, during an ultra your body starts producing a cocktail of hormones en masse that will help you keep running. These include your cliched endorphins, which act like a mild opiate, and cortisol, which aids in fat metabolism. There are also elevated levels of many enzymes, especially creatine kinase which tends to become superelevated in an ultra from the high degree of damage to muscle tissue, but that's another post.
The big player here, though, is vasopression. Also called anti-diuretic hormone, its release encourages the re-absorption of fluids by the kidneys. This is obviously helpful during the event - you conserve water and as a result have to drink less than you would had this hormone not kicked in. Additionally, it means you aren't going to be peeing after every bottle of fluid.
My hypothesis is two pronged. 1), vasopressin is probably, like most other hormones, so elevated during an ultra that it takes a significant amount of time to decay to its resting level, and 2), the body continues to secrete it for up to 48 hours after the event. I believe that the latter is critical to help explain the continued increase in water weight after the race; essentially, the body is still in endurance mode. Perhaps it is keeping you prepared for further running, but I think it may be a survival mechanism. In our evolutionary history, we did not have big barbecues at the finish line with beer and plenty of water. We were still working with scarce resources, and the longer you can conserve water after a big endurance effort, the better.
Does this explain the puffy feet and ankles following an ultra? I think it's sort of a confounding factor. You've been on your feet and engaging your lower legs for 10+ hours, I would expect a little pooling. Perhaps there's also the issue of damaged muscle cells spilling fluids and other items into the muscle tissue, and the body working hard to clean it out.
I don't know, but studying the physiology and biology of ultrarunning would make for a fantastic dissertation.