What's the fastest someone can gain?

[Eat Paydays]

Or fastest gain you've heard of. Pics just to give you something to look at.

[extra_m13]

a normal eating gain has a rate, a medical condition may spur really quick gains but even if that can happen faster i prefer a more permanent arrangement as an overeating habit 

[Pancakes]

Realistically there's only so many calories one can absorb from their gut in any given day. It probably tops out at 5-6k calories a day for most people, though probably varies on any given day. They say ~3500 additional calories adds up to a pound. Assuming your baseline needs are 2000 calories a day, someone could theoretically gain up to 300lbs in a year. Ultimately though, the human body is more complicated than that, and it will put the brakes on, so to speak, as one continues to gain weight, so I'm inclined to say that 150 to 200lbs is the real theoretical limit.

[cheddar]

There are definitely some hard-ish limits in terms of the absorption and metabolic process that turn food into fat. First, there's the enzymes that break down things like fat into soluble bits that can pass through the gut. Second, the proteins that transport those smaller particles to where it's needed and then that energy has to be burnt or stored. The body will increase its metabolic rate the more you eat and due to the number of steps between food and fat cell there's a lot of points where one of those processes can slow things down.

There might be some ways to artificially maximise gains by tricking the body into being more efficient. First, you'd want to look into things like ghrelin that regulate appetite as well as the stomach's stretchiness ('motility'); incidentally, the 'munchies' are a known biological phenomenon due to the effect THC has on the systems that regulate hunger and perception of taste.

Since there's a lag time between the body eating and reacting to its intake, some method of increasing bile acid production (idk if you can somehow buy bile salts) could help, but only if paired with something to increase lipoprotein levels and prevent Hyperlipidemia or excretion of the excess fat.

Insulin is also a problem as levels are going to spike. In some ways, that's good because it's part of the process of storing fat in fat cells but there's obvious health problems alongside that. At the same time, Leptin limits fat storage.  Cold can stimulate fat cell proliferation, depending on the circumstances.

One thing I really would like to see more research on is a thing called neuropeptide y, which might have both positive benefits in terms of weight gain but also in managing its negative side-effects. There's a couple of other things that regulate fat cell growth and proliferation but the research is pretty dense the further you look.

While there's a lot of interesting stuff in the above (it's by no means an exhaustive description of process affecting potential weight gain), messing with it is probably more effort than it's worth, unless somebody wants to explore such things in the name of SCIENCE!

---
TL;DR, there's a lot of barriers limiting rapid weight gain and more extreme types of gain generally involve something affecting the body or some type of predisposition that makes it easier