So I've experienced something of a mirror image of this challenge. I'm (very) skinny, and always have been (as in, I'm 6 feet tall and it was a triumphant moment about two months ago when I discovered I had gotten my weight back up to 135 pounds, from a nadir of just under 120). And I've had body-image issues since early elementary school, because being really skinny isn't manly; strength and muscle are.

My greatest struggle with maintaining weight began for me when I started working full-time after college. I hated the way my job soaked up so much of my time (and I didn't even ever have to work overtime), so I tended not to bother with lunch. And after a while, I started feeling really nauseous every morning, to the point that I couldn't eat the bowl of cereal at breakfast that had, until then, been a mainstay of my eating habits (I would eat a few bites and find myself totally unable to continue eating; funnily enough, there's a disgustingly soggy bowl of frosted mini-wheats sitting on my desk next to me as I type this). So I started working from home basically all the time, and eventually began seeking medical help for the problem.

Unfortunately, the medical industry's concern about eating habits is almost entirely focused on getting people to lose weight; there are almost no resources out there for those of us who want to gain it, and particularly those who are experiencing a reduction in appetite, as in my case (most of the weight-gain advice I've found argues for regularly consuming massive amounts of protein via some kind of shake, which has always been beyond my ability to do--I have a naturally low appetite to begin with, and when I overstuff myself I risk vomiting).

Worse, when I went in for medical tests or imaging procedures, nurses and technicians would always tell me how healthy I was, which was extremely grating when I was there to get an endoscopy to try and find out why the hell I have such trouble eating, and when my APRN had told me I technically qualified as "anorexic" by BMI despite the fact that I have never intentionally starved myself. I still have to renew my FMLA every year so that I have legal protection for taking sick time when I have a bad episode of nausea, and I don't really know what I've been doing differently lately (the weight gain has been pretty much entirely during the pandemic), other than eating more regularly and virtually never engaging in much physical activity anymore. And still, I've gained no more than 15 or so pounds in almost an entire year of this! (And I seem to have stalled out in the mid-130s, I might add--today my weight is 133.8 pounds before eating much of anything.)

(In case you're wondering, I do not have a diagnosis worth a damn; I'm considered to have "irritable bowel syndrome" because I have a ton of gas and other bowel issues, but that's just a catch-all for "the intestines aren't working correctly and we don't know why.")

Even before all of this happened, I had always wished I could weigh more, so I've generally always felt like people's attitudes toward fat people were unfair--if I can't seem to gain weight, then how the hell could it possibly be easy to go the other direction?

Not sure exactly what I'm trying to say here, but as someone with some knowledge of biology I'd like to attack a specific argument: CICO is complete fucking bullshit. Your body is not a dumb machine that ignores its own internal state and mindlessly tacks on pounds when you eat too much (nor one that mindlessly consumes fat when it doesn't get enough calories). The signaling mechanisms of the cell lean heavily on a particular molecule, cyclic AMP (adenosine monophosphate). Those of you who know a thing or two about biology may know of ATP (adenosine triphosphate), commonly called the "energy currency of the cell." Consuming an ATP consists of removing a phosphate from the molecule, converting it to ADP (adenosine diphosphate); consuming an ADP converts it to (you guessed it) cyclic AMP. A ton of cell signaling cascades act in response to the presence of high concentrations of cyclic AMP, because there are a ton of functions that kick on when the cell is low on energy (most of which center on shifting to alternative sources of energy and the conservation of cellular energy). Given all that, why in the name of whatever deity you do or don't worship would you think that the body as a whole just idiotically absorbs whatever energy you dump into it, when there's a perfectly effective method for dumping useless energy, commonly known as 'shitting'?

No, the truth is, our food in this country is absolute garbage, and it's completely fucking up our bodies' internal regulatory mechanisms in ways that we can't begin to fathom as yet, and that is why people have so much trouble modifying their weight. It's completely useless to badger people about "counting calories" or "exercising more" (or, conversely, simply eating more) when in truth their metabolisms are completely out of whack thanks to our awful capitalist food system.