It’s important to notice that while an underlying medical issue is certainly likely in your situation, and that’s hard to work against… There’s no physical way you were actually ingesting 200 daily calories and didn’t lose weight.
This is beyond biology, it’s physical. You were either consuming way more than that, or you were actually losing weight and just didn’t notice. There’s no alternative.
You’re confusing solid food in my mouth with calories ingested.
If for whatever physiological reason your claim is correct, and your digestive system is indeed so fast food goes through unprocessed, you didn’t actually eat. You’ve eaten in the social, pleasurable or psychological sense, but these are not ingested calories, and therefore also completely irrelevant to your metabolism or diet.
If you could take a 1000 calorie burguer, cover it in plastic, swallow it and have it pass through intact… You just ingested zero calories. So you can’t later say “oh I regularly eat 1000 calories per meal and lose weight, but my partner chews a 300 calorie steak and gains weight!”
If you see what I mean.