Hormones. Specifically your thyroid can dramatically change your weight by Hormones production. In my case my thyroid has almost completely shutdown. I could just eat a carrot every day and still gain weight without synthetic hormones. I believe it can sometimes go the other way make it hard to gain weight. I strongly encourage having hormone levels checked and balanced. I wish I’d known about this 20 or 30 years ago.
Sorry but you cannot create weight out of nothing. A single carrot a day will not generate a caloric surplus, and you need sustained caloric surplus to gain weight.
That said, getting your hormones checked is very important
Edit I’m sure whoever downvoted me has news on thermodynamics that the scientific community would be interested in
