Yes there are conditions that lead to weight gain, even if you're presumably consuming food/drink at a calorie deficet...

https://runnersconnect.net/might-not...-eat-less-run/

There are also known medical conditions for weight gain...

https://www.nhs.uk/live-well/healthy...ing-on-weight/

Just for the record, and for obvious reasons, the people who suffer medical conditions have an excuse, but are the vast minority and few and far between. These conditions still don't explain or excuse the level of obesity in America for people that have no other reason to blame other than themselves.