Will i lose weight after open heart surgery?

Likely be advised to: People tend to lose weight after heart surgery more so because of being advised that their bad food habits have to change for them to have a chance of a prolonged life. So if you are having surgery at age 36 then your doctors will definitely talk to you about getting cholesterol under control and any other medical issue like diabetes, stopping cigarette smoking and adding exercise to your routine.

Post op: Usually there is some mild weight loss for a few weeks which is regained if the patient doesn't work at keeping weight down.

Yes: Much of this may be water weight and related to nutrition/appetite. Many patients will often regain the weight within a year or so.