How to gain weight in medical school without dying early

Unfortunately, I don't have the answer but I am looking for an honestly, good answer. Ever since I was a kid, I already had a hard time gaining weight. I already had my thyroid hormones checked (just in case they were the cause), but they turned out normal. So anyway, I would be having my junior internship next year, which means 36 hours of duty, more sleepless nights, more stress and apparently, a great tendency to skip meals. I actually want to gain a decent amount of weight before the next school year so I won't look like a wasted patient or no one would joke about me being emaciated.

If anyone knows a good way to gain weight (pregnancy is not a solution, in case someone might consider suggesting it), please let me know. :)