This is an interesting topic and appreciate Sameer for starting this topic.
I started up almost 7 years back and my weight increased about 13-15 kgs during this time. I had turned myself from a fit/athletic body to fat/lethargic guy! It was only last year I realized what I was doing wrong and had to do a reality check. Since then, I've lost close to 10-12 kgs and trying to regain most of the fitter me back!
Some things I recommend doing to keep yourself fit even when you are stressed:
Talk to your parents / spouse / friends. Spend time with them and don't always think about your startup
Cut down on fatty food. Create a health chart for yourself and allow only one-two cheat days to eat oily / fast food / junk food
Cut down on your sugar intake and you will see easily losing some good amount of weight. You will also feel more alert/attentive once you cut down sugar from your diet
I used to eat a lot when I was stressed. I stopped doing that. Control is really important.
I can say that while I did mess with my health these last years, the recovery has been very welcoming and helpful for me.
Everyone's first priority should be health before their startup. Startups can come and go, but health may never be the same if you don't care for your body.