How do you not put on weight on a plane?

Whenever I travel long haul I always get constipation and seem to have put on weight when I get back. I don't eat hte plane food cause I dont like it. How can I prevent this from happening?How do you not put on weight on a plane?It could be fluid retention due to a mixture of flying and the hot weather,drink plenty of water before and during your flight,nothing fizzy.How do you not put on weight on a plane?Use laxatives/suppositories when you are away. This happens to most people when they are away from home for a week or so. Could be change in water,food or being busy.