From my military experience a diet of freeze dry food can leave you dehydrated and constipated. It seemed to not be a problem if you used a mix, some freeze dry, some canned. Also freeze dry beans take a long time to soften. Simply drinking a lot more water also helped. I understand that MRE's have gotten past the dehydration but cause constipation in some. Perhaps for short periods of time, one or two weeks, but have plenty of water.

I will add that freeze dry and MRE's beat starving.