Hideo Kojima has mentioned that you cannot pause during actual gameplay of Metal Gear Solid V: Ground Zeroes.
Kojima posted more gameplay details about Metal Gear Solid V: Ground Zeroes in a series of tweets. He said the game is like real life so there is no pausing. You have to be quick if you want to change weapons, check your map or listening to the WALKMAN. You can only pause the game during cutscenes. This is when you can go for a break he said.
He concluded with this tweet:
However Snake in GZ can’t have a break, meaning can’t play game in pause status.
Peace Walker is the other Metal Gear Solid game where gameplay was continuous too. If you are playing Metal Gear Solid: Ground Zeroes later this month, make sure go to the bathroom first…