Water Intake: Does Water in Food Count?

Question
I have been debating an issue with my coworker. We all know that water is essential to good health. Does water in food count the same as drinking water? Example: is drinking 8 glasses of water in a day the same to the human body as getting the same 8 glasses of water in mashed potatoes? My coworker believes that the 8 glasses of  water in the mashed potatoes would not satisfy the body's liquid requirement the same as drinking water would. To carry this to an extreme, can the human body thrive if it never had water in liquid form but instead had all of its liquid through food? Is there a difference in getting water from a slice of fruit where the water can be squeezed out vs the same amount of water from a food where the water is more part of the food, like mashed potatoes?

Answer
Yes, Jeff, technically if you could get all the water you needed from food, the body would use it the same--H20 is H20!
But it would be quite difficult.  The average person who eats 2000 calories per day requires 1 gram (cc) of water per calorie to metabolize the food eaten.  2000 cc's of water is about 8 glasses, and this is where the recommendation comes from.

If a banana weighs 200 grams (a bit over 6 ounces)and is 75% water (one of the highest water- content foods) it contains 150 grams of water--just about 5 ounces.  You need most of this to metabolize the calories contained in the banana, so you wouldn't have much left over for other foods that are low moisture--cheese and crackers and meats and oils.

Indeed if it were possible to eat fruits and vegetables all day long, one may get a good percentage of the water needed.  Certainly there are people who never drink water and instead slurp down beverages that make them require more water: sugary sodas and caffeinated coffee and alcoholic beverages.  They manage to live!  But perhaps the functions of water are scaled back in their body so they are not as able to control their temperature, circulate their blood, excrete waste, etc.  They may get symptoms like dizziness, headaches, constipation.  Perhaps people can survive on very little water in addition to their food, but definitely not thrive.