Q: If water supply is very limited during a survival situation, in a hot/dry environment, is it better to delay food intake? I’ve heard both opinions: either digestion would increase water demand (could be lethal), or it would supply water (considering there’s a significant amount in that food).
Interesting question. I am not sure that I can give you a definitive answer.
Water is essential for all body functions. It aids in digestion in both direct and indirect ways. Water is part of the circulation that perfuses the gut so that it works properly. Digestive enzymes, juices if you like, are secreted as water-based liquids. Digestion is also easier and more efficient if the food is pulverized through chewing and is moisturized within the mouth and gut (yes, it is a good idea to drink with meals). Inadequate fluids could therefore impair nutrient absorption. In addition, when a person is fluid deprived, the colon reabsorbs water to help maintain fluid balance thus producing more solid feces and slowing transit time. How much water do we need? The old adage that we all need 8, 8oz (or about 240 mL) glasses of water generally fails to complete the thought, namely that there is an awful lot of water in food. A large part of our fluid needs are met by the water contained within foods that we eat.
So, should you avoid food if water is in short supply? Is the utility of water during digestion really a ”demand” or just a reallocation of resources? I would think that it would depend to a large degree on the food you have. In fact, if you are selective in your choices, you might be able to nearly avoid drinking any water. Dry or dehydrated foods, if not poorly absorbed, would at least be unpalatable. I doubt that you could eat enough of them to cause a problem like worsening dehydration. High salt content could be another problem, but that is a different discussion.