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I've had a pretty interesting discussion beneath this survival question. A source given in the answer to this question states that you shouldn't eat when dehydration can become or is a problem:

(...) (A) simple rule(:) If you have nothing to drink, then do not eat. Eating anything, even watery foods, takes water from your system to create the slurry that will be able to move through your GI tract. Eating without drinking can lead to constipation, or worse, an intestinal blockage. If you are suffering from dehydration, and have watery foods available try squeezing out the liquid through a cloth. I’ll often do this with summer berries to create a drink. Blueberries, raspberries, blackberries, and other watery wild edibles can be mashed and squeezed to make a juice that offers hydration and even a few calories and vitamins, without wasting the water it would take to pass all those skins and seeds.

However, I'm a little bit skeptical if this rule of thumb really applies. Also my discussion partner disagreed with this statement.

I don't believe that's true. (...) So any water your body happens to add plus any water contained in the food will be reabsorbed in your large intestine, the net result being a water gain.

So, is it true to say: "If you have nothing to drink, then do not eat."?

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    Where on earth do you expect to find berries, but not water? – ShemSeger Jul 27 '16 at 15:24
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    @ShemSeger a dry summer/autumn could easily mean that seasonal springs are dry and berries are ripe. Streams could be few and far between if they're mainly fed by snowmelt and the dried-up springs. I've seen this in Corsica though we were never more than a day's hike from a tap - with an injury things could change fast. – Chris H Jul 27 '16 at 15:56
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    @BenCrowell the question is tagged survival. It seems like it might be worth editing it into the titular question. – StrongBad Jul 27 '16 at 18:36
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    Also the question follows the one drinking blood in a survival situation so it seems all connected to that – Erik vanDoren Jul 27 '16 at 20:58
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    @ShemSeger It's also worth noting that just because you can find water, doesn't mean you can drink it safely, whereas you can be pretty confident that certain berries are safe. For example, I know of a place near me where blackberries grow, but the only water is muddy and downstream from the unpleasant end of a field full of cows... – anaximander Jul 28 '16 at 13:13
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When you have no water to drink:

  • You can eat foods high in water and low in proteins, such as fruits, vegetables and milk, which usually contain more than 90% of water.
  • Do not eat foods high in proteins, such as meat, fish, eggs, cheese, nuts and seeds.

PROTEIN IS DEHYDRATING

Every gram of protein you eat results in loss of 8 mL of water with urine (calculated from NAP.edu):

Urea, a major end product of metabolism of dietary proteins and amino acids, requires water for excretion by the kidneys. Renal excretion of 1 g of urea nitrogen (2.2 g of urea) requires 40 to 60 mL of water. Thus, if a person consumes 63 g of protein in a diet that contains 2,100 kcal, the volume of water required increases by 0.4 to 0.6 L/day.

METABOLIC WATER

Amount of water produced in your body from macronutrients (Encyclopedia.com):

  • 100 g of proteins = ~40 mL of water
  • 100 g of carbohydrates = ~55 mL of water
  • 100 g of fats = ~110 mL of water

You should not rely on metabolic water as a source of water, for example, you should not eat 1 kilogram of sugar and hope you'll get 550 mL of water this way. Sugar gets incorporated into glycogen, which binds some water, which does not contribute to the whole body hydration. On the other hand, carbohydrates can improve water retention (Physiology.org).

In conclusion, carbohydrates and fats, while not necessary hydrating, are also not dehydrating.

WATER LOSS THROUGH FECES

With stool you lose about 100-200 mL of water per day (NAP.edu); this should not discourage you to eat fruits or vegetables, from which you can realistically get, let's say, 2 liters of water per day.

"Water moving to the gut during digestion" should also not be a problem because it is only temporary.

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In a survival situation without water you will die in days and without food you will die in weeks. So in general, there is no need to eat in a survival situation. That said, the quick burst of energy you can gain from eating a sugary snack could make all the difference.

There is probably little harm eating a little after one day without water. Your body will still have enough "water" to be able to digest food. Your body will also be in the process of conserving water so the amount of water lost in your stool will be limited. If you manage to live for 4 days without water, you probably should avoid eating as your body may not have enough water to digest the food. Again, a sugary snack might still be helpful in a sharknado.

What is the harm in eating

The chemical reactions for breaking down carbohydrate and fat are different. Hydrolysis is the process by which the chemical bonds in the carbohydrate are broken with the addition of water. The water is "destroyed" in hydrolysis. Dehydration synthesis (a type of condensation reaction is used to breakdown fat. This process "creates" water.

This means that in extreme situations you want to be using fat for energy. Since our body does not store much carbohydrate, in survival situations your body will rely on fat (and to a limited extent on protein). If you are going to eat and you have a bag of sugar and a stick of butter, go with the butter.

  • Actually the fat is one of the things that moves slowly during digestion so whatever water is used to process a stick of butter will be there longer – Erik vanDoren Jul 27 '16 at 21:02
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    Ideally you would want that water to stay in the intestine the least time possible, that was the whole reason of saying "dont eat". However, if memory serves, you dont really have the choice between the two processes as with fat you will have a condensation reaction and a subsequent hydrolysis reaction. Take also into account that there is a co-transport of sodium and glucose and aminoacids in the cells and you need the sodium to drive the absorption process... So, who knows... but butter and sugar taste good together, Id hope there's a slice of bread around too... ;) – Erik vanDoren Jul 27 '16 at 21:26
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    Wait a minute ... simple sugars release water when fully burned in mitochondria. – Joshua Jul 28 '16 at 3:44
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    @Joshua yes, fats and carbohydrate eventually becomes Acetyl CoA and then ATP. Fats just products e more water overall. – StrongBad Jul 28 '16 at 11:10
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    You completely disregard the nature of the food being eaten. Many foods, including meats, contain large amounts of water, and many vegetables are mostly water. In fact, there are documented examples of people surviving at sea for weeks with no fresh water whatsoever by eating fish. So this nonsense about not eating due to lack of free water is just that: nonsense. – Carey Gregory Sep 9 '17 at 20:25
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I think the problem is the generalization. It depends on the food. Watermelon is surely not comparable to trail mix. Also I have my doubts about the idea that berries are a problem and a mash to squeeze out the water is better than eating them because of the skins and seeds... that makes no sense. The way your intestine absorbs water is different through its length, the first part of the small intestine is what needs dilution of food with juices from liver, intestines and pancreas, to bring it to the same osmolarity of blood, that to allow absorption from that part of the gut, the food will already have an high percentage of fluids when it arrives there due to saliva during mastication and stomach juices. (BTW some sport drinks talk about osmolarity etc saying that an highly concentrated juice will sit in your stomach until diluted and that's not true, the stomach doesn't take care of that).

The final part of the small intestine and the colon work a bit differently as they can absorb fluids against osmotic gradient so they don't need that dilution, by design when the food gets to the large intestines will have lost the majority of the fluids, something like 80/90% and the large intestine will absorb more if necessary (it usually does as there's very little fluid in normal feces... obviously diarrhea is a complete different situation).

Seeds and skins that are not digested don't need liquids, you can't dilute something that has not been destroyed by digestion, and you don't need to, they get pushed along and eliminated. I find odd that with the reason of pushing out a few seeds someone would skip the chance to add whatever nutrition could get from the fruit. The way I see it is that mashing and squeezing is done way more efficiently already by our body and the precious fluids I would waste squeezing a bit of berries by hand would be more important than the bit of fluids temporarily added to it by the body.

For the constipation thing... well the more the feces stand in the colon the more water is absorbed from them, it's not that one gets constipated because the colon can't push the waste along because is too dry, it's perfectly able to push that food out, it's not sand. It takes a long time for an otherwise healthy but constipated person to develop the kind of fecal impaction that would lead to a blockage.

If the idea is that the water is not wasted as it gets absorbed anyways but the process is a problem while the food passes through the first portion of the small intestines where it could be diluted then one needs to figure out the advantage of paying the price for that short time, and in that case quantities of food ingested have their importance as extra dilution of an handful is different than dilution of a full pot of food.

A huge portion of our daily fluid intake comes from food, you can't just cut that out because "it's gonna need dilution" without taking in account the types of food available to you, the total quantity you eat and how you spread that food intake through the day.

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    a lot of good points. Do you have an opinion on this: when you are starting to be dehydrated, eating and digesting will require pulling water from other places in your body and keeping it in the digesting system for several hours, possibly making the dehydration worse? – njzk2 Jul 27 '16 at 14:31
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    @njzk2, yes that can be a point thats why in the answer I said that quantity might be a factor, an handful of berries wont sequestrate litres of water for hours, I would probably think twice about eating the whole bag of trailmix. I dont know if somebody ever researched all the possible situations so to put numbers on it to the point one knows what and how much of it is more convenient, there are too many things to take in account. – Erik vanDoren Jul 27 '16 at 14:53
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    @njzk2, what if one would just make the experiment on themselves at home: berries (or fruit) vs other foods vs nothing at all and see the difference? They will end figuring things out on their own – Erik vanDoren Jul 27 '16 at 15:06
  • @njzk2, in an otherwise healthy person and without using laxatives, water goes from the food through the intestinal wall into the blood and not the other way around. This is why you have very dry stool if it stays in your bowel for a long time. – Jan Nov 22 '18 at 18:39
  • IIRC, the proper concentration for transport across the gut wall is comparably high - I'd think it is probably higher than berry juice/mashed berries. (I'm thinking, say, blueberries and blackberries, not raisins) – cbeleites supports Monica Nov 23 '18 at 22:55
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Eat. Because metabolic water:

Metabolic water refers to water created inside a living organism through their metabolism, by oxidizing energy-containing substances in their food. Animal metabolism produces about 110 grams of water per 100 grams of fat, 41.3 grams of water per 100g of protein and 55 grams of water per 100g of starch.

Metabolic water (Wikipedia)

  • From the wikipedia references its not clear if there is any difference between a rabbit and an human in those figures, though – Erik vanDoren Jul 27 '16 at 21:33
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    Good point, different mammals probably metabolize and excrete differently. But in a survival situation, it might be good to remember that your body does produce some water from metabolizing foods. So those berries that contain some moisture or that grilled rattlesnake might turn out to have a net hydration gain overall. – James T Francis Jul 28 '16 at 13:27
  • It is true that macronutrients can generate some mertabolic water, but you need to consider that significantly greater amount of water will be excreted when urea (a breakdown product of proteins) will move from the blood to the urine. So, do not eat meat, seeds, nuts and other high-protein foods when dehydrated. Fruits and vegetables should be OK; most of them contain 90% of water. – Jan Nov 22 '18 at 18:44
  • @Jan: not sure though whether the single rattle snake in survival does actually lead to significant additional urea excretion (considering that the alternative may be to get rid of broken down muscle protein via urea without being able to replace it. I.e. effectively digesting your muscle vs. the snake's muscle). – cbeleites supports Monica Nov 23 '18 at 22:53
  • @cbeleites In my answer to: Can I get fresh water from ocean fish outdoors.stackexchange.com/questions/20613/…, I made some calculation, according to which by eating fish you lose more water through urine (due to urea excretion) than you get it from fish. The calculation may be exaggerated, but, so far, I believe, eating meat, fish, eggs, nuts, seeds and other foods high in protein and relatively low in water results in net water loss. – Jan Nov 24 '18 at 9:06

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