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This question arose out of a discussion about fasting during Ramadan. Which, if you are not aware, involves abstaining from eating and drinking between dawn and dusk. Something which can be quite difficult in the summer time if you live far away from the equator. Particularly concerning is the effects of dehydration. I am interested in knowing how to ameliorate the effects of not drinking water for as much as 20 hours a day.

It occurs to me that the digestive process itself might consume a significant amount of water and that it might be beneficial if the persons observing the fast not eat very much in the short period of time they are allowed to.

But it's just a wild stab in the dark, I am by no means an expert, so I am looking for refutation or confirmation from an expert. Google searches have yielded nothing with all the results being about whether it's OK to drink water with meals. So here we are.

Just to be clear when I say digestive process I mean the processing of food to extract nutrients in the stomach and intestines, not the subsequent conversion of those nutrients to ATP in the cells (which I understand creates water).

I assume (perhaps wrongly) that the production of bile and stomach acid requires water, some of which will be lost during their interaction with the food and the body won't have a use for everything that enters the bloodstream from the digestive tract and some waste will have to be excreted as urine, causing water loss

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  • I've answered the biomedical question embedded in this one since it contradicts the basic premise here, but note that questions about "what to eat" and such are not on-topic here.
    – Bryan Krause ♦
    Apr 17, 2021 at 18:26
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    Drink an adequate quantity of water per day during that 4 hour window and it's probably not going to matter much. I don't see how reducing food intake would help considering that food supplies a substantial percentage of your water intake. The one exception would be extremely hot, arid climates where going without water for 20 hours can actually be dangerous, particularly if you're outdoors and active. Based on personal experience, that's not something I would recommend doing. (A lot of strange customs evolved in the hot, arid Middle East.)
    – Carey Gregory ♦
    Apr 17, 2021 at 20:50

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When plants photosynthesize, the net reactions are to turn water and carbon dioxide into sugars and oxygen, using energy from the sun.

When animals and other aerobic organisms obtain usable energy from sugars and other compounds, the net outcome is the inverse: turning sugars and oxygen into water and carbon dioxide.

Therefore, humans are net producers of water through digestion. See this Biology.SE Q&A and Wikipedia: metabolic water.

Of course water is also used and lost by animals in many ways: it's used in anabolic reactions (just like in plants), used to cool through sweat, excrete waste through urine, etc, and a lot is lost in respiration. There's also a lot of water used in the digestive process, it's just that it is ordinarily reabsorbed in the digestive tract (failure to reabsorb this water causes diarrhea). Therefore, over a span of days it is necessary to intake some additional water beyond that which is obtained through metabolism to replace what is lost.

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    I believe you have misunderstood my question. Only the last two sentences are in any way relevant. I've amended my question to make it clearer
    – kaan_a
    Apr 17, 2021 at 18:55
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    @kaan_a I believe the reason only the last two sentences seemed to answer your question is that Bryan's preceding content was simply an explanation of the underlying processes. Reading your edited question and this answer, I don't see why it doesn't answer the allowable part of your question. The part about how to ameliorate the effects is off topic because it asks for medical advice, which is strictly off topic here. I think this answers your science question. What to do about it now that you know is up to you, but I think accepting and upvoting this answer would be a good first step.
    – Carey Gregory ♦
    Apr 29, 2021 at 0:16

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