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I have heard that it will "wash away the nutrients", that it dilutes stomach acid and all kinds of things. Is this just an old-wives tale?

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    Choking on your food is bad too. If only there were some fluid to wash it down with...
    – Paul
    Commented Apr 7, 2011 at 10:10
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    This was a new brand of silliness to me. I had to go looking for people actually claiming this. Apparently it's a French thing: everydayfrance.com/2007/03/05/…
    – Christian Oudard
    Commented Apr 7, 2011 at 14:35
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    Not drinking enough water is definitely bad for you. It can cause eg. kidney stones.
    – BlueRaja - Danny Pflughoeft
    Commented Apr 7, 2011 at 15:31
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    Most food we eat is made up largely of water anyway.
    – Monkey Tuesday
    Commented Apr 7, 2011 at 17:33
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    Technically, ingesting ANYTHING (including food), except a powerful acid solution, is going to dilute your stomach acid. Trying to ingest a strong acid solution has it's own set of problems. So that's one of those "yeah, so what?" kind of factoids.
    – PoloHoleSet
    Commented Apr 11, 2017 at 21:34

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This positively reeks of old-wives-tale and pseudo-science. It appears to be widespread online, mainly in 'e-health' forums. It makes sense - water dilutes acid, right?

Right. But diluting an acid doesn't make it weaker. It may slow the rate of reaction, but gastric juices are strong stuff: a glass of water will hardly touch them. It takes an alkali or amphoteric like sodium bicarbonate to neutralise them effectively. And it so happens that sodium bicarbonate is exactly what the body secretes to neutralise the gastric juices. It's also what antacid tablets are made of.

See this answer from the Mayo Clinic:

There's no concern that water will dilute the digestive juices or interfere with digestion. In fact, drinking water during or after a meal can actually improve digestion. Water and other liquids help break down the food in your stomach and keep your digestive system on track.

I'd rather believe a qualified gastroenterologist than a web-full of random forum posts.

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    Diluting an acid does make it weaker in any reasonable sense of "weaker", and slowing the rate of reaction is of course important given the finite dwell-time of food in your stomach. You're correct that drinking water is not a problem, but this is not the reason.
    – Rex Kerr
    Commented Apr 7, 2011 at 13:43
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    Good answer. This claim never seems to take into consideration the fact that stomach acid is continually produced in order to meet needs. Barring use of an insanely large amount of strong antacid compounds, there is little the average person could consume in a meal which would alter the acidity of the stomach. Also, nutrients are not absorbed in the stomach, they are absorbed in the intestine, when the digestive process has rendered the food you've eaten alkaline. So this claim just fails on all counts.
    – Monkey Tuesday
    Commented Apr 7, 2011 at 21:15
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    I am definitely inclined to agree with you if not only because those people claiming water is so bad often forget to mention how unhealthy soup is, the most diluted food you can think off, but on the other hand I do think that the evidence you present is very weak. The mayo clinic to me looks like just another website and I see no mention of any studies done to verify their statement. It's not because they are doctors that they can't talk bullshit, often to the contrary (talking about doctors, not the mayoclinic here).
    – user4513
    Commented Aug 17, 2011 at 10:12
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    @RexKerr: "Diluting an acid does make it weaker in any reasonable sense of "weaker"" this is wrong unless you consider chemists' point of view about weak and strong acids as "not reasonable": acid strength is a well defined concept. The strength of an acid does not depend on dilution, it is a material/substance characteristic of the acid. HCl is a strong acid, regardless of dilution. Dilution has no effect on the capacity of the acid-base system: for that the total acid and base equivalents are important. Also the reaction kinetics of ...
    – cbeleites
    Commented Oct 18, 2013 at 17:15
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    @cbeleites - I agree with all the details; my answer to the question was a less detailed version of your point (see below). The technical strong vs. weak acid distiction is not a reasonable one to make in this context, though. If you are kinetically limited, you care about [H+] concentration. If not, you care about total number (not concentration) of [H+].
    – Rex Kerr
    Commented Oct 18, 2013 at 22:24
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Gastric acid production is carefully regulated. Therefore, drinking water (in reasonable quantities) is very unlikely to have a significant negative impact. (After all, any food you eat will also dilute your stomach acid, and the food very likely has pH buffers in it, unlike water.)

Also, "wash away the nutrients" is particularly silly--wash it away to where? It's all still in your stomach, and you have twenty feet of intestine devoted to extracting nutrients from material in your digestive tract.

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