Let's use a little chemistry to see how the acidity changes. It's been a long time since I did pH calculations, so maybe someone else can check my work.
pH is defined (or was, until the 1924 revised definition) as the molar concentration, or molarity, of H+ ions in a solution. Molarity just means the number of mols per unit volume (we'll use liters). If you're curious, a mol is a measure of the number of particles (in this case H+ ions). For our calculation it's not important to know how many particles are in a mol (it's a huge number - think 23 zeros).
pH = log( 1 / molarity )
log is base 10, and
molarity is H+ molarity with unit mol/L. To break it down further:
pH = log( 1 / (mols / volume) )
Before we start, we need to know the approximate amount of gastric acid secreted per meal, and the pH of that acid. Some websites indicate that the stomach secretes 700 ml per meal, some say 400 ml. Averaging these gives us 550 ml. Let's also assume a pH of 2.
First, a meal without water. Note that this maybe be somewhat of an idealized scenario because you're probably not going to eat a lot of food that has absolutely no water. We want to solve for the variable mols:
2 = log( 1 / (mols / 0.55) ) 10^2 = 1 / (mols / 0.55) 1/(10^2) = mols / 0.55 1/(10^2) * 0.55 = mols mols = 0.0055
If we add about 8 oz (0.24 L) of neutral-pH water, the mols will not change, but the volume will be 0.55 + 0.24. So we can use that info to calculate the resulting pH.
pH = log( 1 / (0.0055 / (0.55 + 0.24)) ) pH = 2.16
Aha! It doesn't do as much damage as we might have thought. Gastric acid pH ranges from 1.5 to 3.5 anyway, so we're well within range.
Just remember that the pH scale is logarithmic with base 10, not linear. The volume has to be increased by a factor of 10 to increase the pH by 1 unit. Like so:
pH = log ( 1 / (0.0055 / (0.55 + 5)) ) pH = 3
How much water was that? Five liters! You'll probably get water intoxication before you dilute the stomach acid enough.
This only addresses normal digestion. Things may be different if you have low stomach acid volume or acidity.
One final note: you still may want to delay drinking water during a meal if you have acid reflux or the like. I read that the majority of cases are caused by low stomach acid, not high. So every bit of acidity helps.
EDIT - more thoughts
The above calculations assume another ideal case - that all the stomach acid for the meal is already present before the water is consumed. I don't know the biology behind it, but I suspect that the acid is released gradually; in which case there will be a greater effect if you drink all the water at once. Maybe the stomach also compensates if the pH rises. Also, I think acid is produced as soon as you perceive food, even before you start eating, so it "gets a head start". But I'm probably over-thinking it. If you're concerned, just drink a little water at a time.
Well technically, drinking water will dilute stomach acid, bringing its pH closer to neutral (7). That's just a function of physics and chemistry.
But the question you probably want answered is "does it matter for digestion?". I'd say (from personal experience) that within a reasonable range 1-2 glasses with meals, your body will adapt with no issue. Often, at work, I'm busy so I don't drink when I eat. I notice nothing different from when I drink a couple of glasses.
In general, though, I think that (if you have your health dialed in) you can just trust your thirst signals (like you can trust your hunger signals) to tell you when and how much to drink - don't worry about getting your "8 glasses of water a day", that's as useful as getting your "8-11 servings of hearthealhtywholegrains a day".
Another point to think about that I just heard this week from Mark Sisson: your stomach acid is also useful as a first step protection in your immune system. Say you inhale some virus or bacteria or something like that. Your nose should catch most of that and from there it will drain down your throat to your stomach where it should be killed by the acid before it goes on from there. Drinking too much could dilute that and make it ineffective. It's a new thought to me, so I haven't really looked into it, but it sounds reasonable at first glance.
I would recommend reading this post as it is very relevant: http://high-fat-nutrition.blogspot.com/search/label/Casein%20vs%20gluten ....The gist of it is that pepsin, a hard core protein breakdown enzyme, functions best at a PH of 2. Thats darn low. So for very stubborn proteins that can be problematic i.e. casein, having this enzyme acting optimally is VERY important. It becomes could be quite problematic if such a protein were to reach the small intestines uncleaved (think leaky gut concequences)
How much can that glass of water dilute your stomachs natural acids? I would venture to say its enough to reduce the enzymatic functions that are necessary to break down the most stubborn peptides, but not all of them. Kind of like a, do you feel lucky senerio isn't it? On the safe side I may just skip the water with my protein.
I would avoid large amounts of liquid and especially cold drink just prior to eating, during the meal, and for about an hour after. I've noticed that it increases indigestion. In Ayurveda they say it puts out the digestive "fire." As for if this is literally true and true for everyone, I don't know.
I stick to sipping room temperature drinks around and during meals for the most part.
Something that nobody so far has addressed… if you work in a lab, you know that adding water to a buffered solution will only very slightly change the pH. From what I understand, gastric acid contains bicarbonate, therefore it’s essentially a buffered solution. I’m going to guess that as long as you’re not drinking gallons of water you’re not going to drastically affect the pH of your gastric acid, especially since it works across a range of pHs (which you have to keep in mind works on a log rather than a linear scale).
I am not a doctor nor did I stay in a Holiday Inn Express last night... but I would say not really... since the serious digestion is being done in the small and large intestines anyhow... water should be absorbed almost right away. the stomach is just a prep tank... I could be wrong if Dr. Paleo chimes in and says otherwise...
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