Join 3,363 readers in helping fund MetaFilter (Hide)


Let's get chemical.
September 16, 2008 6:16 PM   Subscribe

Hey Biochemists! Why exactly (in extreme biochemical detail) are pH balancing "Acid/Alkaline diets" a bunch of hooey?

Do tell. Spare no boring detail.

posted by sindas to Health & Fitness (15 answers total)
I'm not a biochemist, but I remember from high school chem that our blood is a buffer. This means that it resists change in pH. The buffer is made from carbonic acid and its conjugate base bicarbonate. Excess base in the blood is neutralized by the carbonic acid and excess acid is neutralized by the bicarbonate. I don't see how these diets could actually change the pH of the fluids in your body. The buffer keeps your blood within a tiny pH range. Outside of this range, enzymes stop working and your body doesn't function properly.
posted by rancidchickn at 6:23 PM on September 16, 2008 [1 favorite]

Your kidney is quite able to maintain your blood pH by excreting urine with different pH. In fact, your blood pH must remain in a fairly narrow range to live.

It's true that diets with different compositions impose requirements for your kidney to excrete more or less acid. That's not a dominant portion of the work that your kidney does. However, urine pH can place you at different risks of kidney stones and infections.

What specific claims do you want rebutted?
posted by a robot made out of meat at 6:35 PM on September 16, 2008

Your body is filled with hell of effective buffers that accept or release proteins to prevent pH change.
posted by demon666 at 6:36 PM on September 16, 2008

The reason why these diets are baloney is because your GI system has an exquisite system to maintain homeostatic control over the chemical environment inside your GI tract. The cells that line your small intestine respond to excess stomach acid by secreting hormones. These hormones signal your pancreas to squirt out some bicarbonate into the intestine which neutralize the extra acid. You can get increased acid secretion if you have an H. Pylori infection or if your vagal nerve is stimulated (i.e. you're hungry, and you're about to grab a cheesesteak from the food cart, your brain preps your GI system by sending a signal down your vagus nerve to your stomach, which squirts out some acid so it's ready to digest when the food comes down your esophagus).

This really has nothing to do with the kind of food that you're eating.
posted by scalespace at 6:37 PM on September 16, 2008

*proteins should be protons

(some proteins act as buffers)
posted by demon666 at 6:38 PM on September 16, 2008

I am not a chemist, but the person who wrote this page is.
posted by Sidhedevil at 6:39 PM on September 16, 2008 [1 favorite]

I'm a biochemist but I've never heard of these "pH balancing" diets. However, rancidchickn is correct in stating that your blood is buffered to stay within a narrow range of pH and if it drifts outside that range you're in trouble. (See acidosis and alkalosis) You don't need a special diet to buffer your blood; it's a normal physiological process.

Your stomach contains HCl that brings the pH down to about 2 (very acidic), which is necessary to help digest your food properly. The pH in your intestines is brought back up to ~ 7 - 8 by bile secreted from your liver, so there again your body is adjusting pH as needed without requiring a special diet. Here is a Wikipedia link about digestion which talks a little about pH, and this looks like a pretty good overview of the GI tract.

Basically, the human body does just fine adjusting the pH of various fluids and tissues, working on ordinary food. Again, I don't know what claims these diets are making, but if you could let us know specifically what you're interested in maybe we can help.

(I'm slow to preview so I won't cite all the preceding answers, but they're on track too.)
posted by Quietgal at 7:02 PM on September 16, 2008 [1 favorite]

The claim is that by adjusting the PH balance in your cells, you can prevent diseases. It's something that's so wrong it's not really possible to go into too much detail about why it's wrong, but the basic idea is that your body always has the same pH level, which is also the same pH level for all DNA based cells, otherwise they wouldn't work at all (IIRC and I'm sure there are some exceptions)
posted by delmoi at 7:12 PM on September 16, 2008

delmoi has the claim exactly. Thats what I want to be able to refute more effectively. I have a basic understanding of biochem, and an advanced understanding of human nutrition, but what I don't have is a silver bullet to prove my point on this specific subject.
posted by sindas at 7:52 PM on September 16, 2008

For an extensive description of acid-base physiology, see here, especially Chapter 2.

There are some review articles out there about the role of diet in acid-base balance that you can find with clever searching on PubMed, but most are inaccessible without a license. Here's one, though, that seems to cover the issue well. Basically, it's not as simple as some of the answers above indicate. Yes, diet doesn't shift the body's pH because it's buffered by things like the minerals in bone. But it's possible that a prolonged diet out of pH-balance might continually drain those buffer reserves.

Essentially, nutrition and human physiology is not as well understood as we would like to think, and a lot more science needs to be done before some of these issues can be put to rest.
posted by Durin's Bane at 8:40 PM on September 16, 2008 [1 favorite]

Would it help to point out that acidosis or alkalosis can be respectively caused by hypoventilation and hyperventilation? Under normal conditions, simply breathing gives your body basically unlimited buffering capacity.

"In a general way, then, the lungs try to compensate for a metabolic acidosis or alkalosis by doing their thing, changing the pCO2; the kidneys try to compensate for a respiratory acidosis or alkalosis by doing their thing, changing the HCO3- . The lungs compensate by decreasing pCO2 in metabolic acidosis and increasing pCO2 in metabolic alkalosis. The kidneys compensate by increasing HCO3-, in respiratory acidosis and decreasing HCO3- in respiratory alkalosis."
posted by demon666 at 9:31 PM on September 16, 2008

"But it's possible that a prolonged diet out of pH-balance might continually drain those buffer reserves."

Would the standard western diet already be out of balance one way or the other?
posted by dws at 9:40 PM on September 16, 2008

what I don't have is a silver bullet to prove my point on this specific subject.

Since the claim is so silly, a silly silver bullet might be sufficient:

It clearly doesn't matter what the acidity of the food is, because your stomach dissolves it all in acid anyway, so not a shred of that carefully balanced pH ever even reaches the digestive system, let alone has a snowball's chance in hell of changing your body's chemistry.

There are obvious counter arguments to this, but at that point it's like arguing whether the death star would beat the borg cube - whatever tenuous connection to reality remains just isn't part of the program.
posted by -harlequin- at 2:19 AM on September 17, 2008

Well, the argument seems to be that pathogens work because all DNA-based cells need the same pH environment. So if this is true, your own cells won't work if you change their pH. QED. You can fling this back at your friends.

In truth, however, even inside your cells there are different pHs. The cytosol is close to neutral, although a bit on the alkaline side (pH 7.4ish, similar to blood), whereas lysosomes, digestive organelles inside the cell, are fairly acidic at pH 4.8.

As you might expect, cells have to work constantly to maintain these different pH zones in such close proximity to each other. Recall your high school chemistry: pH is a measure of proton (H+) concentration. Protons can diffuse around like any other ion, and thermodynamically they will tend to diffuse from regions of high concentration (acidic pH) to regions of lower concentration (basic or alkaline pH). This is entropy, the great destroyer, at work.

To fight entropy requires energy. Thermodynamics is a bitch. Cells have evolved a class of proteins called "proton pumps" which expend energy to push the protons back where the cell wants them. The gastric hydrogen/potassium ATPase is the proton pump that keeps your stomach at around pH 2. The pump has to use up one molecule of ATP to push one proton into your stomach (I think that's the ratio - I don't have time to find a citation right now). The vacuolar H+ ATPase does the same job in your lysosomes and other organelles.

(ATP is the main "energy currency" in your cells. A large part of cellular metabolism is devoted to burning glucose and using the energy liberated to drive the production of ATP: see oxidative phosphorylation and glycolysis).

Anyway, the point is that cells do a lot of work and spend a lot of ATP to maintain the various pH environments that they need to function properly. There are powerful homeostatic control systems in place to keep the pH where the cell needs it. Nothing you'd want to put in your mouth is going to change the pH inside your cells and organelles.

Hope this helps - sorry for the dissertation but biochemistry is complicated as well as fascinating and "sound bites" don't do justice to the elegance of living systems.
posted by Quietgal at 8:07 AM on September 17, 2008 [3 favorites]

the basic idea is that your body always has the same pH level, which is also the same pH level for all DNA based cells, otherwise they wouldn't work at all

Is that even TRUE? Aren't there potentially nefarious DNA-based cells thriving at a variety of pH levels (like E. coli in acidic environments)? Simply changing the pH of one's body to be less welcoming to certain diseases might theoretically make the body more vulnerable to other diseases. Wouldn't that alone make the whole endeavour kind of a wash?

I like Quietgal's answer -- because even if you were able to alter the pH of your cells through diet, why would you want to? Case in point. In a normally-functioning body, forcing a pH change isn't likely to accomplish anything good, and would have terrible side-effects.

But if you're talking about the pH of the food directly impacting your body's pH, it's as simple as: your food gets processed through a giant vat of hydrochloric acid and then gets squirted with bicarbonate before the components even reach the your blood and the rest of your cells -- each of which has its own complex pH-balancing mechanisms. Ultimately, the initial pH of the food doesn't matter at all. (But you know this since you study nutrition.)
posted by peggynature at 2:23 PM on September 17, 2008

« Older Which theaters offer quality i...   |  Please suggest a learning plan... Newer »
This thread is closed to new comments.