Acid-forming foods: it definitely sounds alarming, and readers of Loren Cordain’s original book, The Paleo Diet, might remember the importance of an acid-base balance as something connected to avoiding modern illness. The basic idea is that the human body is designed to function at a certain balance between acidic and non-acidic (also called basic or alkaline) foods, and that upsetting this balance is a major contributor to chronic disease. There’s even a special Alkaline Diet based exclusively on avoiding “acid-producing” Western foods and prioritizing “alkaline” foods like fruits and vegetables.
This seems like a very Paleo-friendly hypothesis – after all, we know that the dysregulation of other ratios (like the O6:O3 ratio) can contribute to all kinds of problems. Dr. Cordain cites the original human diet as being largely alkaline-producing, and some of the most acid-producing foods are grains, so it seems fairly plausible that humans were meant to eat fewer acid-producing foods than we do today.
Unfortunately, this is one theory that just doesn’t hold water. As it turns out many modern hunter-gatherer groups (our best estimates for what an actual Paleo diet might have looked like) have a net acid-producing diet, rather than a net alkaline diet. Various papers have gone back and forth on this issue, but the real question for humans living in the 21st century is whether or not an acidic diet contributes to disease or poor health, regardless of what anyone did in the Paleolithic. And there just doesn’t seem to be much evidence that a net acid load diet by itself contributes to any particular damage – instead, the diet seems to be healthful because it restricts grains and sugar and promotes a high intake of fruits and vegetables.
The Acid-Base Hypothesis
A quick review of the terminology: pH is a measure of how acidic a liquid is (how much hydrogen it contains), ranging from 0 to 14. The lower the pH, the more acidic the liquid, so battery acid has a pH of around 1, distilled water has a pH of 7 (perfectly neutral), and bleach has a pH of 13. Normally, human blood has a pH around 7.4, so it’s very slightly toward the alkaline side of the scale.
This is the basic biological background. To understand the basis for the alkaline diet, it’s also important to know that “acid” or “alkaline” foods aren’t judged according to their pH when they’re in your mouth. This doesn’t matter, since they all get drowned in acid in your stomach, and then alkalinized again in your intestine. What really matters is the pH of the residue, or ash, that they leave in your blood after being metabolized (more on this below). This is why apple cider vinegar, for example, is acidic to the taste, but counts as an “alkalizing” food in your body, while grains aren’t at all acidic but still count as “acid-forming.” As well as grains, acid-forming foods include dairy, legumes, sugar, some oils, and animal protein.
In its simplest terms, the acid-base theory of nutrition holds that these acid-producing foods are dangerous through one of two pathways:
- Acid foods lower the blood pH, creating an “acidic environment” where diseases like cancer can thrive.
- Acid foods do not raise the blood pH for long, but the effort of buffering the blood against them leaches calcium from the bones, causing bone disease and osteoporosis.
Since these seem to be two different claims, this article takes a look at them individually, to see what kind of evidence supports them and whether they actually seem plausible.
Claim #1: Acid Foods Lower Blood pH
One claim by proponents of the acid/alkaline balancing hypothesis is that an overload of acid foods causes the pH of the blood to change. The result is a more acidic environment (a lower pH), which supposedly makes you more prone to all kinds of chronic diseases, especially cancer.
This one is fairly easy to debunk. The body has a very complex system for regulating pH, and if pH strays outside a tightly regulated range (7.35-7.45), you’ll probably end up in the hospital. Anything below approximately 7 or above approximately 7.7 is a recipe for a fast and painful death. For example, in diabetic ketoacidosis (not to be confused with ketosis), a life-threatening complication of Type 1 Diabetes, a blood pH of 7 or below is a mark of an attack so severe that the patient is unconscious.
Since it’s so critical to keep blood pH in balance, the body doesn’t leave much to chance. The most important regulator of pH in the blood is simply breathing. Remember that when you inhale, you breathe in oxygen, and breathe out carbon dioxide. Carbon dioxide is acidic in the blood, so the rate of your breathing controls your body’s pH by regulating the balance of carbon dioxide and oxygen.
The kidneys also play an important role in this process because they control what substances get eliminated in the urine or reabsorbed into the body. If the blood pH is too low (too acidic), they can excrete more acid or reabsorb more bicarbonate (which is alkaline). If the blood pH is too high (too basic), they can excrete more bicarbonate and less acid. Acid in the blood can also be “buffered” by various different substances. These include bicarbonate, calcium, and other minerals.
This triple system of blood pH control (by the lungs, the kidneys, and buffers in the blood) ensures that blood pH doesn’t change much in response to dietary intake of acid/alkaline foods or anything else. If your blood pH undergoes any kind of drastic change, you’re going straight to the ER.
A lot of people get confused here, because they find plenty of evidence that a more acidic diet can alter urine or salivary pH. This is completely true, but remember that urine and saliva are not blood. Your kidneys do filter out excess acid and get rid of it through urine, so urinary pH is actually highly variable compared to blood pH, and a little more acidic overall. This doesn’t tell you anything about your blood. In fact, the changes in urine pH are a sign that your body’s pH regulation system is working well and your blood pH is staying constant, since any excess acid is being excreted.
Right off the bat, this need to keep blood pH fairly constant gives the lie to a lot of exaggerated health claims made by the Alkaline Diet. For example, proponents claim that a low-acid diet prevents cancer, because cancer cells can’t survive in an alkaline environment. This is true in test tubes, but the levels of alkalinity used in these experiments would kill you a lot faster than the cancer. You certainly can't get there by eating or drinking any "alkalinizing" foods. In other words, the theory that your blood pH actually shows significant changes in response to eating or drinking too much of anything is a complete myth.
Claim #2: Acidic Diets Leach Calcium from your Bones
This theory of acid and disease is a little more plausible on the surface. Proponents understand that the body has to maintain a pH within a very limited range – they aren’t arguing that acidic foods suddenly cause your blood pH to drop to dangerous levels. Instead, the claim is that the digestion and metabolism of “acid-forming” foods results in acidic by-products (most importantly phosphate), which your body has to “neutralize” to keep a healthy pH. This involves leaching calcium (which is alkaline, or basic) out of your bones to balance out the acid of the foods.
A few words about calcium: before diving into the theories about calcium and pH, remember that calcium absorption is a very complicated process. For one thing, it depends on several other micronutrients including Vitamin D, Vitamin K2, and magnesium – this is why many people take just a calcium supplement, but still suffer from osteoporosis or other bone problems. Just putting enough calcium in your mouth doesn’t guarantee it will get absorbed.
Similarly, if you have a high rate of calcium excretion in your urine, it doesn’t necessarily mean that calcium is “leached” from your bones; it could also be a sign that your body isn’t absorbing as much of the calcium you’re eating.
With that in mind, let’s take a look at the research that’s actually been done on one of the “net acid” foods (animal protein) and calcium levels in the body. It’s more useful to look at protein than other “acid-forming” foods because many of these foods (like grains and seed oils) also contain antinutrients that can bind to calcium and make it unavailable to the body. This can obviously influence study results – for example, a study showing that wheat caused osteoporosis might just as easily be measuring the damage of the phytic acid in the wheat, and not necessarily the acid load.
The research into protein levels and calcium absorption at first seems to support the acid/alkaline theory (high-protein diets increase calcium excretion in the urine). But the wrench in the monkey machine here is that this higher excretion doesn’t have a negative effect on bone health. A 2002 study found that dietary protein did increase calcium loss in the urine, but also showed a net positive effect on bone health – in other words, there’s more going on than simply robbing your bones of nutrients. Interestingly, the study reported that protein and calcium seem to act together: in a three-year trial, subjects with high protein intake and calcium supplements gained bone mass, but those with low protein intake and calcium supplements lost bone mass.
This isn’t surprising given that 50% of your bone mass is protein. Bones have a complicated internal structure – just shoving more calcium in there isn’t doing anyone any good. What your body actually needs is the building blocks to re-create that structure, one of which is protein.
Another study (from 2005) directly addressed the issue of acid buffering by calcium from bones, and found that a high-protein diet did increase calcium content of the urine, but also increased bone density. They noted that dietary protein increased intestinal absorption of calcium, and that most of the calcium excreted in the urine of the high-protein group was from the diet, not from the bones.
These studies aren’t conclusive by themselves, but they do suggest that in the absence of other food toxins, “acid-forming” foods by themselves actually don’t leach calcium from bones. In the same way that urinary pH is not an accurate measure of blood pH, urinary calcium excretion isn’t a good measure of bone health.
Looking at dietary acid load more generally, instead of just meat, there still doesn’t seem to be any relationship between acidic or basic foods and osteoporosis. A meta-analysis showed that an increase of dietary phosphate (the most important acidic ion produced by “acid-forming” foods.) was actually correlated with a decrease in urinary calcium – in other words, the body was losing less calcium through urine, not more. Another meta-analysis of overall dietary acid load concluded that no relationship could be determined between a dietary acid load and osteoporosis, and yet another one concurred.
Dairy and Ostoporosis
One frequently-cited piece of evidence in favor of the “acid leaches calcium from your bones” theory deserves special mention. This is the fact that the United States has an exceptionally high consumption of dairy products (which contain calcium), but also an exceptionally high rate of osteoporosis (caused when calcium leaches out of bones, making them brittle and weak). In fact, many countries with very high dairy intakes have higher rates of osteoporosis than countries with lower intakes. This is often used to "prove" that the net acid balanced produced by the dairy products neutralizes all the good effects of the calcium.
However, as the meta-analyses linked above show, there really isn’t a relationship between net acid load and osteoporosis. Another study specifically on dairy also failed to find a relationship. So how can we explain the apparent link between dairy and osteoporosis in other terms? The answer is: it’s complicated, because there are so many differences between countries besides just their dairy intake. Remember that just adding calcium to the diet itself doesn’t do much; it requires several other nutrients to be absorbed effectively. What about the differences in intake of Vitamin K2, magnesium, or Vitamin D? Also, many dairy products are actually sweetened “yogurts,” milkshakes, or ice cream: not exactly a big glass of milk straight from the cow. For example, take a look at the sources of cheese in the U.S. diet: fully ¼ of the pie chart is “Cheese on Pizza.” If dairy intake comes with these “add-ons” attached, it’s hard to claim that dairy itself is the culprit.
If we control for some of these other factors, the link between milk and osteoporosis starts to look less promising. For example, when Weston A. Price visited the Maasai, a tribe in Africa that lived mainly on milk and blood, he found that their teeth were nearly perfect. These people were obviously not eating milkshakes or ice cream full of sugar and other toxins, and they were getting plenty of Vitamin D (from the sun) and other nutrients from an overall healthy diet.
Theories #1 and 2: Protein, Calcium, and the Kidneys
One of the common points to both acid/alkaline theories is that a high acid-load diet stresses your kidneys. Kidney stones are the major concern here: since kidney stones are primarily made of calcium, the idea is that a high acid load fills your blood with calcium (to buffer the acid), and it’s hard for the kidneys to effectively filter all the calcium out. This is one of the reasons why a low-protein diet is recommended for people with kidney stones, because animal protein is supposed to be “acid-forming.”
In this case, there is some truth to this theory, but it only applies to people who already have kidney problems. People with healthy kidneys see no risk from increasing protein (within reasonable limits - protein is great, but it shouldn’t be the bulk of your calories).
Even for people with pre-existing kidney problems, alkalinizing the diet is not necessarily a miracle cure, because “kidney stones” are not a one-size-fits-all problem. For the most common type (calcium oxalate stones), patients are advised to alkalinize their diet, but for two other types (calcium phosphate and struvite stones), it’s actually beneficial to acidify the urine.
On the other hand, in some patients with chronic kidney disease, it might be useful to reduce the net acid load. Another study showed that kidney transplant patients might benefit from a diet low in animal protein and high in fruits and vegetables to reduce the acid load on the kidneys. Again, though, this could have something to do with the other nutrients in the fruits and vegetables rather than the acid/base balance.
The upshot is that if you have a pre-existing kidney problem, you should talk to a doctor about your specific situation, to see if restricting protein or anything else might be beneficial. If you don't, there's no reason to worry about a reasonable amount of dietary protein giving you kidney stones.
The Grain of Truth
As dubious as the acid-base hypothesis seems, if it were completely bunk, there wouldn’t be so many studies and so much anecdotal evidence suggesting improvement in bone health and various other physical processes with a lower acid diet. So what gives? To quote from one of the studies: “Intakes of potassium, magnesium, fruit, and vegetables have been associated with a more alkaline environment in the human body and a beneficial effect on bone health.” In other words, a low-acid diet is one without processed grains, sugar, and legumes, but with plenty of fruits and vegetables supplying valuable micronutrients – does this sound familiar?
The major health benefit of a low-acid load diet probably has more to do with cutting out processed foods and eating plenty of nutrient-dense vegetables than anything else. Fruits and vegetables do contain some nutrients that increase absorption of calcium (like magnesium, for example), so these foods probably are good for bone health, and every other kind of health as well. But it’s not because they’re re-balancing the body’s acid levels, it’s because they’re supplying essential aids to calcium absorption from green vegetables and other plant sources of calcium.
In fact, a whole-foods alkaline diet is quite close to a basic Paleo diet. Although most of us associate meat with protein, it’s also a source of fat, and in fact most of the calories on a Paleo diet come from fat, not protein. Paleo conforms quite well to alkaline diet guidelines, but that’s not why it’s so healthful, and there’s no reason to make special effort to “balance out” acidic foods or take alkaline supplements.
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