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The basic premise of the acid-alkaline theory is flawed. The evidence doesn’t support the idea that a net acid-forming diet is harmful to bone health.
Part 1

Many of you have probably heard of the 'alkaline diet'. There are a few different versions of the acid-alkaline theory circulating the internet, but the basic claim is that the foods we eat leave behind an 'ash' after they are metabolized, and this ash can be acid or alkaline (alkaline meaning more basic on the pH scale).

According to the theory, it is in our best interest to make sure we eat more alkaline foods than acid foods, so that we end up with an overall alkaline load on our body. This will supposedly protect us from the diseases of modern civilization, whereas eating a diet with a net acid load will make us vulnerable to everything from cancer to osteoporosis. To make sure we stay alkaline, they recommend keeping track of urine or saliva pH using handy pH test strips.

In this two-part series, I will address the main claims made by proponents of the alkaline diet, and will hopefully clear up some confusion about what it all means for your health. Will eating an alkaline diet make you and your bones healthier?

Foods can influence our urine pH

Before I start dismantling this theory, I want to acknowledge a couple things they get right. First, foods do leave behind acid or alkaline ash. The type of 'ash' is determined by the relative content of acid-forming components such as phosphate and sulfur, and alkalis such as calcium, magnesium, and potassium. (1, 2) In general, animal products and grains are acid forming, while fruits and vegetables are alkali forming. Pure fats, sugars, and starches are neutral, because they don't contain protein, sulfur, or minerals.

It's also true that the foods we eat change the pH of our urine. (3, 4) If you have a green smoothie for breakfast, for example, your pee a few hours later will likely be more alkaline than that of someone who had bacon and eggs. As a side note, it's also very easy to measure your urine pH, and I think this is one of the big draws of the alkaline diet. Everyone can probably agree that it's satisfying to see concrete improvements in health markers depending on your diet, and pH testing gives people that instant gratification they desire. However, as you'll see below, urine pH is not a good indicator of the overall pH of the body, nor is it a good indicator of general health.

Foods don't influence our blood pH

Proponents of the alkaline diet have put forth a few different theories about how an acidic diet harms our health. The more ridiculous claim is that we can change the pH of our blood by changing the foods we eat, and that acidic blood causes disease while alkaline blood prevents it. This is not true. The body tightly regulates the pH of our blood and extracellular fluid, and we cannot influence our blood pH by changing our diet. (5, 6) High doses of sodium bicarbonate can temporarily increase blood pH, but not without causing uncomfortable GI symptoms. (7, 8) And there are certainly circumstances in which the blood is more acidic than it should be, and this does have serious health consequences. However, this state of acidosis is caused by pathological conditions such as chronic renal insufficiency, not by whether you choose to eat a salad or a burger. In other words, regardless of what you eat or what your urine pH is, you can be pretty confident that your blood pH is hovering around a comfortable 7.4.

A more nuanced claim has been proposed specifically regarding bone health, and this hypothesis is addressed somewhat extensively in the scientific literature. It supposes that in order to keep blood pH constant, the body pulls minerals from our bones to neutralize any excess acid that is produced from our diet. Thus, net acid-forming diets (such as the typical Western diet) can cause bone demineralization and osteoporosis. This hypothesis, often referred to as the 'acid-ash hypothesis of osteoporosis,' is what I will discuss for the rest of this article. I'll address some of the other health claims in part two.

The kidneys - not bone - regulate blood pH

While more reasonable than the first claim, the acid-ash hypothesis seems to completely disregard the vital role the kidneys play in regulating body pH. The kidneys are well equipped to deal with 'acid ash.' When we digest things like protein, the acids produced are quickly buffered by bicarbonate ions in the blood. (7) This reaction produces carbon dioxide, which is exhaled through the lungs, and salts, which are excreted by the kidneys. During the process of excretion, the kidneys produce 'new' bicarbonate ions, which are returned to the blood to replace the bicarbonate that was initially used to buffer the acid. This creates a sustainable cycle in which the body is able to maintain the pH of the blood, with no involvement from the bones whatsoever.

Thus, our understanding of acid-base physiology does not support the theory that net acid-forming diets cause loss of bone minerals and osteoporosis. But just for argument's sake, let's say that our renal system cannot handle the acid load of the modern diet. If bones were used to buffer this excess acid, we would expect to see evidence of this taking place in clinical trials. Alas, that is not the case.

Clinical trials do not support the acid-ash hypothesis of osteoporosis

At first glance, some of the studies may look convincing, because higher acid diets often increase the excretion of calcium in the urine. Some researchers assumed that this extra calcium was coming from bone. (8) However, when calcium balance (intake minus excretion) was measured, researchers found that acid-forming diets do not have a negative effect on calcium metabolism. (9) Some studies found that supplementing with potassium salts (intended to neutralize excess acid) had beneficial effects on markers for bone health, which would tend to support the acid-ash hypothesis. However, these results were only observed in the first few weeks of supplementation, and long-term trials did not find any benefit to bone health from these alkalizing salts. (10)

Finally, even though the hypothesis holds that higher intakes of protein and phosphate are acidifying and therefore detrimental to bone health, multiple studies have shown that increasing protein or phosphate intake has positive effects on calcium metabolism and on markers for bone health. (11, 12) Summarizing the clinical evidence, two different meta-analyses and a review paper all concluded that randomized controlled trials do not support the hypothesis that acidifying diets cause loss of bone mineral and osteoporosis. (13, 14, 15)

So, it appears that neither physiology nor clinical trials support the acid-ash hypothesis of osteoporosis. But again, just for argument's sake, let's suppose that these trials are imperfect (which they are, of course; no science is perfect!), and thus we can't depend on their conclusions. If the acid-ash hypothesis of osteoporosis were true, we would expect to see an association between net acid-producing diets and osteoporosis in observational studies. Yet again, this is not the case.

Observational studies do not support the acid-ash hypothesis of osteoporosis

Observational studies have not found a correlation between dietary acid load and bone mineral density (BMD) or fracture risk, nor have they found a correlation between urine pH and BMD or fracture risk. (16, 17, 18) Additionally, higher protein intakes are correlated with better bone health in multiple studies, even though high-protein diets are generally net acid forming. (19) In fact, animal protein in particular (the most acid-forming food of all) has been associated with better bone health. (20, 21) Imagine that! One study included in a recent meta-analysis did find an association between higher protein intake and greater risk for fracture (22), but compared to the numerous more recent studies showing the opposite, this evidence isn't very strong. Overall, the acid-ash hypothesis of osteoporosis is not supported by physiology, clinical trials, or observational data.

Hopefully I've given you a decent understanding of how our bodies handle pH balance, and have reassured you that you don't need to worry about the acidity of your urine with regards to bone health. Stay tuned for part two where I'll tackle some of the other claims of the alkaline diet!

Part 2

In Part 1 of this series, I talked about why the basic premise of the acid-alkaline theory is flawed, and I showed that the evidence doesn't support the idea that a net acid-forming diet is harmful to bone health. Now I want to look at the effect of dietary acid load on other health conditions.

Can the acidity or alkalinity of your diet affect your risk for muscle loss, cancer, and more?

Muscle wasting

There is some research claiming that acid-forming diets cause muscle wasting, and the proposed mechanism is similar to that of the acid-ash hypothesis of osteoporosis. Some researchers hypothesize that in order to eliminate excess acid and maintain homeostasis, the kidneys must steal amino acids from muscle tissue. (1, 2) Just as a higher acid load increases calcium in the urine, it also increases nitrogen in the urine, leading some to believe that an acid-forming diet causes net nitrogen loss. However, some of these studies neglect to measure nitrogen balance, so this is not necessarily true. (3, 4) In fact, one study showed that a higher acid diet improved nitrogen balance! (5) This theory also does not acknowledge that protein, although it's acid forming, actually increases the body's ability to excrete acid. (6) Finally, the one observational study concluding that alkaline diets improve lean muscle mass didn't even measure the overall acid load of the diet. (7) Instead, they used potassium intake as an approximate measure, and just assumed that the observed improvement in muscle mass was due to the diet being more alkaline. This, in addition to the limitations that always accompany observational data, makes the evidence less than convincing, especially since the clinical trials have conflicting results.

Cancer

One of the more popular claims of the alkaline diet is that it can cure cancer. Proponents say that because cancer can only grow in an acidic environment, a net-alkaline diet can prevent cancer cells from growing, and can eliminate existing cancer cells. This theory is incorrect for a few reasons. First of all, the hypothesis depends on the ability of food to substantially change the pH of the blood and extracellular fluid, which I've already shown is not the case. (8, 9, 10) Second, cancer is perfectly capable of growing in an alkaline environment. The pH of normal body tissue is 7.4, which is slightly alkaline, and in almost every experiment done with cancer cells, they are grown in an environment at that pH. (11)

Now, cancer cells do tend to grow better in an acidic environment, but the causality is reversed. Once a tumor develops, it creates its own acidic environment through up-regulated glycolysis and reduced circulation, so the pH of the patient's blood no longer determines the pH of the cancer. (12) It's not the acidic environment that causes the cancer; it's the cancer that causes the acidic environment. To top it all off, the only comprehensive review on 'diet-induced' acidosis and cancer did not even acknowledge this as a valid mechanism by which an acid-forming diet could increase cancer risk. They discuss a few biological pathways that could potentially link dietary acid load and cancer, but they admit that it's mostly speculation and there's no direct link. (13)

Other effects

There are a few observational studies attempting to link acid-forming diets with hypertension, but the results are mixed. (14, 15) There's also limited observational data associating higher acid loads with things like high cholesterol, obesity, and insulin resistance, but there are no proposed mechanisms or clinical studies to validate the hypotheses. (16, 17)

There are a few review papers examining the effect of acid-forming diets and health, but as you've seen above, the evidence they have to review is sparse. (18, 19, 20, 21, 22) If you read these papers, you'll notice that whenever they cite trials showing the deleterious effects of acidosis, those trials were done on patients with chronic kidney disease or diabetes-induced acidosis. In the studies done on healthy people, they're given ammonium chloride to induce acidosis. What you won't see are clinical trials showing health consequences from purely 'diet-induced' acidosis. (Perhaps because 'diet-induced' acidosis doesn't exist!) You'll also notice that the strongest two hypotheses deal with osteoporosis and muscle wasting, and that links with other diseases are speculative or based on observational data. And although conflicts of interest don't necessarily mean their conclusion can't be trusted, it's interesting to note that one of these reviews was funded by "pH Sciences®," which "develops and manufactures patent-protected ingredients that safely and effectively manage biological pH levels." (23)

In sum, I am not convinced that an acid-forming diet has negative effects on healthy people, based on the science. But just to be sure, it's always a good idea to observe healthy cultures to see if there's any anthropological evidence to support or refute the hypothesis.

Evolutionary data

There are a few studies where researchers attempted to approximate the net acid load of Paleolithic diets. One estimated that 87% of pre-agricultural people ate net-alkaline diets, and proposed this discrepancy with our modern diets as a possible reason for our declining health. (24) However, a more recent study estimated that only half of the world's hunter-gatherer societies eat net-alkaline diets, while the other half are net acid-forming. (25) They reason that the other estimate is likely accurate for our earlier ancestors, because their tropical habitat would've provided ample fruits and vegetables. This idea is confirmed by another analysis that showed increasing acid load with increasing latitude. (26) Even without the study, it stands to reason that as humans moved into less hospitable environments, the animal content (and acid load) of their diet increased.

Given the subpar clinical science on this topic, I think the evolutionary argument is far more convincing. If half of the world's hunter-gatherer populations avoid the 'diseases of civilization' on an acid-forming diet, it would seem that acid load has little to no bearing on overall health. For some case studies, we can always look to Weston Price's work to see quite clearly that acid-forming diets are not detrimental to health. Based on Price's descriptions, many of the traditional diets he studied would have been primarily acid-forming, including the Swiss, the Masai, and the Inuit. Yet despite their high intake of animal foods or grains and their comparatively low intake of fruits and vegetables, they maintained excellent health.

Conclusion

I don't deny that many people have seen significant health improvements when switching to an alkaline diet, but there are many possible reasons for this not having to do with pH balance. Eating more fresh produce is rarely a bad idea, especially when it displaces nutrient poor processed foods. A person switching to an alkaline diet would significantly reduce their consumption of grains, which could cause dramatic health improvements for somebody with a leaky gut or gluten sensitivity. Dairy would also be minimized, which would help those with dairy sensitivities. And although pure sugar isn't an acid-forming nutrient, many laypeople claim that it is, so alkaline diets tend to contain far less sugar than a standard Western diet.

Between the scientific evidence (or lack thereof) and the anthropological research, I think we can be confident that the acid load of our diets doesn't negatively impact healthy people. For those with renal failure or similar conditions that affect kidney function, it's a different story - there's certainly room for manipulation of urine pH in the treatment of those conditions. But for someone with functioning kidneys, there should be no concern that an acid-forming diet will harm health.