Are Kombucha and Other Fermented Foods Toxic Because of Their Aldehyde Content?

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In my last post, I addressed the first of two questions raised by a reader in response to a newsletter circulated by a representative of Biotics Research Canada, citing articles from Dr. Lawrence Wilson and Dr. Mark Hyman. In that post, I addressed the question of whether we should avoid animal protein in order to optimize an important health-related process known as methylation. In this post, I will address the question of whether we should avoid kombucha and other fermented foods because their content of chemicals known as aldehydes makes them toxic.

This idea comes from the article by Dr. Wilson, which states that fermented foods such as kombucha, kimchi, and most fermented grains contain aldehydes that are “quite toxic for the liver.”  Wilson allows, by contrast, yogurt, kefir, miso, and “most good quality cheeses” as “safe to eat in moderation.” Following the links in the article reveals that Wilson uses as a prototypical example the acetaldehyde content of kombucha.

Googling “kombucha acetaldehyde” without the quotes reveals that this concept has been discussed on many different websites and is quite controversial within the diverse array of alternative health communities, with more speculation and opinion being circulated than hard evidence.  I am afraid that hard evidence in this area is wanting; what I will provide in this post is not hard evidence, but rather some context and perspective that seems to be missing in the other posts I have read on this topic.

Dr. Wilson’s article links back to another article of his, “Kombucha Tea – Please Avoid This Poison,” which links in turn to another article of his, “Aldehyde Poisoning,” which contains the references he uses for his information about aldehydes. These references are primarily about aldehyde toxicity, not about the concentration of acetaldehyde in kombucha, nor about the degree to which acetaldehyde in kombucha is absorbed into the human body, nor on any negative health effects that directly result from drinking kombucha. Although I have not searched exhaustively, I did peruse the literature and I was unable to quickly locate anything that actually reports the concentration of acetaldehyde in kombucha, let alone the degree to which acetaldehyde is absorbed from kombucha into the human body. If you know of any such evidence, please post it in the comments.

In perusing the literature, I did find articles reporting the contents of ethanol (the alcohol we drink in alcoholic drinks) and acetic acid. These articles are easy to find by keyword searching, but can also be found listed in the Body Ecology article, “4 Surprising Reasons to Ditch Kombucha.”  I believe the likely reason that articles have reported the content of ethanol and acetic acid without reporting the content of acetaldehyde is probably because kombucha does not contain very much acetaldehyde, and this would be one reason it could be very hard to measure. And indeed, I would not expect kombucha to contain very much acetaldehyde. For the same reasons I would hold this expectation, I would also expect that for most people who consume kombucha in the manner in which it is typically consumed the acetaldehyde content is nothing to worry about.

Let us back up a moment and discuss what an aldehyde actually is. The picture below depicts acetaldehyde in two different ways. This will help us discuss aldehydes in general as well as acetaldehyde in particular.

The figure on the left emphasizes the bonding patterns of the specific atoms within acetaldehyde. The figure on the right emphasizes the arrangement of the atoms in space and realistically depicts the proportion of space taken up by each atom. The key feature that makes acetaldehyde an aldehyde is that one of the carbons (C) is double-bonded to oxygen (O) and on one side is only bound to a hydrogen atom (H). On the right, carbons are shown in black, hydrogens are shown in gray, and the oxygen is shown in red. This pattern is called an aldehyde group.

The reason that aldehydes have the potential for toxicity is because their aldehyde groups make them highly reactive, and while some of the reactions they engage in may actually be helpful to us, many of them may be harmful to us. Let us briefly consider why they are so reactive. All of these atoms are balanced by positive and negative charges. Positive and negative charges are attracted to one another and when they are fully balanced within a molecule the molecule tends to be relatively inert. When carbons and hydrogens are bound to one another, they share electrons and share those electrons relatively equally between one another.  By contrast, when carbon is bound to oxygen, oxygen holds on to the shared electrons much more tightly than carbon does. Electrons carry negative charges, so this makes the oxygen relatively more negatively charged and the carbon relatively more positively charged. Thus, most of the acetaldehyde molecule is inert but the separation of charge between the carbon and oxygen allows the relatively positively charged carbon to be strongly attracted to negative charges within other molecules.

Many molecules contain both carbon and oxygen and these include even fats, proteins, and carbohydrates. What makes the aldehyde group unique is that on one side there is only a hydrogen atom. You can see in the picture above on the right that hydrogen is the smallest atom. Were there any larger atom in its position, that larger atom would get in the way of any molecules that would want to react with acetaldehyde. Since the aldehyde group has nothing in the way except this measly hydrogen atom, it is free to react with any of the molecules to which it is attracted, regardless of whether such a reaction would be helpful for us or harmful to us.

For this reason acetaldehyde, and aldehydes in general, have a high potential for toxicity. But they are no more toxic to us than they are to the microorganisms that make them. These microorganisms, just like us, make aldehydes primarily as intermediates between one thing and another. These microorganisms, just like us, try to avoid these intermediate aldehydes from accumulating. If they did not, they too could suffer from the toxicity risk that aldehydes pose.

In kombucha, most of the acetaldehyde is made by acetobacter. Acetobacter makes it as an intermediate in the two-step conversion of ethanol to acetic acid. If one follows up any of the references on this matter listed in the Body Ecology posts that I linked to above, one will find that these sources describe kombucha as primarily an acetic acid ferment. Kombucha can become alcoholic, but the kombucha that is sold commercially in stores is consistently low enough in ethanol to be considered nonalcoholic. Home ferments can reach 1% or 2% ethanol, but this is significantly less alcoholic than what most people would consider alcoholic drinks. If someone ferments kombucha for too long, it becomes like vinegar because of its high acetic acid content; it does not become strongly alcoholic. Regardless, because acetaldehyde is toxic to acetobacter, the sum of ethanol and acetic acid will always greatly prevail over acetaldehyde, whose concentration will always be small.

We humans, just like acetobacter, convert ethanol in a two-step process first to acetaldehyde and then to acetic acid. Because acetaldehyde is toxic to us, we make it only as an intermediate and do not let its concentration accumulate. Most acetaldehyde in our bodies will be from the ethanol be consume.  If we experience a rise in the amount of acetaldehyde that passes through our bodies from drinking kombucha, it will most likely predominantly result from the ethanol in the kombucha rather than the acetaldehyde, simply because there is presumably much more ethanol in the kombucha then there is acetaldehyde. This risk, then, would be quite low when the kombucha is fermented in the typical manner that results in a low concentration of ethanol, and it would always be far lower with kombucha than with typical alcoholic drinks.

Some people are more vulnerable to this than others. It is thought that one reason many Asians have a low tolerance for alcohol is because they are more likely to be genetically predisposed to convert ethanol to acetaldehyde more quickly than they convert acetaldehyde to acetic acid.

But ethanol is hardly the only precursor to an aldehyde within the human body. In fact, the production of aldehyde intermediates is a universal feature of energy metabolism.

The bulk of our energy supply is generally met by utilizing fatty acids or carbohydrates for fuel, depending on our diet and whether we are in the fed state or the fasting state.  In order to burn carbohydrates for energy, we first split glucose in half to form glyceraldehyde. Glyceraldehyde is a potentially toxic aldehyde. We therefore quickly convert it to pyruvate, which is almost half of a glucose molecule but has the potentially dangerous aldehyde group removed.

When we burn fatty acids we make ketone bodies. These include acetone, which is the principal ingredient in paint thinner and nail polish remover and is responsible for “ketone breath.” When we are burning fatty acids, it is partly because the supply of glucose is low, and this generally means that we need to synthesize new glucose to meet the needs of red blood cells and certain cells within the brain that are unable to use fatty acids or ketone bodies. Although we primarily make glucose from amino acids during these conditions, we can also make it from acetone. Doing so requires us to make pyruvaldehyde (also known as methyloglyoxal) as an intermediate. Pyruvaldehyde is a potentially toxic aldehyde. We therefore quickly convert it to pyruvate, which we can use to make glucose.

For further information and references about these topics please see these other posts of mine.

I therefore agree that drinking kombucha could increase the flux of aldehydes through the body, but I do not see it as intrinsically any more dangerous than meeting our essential caloric needs by consuming carbohydrates and fat. The main problem is that in any of these pathways things can go wrong. When energy metabolism becomes dysfunctional, when we have deficiencies in the nutrients needed to support these pathways, or when we have genetic predispositions to tolerate certain dietary factors poorly, the aldehydes may accumulate instead of being quickly metabolized to other safer molecules. This could allow them to have toxic effects.

I personally see the practical implications of this material as follows:

• If you feel flushed and uncomfortable when you drink kombucha, or if drinking it gives you a hangover, I would avoid drinking kombucha and other even slightly alcoholic drinks. You may be producing acetaldehyde from the alcohol at a faster rate than you convert the acetaldehyde to acetic acid. Alternatively, if this is true but you nevertheless really enjoy kombucha, I would use it with a level of moderation that allows you to get enjoyment without risking negative effects that you don’t think are worth enduring in order to get that enjoyment. I do not see any difference between considering kombucha in this case, and considering other drinks that contain alcohol.
• If you quantify your performance on some health-related issue, athletic performance for example, and you find that drinking kombucha seems to hurt this performance, then I would impose a level of moderation on kombucha consumption that is needed to continue meeting your performance goals.
• If you currently drink kombucha, have no health problems, and feel that you are meeting all of your goals that relate in any way to the performance of your body, then I would continue to drink kombucha without dwelling on it any further.
• If you do not currently drink kombucha, and you’re thinking of starting, as long as you do not have a particular reason to avoid even slightly alcoholic drinks, I would try it without worrying too much about any of this.

It is easy to find information about the benefits of kombucha all over the Internet. I do not think any of this is invalidated by the concern about aldehydes, nor do I think it makes much sense to single out kombucha or other fermented foods for this concern. The bullet points above are not meant to suggest that there is anything particularly risky about drinking kombucha; everyone is different, however, so with anything it is always advisable to try to listen to your body and take into account how it responds.

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