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Regardless of whether we hate it or find it very funny, farting from the intestine, commonly called “farting”, is part of our lives. They are a completely natural and normal part of the functioning of our digestive system and will always be present, no matter how much we try to control them. However, it is evident and well known that not all are equal. Some produce a distinctive sound and some do not, some are characteristically flammable and some are not, and most have different odors depending on our diet and state of intestinal health.
The above evidence leads us to conclude that not all farts (or flatulence, as doctors and more refined people call it) have the same chemical composition. This, in turn, leads us to wonder what this chemical composition is and what chemical substances are part of intestinal gases, giving them their characteristic and well-known properties.
In the following sections, we will discuss everything related to the chemistry of flatulence, its composition and the origin of the chemical substances that compose it .
The chemical composition of the average fart
As mentioned in the previous paragraphs, the fact that some farts are flammable and others are not, and that some farts smell very bad and others not so bad, are evidence of a difference in the chemical composition of these embarrassing gases. However, most flatulence contains the same class of components, although possibly in different proportions.
The average fart is made up of the following gaseous chemicals (concentrations are reported in terms of mole percent):
| Chemical substance | Concentration |
| Nitrogen ( N2 ) | 20-90% |
| Hydrogen ( H2 ) | 0-50% |
| Carbon dioxide (CO 2 ) | 10-30% |
| Oxygen ( O2 ) | 0-10% |
| Methane (CH 4 ) | 0-10% |
| Mercaptans (C x H 2x+1 SH) | <1% |
| Hydrogen Sulfide (SH 2 ) | <1% |
| volatile amines | <1% |
Not all farts contain all of the compounds mentioned above. For example, some do not contain methane, others do not contain hydrogen, and in all cases, the amount of nitrogen. oxygen and carbon dioxide, the main components of air, is highly variable.
The flammability of farts is due to the presence of methane and/or molecular hydrogen
Of all the main components of flatulence, methane and molecular hydrogen are the only ones that are flammable. In fact, it is due to one or both of these compounds that some farts are flammable. This flammability can be dangerous and, in some cases (very rare, by the way), have caused explosions during intestinal surgeries in which a hot soldering iron (soldering iron) has been used to cut the intestine.
Mercaptans are organic compounds so they are also flammable. However, his concentration in most farts is very low. For this reason, in the absence of hydrogen and methane, a fart is unlikely to be flammable, even if it has a relatively high concentration of mercaptans.
Mercaptans, hydrogen sulfide and some amines are responsible for the bad smell
One of the most noticeable (and unpleasant) characteristics of most flatulence is its foul odor. This is mainly due to the presence of mercaptans, produced by the breakdown of proteins present in food by some bacteria of the intestinal flora.
Mercaptans, also called thiols, are a family of organic chemical compounds very similar to alcohols, but instead of a hydroxyl group, they have a sulfhydryl or –SH group. They have the general formula C x H 2x+1 SH, where X represents the number of carbon atoms. For example, methanethiol or mercaptomethanol has the formula CH 3 SH and mercaptoethanol or ethanethiol has the formula C 2 H 7 SH. These compounds have a very intense fecal odor. We tend to produce more mercaptans when we eat sulfur-rich foods, such as some animal and plant proteins.
Additionally, hydrogen sulfide or H 2 S is the compound responsible for the smell of rotten eggs, and is also produced by many of the bacteria in the intestine.
On the other hand, depending on the diet, some gases may contain variable amounts of some volatile amines. Many of these compounds have very strong and characteristically unpleasant odors. For example, trimethylamine is responsible for the smell of decaying fish (rotten fish).
Most of the components are harmless and odorless
The other gaseous substances present in intestinal gases are inert and odorless substances. Nitrogen, oxygen, carbon dioxide are the main components of air, which normally does not have an odor. On the other hand, hydrogen and even methane, despite their chemical reactivity and flammability, are both completely odorless substances.
Origin of the components of flatulence
Since we know what intestinal gases contain, the next logical question is where do these gaseous chemicals come from? Doctors identify three main sources:
1 swallowed air
Part of the components of the farts corresponds to the same components of the air. The reason behind this is that part of the gases that enter our intestine and that we then expel come from small amounts of air that we ingest along with our food. This air is compressed in the colon due to the movements of the stool.
In addition, when ingesting carbonated drinks, they release large amounts of carbon dioxide upon contact with stomach acids. A part of these gases is usually released in the form of belching, but the rest passes to the intestine and then becomes part of the flatus.
2 Diffusion of gases from the blood
Another source of intestinal gases is the process of passive diffusion of gases from the bloodstream, driven by the difference between the partial pressures of these gases in the blood and in the lumen of the intestine, that is, the interior space of the intestine. The intestine is lined with countless villi irrigated by blood capillaries, whose function is to efficiently absorb the nutrients present in our food. This absorption process can occur passively as a consequence of the difference in the concentration of nutrients inside and outside the epithelial cells of the intestine. However, this same process can also occur in the opposite direction, especially with nonpolar gases such as nitrogen and carbon dioxide, which can cross the cell membrane passively,
As the concentration or partial pressure of these gases in the lumen of the intestine is lower than in the blood, a part of the aforementioned gases can diffuse from the blood, through the epithelial cells and into the interior of our intestine, accumulating and then desorbing to pass from the solution to the gaseous state.
food fermentation
Finally, three quarters of the total volume of intestinal gases in a healthy person come from bacterial fermentation of food and from the action of some endogenous glycoproteins present in the intestine. In fact, one of the main sources of bad odors in flatulence is fermentation.
Every healthy person has a gut flora made up of many different species of symbiotic bacteria that help break down and break down the large protein molecules in our food into more manageable chunks that are easier for the epithelial cells in our gut to absorb. Without this bacterial flora, we simply could not digest most of our food and would quickly become malnourished.
This bacterial spoilage process is called fermentation. There are different types of fermentation characteristic of different types of bacteria. Each type of fermentation produces different byproducts, some of which become part of the farts.
For example, most people have in their intestines a class of microorganisms called archaea, which are methanogenic species that carry out anaerobic fermentation that produces methane.
On the other hand, some of the bacteria break down sulfur-containing amino acids such as methionine and cysteine to produce the mercaptans responsible, in part, for the characteristic odor of some farts. The same can be said of hydrogen sulfide.
Finally, molecular hydrogen is also produced by bacterial fermentation. This flammable substance can be produced in large quantities when we eat some fruits that contain carbohydrates that we cannot digest, and these are not absorbed before reaching the large intestine. Most methanogenic microorganisms are also responsible for the production of hydrogen in the intestine. However, 10% of people have methanogens that do not release molecular hydrogen.
References
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International Clinic. (2021, April 13). What are intestinal bacteria and what is their importance? https://www.clinicainternacional.com.pe/blog/bacterias-intestinales-importancia/
Gotfried, J. (2022, April 4). Gas related disorders . MSD Manual Professional Edition. https://www.msdmanuals.com/en/professional/gastrointestinal-disorders/s%C3%ADntomas-of-gastrointestinal-disorders/gas-related-disorders
Helm Clark, CM (2017, September 30). Human Fart Gas Composition, Farting Habits and the Cause of Fart Odors . The Gnarly Science Blog. http://gnarlyscience.com/human-fart-gas-composition/
Marquez Moreno, MD (2013). Fish odor syndrome: trimethylaminuria . Active Training in Primary Care Pediatrics, 6(4). https://fapap.es/articulo/270/sindrome-del-olor-a-pescado-trimethylaminuria
Nemours Kids Health. (nd). What is a fart? (for Children) . https://kidshealth.org/es/kids/fart.html
