Page 24 - Science Focus (issue 15)

Myths and Facts about Farts

Question 1: “Loud Farts are Not Smelly, Smelly Farts are Silent”(「響屁不臭，臭屁不響」)?

Have you ever farted so loud that your classmates kept laughing at you, so you try to
defend yourselves by saying “It wouldn't be smelly”? Or you are taking a test where you didn’t hear
anything but suddenly smell your classmate’s silent “biochemical weapon”? In fact, loudness
and odor of the farts are independent of each other.
Loudness is determined by the amount of gases produced. People who fart loudly are
probably experiencing indigestion – undigested carbohydrates are fermented by the bacteria
in the gut, producing gases such as carbon dioxide and nitrogen. These gases are mostly odorless.
Most of the time, the odor in farts comes mainly from hydrogen sulfide, a gas with rotten egg smell [1].
Individuals who eat more dairy products and meat typically produce more hydrogen sulfide because
those foods contain proteins that are rich in sulfur. Interestingly, the hydrogen sulfide produced due to
large consumption of sulfur-containing foods is always smaller in volume compared with the carbon
dioxide produced by carbohydrate fermentation. Therefore, smelly farts usually build up less gas
pressure and are quieter. However, the farts can be both smelly and loud if a person is suffering from
indigestion but at the same time consumes too much proteins. Therefore, this saying is not totally
correct.

Question 2: Vegetarians Fart More?
In terms of frequency, this is correct. Vegetables and fruits are rich in dietary fiber. Speaking of
dietary fiber, there are many kinds apart from cellulose that you may have heard of. Non-digestible
1
oligosaccharides are a kind of dietary fiber that humans don’t have enzymes to digest, so they will
end up undigested in the large intestine. Interestingly, certain types of bacteria in the large intestine do
have the ability to digest these oligosaccharides, and the fermentation process does produce gas, so
it is possible that vegetarians generally fart more.
Are vegetarians’ farts smellier then? Apart from meat and dairy products, some vegetables, like
cabbage and broccoli, are also rich in sulfur [2]. Since odor depends on the amount of hydrogen
sulfide produced, if vegetarians eat a lot of vegetables that are rich in sulfur, their farts could be smelly
too. In other words, flatulence from vegetarians and omnivores can be equally smelly!
Question 3: Smelling Farts is Good for Health?

This saying sounds suspiciously like nonsense, but there are some interesting facts that led to the
perpetuation of this myth.
As mentioned above, hydrogen sulfide is one of the gases that gives the distinctive smell of farts.
Interestingly, extensive research suggested a range of beneficial effects of hydrogen sulfide when
it is made within a cell. Hydrogen sulfide can modulate reactions which occur in mitochondria to
provide protections to cells under certain conditions, such as high blood glucose caused by diabetes
[3]. Mitochondrion is often known as the “powerhouse” of cells because it is the key organelle of
energy production in a cell. Mitochondria also play a key role in regulating cell survival. Inspired
by the natural protective mechanism, scientists in the University of Exeter have identified a small
molecule, AP39, that can direct hydrogen sulfide to the mitochondria of the stressed cells, and
release the gas in a very slow manner [3]. This enables us to administer hydrogen sulfide from
outside the body and mimic the natural process.
Notably, the scientists remarked at end of their press release that they have never claimed that
sniffing hydrogen sulfide can bring any health benefits. This is because for the treatment to be
effective, the hydrogen sulfide must be delivered to the right cells at the right dose. Smelling farts
is likely not at all effective or beneficial as a form of disease treatment or prevention, because
we can neither lead the hydrogen sulfide to the mitochondria of the stressed cells nor
control the dose.
Next time when you fart, while having a laugh about it, you should also
pay attention to them. If you are constantly farting too much or your farts are
abnormally smelly, it may be warning signals from your body that warrant a visit
to the doctor’s office.

1 Oligosaccharide: A saccharide polymer that consists of 3-10
monosaccharides as monomers