Short answer: Take the water out of your poop and about 55% of what is left is gut bacteria. You carry about 38 trillion of them, and they work for you. Antibiotics and some packet-food preservatives can knock them down. Regular exercise helps them.
In this article
- About 55% of the dry part of stool is bacteria.
- You carry about 38 trillion gut bacteria, about 200 g, roughly 1.3 for each of your own cells.
- They turn fibre you cannot digest into short-chain fatty acids your body uses.
- One antibiotic course changed the gut within 3 to 4 days, and it did not fully come back.
- Exercise 3 days a week for 6 weeks raised the fuel-making bacteria in lean adults; the gain faded when they stopped.
How many bacteria live in you?
Take the water out of stool and weigh what is left. In a careful study of nine healthy adults, about 55% of that dry part was bacteria. Not leftover food: bacteria.
A 2016 recount put the number at about 38 trillion bacteria in a 70 kg man, about 1.3 for every one of his own cells. Most live in the large gut. All together they weigh about 200 g. The paper says older figures of 1 to 3 kg of bacteria should be replaced.
Many of them are alive when they leave. When stool bacteria were sorted one by one, 49% were intact, 19% were injured and 32% were dead. So the bacteria you pass each day are a living part of you, not just waste.
They work for you
Your gut bacteria eat the fibre your own body cannot digest. They turn it into short-chain fatty acids, which your body takes in and uses for energy. That is why fibre from dal, vegetables and whole grains feeds them, and why they are worth looking after.
Antibiotics hit them hard
Antibiotics kill bacteria, and they do not pick only the bad ones. A Stanford team followed three people for ten months through two courses of the antibiotic ciprofloxacin. The gut bacteria lost variety and changed within 3 to 4 days of starting. They began to return about a week after each course, but the return was often incomplete, and the mix settled in a new state.
When you need an antibiotic, take it as your doctor prescribes and finish the course. Do not ask for one for every cold. If you are not sure whether you need one, ask your doctor.
Packet food and preservatives
Preservatives are added to packet food to stop germs from growing. Three common ones are sodium benzoate (E211), sodium nitrite (E250) and potassium sorbate (E202). In a lab test, they slowed the growth of bacteria taken from the human gut, and some of the friendly ones were the most sensitive.
In a second study, mice carrying human gut bacteria drank these three preservatives from birth, at the most that people in Europe are thought to eat in a day. Their gut bacteria shifted in an unhealthy direction. This result came from mice, not from people.
One honest note: these are lab and mouse results. No study in people has yet shown the same. Still, if packet food is a daily habit, your gut bacteria may be getting a small dose of germ killer every day. Read the label, and keep fresh, home-cooked food as the base.
Exercise helps them
In a 6-week study of 32 adults who had not been exercising, everyone did supervised endurance training 3 days a week, building from 30 to 60 minutes. In lean adults, the bacteria and genes that make short-chain fatty acids went up, and so did the acids themselves. The rise was not seen in adults with obesity.
Then they stopped for 6 weeks, and the changes mostly went away. So the gain lasts only as long as the habit does. A physician supervised exercise intervention, 3 days a week, keeps the habit going safely. Physician supervised, exercise based cardiac rehab is built on exactly this kind of steady, regular session.
What to do
Eat fibre every day. Take antibiotics only when your doctor prescribes them. Read packet labels and keep packet food as the exception.
Exercise on most days. If you have heart disease, diabetes or high blood pressure, start with your doctor. The runner’s side of this story is on the Telangana Runners blog: running and gut bacteria.
Your questions, answered
Is it true that half of poop is bacteria?
For the dry part, yes. In healthy adults on a measured diet, bacteria made up about 55% of the solids left once the water was removed (Stephen 1980). Fresh stool is mostly water, so the share of the whole wet stool is smaller.
How much do my gut bacteria weigh?
About 200 g in a 70 kg man, by the 2016 recount (Sender 2016). Older books said 1 to 3 kg, which the authors say should be replaced. That is still about 38 trillion bacteria, a little more than the number of your own cells.
Do antibiotics harm good gut bacteria?
Yes, they can. In one study, a single course of ciprofloxacin changed the gut bacteria within 3 to 4 days, and after a second course the gut did not fully return to how it was (Dethlefsen 2011). Take them when they are needed, as your doctor prescribes.
Does exercise improve gut bacteria?
In one 6-week study, supervised exercise 3 days a week raised the bacteria that make short-chain fatty acids in lean adults. The change faded after 6 weeks without exercise (Allen 2018). Regular, lasting exercise is what counts, so pick a plan you can keep for months, not weeks.
Keep exercise regular, with your doctor
A physician supervised exercise intervention, 3 days a week, builds a habit that lasts. General advice: your own doctor knows your health.
info@cardiacrehab.com · www.CardiacRehab.com
Cardiac Rehab, 7B Journalist Colony, Film Nagar, Jubilee Hills, Hyderabad, Telangana 500096
References
- Stephen AM, Cummings JH. The microbial contribution to human faecal mass. J Med Microbiol. 1980. pubmed.ncbi.nlm.nih.gov/7359576
- Sender R, Fuchs S, Milo R. Revised estimates for the number of human and bacteria cells in the body. PLoS Biol. 2016. pubmed.ncbi.nlm.nih.gov/27541692
- Ben-Amor K, et al. Genetic diversity of viable, injured, and dead fecal bacteria assessed by fluorescence-activated cell sorting and 16S rRNA gene analysis. Appl Environ Microbiol. 2005. pubmed.ncbi.nlm.nih.gov/16085863
- den Besten G, et al. The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism. J Lipid Res. 2013. pubmed.ncbi.nlm.nih.gov/23821742
- Dethlefsen L, Relman DA. Incomplete recovery and individualized responses of the human distal gut microbiota to repeated antibiotic perturbation. PNAS. 2011. pubmed.ncbi.nlm.nih.gov/20847294
- Hrncirova L, et al. Human gut microbes are susceptible to antimicrobial food additives in vitro. Folia Microbiol. 2019. pubmed.ncbi.nlm.nih.gov/30656592
- Hrncirova L, et al. Food preservatives induce Proteobacteria dysbiosis in human-microbiota associated Nod2-deficient mice. Microorganisms. 2019. pubmed.ncbi.nlm.nih.gov/31548508
- Allen JM, et al. Exercise alters gut microbiota composition and function in lean and obese humans. Med Sci Sports Exerc. 2018. pubmed.ncbi.nlm.nih.gov/29166320