What the gut is made of, what it does every day, what lives in it, and why fiber is an important part of a daily diet.
We usually only think about our gut when something goes wrong: bloating, irregular bowel movements, or feeling heavy after a meal. However, the gut works without interruption. It digests food, absorbs water and nutrients, and tens of trillions of bacteria live in the large intestine.
In this article, we rely on scientific reviews, European Food Safety Authority (EFSA) documents, and National Health Service (NHS) recommendations. You will find the sources at the end of the article, and they are marked in the text with numbers, for example [1]. We write more about the role of fiber in the article "Fiber for the Gut".
What is the gut?
The gut is the longest part of the digestive system. It starts behind the stomach and ends at the anus. The small and large intestines together are about 5 meters long, with the small intestine making up about two-thirds of this length [1]. Length varies significantly between individuals, so these figures are averages.
Even more interesting is the internal surface area of the gut. Textbooks used to state that it was equivalent to a tennis court, but measurements taken in 2014 showed that the total surface area of the digestive tract mucosa is about 32 m², roughly half of a badminton court [1]. The greatest area is provided by the folds, villi, and microvilli of the small intestine.
Small intestine
Most digestion takes place in the small intestine. Here, pancreatic juices and bile, together with intestinal juices, finish breaking down proteins, carbohydrates, and fats, and the intestinal wall absorbs water and nutrients [10]. The small intestine has three parts: the duodenum, the jejunum, and the ileum.
Large intestine
The large intestine receives what the body has not digested, primarily fiber. Water is absorbed here and stool is formed, while bacteria help break down the remaining nutrients and produce vitamin K [10]. The surface area of the large intestine is much smaller than that of the small intestine, about 2 m² [1], but it is precisely where the majority of gut bacteria reside [2].
| Part | Size | Main role |
|---|---|---|
| Entire digestive tract | mucosal surface area about 32 m²; gut length about 5 m | Food breakdown, absorption, and elimination [1] |
| Small intestine | about two-thirds of gut length | Digestion and absorption of nutrients and water [1, 10] |
| Large intestine | mucosal surface area about 2 m² | Water absorption, stool formation, bacterial activity [1, 10] |
What does the gut do?
- Digests and absorbs. It extracts from food what is needed for energy and body functions [10].
- Absorbs water. Water is absorbed in both the small and large intestines [10].
- Eliminates waste. Stool is formed from undigested residue in the large intestine [10].
- Collaborates with the immune system. The intestinal wall contains many immune cells, and substances produced by bacteria interact with them [4].
- Exchanges signals with the nervous system. This is why stress is often felt in the stomach.
Gut microbiota: what lives in the gut?
The collective of microorganisms living in the gut, primarily bacteria, is called the gut microbiota. For a long time, it was repeated that there are 10 times more bacteria in the body than human cells. In 2016, researchers recalculated this number and found that the ratio is closer to 1:1 [2].
| Indicator | Estimate |
|---|---|
| Number of bacteria in a 70 kg man | about 38 trillion |
| Number of human cells | about 30 trillion |
| Ratio of bacteria to human cells | about 1.3:1 |
| Mass of all bacteria | about 0.2 kg |
| Bacteria in stomach and upper small intestine | 1,000–10,000 per milliliter |
| Bacteria in large intestine | about 90 billion per gram of content |
Researchers explain why the number of bacteria differs so much: the environment in the stomach is very acidic, and contents move through the small intestine quite quickly, making it difficult for bacteria to colonize. In the large intestine, contents move more slowly, and that is where the majority of bacteria are concentrated [2]. Each person's microbiota is unique and depends on diet, age, lifestyle, and other factors.
What happens when bacteria break down fiber?
Human enzymes do not break down most fiber, so it reaches the large intestine undigested. There, some of it is broken down by bacteria. This process is called fermentation, and it produces short-chain fatty acids: acetate, propionate, and butyrate. These three acids make up 90–95% of all short-chain fatty acids in the large intestine, and acetate accounts for more than half of their amount in stool [4].
- Butyrate is the primary energy source for cells in the large intestine wall [4].
- Propionate mostly travels to the liver, where it is used for glucose production [4].
- Acetate is the most abundant: it is found in the highest concentrations in both the large intestine content and the blood [3, 4].
A classic study, which examined the contents of the human large intestine and blood, shows where these acids are produced the most and how quickly they are absorbed [3].
| Location | Amount of short-chain fatty acids |
|---|---|
| End of the ileum | 13 mmol/kg of content |
| Cecum (beginning of large intestine) | 131 mmol/kg of content |
| Descending colon | 80 mmol/kg of content |
| Portal vein blood | 375 µmol/l |
| Peripheral blood | 79 µmol/l |
The amount increases sharply at the beginning of the large intestine, where bacteria most actively ferment fiber. Further down, it decreases because the acids are absorbed. In the same study, the pH at the beginning of the large intestine was 5.6, and in the descending colon, it was 6.6 [3]. Researchers also noted that the large intestine wall mostly takes up butyrate, and the liver takes up propionate [3].
What is a prebiotic from a scientific point of view?
The International Scientific Association for Probiotics and Prebiotics (ISAPP) defined a prebiotic in 2017 as a substance that is selectively utilized by host microorganisms and confers a health benefit [5]. It is important that according to this definition, the benefit must be proven by specific studies, so not every fiber is automatically a prebiotic. We discuss the difference between probiotics and prebiotics in more detail in the article "Probiotics and prebiotics: what is the difference?".
Important: these study data are about gut microbiota and fiber fermentation in general. They are not a description of the effects of the fruttberry product and do not replace a doctor's consultation.
How much fiber is needed per day?
EFSA considers 25 g of fiber per day to be an adequate amount for normal gut function in adults [6]. Most European countries recommend 25–35 g per day, but in reality, adult men in Europe consume an average of 18–24 g, and women 16–20 g [7].
| Indicator | Amount per day | Source |
|---|---|---|
| EFSA adequate intake for adults | 25 g | [6] |
| Most European countries recommend | 25–35 g | [7] |
| Recommendations for adult women | 25–32 g | [7] |
| Average consumption for men in Europe | 18–24 g | [7] |
| Average consumption for women in Europe | 16–20 g | [7] |
The British Dietetic Association notes that a variety of fiber in the diet is associated with greater microbiota diversity [8]. Therefore, not only quantity but also variety is important: vegetables, fruits, berries, legumes, whole grains, nuts, and seeds. We write about where to find fiber in the guide "Fiber: what is it and where is it abundant?".
Why do so many people not meet the recommendations? Usually, because the daily diet contains many refined products, and too few vegetables, legumes, and whole grains. The difference between the recommended and actual amount often reaches several or even more than ten grams per day [6, 7].
Daily habits that help the gut
- Fiber every day. Aim for at least 25 g per day from various plant-based foods [6].
- Increase the amount gradually. Suddenly increasing fiber intake can cause bloating and gas [8]. Increase every 3–4 days.
- Drink enough water. Fiber draws water into the gut, so drink more fluids when increasing your intake [8].
- Variety on your plate. The more diverse the plant-based food, the more diverse the fiber that reaches the large intestine [8].
- Movement. Regular physical activity also helps gut function.
- Sleep and calm eating. Eating slowly and chewing well makes it easier for the digestive system.
If you feel bloated after starting to eat more fiber, read the article "Bloating: common causes and daily habits".
When to see a doctor?
Occasional bloating or irregular bowel movements are usually related to diet and habits. However, the NHS points out symptoms that should always be evaluated by a doctor [9]:
- changes in bowel habits that are unusual for you (loose or hard stools);
- blood in your stool, which may be red or black;
- frequent urge to have a bowel movement even right after having one;
- abdominal pain or a lump in the abdomen;
- unexplained weight loss;
- severe fatigue or shortness of breath.
If your stool is black or dark red, or you have diarrhea with blood, the NHS advises seeking urgent medical attention [9].
Fiber for your daily life: fruttberry fiber mix
We created fruttberry out of a personal need, as we ourselves wanted a fiber mix that was pleasant to drink daily. If it is hard to get enough fiber from food every day, the mix can help supplement your daily intake.
Ingredients
- Psyllium husk is the basis of the mix. We write more about it in the article "Psyllium: what is it and how to use it?".
- Inulin from agave is a soluble fiber fermented in the gut.
- Freeze-dried strawberries (12%) and blackcurrants (8%) provide flavor and color from real berries, not from flavoring.
- Glucomannan from konjac tubers is another soluble fiber.
How much fiber per teaspoon?
| Indicator | Value |
|---|---|
| Fiber content per 100 g of mix | 77 g |
| Fiber per teaspoon (5 g) | about 3.86 g |
| Energy value per serving | 11 kcal |
In an independent study by the KTU Food Institute, our mix sample was found to contain as much as 84.4 g of fiber per 100 g.
The mix has no added sugar, gluten, lactose, or preservatives, and it is suitable for vegans. Mix one teaspoon in 200–250 ml of water and drink immediately, increasing the amount every 3–4 days. Since glucomannan and psyllium swell, always drink the mix with enough water: if not enough liquid is consumed, there is a risk of choking. For people who have difficulty swallowing, such fibers are not recommended.
The most convenient for home use is the 250 g pack, and for work and travel, 30 portions of 5 g.
All fruttberry dietary fibers in one place.
Frequently asked questions about the gut
How long is the human gut?
The small and large intestines together are about 5 m long, with the small intestine making up about two-thirds of this length [1]. Length varies between individuals.
Is it true that there are 10 times more bacteria in the body than human cells?
No. Newer calculations show that bacteria and human cells are similar in number, with about 1.3 bacteria per human cell [2].
What are short-chain fatty acids?
These are substances produced when large intestine bacteria ferment fiber. The main ones are acetate, propionate, and butyrate [4].
Why are bacteria most abundant in the large intestine?
The environment in the stomach is very acidic, and contents move through the small intestine quite quickly, so there are only 1,000–10,000 bacteria per milliliter there. In the large intestine, contents move more slowly, and there can be about 90 billion bacteria per gram of content [2].
Is all fiber fermented the same way?
No. Bacteria break down some fibers quickly, others slowly or only partially. Larger amounts of rapidly fermentable fibers can cause gas and bloating, especially in more sensitive individuals [8]. Therefore, it is beneficial to eat a variety of fibers and increase their intake gradually.
What food is good for the gut?
Diverse plant-based foods with fiber: vegetables, fruits, berries, whole grains, legumes, seeds. Also, enough water [6, 8].
Sources
- Helander HF, Fändriks L. Surface area of the digestive tract – revisited. Scandinavian Journal of Gastroenterology. 2014;49(6):681–689.
- Sender R, Fuchs S, Milo R. Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biology. 2016;14(8):e1002533.
- Cummings JH, Pomare EW, Branch WJ, Naylor CP, Macfarlane GT. Short chain fatty acids in human large intestine, portal, hepatic and venous blood. Gut. 1987;28(10):1221–1227.
- Ríos-Covián D, Ruas-Madiedo P, Margolles A et al. Intestinal Short Chain Fatty Acids and their Link with Diet and Human Health. Frontiers in Microbiology. 2016;7:185.
- Gibson GR, Hutkins R, Sanders ME et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nature Reviews Gastroenterology & Hepatology. 2017;14(8):491–502.
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for carbohydrates and dietary fibre. EFSA Journal. 2010;8(3):1462.
- Stephen AM, Champ MM-J, Cloran SJ et al. Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health. Nutrition Research Reviews. 2017;30(2):149–190.
- British Dietetic Association. Fibre: Food Fact Sheet. 2021.
- NHS. Symptoms of bowel cancer. Accessed 2026.
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Your Digestive System & How It Works. 2017.
The information provided is general in nature and does not replace a consultation with a doctor or dietitian. Scientific data is provided regarding fiber and nutrition in general and is not a description of the properties of a specific product. If you have chronic illnesses, are taking medication, are pregnant, or are breastfeeding, consult a healthcare professional before taking food supplements. A food supplement does not replace a varied and balanced diet.
