Here is a bit of gut trivia: your small intestine is the long one. Small and large refer to width, not length, and the two do very different jobs. The small intestine absorbs most of the nutrients from your food, while the large intestine reclaims water, forms stool and is home to most of your gut bacteria.
The small intestine is the main site of nutrient absorption and is several metres long in adults. The large intestine is shorter but wider; it reclaims water and salts, forms stool and hosts most of the gut's bacteria, which ferment fibre the small intestine cannot digest.
What does the small intestine do?
The small intestine runs from the stomach to the large intestine. It is several metres long in adults, although reported measurements vary depending on the person and how it is measured [1]. It has three sections: the duodenum, where digestive juices from the pancreas and gallbladder join in, the jejunum and the ileum.
Its main job is absorption. Most nutrients from your food, including sugars, proteins, fats, vitamins and minerals, pass into the bloodstream here [2]. Its lining is folded into tiny projections called villi and microvilli, and researchers estimate the lining of the whole digestive tract has a surface area of roughly 30 square metres [3]. Food usually spends a few hours here, as our guide to gut transit time explains.
What does the large intestine do?
The large intestine is shorter, often around 1.5 metres although this varies, but wider [1]. It includes the caecum, the colon and the rectum. Its jobs include reclaiming water and salts, forming stool and hosting the bacteria that ferment fibre your body could not digest earlier. That fermentation produces short chain fatty acids, which may help nourish the cells lining the colon [4].
| Small intestine | Large intestine | |
|---|---|---|
| Rough length | Several metres [1] | Around 1.5 metres [1] |
| Main job | Absorbing nutrients [2] | Reclaiming water, forming stool, fermenting fibre [4] |
| Bacteria | Relatively few, rising further down [5] | Most of the gut's bacteria [5] |
| Pace | Usually hours | Often a day or more |
Figures are approximate and vary from person to person.
Where do your gut bacteria live?
Most of the bacteria in the digestive tract live in the colon, where slower movement and low oxygen conditions support a large and diverse community [5]. The stomach and upper small intestine host relatively few, partly because of acid and the speed at which things move. The exact make up varies between people and along different parts of the gut [5]. Our guides to a healthy gut microbiome and what is living in your gut explore that community in more depth.
What this means for you
Because the two sections do different jobs, they may respond to different habits. A varied diet supports the small intestine's absorbing work, while fibre from a range of plants feeds the bacteria further down. Some people aim for 30 different plants a week as one optional way to add variety; if you try it, build up gradually and choose foods you tolerate. Drinking enough water may also help the large intestine with its water balancing job, and our piece on probiotics and prebiotics covers the difference between the two.
What reaches the exit is largely the large intestine's handiwork, so it is the easiest part of the system to notice. Gutsi, a gut health tracker, is designed to help you notice patterns in frequency and consistency over time. It does not measure absorption or your microbiome, but it can help you learn your personal baseline.
References
- Hounnou, G., et al. (2002). Anatomical study of the length of the human intestine. Surgical and Radiologic Anatomy, 24(5), 290-294. https://doi.org/10.1007/s00276-002-0057-y
- Kiela, P. R., & Ghishan, F. K. (2016). Physiology of intestinal absorption and secretion. Best Practice & Research Clinical Gastroenterology, 30(2), 145-159. https://doi.org/10.1016/j.bpg.2016.02.007
- Helander, H. F., & Fändriks, L. (2014). Surface area of the digestive tract, revisited. Scandinavian Journal of Gastroenterology, 49(6), 681-689. https://doi.org/10.3109/00365521.2014.898326
- den Besten, G., et al. (2013). The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism. Journal of Lipid Research, 54(9), 2325-2340. https://doi.org/10.1194/jlr.R036012
- Donaldson, G. P., Lee, S. M., & Mazmanian, S. K. (2016). Gut biogeography of the bacterial microbiota. Nature Reviews Microbiology, 14(1), 20-32. https://doi.org/10.1038/nrmicro3552
Gutsi provides lifestyle and wellness insights only. It is not intended to diagnose, treat, cure or prevent any disease, and should not be used as a substitute for professional medical advice.
