Types of Laxatives
by Cameron Craw, ND
Among the various foods and medicines that one can ingest throughout each day, there is a special group that we classify as laxatives, meaning they increase the frequency and/or ease with which bowel movements occur. There are many different mechanisms by which this laxative effect can be achieved. Some of the more common laxative types are reviewed below.
Bulk-forming Laxatives
As the name suggests, bulk-forming laxatives increase the total volume of stool by virtue of the water absorbing capacity of soluble fiber. Soluble fiber's water absorbing capacity can be quite impressive. For example, chia seeds contain enough soluble fiber to hold up to 12 times their own weight in water (Das, 2017). The increase in total stool volume achieved by increasing soluble fiber intake provides the muscular lining the intestines something more voluminous to push against as it squeezes the bulk of a meal through the gastrointestinal (GI) tract. The added water content in the stool can also help to make stool more flexible which may make it easier to pass (functioning similarly to a "stool softener").
Bulk-forming laxatives tend to be quite safe and have the advantage of being available as whole food substances. Adding more water absorbing soluble fiber into one's diet will require a proportional increase in water intake to ensure one's stool is well hydrated! This is why bulk-forming laxatives are best administered concurrently with a large glass of water. Failing to hydrate with the use of a bulk-forming laxative can lead to bloating and carries the risk of bowel obstruction (Leung, 2011).
Osmotic Laxatives
Osmosis involves the drawing of fluid through a membrane by means of an "electrochemical gradient." It can be helpful to think of an electrochemical gradient as a force, like gravity. Imagine pouring fluid into a coffee filter (think membrane). With the help of gravity, the fluid is drawn through the coffee filter into the container below—similar to how an electrochemical gradient draws fluid across a membrane. The special thing about osmosis is that the electrochemical force that draws fluid across membranes will flip to the side of the membrane that has the higher electrochemical potential. This fact is used to our advantage with osmotic laxatives.
When it comes to the colon (large intestine), the osmotic membrane of interest is the inner lining of the colon itself. One of the colon's most important jobs is to reabsorb the fluid that was mixed into your food in the stomach and small intestine during digestion. For many different reasons, the colon's reabsorption of fluid can be too successful leading to dry, firm, hard to pass stools. By introducing a poorly absorbed electrochemical substance (A.K.A. osmotic laxative) into the GI tract, we can use the power of osmosis to draw more fluid across the colon's inner lining and into the stool; thereby, prevent stool from becoming too dry. This can both help to preserve stool volume (if there is soluble fiber to absorb the fluid) while also helping to reduce friction as the stool passes along the colon wall.
Stool Softeners
Stool softeners are also called emollient laxatives. Emollients are substances most often used in skin care in order to help soften the skin. In the case of an emollient laxative, what is being softened is the stool. This is achieved by making the stool more receptive to water by reducing the surface tension (the self-attracting/other-repelling tendency of fluids) of water and oil. Reduced surface tension leads to greater mixing of water and oil in the stool helping the stool stay hydrated and soft. Unlike most other laxatives, stool softeners can have a delayed onset and may take about 3 days to fully take effect.
Lubricant Laxatives
Lubricant laxatives coat the walls of the colon to reduce friction. In order for a lubricant laxative to work well, it must be poorly absorbed so that it can adequately coat the colon. Mineral oil is often used for this reason. As you may remember from Chemistry 101, oil and water do not mix well. This means a colon coated with mineral oil will not only provide lubrication, but will also have the effect of limiting water absorption through the wall of the colon due to its water repellent nature.
Stimulant Laxatives
Stimulant laxatives act on the nerves that control peristalsis (A.K.A. the intestinal muscle contractions that propel stool through the GI tract). By stimulating peristalsis, bowel movements tend to be more frequent and/or complete. Stimulant laxatives also increase intestinal secretions (Tack, 2009) leading to a more lubricated GI tract. Amazingly, stimulant laxatives are also capable of decreasing the absorption of water through the bowel wall (Leung, 2011) leading to more hydrated stool.
Because stimulant laxatives work more directly on neuromuscular function than other laxative types, they may succeed where other laxatives have failed. Stimulant laxatives are often more effective because they are more aggressive in their mechanism of action. This carries some risk of negative long-term effects. For example, stimulant laxatives are often irritating to the GI tract and can lead to tolerance (meaning they require higher and higher doses to achieve the same result). For these reasons, stimulant laxatives are not usually a doctor's first choice and are generally best used for short periods of time.
References
1. Das A. Advances in Chia Seed Research. Adv. Biotechnol. Microbiol. 2018;5:5–7. doi: 10.19080/AIBM.2017.05.555662.
2. Leung L, Riutta T, Kotecha J, Rosser W. Chronic constipation: an evidence-based review. J Am Board Fam Med. 2011;24(4):436-451. doi:10.3122/jabfm.2011.04.100272
3. Tack J, Müller-Lissner S. Treatment of chronic constipation: current pharmacologic approaches and future directions. Clin Gastroenterol Hepatol. 2009;7(5):502-496. doi:10.1016/j.cgh.2008.12.006
Cameron Craw, ND, is Hive Mind Medicine’s resident for the 2023/24 academic cycle. He received his doctorate in Naturopathic medicine at the National University of Natural Medicine (NUNM) in 2023. He is currently completing a master’s degree in acupuncture. Prior to graduation, Dr. Craw spent three terms on Dr. Sandberg-Lewis's Gastrointestinal Mentor Rotation and is now helping Dr. SS-L supervise the shift.
Hive Mind Medicine blog posts are for educational purposes only and are not intended as medical advice. Please consult with your health care practitioner for personalized guidance.