Inulin vs Psyllium husk: Which is better for constipation?

Let's start with the mechanism of action of each. 

Psyllium husk:

This is the main component of many fiber powder based brands (Metamucil powder, Pinch, etc.) 

Psylium husk is a soluble, viscous, non-fermentable fiber. It absorbs water and forms a gel-like mass in the intestines. You can already see the gel forming as you start to mix it into water. The gel created by psyllium adds bulk, stimulating the intestines to contract and move waste along. This softens stool, making it easier to pass. 

Inulin: 

Figure 1. Examples of inulin. 

This is the main component of many gummy fiber brands (Metamucil gummies, Target) that aim to "improve regularity" as well as Olipop (what I'd say is the healthier pop alternative to Coke). 

Inulin is a type of fiber found in vegetables, fruits, and whole grains. Unlike other foods, inulin isn't broken down in the stomach or small intestine. Instead, it travel all the way to the large intestine, where it become food for gut bacteria.

When gut bacteria ferment (or break down) these fibers, they produce gas (which is why some people get bloated) and help increase the number of bacteria in the gut. Since bacteria hold a lot of water, this makes stools softer and easier to pass.

To me, the mechanism of action for inulin sounds like more of a round about way to improve constipation as opposed to psyllium husk. Which is why, anecdotally, I feel it does not work as well. 

Does the evidence support this?  As you can imagine, there is not much incentive (money) for people to run clinical trials comparing psyllium husk to inulin and assess for improvement in constipation. 

Luckily, I did find this one study from 2022. Lancaster et al assessed the effects of different fibers on the microbiome. This included a comparision of arabinoxylan (AX, common in whole grains and the hemicellulose of other plants and found in a high amount in psyllium husk) to long chain inulin (LCI). Stool consistency as measured by increase in Bristol stool score improved in a statistically significant manner (p < 0.05) compared to no change for LCI (Figure 2). There were also larger effect sizes for AX compared to LCI for stool frequency increase over time, and stool volume increase. Important to note that there was a statistically significant increase in bloating with LCI compared to AX, as well as increase in flatulence with both fibers but greater effect size with LCI. This also supports the claim the hypothesis that inulin is readily fermentable and can lead to more bloating. 

Figure 2. From supplementary section of 2022 paper by Lancaster et al. Compare the red line (AX, commonly found in psyllium husk) and blue line (inulin) in various clinical parameters (Bloating, bowel movements per day, Bristol Stool Score, etc). 

Compared to inulin, psyllium husk does have its downsides. Many complain about the texture and it's gelatinous nature (good for poop, perhaps worse for texture and palatability). It also can lead to bloating. This side effect is usually mitigated with gradually increasing the dosage. Inulin, however, can also lead to bloating (and perhaps more of it). 

Based on the mechanism of action, anecdotal evidence, as well as the available evidence, if you are struggling with constipation, I would generally recommend psyllium husk over inulin based products.