Plenty of foods are labeled ‘superfoods’, but fiber is as close to the real deal as it gets. In fact, fiber is one of the main reasons why plant food is so good for you, and there’s growing evidence that fiber reduces your risk of several chronic diseases, including heart disease, stroke, diabetes, and several types of bowel cancer. But the health benefits of fiber may vary from fiber to fiber, and from individual to individual.

Fiber is essentially a type of carbohydrate, but in human nutrition, it’s not regarded as a carbohydrate because the human body can’t normally digest it. But even if the fiber just passes through the body undigested, it can help regulate the body’s use of sugar, keeping both our hunger cravings and our body’s sugar usage in check, and reducing cholesterol.

But it gets even more interesting: fibers can be selectively metabolized by gut microbes. In other words, they are fermented in the colon by some beneficial bacteria. Researchers suspect that this process is helpful to the lining of the colon, which may be why fiber reduces the risk of colon cancer. In addition, the environment that results from this fermentation may help the body fight pathogens.

But chemically, fibers can be quite different from one another. Some are long, some are short; some are soluble in water, some are not; some branch out, and some don’t. It seems plausible then, that they also have different effects on the human body. To put that to the test, a team of researchers led by Samuel Lancaster and Brittany Lee-McMullen at Stanford set out to understand how different fiber components affect the same group of participants.

The team, which also included corresponding author Michael Snyder, a geneticist at Stanford School of Medicine, selected two common and structurally distinct soluble fibers: arabinoxylan (which is common in whole grains and psyllium husks), and long-chain inulin, (which is found in onions, garlic, asparagus, and bananas).

“We know that fiber is good for you but they are all quite different. Reports about the benefits of specific fibers have been highly conflicting,” Snyder told ZME Science. “We thought it was likely due to the fact that people were testing different mixtures that did not contain purified fibers. Therefore, we decided to test two highly purified fibers. The two that were selected, arabinoxylan and inulin, are probably the two most popular fibers consumed—they are quite common.”

The study, which was carried out on a relatively small sample size of 18 participants, found surprising results. The participants were given 10 grams of fiber per day during the first week, 20 grams per day during the second week, and 30 grams per day during the third week. The results were pretty surprising.

Despite what previous work suggested, arabinoxylan had no effect on glucose levels, but it did reduce bad cholesterol. It most likely does this through the production of bile acids and by supporting the microbiome, Snyder told ZME Science. Meanwhile, inulin produced a modest decrease in inflammation and caused an increase in the abundance of Bifidobacterium (“good” bacteria in the gut), but had no effect on either cholesterol or glucose on average — in fact, for a few people in the highest fiber intake group, it even caused a bit of inflammation. However, these were just the averages. Perhaps the most striking find was that different people reacted in different ways to the fiber intake.

“There were some people who did not respond to arabinoxylan but did have their cholesterol lowered with inulin — so we are all different,” Snyder explained, commenting on the findings. “It definitely varies from person to person, but if you are trying to lower your cholesterol—go with arabinoxylan foods as a first start,” the researcher added.

There are significant limitations to this study, the researchers note; one is the reduced number of participants, the other is the relatively short time over which participants were tracked. But the study does suggest that the benefits of fiber are dependent both on the type of fiber and the differences in the human body. So, if you’re trying to increase your fiber intake (which you almost certainly should), it’s probably best to keep track of what effect it has on your body.

“After starting fiber for a particular goal–lowering cholesterol or glucose, be sure to see if it has the desired effect. What is good for someone else may not be good for you,” Snyder concludes

The study was published in Cell Host & Microbe. Journal Reference: Lancaster and Lee-McMullen et al. “Global, distinctive, and personal changes in molecular and microbial profiles by specific fibers in humans,” https://www.cell.com/cell-host-microbe/fulltext/S1931-3128(22)00166-4