The current body of research literature around the health impacts of different types of dietary fibers largely deals with whole-diet interventions, whereas the impact of purified fibers on the gut microbiome has been tested in a limited number of human intervention studies, noted researchers of the study, which was funded by the National Institutes of Health and the National Center for Complementary and Integrative Health and published in the journal Cell Host & Microbe.
"They are usually studied as complex mixtures from their plant source," said senior study author Michael Snyder, a geneticist at Stanford School of Medicine.
"There is a need for determining the unadulterated effects of individual fibers on the microbiome and for establishing associated health biomarkers, ideally by testing different fibers on the same individuals."
Therefore, researchers set out to study individual "purified" dietary fibers -- which are diverse in a number of properties including length, branching, and solubility -- and their particular impact on the gut microbiome.
Studying two distinct dietary fibers
"Fiber is associated with improved metabolic and cardiovascular health, but understanding the effects of individual fibers on microbial and metabolomic response has not been studied using a multiomics dataset," noted Snyder.
To do this, researchers investigated two structurally distinct soluble fibers: arabinoxylan (AX), commonly found in whole grains, and long-chain inulin (LCI), found in onions, chicory root, and Jerusalem artichokes.
Over the course of the three weeks, 18 study participants (average age of 56.9 years) who were randomized into two groups, consumed 10g of one of the fiber supplements (either AX or LCI) during the first week, 20g per day during the second week, and 30g per day during the third and final week.
Researchers used stool metagenomics, plasma proteomics, metabolomics, lipidomics, and analyzed serum cytokines and clinical values to evaluate the health effects of dietary fiber intake of all 18 participants.
Clear differences in physiological responses to arabinoxylan and inulin
Initial results revealed clear differences in the physiological responses based on the type of fiber and dose given.
On average, AX consumption was associated with a significant reduction in low-density lipoprotein (LDL), known as the 'bad cholesterol', and an increase in bile acids, which may be contributing to the cholesterol reduction. Yet individual responses varied, and some participants saw little to no change in cholesterol levels, researchers noted.
Meanwhile, LCI was associated with a modest decrease in inflammation markers and an increase in the abundance of Bifidobacterium -- a generally beneficial type of gut microbe known to produce healthy short-chain fatty acids.
However, at the highest dose of LCI, there was an increase in inflammation and levels of a liver enzyme called alanine aminotransferase, suggesting that too much of this fiber may be harmful. Again, these potentially negative responses were variable across the participants, added researchers.
New possibilities for personalized nutrition
While researchers did comment on some limitations, including the study's short duration and limited number of participants, the results could lead to some exciting prospects in the growing area of personalized nutrition, noted Snyder.
"Our results demonstrate that the physiological, microbial, and molecular effects of individual fibers differ substantially," said Snyder.
"Further, our results demonstrate the tantalizing prospect of using targeted fibers, mediated by the microbiome, to drive health and systems biology in a predictable, personalized direction."
According to researchers, the study provides specific insights into mechanisms behind fiber-induced cholesterol reduction, reveals potentially deleterious effects of high inulin consumption, and highlights the association of individual, purified fibers with the microbiome.
"Overall, our findings show that the benefits of fiber are dependent on fiber type, dose, and participant -- a landscape of factors resulting from interactions between fiber, the gut microbiome, and host. These results have important implications in personalized response and interventions," added Snyder.
BENEO: 'The design of the study has several shortcomings'
BENEO, a global ingredient supplier and manufacturer of Orafti Inulin, took issue with the study design, and its conclusion that high doses of inulin could cause inflammation and liver damage.
In a statement to FoodNavigator-USA, Anke Sentko, vice president regulatory affairs and nutrition communication at BENEO, commented: "BENEO is of the opinion that the design of the study has several shortcomings, some of them already mentioned by the authors themselves, i.e. the short duration and the low number of participants. This should not be the basis for strong conclusion.
"The conclusion of increase in risk of liver damage, which is based on observed changes of three individual participants, is not justified."
Sentko argued that the changes some participants experienced could be explained by other factors such as extreme weight changes during the study intervention, the presence of insulin resistance, and high background cholesterol and LDL levels.
"In BENEO’s view, there is thus no proof for a causative effect of long chain inulin on inflammation and liver damage, even at extremely high dosages. We therefore think the study findings are overinterpreted."
Sentko added that inulin is an established ingredient with proven and studied health benefits.
"We know, for example, that it plays a role in selectively promoting the growth of good bacteria, contributing to a healthy gut in general and also supporting a person’s inner defense system.
"Increase in Bifidobacteria in human intervention studies is seen from a daily intake of 3 to 5g per day, the support of regularity is demonstrated for an intake of 12g/d, the improvement of calcium absorption is demonstrated at an intake level of 8g/d."
Source: Cell Host & Microbe
Global, distinctive, and personal changes in molecular and microbial profiles by specific fibers in humans
Authors: Samuel M. Lancaster, et al.