- High fiber diets have many health benefits, including reducing the risk of heart attack and stroke.
- Many different types of dietary fiber can be eaten as part of a varied diet or as dietary supplements.
- Different fibers may have different effects on our gut microbiome.
- A new study suggests that using targeted dietary fibers may benefit health.
Fiber is an essential part of our diets. Otherwise known as roughage, it is the indigestible part of plant foods that helps reduce the risk of health conditions such as heart disease and type 2 diabetes.
There are two types of fiber, both of which are non-starch polysaccharides that people cannot digest:
- Insoluble fiber provides bulk to the diet and moves waste through the body, keeping the gut healthy and helping prevent constipation.
- Soluble fiber forms a gel-like substance that is broken down by gut bacteria. It can lower cholesterol levels and help regulate blood sugar levels.
But not all dietary fibers are equal. A new study published in Cell Host & Microbe has found that health benefits vary between individuals and may depend on the type of fiber, the dose consumed, and the individual’s microbiome.
A small, crossover trial
Researchers from Stanford School of Medicine tested how two purified soluble fibers — arabinoxylan (AX) and long-chain inulin (LCI) — affected a group of 18 participants.
AX is found in whole grains, such as rye, wheat, oats, and rice; LCI is found in onions, chicory root, garlic, and Jerusalem artichokes. Both types of fiber can also be taken as dietary supplements.
The participants in the study had an average age of 56.9 years. Of the 8 men and 10 women, 14 had overweight or obese, and 11 were insulin sensitive. The researchers separated them randomly into 2 groups for three crossover trials. One group started with AX, the other with LCI then switched over. Both groups finished with a mixture of fibers consisting of AX, LCI, acacia gum, glucomannans, and resistant starch.
Each trial lasted 3 weeks. In the first week, the participants consumed 10g of fiber per day, rising to 20g in the second week and 30g in the third. The participants then had a 6-8 week break between the 3 trials.
“This is a VERY small study of 18 participants who are ‘free-living’ — meaning their food is not being controlled — so between the food and the sample size, it’s extremely difficult to draw meaningful conclusions. Like almost all good research I read on the microbiome, this raises as many questions as it answers.”
– Kate Cohen, M.S., R.D.N., of the Ellison Institute for Transformative Medicine at Providence Saint John’s Health Center in Santa Monica, CA, speaking to Medical News Today.
Measurements
The researchers collected plasma, serum, and stool samples from all participants at the start of the trial, then at the end of each week. They also measured their heart rate and blood pressure.
They measured changes in lipids, including cholesterol, the genetic material in the stool samples (to identify gut bacteria), plasma proteins, metabolites, and cytokines. Cytokines are inflammatory markers indicating inflammation in the body.
Different fibers, different effects
When taking AX, most participants had a significant drop in low-density lipoprotein (LDL), or ‘bad’ cholesterol, and an increase in bile acids. The authors suggest that the increase in bile acids may contribute to the reduction in LDL. However, some participants saw no change in LDL levels.
For LCI, most, but not all, people saw a small decrease in inflammatory markers and an increase in Bifidobacterium. This gut microbe is generally regarded as beneficial to gut health. However, the highest dose of LCI (30g per day) reversed this effect. At this dose, participants saw increased inflammation and elevation in alanine aminotransferase, an enzyme associated with liver damage.
Mixed fiber supplementation yielded fewer significant changes.
The authors note that responses were not consistent for all people for either type of fiber, suggesting that each person’s microbiome may determine responses.
“Our results demonstrate that the physiological, microbial and molecular effects of individual fibers differ substantially.”
– Dr. Michael Snyder, senior study author, said in a press release.
Kate Cohen was excited to see where the authors would go next: “Uncovering how different fibers interact with the microbiome is an essential step toward making personalized nutrition a reality. This research is also laying the groundwork for using food-as-medicine in a truly prescriptive way. This study confirms once again that the microbiome holds enormous potential for understanding human health.”
The current recommended fiber intake is 14 grams for every 1,000 calories consumed, according to the American Academy of Nutrition.
Experts say it is best to get your fiber from food sources before using supplements.
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