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Fiber supplements are known for a range of benefits, including the improvement of bowel regularity, regulation of healthy blood sugar levels, and reduction of appetite. Another amazing benefit of some fibers is the ability to favorably alter the balance of gut bacteria. A recent study published in the journal Microbiome analyzed the ability of two types of fiber—polydextrose and soluble corn fiber—to alter the gut bacteria of a group of healthy men.1

Twenty healthy men eating an average of 14 grams of daily fiber were given snack bars containing 21 grams of polydextrose fiber, bars containing 21 grams of soluble corn fiber, or bars with no fiber for 21 days. Stool samples were collected and analyzed using a method of DNA analysis called 454 pyrosequencing, which examines the full range of genetic information to avoid missing certain bacterial groups that sometimes go undetected using 16S DNA sequencing.

The researchers found that those participants who ate bars supplemented with polydextrose or soluble corn fiber had increased amounts of Bacteroidetes and decreased amounts of Firmicutes. These changes have also been found in studies evaluating the effects of weight loss and obesity on the gut microbiota.2

“This was of particular interest to us because other research has shown that having more Bacteroidetes may be beneficial because the higher that proportion is, the individual tends to be leaner,” noted Hannah Holscher, PhD, RD. “With higher Firmicutes, that individual tends to be more obese. It’s an exciting shift and helps to drive researchers to study these fibers as part of a weight-loss diet.”

After the fiber was discontinued the bacteria levels went back to their previous levels, suggesting that fiber supplementation needs to be continued to maintain healthy changes in gut bacteria. And of course, the researchers recommend that a healthy diet, high in fibrous foods like fruits, vegetables, and legumes be eaten daily to fully support a healthy balance of gut bacteria.

Soluble corn fiber has been found to have prebiotic effects in a previous study by the same research group.3 Other soluble fibers are also known for their prebiotic effects. Acacia, inulin, and oligofructose fibers also favorably alter the bacteria in the gut.4,5 Soluble fibers are rich in polysaccharides that act as food for the good gut bacteria, helping to increase their populations and produce beneficial compounds such as short chain fatty acids (SCFAs), which act as fuel for the cells that line the intestine.

Some soluble fibers, such as the ones in this study and acacia fiber, are better tolerated because they cause less gas, bloating, and abdominal discomfort than inulin-type fibers. In addition to the SCFAs feeding the gut lining, they also sit on receptors of the white blood cells and modulate immunity in a very beneficial and profound way.6


  1. Holscher HD, Caporaso JG, Hooda S, et al., “Fiber supplementation influences phylogenetic structure and functional capacity of the human intestinal microbiome: follow-up of a randomized controlled trial.” Am J Clin Nutr. January 2015;ajcn.092064.
  2. Ley RE, Turnbaugh PJ, Klein S, et al., “Microbial ecology: human gut microbes associated with obesity.” Nature. 2006 Dec 21;444(7122):1022-3.
  3. Hooda S, Boler BM, Serao MC, et al., “454 pyrosequencing reveals a shift in fecal microbiota of healthy adult men consuming polydextrose or soluble corn fiber.” J Nutr. 2012 Jul;142(7):1259–65.
  4. Cherbut C, Michel C, Virginie R, at al., “Acacia gum is a bifidogenic dietary fibre with high digestive tolerance in healthy humans.” Microbiol Ecol Health Dis. 2003; 15(1):43–50.
  5. Kolida S, Tuohy K, and Gibson GR, “Prebiotic effects of inulin and oligofructose.” Br J Nutr. 2002 May;87 Suppl 2:S193–7.
  6. Maslowski KM and Mackay CR, “Diet, gut microbiota and immune responses.” Nat Immunol. 2011 Jan;12(1):5–9.