Soluble corn fiber, polydextrose and other added fibers: a Q&A

A snack bar with 12 grams of fiber used to mean a lot of bran. Today it might mean a fiber that was extracted or made separately and added as a powder. Those ingredients can be useful. They can also explain why a "healthy" snack leaves you uncomfortable.

If you've already learned to spot inulin and chicory root, these are the next names to know.

What counts as an added fiber?

Added fibers are isolated or manufactured fibers mixed into foods to raise the fiber count, replace sugar or improve texture. Common names on labels include:

• Soluble corn fiber, sometimes called soluble gluco fiber
• Polydextrose
• Resistant dextrin, wheat dextrin or resistant maltodextrin
• Galactooligosaccharides (GOS)
• Inulin, chicory root fiber and FOS
• Partially hydrolyzed guar gum (PHGG) and acacia fiber
• Oat fiber and cellulose

These isolated fibers can count toward the dietary fiber grams on the Nutrition Facts label. So a big fiber number doesn't tell you what kind of fiber you're getting.

Do they all affect the gut the same way?

No. Fibers differ in how quickly gut bacteria ferment them and how much they do so. Fast, intense fermentation is more likely to mean gas. That's why inulin can be a problem for a bloating-prone gut while some other fibers are easier to tolerate.

How well do people tolerate these fibers?

Soluble corn fiber. In a double-blind, placebo-controlled study in 20 healthy volunteers, a branded soluble corn fiber was well tolerated up to 40 g taken at once and 65 g spread across a day. In a companion test in 18 volunteers, 20 g of inulin already raised digestive symptom scores significantly, so the corn fiber was tolerated at about twice the amount of inulin (Housez et al., 2012).

Polydextrose. A review of nine clinical studies concluded that polydextrose was better tolerated than most other low-digestible carbohydrates, such as sugar alcohols. The authors attribute this to its larger molecules and only partial fermentation. Expert food safety committees put its average laxative threshold at about 90 g a day, or 50 g in a single dose (Flood et al., 2004).

Resistant or wheat dextrin. In a trial in 20 healthy men, a wheat-based resistant dextrin was well tolerated up to 45 g a day. At 30, 60 and 80 g a day, flatulence became more frequent than on placebo, and bloating increased at 80 g. None of the doses caused diarrhea (van den Heuvel et al., 2004). Worth noting: the study only included men, and the manufacturer commissioned it.

Those amounts sound huge. Do they apply to me?

Not directly. These tolerance studies were done in healthy volunteers, and a sensitive gut can react to much less. FODMAP researchers have also pointed out that adding prebiotic fibers to foods and supplements can push fermentable carb intake well above what a normal diet provides. In animal studies, rapid fermentation from high intakes caused problems such as gut barrier dysfunction and increased sensitivity, though whether that applies to people still needs study (Gibson et al., 2020).

The practical point: several "high-fiber" products in one day can add up quickly, even if each one seems modest.

Has any added fiber been studied for bloating?

PHGG has. In a randomized, double-blind trial in people with IBS, 108 patients had complete data. After 12 weeks, those taking 6 g of PHGG a day had a significantly greater improvement in their bloating score than those on placebo, and the effect lasted at least 4 weeks after stopping. PHGG didn't change other IBS symptoms or overall severity and quality-of-life scores, and it caused no significant side effects (Niv et al., 2016).

How should I use this at the store?

1. Read the ingredient list, not just the fiber grams.
2. Note which added fibers appear, and in how many products you eat daily.
3. If you're adding fiber on purpose, pick one type and start with a small amount.
4. Increase slowly over a couple of weeks and drink enough water.
5. If bloating gets worse, go back to the last amount that felt fine, or try a different fiber.

Gentle Fiber, for example, contains 6 g of Sunfiber PHGG per stick, with no inulin, psyllium or sugar.

When should I see a doctor?

If bloating is new and happens most days, especially with early fullness, pelvic pain or urinary urgency, or if you notice blood in your stool, unexplained weight loss or a lasting change in bowel habits, see your doctor before experimenting with fiber.

Sources

1. Housez B, Cazaubiel M, Vergara C, et al. Evaluation of digestive tolerance of a soluble corn fibre. J Hum Nutr Diet. 2012;25(5):488-96. PMID 22672058
2. Flood MT, Auerbach MH, Craig SA. A review of the clinical toleration studies of polydextrose in food. Food Chem Toxicol. 2004;42(9):1531-42. PMID 15234083
3. van den Heuvel EG, Wils D, Pasman WJ, Bakker M, Saniez MH, Kardinaal AF. Short-term digestive tolerance of different doses of NUTRIOSE FB, a food dextrin, in adult men. Eur J Clin Nutr. 2004;58(7):1046-55. PMID 15220947
4. Gibson PR, Halmos EP, Muir JG. Review article: FODMAPS, prebiotics and gut health: the FODMAP hypothesis revisited. Aliment Pharmacol Ther. 2020;52(2):233-246. PMID 32562590
5. Niv E, Halak A, Tiommny E, et al. Randomized clinical study: partially hydrolyzed guar gum (PHGG) versus placebo in the treatment of patients with irritable bowel syndrome. Nutr Metab (Lond). 2016;13:10. PMID 26855665

This article is for general information and isn't medical advice. If you have a digestive condition or take medication, check with your healthcare provider before changing your fiber intake.