Fructo-oligosaccharides (FOS) are starches that the human body cannot fully digest. Inulin and galacto-oligosaccharides (GOS) are similar substances also discussed in this article.

When a person consumes FOS, the undigested portions provide nourishment for bacteria in the digestive tract. “Friendly” bacteria ( ]]>probiotics]]> ) may respond particularly well to this nourishment. Because FOS feed probiotics, they are sometimes called a “prebiotic.”

Low doses of FOS are often provided along with probiotic supplements to aid their growth. High doses of FOS (and related substances) have been advocated for a variety of health conditions. However, currently, the available scientific evidence for benefit remains more negative than positive.



There is no daily requirement for FOS. FOS and related substances are found in asparagus, Jerusalem artichoke, leeks, onions, and soybeans, among other foods.

Therapeutic Dosages

When taken simply for promoting healthy bacteria, FOS are often taken at a dose of 4-6 g daily. When used for therapeutic purposes, the typical dose of FOS is 10-20 g daily, divided into three doses and taken with meals. Side effects are common at a daily intake 15 g or more (see Safety Issues]]> ).


Therapeutic Uses

Animal studies hint that FOS, GOS, and inulin can significantly improve cholesterol]]> profile; however, study outcomes in humans have been inconsistent at best. ]]>1-7]]> One study found that while inulin might produce a short-term benefit, any such benefit disappears after six months of use. ]]>18]]>

At most, it appears that FOS might improve cholesterol profiles by 5%, an amount too small to make much of a difference in most circumstances. These relatively poor results might be due to that fact that humans cannot tolerate doses of FOS much above 15 g daily without developing gastrointestinal side effects.

FOS has also been suggested for preventing ]]>traveler’s diarrhea]]> . However, in a large (244-participant) double-blind study, FOS at a dose of 10 g daily again offered only minimal benefits. ]]>8]]>]]>Probiotics]]> themselves might be a better bet.

Another study found that use of FOS might help reduce incidents of diarrhea, flatulence, and vomiting in preschoolers. ]]>16]]>

According to most studies, FOS at 10-20 g daily do not improve blood sugar control in people with type 2 ]]>diabetes]]> . ]]>9,10]]>

FOS have been advocated as a treatment for ]]>irritable bowel syndrome]]> . However, research results are currently inconsistent at best. For example, a 6-week, double-blind study of 105 people with mild irritable bowel syndrome compared 5 g of fructo-oligosaccharides daily against placebo, and returned conflicting results. ]]>19]]> According to some measures of symptom severity employed by the researchers, use of FOS led to an improvement in symptoms. However, according to other measures, FOS actually worsened symptoms. Conflicting results, though of a different kind, were also seen in a 12-week, double-blind, placebo-controlled study of 98 people. ]]>11]]> Treatment with FOS at a dose of 20 g daily initially worsened symptoms, but over time this negative effect wore off. At no time in the study were clear benefits seen, however. On a positive note, one study did find benefit with a combination prebiotic-probiotic formula, ]]>20]]> and another study found the combination beneficial for women with constipation when taken in yogurt. ]]>22]]>

Small double-blind studies found that FOS at a dose of 10 g daily may improve ]]>magnesium]]> absorption in postmenopausal women. ]]>12]]> Whether this is beneficial remains unclear, since magnesium deficiency is not believed to be a widespread problem. FOS may also slightly increase ]]>copper absorption]]> , but does not appear to affect absorption of ]]>calcium]]> , ]]>zinc]]> , or ]]>selenium]]> . ]]>13,14]]>

A randomized, placebo-controlled trial, involving 134 infants less than 6 months old whose parents suffered from allergies, found that those fed a prebiotic combination of FOS/GOS experienced a significant reduction in both allergy symptoms and minor infections that lasted at least through age 2. The researchers suggested that the favorable effects of prebiotics on intestinal bacteria early in life may produce lasting benefits to the immune system. ]]>21]]>

One study found that use of inulin promoted growth of probiotic bacteria in the bifidobacteria family. ]]>17]]>


Safety Issues

FOS appear to be generally safe. However, they can cause bloating, flatulence, and intestinal discomfort, especially when taken at doses of 15 g or higher daily. 6,7]]> People with lactose intolerance may particularly suffer from these side effects. ]]>15]]>