Fructo-oligosaccharides (FOS) are non-digestible plant components that comprise of fructose residues linked by β(2-1), creating fructosyl to fructose chains of various lengths. Fructo-oligosaccharides are classed as fructans, and if the polymerisation increases further, inulin is formed. Because FOS are not digested during passage through the small intestine, they pass to the colon where the sugars are hydrolysed and fermented by gut bacteria. The resulting formation of short-chain fatty acids decreases the pH of the colon and the fatty acids are then absorbed and pass to the circulation. The lower pH of the colon is though to provide health benefits, one of which might be an increased absorption of certain minerals. Some studies have reported increases in whole body mineral content and mineral density with FOS supplementation and some studies have reported that mineral absorption is also increased. However, the evidence remains controversial as some negative findings have been reported.
For example, researchers1 have evaluated a moderate dose of fructo-oligosaccharides (9g/d) as part of a healthy controlled diet in adolescent girls in order to investigate the effects on calcium absorption. The study had a randomised double blind crossover design and the FOS was provided by a cereal that also contained calcium fortification (1500mg/d). The control group had an identical calcium fortified cereal but it contained no FOS. The study consisted of two 3-week periods separated by a 2-week washout period. The results showed that there was no difference between the calcium absorption or calcium retention between the groups. Calcium absorption was 67% in the treatment group and 66% in the control group, and calcium retention was 409mg/d in the treatment group and 464mg/d in the control group. Therefore FOS at 8mg/d did not improve calcium status in adolescent girls.
It is not clear why these results differ from previous studies where positive effects on mineral absorption have been observed. One possible reason for the discrepancy is the difference in the amount of calcium and FOS contained within the diet, which has varied slightly between studies. One study that found positive results used twice the concentration of FOS. Another possible explanation is that the benefits of FOS are blunted when taken as part of a cereal food, perhaps because other components in the fibre bind the minerals and prevent absorption. Because FOS has shown benefits in mineral absorption in previous studies, care should be taken before dismissing the findings in this study. However, these results do illustrate that the relationship between FOS and mineral absorption are not fully understood and more research is needed before guidelines can be given to induce benefits.
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