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Health benefits of fructo-oligosaccharides

Robert W Welch
BSc PhD FIFST RNutr
Senior Lecturer in Nutrition and Food Science
Northern Ireland Centre for Diet and Health
School of Biomedical Sciences
University of Ulster
Coleraine

Fructo-oligosaccharides (FOS) are indigestible carbohydrates found naturally in a number of plant foods, including artichokes, asparagus, bananas and onions.(1) FOS are also found in chicory, and are extracted from this source for use as a food ingredient.(1,2) There is increasing evidence that FOS confer a number of specific health benefits (Table 1). Thus FOS may be considered a functional food component, and foods that contain FOS may be considered functional foods.(2)

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What are fructo-oligosaccharides?
Oligosaccharides are molecules composed of a few (Greek "oligos") sugar units (saccharides). In fructo- oligosaccharides, the major sugar unit is fructose (fruit sugar), although many FOS also contain glucose.(3)
The FOS found in food sources is also called inulin, whereas FOS isolated from these sources or synthesised for ingredient use is also known as oligofructose.(2)  Fructan is an alternative term for FOS.
FOS have from 2 to 65 sugar units per molecule, linked by b(2fi1) bonds.(2) These bonds cannot be broken down by the secretions of the digestive tract. Thus, like dietary fibre, FOS are indigestible. However, FOS are water-soluble, and the molecules are much smaller than the polysaccharide molecules that mainly comprise dietary fibre. The advent of suitable methods for FOS analysis and the recognition that FOS can exert beneficial effects have led recently to the proposal that FOS should be considered part of dietary fibre.(3,4)

Probiotics and prebiotics
Although FOS are indigestible, they are completely degraded by the bacteria in the large intestine.(2) Currently our understanding of the role that this often-neglected organ can play in health is being reappraised.
It is estimated that human adults have over one kilogram of bacteria in the large intestine.(5) Thus these bacteria are extremely numerous and represent over 90% of the cells found in the body. Furthermore, these bacteria account for over 60% of faecal solids.(5) Thus they are being produced continually, in large quantities, using undigested dietary residues such as fibre components as a substrate.
There are at least 500 different bacterial species in the large intestine.(5) Most of these bacteria are benign. Indeed, those such as the bifidobacteria and lactobacilli, which produce lactic acid, may be beneficial to health. These are known as probiotic bacteria.(5) They are provided in a range of so-called bioyogurts that are on the market. However, it is not clear whether the probiotic bacteria in these yogurts can survive the passage through the stomach and small intestine in sufficient numbers for them to compete successfully with the bacteria that reside in the large intestine.(5)
An alternative approach is to use prebiotics. Prebiotics are indigestible diet components that act as substrates for, and thus enhance the growth of, these beneficial bacteria.(5) FOS are the most widely evaluated and most effective prebiotic diet components.

FOS and gut health
Perhaps the most obvious beneficial effect of FOS is on stool weight and laxation. Human studies have shown that faecal weight is increased by 1.5-2g per gram of FOS in the diet, with increases in stool frequency and relief of constipation.(2)
A prebiotic effect for FOS has been shown in a number of studies where doses of FOS ranging from 4g to 40g per day increase the beneficial bifidobacteria tenfold, with concomitant decreases in bacteroides and clostridia in some instances.(2,6) These changes in the gut microbial population may confer a number of benefits, including an enhanced resistance to pathogenic bacteria and an increase in endogenous vitamin formation.(2,5,6) Other associated effects such as decreased pH, carcinogen inactivation and the induction of protective enzymes may, in the longer term, decrease the risk of colon cancer.(2,7,8)

Other beneficial effects of FOS
Although the data are not entirely consistent, doses of FOS of 15-40g per day have been shown to increase the absorption of calcium and magnesium in humans.(2,9,10) Animal studies show that FOS can enhance the uptake of calcium, magnesium and iron, and increase bone density and bone mineral content.(2,9) FOS may thus help to reduce the risk of osteoporosis. The increased mineral uptake appears to occur in the large intestine, and may be mediated by the decreased pH, enhanced mucosal conditions or by stimulation of mineral binding proteins.(9) Effects of FOS on plasma lipids have been studied using doses ranging from 8g to 52g per day. Although some studies have shown reductions in total and LDL-cholesterol and triglycerides, the results were inconsistent.(2,11) Thus further rigorously designed trials are needed to evaluate the potential effects of FOS on these heart disease risk factors.

Perspective
In 1900, scientists had just begun the work that was to lead to the discovery of the vitamins and their role in the prevention of deficiency diseases. Now, after long neglect, scientists are increasingly turning their attention to the indigestible dietary constituents. This work is showing that indigestible components such as FOS can play significant roles in the prevention or alleviation of a number of disease states common in Western industrialised regions. However, currently the intake of FOS from food sources in these regions is modest, ranging from 1g to 10g per head per day.(1) Comparisons with the intakes shown to give beneficial effects indicate that FOS intake is likely to be suboptimal for a large proportion of the population.

References

  1. Van Loo J, Coussement P, Deleenheer L, Hoebregs H, Smits G. On the presence of inulin and oligofructose as natural ingredients in the western diet. Crit Rev Fd Sci Nutr 1995;35:525-52.
  2. Van Loo J, Cummings J, Delzenne N, et al. Functional food properties of non-digestible oligosaccharides: a consensus report from the ENDO ­project (DGXII AIRII-CT94-1095). Br J Nutr 1999;81:121-32.
  3. Prosky L. Inulin and oligofructose are part of the dietary fibre complex. J AOAC Int 1999;82:223-6.
  4. Flamm G, Glinsmann W, Kritchevsky D, Prosky L, Roberfroid M. Inulin and oligofructose as dietary fiber: A review of the evidence. Crit Rev Fd Sci Nutr 2001;41:353-62.
  5. Gibson GR, Williams CM. Gut fermentation and health advantages: myth or reality? Br J Nutr 1999;81:83-4.
  6. Roberfroid M. Fructo-oligosaccharide malabsorption: benefit for gastrointestinal functions. Curr Opin Gastroenterol 2000;16:173-7.
  7. Jenkins DJA, Kendall CWC, Vuksan V. Inulin, oligofructose and intestinal function. J Nutr 1999;129:1431S-3.
  8. Wollowski I, Rechkemmer G, Pool-Zobel BL. Protective role of probiotics and prebiotics in colon cancer. Am J Clin Nutr 2001;73(suppl):451S-5.
  9. Scholz-Ahrens K, Schaafsma G, van den Heuvel, EG, Schrezenmeir J. Effects of prebiotics on mineral metabolism. Am J Clin Nutr 2001;73(suppl):459S-64.
  10. Tahiri M, Tressol JC, Arnaud J, et al. Five week intake of short-chain fructo-oligosaccharides increases ­intestinal absorption and status of magnesium in postmenopausal women. J Bone Miner Res 2001;16:2152-60.
  11. Delzenne NM, Kok N. Effects of fructans-type prebiotics on lipid metabolism. Am J Clin Nutr 2001;73(suppl):456S-8.