Fructooligosaccharides (FOS) is a type of oligosaccharide. As a natural ingredient, it exists in onions, garlic, chicory and other foods. It was discovered by scientists in 1950 and was named kestose oligosaccharide.  In 1990, it was confirmed that FOS can be fermented by bifidobacteria.  Japan successfully carried out the industrial production of FOS for the first time after the 1990s. As the understanding of the impact of intestinal flora on health deepens, the study of oligosaccharides has increasingly attracted the interest of scientists.

Dry fructooligosaccharide is a white or slightly yellow powder, which is easily soluble in water and appears as a colorless or light yellow, transparent and viscous liquid. The sweetness of FOS is 30% to 60% of sucrose, its taste is refreshing and purer than sucrose, and its water retention is higher than sucrose. The caloric value of FOS is very low, with an average of 7.8kJ/g, less than half of ordinary carbohydrates. As a non-reducing sugar, FOS does not undergo the Maillard reaction. When pH>3 and heated to 130°C, FOS properties are very stable; when in an acidic environment with pH<3, it is stable at low temperatures and easily decomposes at high temperatures.

Main food sources.

FOS, especially high molecular weight inulin, naturally exists in the roots, tubers and fruits of Asteraceae, Amaryllidaceae, Liliaceae, Gramineae and other plants. The main foods include: rye, wheat, barley, oats and vegetables such as onions, leeks, asparagus, garlic, endive, lettuce, tomatoes, and fruits such as bananas. Among them, onions have the highest FOS content, accounting for 25% to 40% of dry weight. The FOS content in garlic and chicory accounts for 25% to 35% and 15% to 20% of their dry weight respectively.

FOS can improve intestinal flora and relieve constipation.

Human intervention trials of FOS have shown that FOS can be utilized and fermented by bifidobacteria, lactic acid bacteria, etc. in the colon, thereby significantly stimulating the production of colonic probiotics and improving the intestinal microecological environment. The short-chain fatty acids produced by FOS fermentation are an important material basis for its functions of laxative, enhancing intestinal immunity, and preventing colitis.

FOS is a water-soluble dietary fiber with a small molecular weight, which can absorb water and make stool thinner; and the short-chain fatty acids produced by fermentation of intestinal flora reduce the pH value of the colon, stimulate colon movement, and thus promote defecation. The results of the study found that infants had good tolerance to the cereal diet supplemented with 3g/d fructooligosaccharides. The average number of defecation times per day was 1.99, which was more than that of the control group supplemented with maltodextrin. The feces were softer and smoother, and there was no occurrence of diarrhea.

Absorption and metabolism.

FOS can hardly be broken down by sucrase and maltase, so it can hardly be digested and absorbed in the human stomach and small intestine. Instead, it directly enters the colon and is fermented by intestinal flora. Therefore, it has a lower energy value and is also a component of dietary fiber. The main metabolites after FOS fermentation are lactic acid and volatile short-chain fatty acids, including butyric acid, acetic acid, propionic acid, etc., and also produce H2 and CO2. Some short-chain fatty acids are taken up and utilized by colon epithelial cells, while the rest enter the liver and peripheral tissues, are metabolized to produce CO2 and water, and are excreted through exhalation, urine, sweat glands, etc. Human experiments show that when the FOS intake is ≤15g/d, almost all of it is fermented in the colon. Therefore, when the intake is greater than this, part of it will be excreted through the feces.

Specific recommendations and tolerable maximum intake levels.

In 2000, the U.S. FDA included FOS in the GRAS list (Generally Recognized as Safe Substances), and set the amount of FOS added in dairy products, beverages, confectionery, pastries, meat products, etc., up to 15.4%. The maximum amount allowed in recipes given by the European Union is 15%. Japan allows FOS to be used in health foods with a content of up to 37.5%. China allows FOS to be used in milk powder for infants and pregnant women, with the total amount not exceeding 6.45%; the amount of FOS used in approved health foods ranges from 4 to 30g/d.

Since FOS is basically not absorbed by the human intestine, it is easy to produce intestinal symptoms of intolerance. Some studies have reported that healthy adults will not develop any intolerance symptoms when their intake is ≤15g/d.