Earlier this year, Spanish researchers unveiled coffee as a notable source of soluble fiber. Now, a team in Germany confirms the finding and shows that beneficial gut microbes can easily digest the coffee-bean fiber left in brewed liquid and extract its energy for their growth. Because the waste products of that digestion—also called fermentation—can repel some disease-causing bacteria, the new data suggest that coffee drinking might represent more of a benefit than a vice.
However, be forewarned: Coffee won't supply your recommended daily intake of fiber—almost 30 grams. Four cups of coffee provide only 7 percent of that. Two cup's worth does provide the soluble-fiber equivalent of an apple, banana, plum, or half-cup of cooked lentils. Unlike coffee, those other foods also provide substantial amounts of insoluble fiber, the type that serves as roughage to scour out the digestive plumbing. Still, coffee may provide a good start for accumulating the day's fiber needs.
For its new study, the German team filter-brewed coffee, freeze-dried the liquid, and then used chemical techniques to extract its microscopic fibers. Upon analysis, most of the fibers turned out to be indigestible polysaccharides. These are complex chains of sugar molecules that serve as building blocks of plants' cell walls.
To find out whether beneficial bacteria in the gut would consume and thrive on this fiber, the researchers isolated a mix of those bacteria from human feces and placed them into a fermentation vessel with a growth broth. These microbes possess enzymes that can, in the absence of oxygen, ferment energy-rich materials into organic acids. The researchers then added 1.5 milligrams of coffee fibers per milliliter of broth to some of the fermentation vessels.
In an upcoming issue of the Journal of Agricultural and Food Chemistry, the researchers report that gut-friendly bacteria belonging to groups called Bacteroides and Prevotella got a 60 percent growth boost during the 24 hours after encountering coffee fibers. Other beneficial bacteria showed no such growth spurt, indicating they didn't find the coffee fiber particularly appetizing.
The primary fermentation products of microbes dining on coffee fibers were short-chain fatty acids—notably acetic acid, propionic acid, and butyric acid. Previous research has indicated that such substances "play a key role in maintaining gut health," notes team leader Mirko Bunzel, who recently moved to the University of Minnesota.
"The most important message of this paper," he says, "is that coffee's dietary fiber is really a good [food] for gut microflora." In terms of health, he concludes, ingesting coffee's fiber is "a good thing."
Likely gut reactions
Bunzel's group identified many of the microbes fermenting coffee fiber as Bacteroides. These bacteria typically reside in the oxygenfree environment of the colon, where they help break down food and even supply some vitamins and other nutrients that the body can't make on its own. The most numerous of colon dwellers, the microbes also break down bile acids: compounds that the liver releases to absorb fats in the gut.
Bacteroides can even prove beneficial simply by edging out other germs. As the friendly bugs cover intestinal walls, they leave little room for harmful bacteria to colonize. By churning out acids, Bacteroides also lower the gut's pH to an acidity that many other germs can't abide.
"It's not really surprising that Bacteroides really like coffee's fibers, because they're the most common bacteria involved in polysaccharide breakdown," Bunzel told Science News Online.
Research by other investigators has been pointing to a host of health benefits from diets rich in polysaccharides. For instance, soluble dietary fiber improves insulin sensitivity—at least in animals with diabeteslike symptoms. Soluble polysaccharide fibers also form gels that make the gut feel full and a dieter less likely to overeat.
Animal and test-tube studies have both indicated that the butyric acid produced by many fiber-noshing microbes—including Bacteroides—decreases the risk that colon cancer cells will turn cancerous. Finally, animal studies have linked consumption of soluble fiber to a heightened ability of the gut to absorb calcium and of the body to transform the pigments in some consumed vegetables into vitamin A.
All of this should be good news to coffee drinkers. But will tea drinkers like me derive a similar benefit from our favorite beverage? That's unlikely, Bunzel says, because brewed tea doesn't seem to have fiber. However, he points to a paper that the Spanish group published last year in the American Journal of Enology and Viticulture that might offer an alternative to both tea and coffee: wine.
I prefer red wines, and that's the type that M. Elena Díaz-Rubio and Fulgencio Saura-Calixto identified as containing the most soluble polysaccharides. Indeed, the five types of red wine they tested boasted 3.5 to 5 times as much fiber as the three white vintages in the study. However, for perspective, even the most fiber-rich red contained only about three-quarters as much soluble fiber as an equivalent volume of filter-brewed coffee.
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University of Minnesota
Department of Food Science and Nutrition
225 Food Science and Nutrition
1334 Eckles Avenue
St. Paul, MN 55108-1038
M. Elena Díaz-Rubio and Fulgencio Saura-Calixto
Department of Metabolism and Nutrition
Instituto del Frio
Consejo Superior de Investigaciones Cientificas
C/José Antonio Novais
10 Ciudad Universitaria
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