"This observation may be valuable because flavonoids may prevent various diseases associated with the Western-type diet and should be an integral ingredient of a common diet," wrote lead author Zenon Zdunczyk from the Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences.
Grapefruit contain flavonoids, which have received much attention because of their ability to scavenge free radicals. Recently, American and Chinese researchers reported that one specific flavonoid, naringenin, has anti-cancer effects beyond that of an antioxidant (Journal of Nutritional Biochemistry, Vol. 17, pp. 89-95).
Another study from Israeli scientists reported that eating a red grapefruit daily could lower blood cholesterol by 15 per cent (Journal of Agricultural and Food Chemistry published on-line, doi:10.1021/jf058171g).
However, the researchers behind the new study, published on-line in the journal Nutrition (doi: 10.1016/j.nut.2006.05.010), report that various polyphenols both inhibit and stimulate bacterial growth and thus affect metabolism.
The researchers, from Poland and the Instituto de Fermentationes Industriales in Spain, supplemented the diets of male Wistar rats with 0.3 per cent flavonoid, extracted from grapefruit, or 5 or 10 per cent inulin, or a combination of both supplements.
The metabolism in the large bowel was based on the following measurements: bulk effect, pH, microbial enzyme activity and short chain fatty acid (SCFA) production.
After 28 days, supplementation of the standard diet with grapefruit flavonoids only resulted in an increase in pH, indicating more alkaline conditions. Improved gut health is associated with lower pH, said the researchers.
Supplementation with grapefruit flavonoids only also decreased the activity of certain microbial enzymes, notably the beta-glucosidase and beta-a and alpha-galactosidases.
Addition of inulin to the flavonoid decreased pH in the large bowel, increased the activity of the three microbial enzymes, and normalized the water content of digested mass.
While flavonoid supplement was associated with an increase in SCFA production, mostly acetic acid, no change in production was observed when inulin was included with the grapefruit extract.
"In our study, the flavonoid extract from grapefruit used as a single dietary supplement did not display a beneficial influence on cecal metabolism, increased pH, hydration of digesta, and lowered bacterial activity and SCFA concentration.
"However, our results suggested that the simultaneous intake of inulin and polyphenols could reduce the detrimental effects of the latter on cecal fermentation," said the researchers.
The mechanism behind the effect of flavonoids and their metabolites is not well understood, said the researchers, and it is clear that significant additional research.
It should also be stressed that this was an animal study and the effect observed may not directly translate to the human intestinal microflora.
Despite these limitations, the researchers concluded: "Our results suggested that simultaneous intake of inulin and polyphenols can decrease the detrimental effects of the latter on cecal fermentations."