Grapefruit and oranges contain flavonoids, which have received much attention because of their ability to scavenge free radicals. American and Chinese researchers have now reported that one specific flavonoid, naringenin, has anti-cancer effects beyond that of an antioxidant.
The study, published in the February issue of the Journal of Nutritional Biochemistry (vol. 17, pp. 89-95) looked at the effect of naringenin on DNA repair in human prostate cancer cell cultures (cell line LNCaP).
DNA repair is an important factor in the prevention of cancer since it prevents the proliferation of mutations in the cells. The risk of prostate cancer, the most commonly diagnosed cancer amongst men in the US, increases with age. As men get older, their cells become more susceptible to oxidative stress, which in turns leads to increased DNA damage.
"Induction of DNA repair by naringenin may contribute to the cancer-preventive effects associated with an increased dietary intake of fruits containing flavonoids," wrote the scientists from UCLA and Sun Yatsen (Zhongshan) University.
The degree of repair was measured by the ratio between deoxyguanosine (dG) and 8-hydroxydeoxyguanosine (8-OH-dG). The former is found in normal DNA, while the latter is the product of hydroxyl radical attack - found in damaged DNA.
After 24 hours of exposure to 80 micromoles per liter of naringenin, the ratio of 8-OH-dG to dG had decreased by 24 per cent, showing that the flavonoid had stimulated DNA repair.
The naringenin is proposed to function by stimulating the so-called Base Excision Repair (BER) cellular mechanism that repairs DNA during the replication stage. This was supported by measurable and significant increases in two of the main enzymes in the BER pathway, DNA poly-beta and hOGG1.
Between two and 15 per cent of ingested flavonoids can be absorbed in the GI tract, highlighting their limiting bioavailability. A study published in the European Journal of Clinical Nutrition (Vol. 56, pp. 891-898) reported that plasma concentrations after eating flavonoid-rich foods is in the range 0.5 to 1 micromole per liter.
The scientists in the new study exposed the cell cultures to 80 micromoles per liter, a figure that is not possible physiologically. However, "we demonstrated that the intracellular concentration is 0.5 percent of the naringenin concentration in the medium," wrote the researchers.
"Therefore, the intracellular concentration at which these changes occur is in an achievable physiological range."
This study comes hot on the heels other research reporting the benefits of grapefruit. Israeli scientists showed 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).