Palm oil, unlike many other
dietary oils, does not increase the yield of chemically-induced mammary
tumors in rats when fed at high levels in the diet. This difference appears to be due to the
vitamin E fraction of
palm oil, which is rich in
tocotrienols, since
palm oil stripped of this fraction does increase
tumor yields. Experiments in our laboratory have shown that
tocotrienols inhibit proliferation and growth of both MDA-MB-435 and MCF-7 cells in culture much more effectively than a-
tocopherol. In addition, it was found that combinations of
tocotrienols with
Tamoxifen, a
drug widely used for treatment of
breast cancer, inhibit these cells more effectively than either
tocotrienols or
Tamoxifen alone. The present studies have now shown synergistic effects between
tocotrienols and a number of other
flavonoids from various plant sources, including citrus fruits, in the inhibition of both MDA-MB-435 and MCF-7 cells (IC50s 0.05-25 and 0.02-5 μg/mL respectively). In the MCF-7 cells, 1:1:1 combinations of
tocotrienols,
flavonoids and
Tamoxifen were even more effective, with the best combination being d-
tocotrienol,
hesperetin and
Tamoxifen (IC50 0.0005 μg/mL). These results suggest that diets containing
palm oil may reduce the risk of
breast cancer, particularly when eaten with other plant foods containing
flavonoids, and may also enhance the effectiveness of
Tamoxifen for treatment of
breast cancer.