We previously reported that garlic grown in a
selenium-fertilized medium (
selenium-enriched garlic) is superior to regular garlic in
mammary cancer prevention in an animal model (Nutr
Cancer 17, 279-286, 1992). The present study was designed to evaluate the nutritional bioavailability of
selenium from this garlic with use of two liver selenoenzymes as
biomarkers:
glutathione peroxidase and
type I 5'-deiodinase. Rats were fed a
selenium-deficient diet (0.01 ppm Se) from weaning for four weeks to deplete both
enzymes. They were then supplemented with nutritional levels of
selenium (0.1-0.5 ppm) in the form of
sodium selenite (positive control) or
selenium-enriched garlic. Our results showed that
selenium-enriched garlic was just as effective as
selenite in restoring the activity of both selenoenzymes. This was demonstrated in a time course repletion experiment as well as in a dose-response experiment. Thus the
selenium in
selenium-enriched garlic has potent nutritional and anticancer efficacy. The
type I 5'-deiodinase enzyme catalyzes the conversion of
thyroxine (T4) to 3,5,3'-triiodothyronine (T3) and is responsible for most of the circulating T3. Because
cancer chemoprevention by
selenium usually requires pharmacological levels of
selenium, we also examined the possible modulation of
type I 5'-deiodinase by long-term feeding of
selenium-enriched garlic at 3 ppm Se in the diet. The observation that a high intake of
selenium-enriched garlic did not affect
5'-deiodinase activity suggests that its
anticarcinogenic effect is unlikely to be mediated by an imbalance in the blood T4-to-T3 ratio.