Selenium is an important trace mineral in humans, which is incorporated into a number of selenoproteins such as glutathione peroxidase, iodothyronine deiodinase and thioredoxin reductase (here). It is known that supplementing with 200 µg of selenium a day reduces the risk of developing all types of cancer by roughly 50 %. However, the exact metabolic fate and tissue distribution of dietary selenium remains unclear. To help elucidate the absorption, distribution and excretion of selenium, researchers fed healthy subjects beef and rice diets containing 47 µg per day of selenium, while they were housed in a metabolic research unit1. After 21 days the subjects diets were changed to either a high selenium diet (297 µg per day), or a low selenium diet (14 µg per day) for the remaining 99 days. During this time the selenium status of the subjects was monitored.
The pre-study selenium intakes of the subjects were estimated to be 48-242 µg per day. The overall selenium retention of the subjects (intake minus excretion) was negative during the initial period of the baseline diet. This was because the baseline diet was deficient in selenium compared to the subjects original dietary intake, but excretion rates were still relatively high. However, by the end of the 21 day baseline diet period, selenium status was balanced, suggesting that the subjects had adapted to the lower level of selenium intake by decreasing excretion. On day 22, when subjects were fed the high selenium diets, retention increased sharply as excretion levels took time to adjust. In contrast, those on the low selenium diets had a further reduction in selenium retention as excretion remained relatively high, but intakes dropped further.
This study shows that in both groups, a metabolic adaptation to a homeostatic level was an ongoing process. As intakes rose, excretion rates slowly adapted to the higher intakes with raised urinary excretion, and as selenium intakes fell, a period of negative selenium retention occurred as the body took time to adjust the urinary excretion to the lower dietary intake levels. In the study, the high selenium group showed increases in glutathione peroxidase in red blood cells (66 %) and plasma (85 %), as well as skeletal muscle (38 %). In contrast the low selenium group had a fall in selenium concentration in the same tissues. Hair mineral analysis was slow to react to dietary selenium changes, but by the end of the study there was a 2.5 times higher concentration of selenium in the hair of the high selenium diet subjects.