Effects of long-term selenium yeast supplementation on selenium status studied in the rat

Abstract

To investigate the selenium status during long-term dietary supply of selenium yeast, 30-day-old male rats were fed for 379 days a methionine-adequate low-selenium diet supplemented with 0.2 mg Se/kg (selenium-adequate diet) or 1.5 mg Se/kg (high-selenium diet) in the form of selenium yeast that contained 60% of the element as l-selenomethionine. Their selenium load was determined at several intervals by neutron activation analysis of the selenium concentrations in the main selenium body pools, skeletal muscle and liver. After 64 days the tissue selenium concentrations plateaued in both groups and then stayed at that level. Compared with the selenium-adequate group, elevated tissue selenium concentrations were found in the high-selenium group, but the increase by a factor of 3.5 in the muscle and by a factor of 2.3 in the liver was smaller than the 7.5-fold increase in the selenium intake. In the selenium-adequate group about 50% of the muscle selenium and 30% of the liver selenium and in the high-selenium group about 85% of the muscle selenium and 70% of the liver selenium were estimated to be present in non-selenoprotein forms. During selenium depletion the liver glutathione peroxidase activity in the high-selenium group remained unaffected for 4 weeks and then decreased more slowly than that in the selenium-adequate group. From these results it can be concluded that selenium incorporated from the selenium yeast diet into non-selenoprotein forms can serve as an endogenous selenium source to maintain selenoprotein levels in periods of insufficient selenium supply.

Introduction

Selenium-enriched yeast is frequently taken as a dietary supplement or is applied in relatively large doses as a preventive or therapeutic measure. It contains most of its selenium as selenomethionine (SeM) but there are also some other selenium compounds [1]. In several early investigations it was shown that the tissue retention of selenium administered as SeM differs from that after ingestion of selenite, selenate or selenocystine [2], [3], [4], [5]. This is due to the fact that SeM not only provides selenium for incorporation into the specific selenocysteine-containing selenoproteins as the other three forms, but, because of its chemical similarity to methionine, is also incorporated directly non-specifically into proteins in place of methionine [6]. The part that follows the metabolic pathway of methionine appears to be dependent only on the ratio of SeM to methionine in the diet. Accordingly, as the remaining non-protein-bound surplus of the ingested amount is excreted, higher selenium concentrations are found in the tissues when larger doses of the element are administered as SeM or SeM-containing selenium-enriched yeast instead of selenite, selenate or selenocystine.

The rat has frequently been used as a mammalian model to investigate the retention of selenium in the tissues after increased intake of SeM. In these studies, of which some are listed here as examples, the maximum supplementation periods were, however, only in the range of 8–12 weeks [6], [7], [8], [9], [10], [11], [12]. Relatively few investigations have been concerned with selenium yeast. In order to obtain information on the selenium body load after long-term supplementation with selenium yeast, we fed rats with a selenium-adequate or a high-selenium diet that contained the element in that form and determined the selenium concentrations in the main body pools, muscle and liver, at several intervals during an experimental period of more than a year. In addition, we also wanted to find out to what extent the part of selenium present in the organism in non-selenoprotein forms can be reutilized and may thus serve as an endogenous selenium pool to be used in the formation of biologically important selenoproteins during insufficient selenium supply. As the non-specific incorporation of SeM is influenced by the methionine concentration in the diet [13], [14], [15], the study was carried out on rats with an adequate methionine supply.