Selenium absorption and bioactivity in dogs are affected by dietary format

Introduction: The main function of the essential trace mineral selenium (Se) is the protective effect against oxidative stress. Raw materials of dog foods vary greatly in their Se concentration and availability and canned diets are formulated with different raw materials than kibble diets. These dietary formats (canned and kibble) also undergo a different type of processing. However, the current recommendation on Se in dog foods is not format specific. Our previous in vitro study already identified dietary format (canned vs. kibble) and protein digestibility as the main determinants of in vitro Se accessibility in dog foods. The present study was designed to evaluate if these findings are reflected in in vivo Se absorption, i.e. the fraction of dietary Se that reaches the systemic circulation, and Se bioactivity, i.e. the amount of Se that can be used for incorporation into enzymes. Material and methods: Twenty-four Labrador retrievers were randomized to four balanced groups in an incomplete cross-over design in which they were fed 4 of 8 diets during 6 transition days and 29-43 experimental days. The 8 diets were commercially available and varied in format (canned or kibble) and crude protein (CP) concentration. For every format, diets with an intended concentration of 9.6, 14.3, 19.1 and 23.9 g CP/MJ ME were selected. At the end of every feeding period, blood, urine and faecal samples were collected. Blood was analysed for whole blood glutathione peroxidase (GPx) and serum Se. Urine samples were analysed for total Se and creatinine (CT) and faeces for Se. Apparent Se absorption was calculated as the percentage of Se intake that was not found in the faeces. Data were analysed using linear mixed effects models to investigate the effects of actual CP intake, format and their interaction. Results and discussion: The average dietary Se content of the canned and kibble diets was 40.6 and 22.3 µg/MJ ME, respectively. Canned diets also had a higher dietary Se content in our in vitro study; 34.8 µg/MJ ME in canned (n=20) vs. 22.5 µg/MJ ME in kibble diets (n=23). This indicates that a higher dietary Se content is inherent to canned diets. Both apparent Se absorption (as % of Se intake) and urinary Se excretion corrected for the absolute amount of absorbed Se were lower in canned compared to kibble diets (p<0.001 and p<0.001, resp.). The former is in accordance with our in vitro Se accessibility findings(3). In contrast to the in vitro study, CP did not have a significant impact on Se absorption (p=0.753). The potential bioactivity was higher in canned than in kibble diets and decreased with increasing CP intake as measured by GPx (p<0.001 and p