Selenium in dog foods

Selenium is an essential trace mineral involved in many biological processes, such as anti-oxidant, thyroid and immune function. Current recommendations for selenium inclusion in dog foods lack knowledge on the requirements of adult dogs and the effect of various factors on its bioavailability (i.e. the fraction of the dietary selenium that reaches the systemic circulation) and bioactivity (i.e. the selenium that is incorporated into selenoproteins). Therefore, the objective of this PhD project was to identify factors that contribute to the biological utilisation of dietary selenium, as a first step towards correct selenium inclusion levels in dog foods to prevent long-term selenium related diseases. An in vitro study, assessing various factors of 62 pet foods, identified diet type and crude protein digestibility as most important factors to affect selenium accessibility (i.e. the amount of dietary selenium that is potentially available for absorption after in vitro digestion). A consequent in vivo study evaluated the effect of these factors on selenium bioavailability and -activity. Twenty-four dogs, four canned and four kibble diets with a range in crude protein concentration were used in a randomised cross-over study. Selenium bioavailability was higher from kibble than canned diets and was not affected by crude protein intake in both diet types. The biological activity of selenium, as measured by whole blood glutathione peroxidase activity, was higher in dogs fed the canned diets than in dogs fed the kibble diets and decreased with increasing crude protein intake. These results suggest that there were indeed differences in the bioavailability and -activity of selenium from canned and kibble diets, which can be caused either by the difference in ingredients used, by the selenium species or by the processing conditions. Another in vivo study with 24 dogs assessed selected biomarkers on their suitability to detect a decrease in dietary selenium in adult dogs within eight weeks. All dogs received a semi-purified diet with an adequate amount of selenium for eight weeks. After this, 12 of the dogs remained on this diet for another eight weeks and the other 12 dogs received a semi-purified diet with a low selenium concentration (6.5 µg/MJ; 31% of the FEDIAF minimum). Of the biomarkers measured, urinary selenium:creatinine ratio, serum selenium and serum glutathione peroxidase activity reacted most quickly and should be used in epidemiological trials, to determine their association with the occurrence of long-term selenium related diseases and could eventually be used in trials to determine the minimum selenium requirements of healthy adult dogs. In conclusion, the studies included in this PhD thesis showed that diet type was associated with selenium bioavailability and diet type and dietary crude protein concentration with selenium bioavailability and -activity. Selenium is generally more bioavailable from kibble diets, but selenium from canned diets results in an absolute higher bioactivity. Future studies may use the urinary selenium:creatinine ratio, serum selenium and serum glutathione peroxidase activity to assess the selenium requirements of healthy adult dogs