Improving bioavailability of trace elements in dog feed: The role of organic sources

Improving bioavailability of trace elements in dog feed: The role of organic sources

Margarida Pereira

(PhD final thesis)

Trace minerals are microminerals that, although required in small amounts, are essential for a proper body function. In dog foods, essential trace elements are sourced by the raw ingredients and usually supplemented to meet dog needs, simultaneously respecting the European legal impositions. The level of intake of essential trace elements supplied in complete dog food does not correspond to the amount absorbed by the gut and made available for metabolic use and storage (bioavailability). The bioavailability depends upon several factors, e.g., the status of the trace element and the interaction with food components. Inorganic trace elements are more commonly used for dog food supplementation. However, in theory, organic trace elements have higher bioavailability compared to inorganics since they are protected from interactions within the gastro-intestinal tract. Although already extensively studied in farm animals, data on comparison of organic and inorganic sources for supplementation of pet foods is scarce. The content of essential macro, trace, and non-essential elements was determined in 26 commercially available dog food samples from different brands and market segments. Results showed that the content of macro elements was higher in dog foods from low market segments, whereas content of trace elements was not correlated with market segment to which dog foods belonged. An oversupply of essential macro elements was observed. Moreover, except for selenium, all essential trace elements were supplied above the nutritional requirements of adult dogs and some were above the legal limits (selenium, zinc, and copper).

The non-essential elements did not surpass the safe upper limits. To study the influence of selenium-enriched yeast (Saccharomyces cerevisiae; Selplex®, Alltech; organic source) and sodium selenite (inorganic source) as supplementation sources for dog foods in puppies’ health and growth, two complete dry dog foods were formulated with similar composition differing only in the source of supplemental selenium (same level, 0.1 mg/kg added of 0.56 mg/kg dry matter, DM, total content analyzed). The study was performed as a randomized block design, with a duration of 40 weeks. Twelve puppies (6 males and 6 females; 12 weeks of age), from 3 litters were distributed into 6 blocks according to body weight, gender, and litter. Puppies were randomly allocated to one of the diets, and sample collection was performed at specific time points, defined according to the parameters under evaluation. Puppies fed organic selenium had a higher body weight gain and lower freeT4:T3, suggesting an improved metabolism of thyroid hormones. Also, urinary content and the fractional clearance of selenium were higher in puppies fed the inorganic selenium, suggesting a better retention of organic forms and potentially less waste. Moreover, organic selenium promoted a reduction of pro-inflammatory cytokines, namely, tumor necrosis factor alpha (TNF-α), interferon-gamma (IFN-γ) and interleukin-10 (IL-10, anti-inflammatory cytokine) in culture supernatants of peripheral blood mononuclear cells stimulated with mitogens ix (lipopolysaccharides and concanavalin A), and a lower basal production of superoxide and other reactive oxygen species in polymorphonuclear cells. These results suggest that dietary supplementation of organic selenium can modulate the inflammatory response and, thus, prevent chronic inflammation. Organic selenium also showed a probiotic effect, being associated with a higher production of total volatile fatty acids, and higher concentrations of butyrate and propionate, which most likely reflect the higher fecal DNA concentration of Lactobacillus, and a constant relative abundance of Coriobacterialles and Bifidobacterium, through growth. Also, the DNA concentration of E. coli, an important enteropathogen, tended to be lower in feces of dogs fed diets supplemented with organic selenium.

To test supplementation strategies to enhance zinc bioavailability, it was performed a study consisting in 3 Latin Squares 4 × 4 with a 2 × 2 factorial arrangement of treatments, with 4 periods, 4 diets, and 12 young adult Beagles. Each period lasted 5 weeks, and the sample collection took place in the last week of each period. The diets had the same ingredient composition and zinc level (75 mg/kg added of ≈ 147 mg/kg DM total content analyzed), differing only in the source of zinc – zinc sulfate (inorganic source) and zinc proteinate (Bioplex®, Alltech; organic source). A commercial solid-state fermentation product of Aspergillus niger (Synergen®; Alltech) with residual enzymatic activity was either not added or added at a level of 200 g/kg to the former diets. The results showed that the selected biomarkers commonly described for the evaluation of zinc status (e.g., the concentration of zinc in plasma, urine, hair, coat quality, activity of zinc-dependent enzymes, and serum
concentration of C-reactive protein) did not respond to either zinc source. However, the CD4+T lymphocyte subset was positively affected by organic zinc supplementation, suggesting an improvement in immune function. Concerning gut microbiome, zinc source affected the beta diversity. Both sources were associated with beneficial bacteria. The fecal end-fermentation products were unaffected by zinc source. In turn, the solid-state fermentation product of A. niger did not enhance zinc bioavailability, nor affected diet digestibility or the gut microbiome.

The lack of results might be explained by the inadequacy of the product according to the anatomy and physiology of dogs’ digestive system or by an insufficient level of supplementation.

Overall, the trace element supplementation of dog foods has a scope for improvement. Finding innovative strategies to support the nutritional needs, respect the legal limits, and improve the value of pet foods are required. The supplementation with selenium enriched yeast over sodium selenite brought benefits for puppies. The supplementation of dog food zinc proteinate in detriment of zinc sulfate was a better strategy to enhance zinc bioavailability than the addition of a solid-state fermentation product of A. niger.


Doctoral Thesis in Animal Science, Specialty in Nutrition

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