## 4 Discussion  
We have shown that the addition of _Spirulina_ and _Chlorella_ generally improves the protein bioavailability of kefir compared to the control (without added algae). At a concentration of 0.5%, _Spirulina_ showed particularly high protein bioavailability. At higher concentrations, the protein bioavailability of both algae was still good. Research indicates that protein from _Spirulina_ and _Chlorella_ is generally well digested and absorbed [22; 23], although this may vary depending on the method of processing the algae (e.g., breaking down the cell walls may improve bioavailability). Microalgae such as _Arthospira_ and _Chlorella_ can increase the amount of protein released in food products. For example, [PERSON] et al. (2010) [40] observed an increase in the amount of protein released from pasta enriched with these algae. [PERSON] et al. (2013) [41] indicated that _Spirulina_ protein has a better availability than _Chlorella_ protein due to the lack of cellulose in its cell wall. Similar results were reported for fermented dairy products. In the study by [PERSON] et al. (2019) [42], the addition of dried _Spirulina_ to yogurt increased the protein content. [PERSON] et al. (2020) [43] also observed an increase in the protein content of yogurt from 5.4% to 6.4% after the addition of 1% _Spirulina_. _Spirulina_ was also added to buttermilk [44], where it also increased the protein content and other nutrients. Our results confirm that the addition of _Spirulina_ or _Chlorella_ can effectively improve the bioavailability of protein in kefir, especially at lower microalgae concentrations. There may be interactions between milk proteins (e.g., casein) and algal proteins in the product, which may affect the digestibility and absorption of individual protein fractions, but the overall bioavailability remains high. However, it should be remembered that protein availability can be modified by the presence of antinutritional components such as polyphenols and polysaccharides. Oxidized phenols can react with proteins to form insoluble complexes that are more difficult to enzymatically digest [5; 9]. On the other hand, the fermentation process of kefir may partially break down proteins, potentially making them easier to digest and absorb, which could work synergistically with algal protein. Our results indicate improved protein bioavailability after adding _Spirulina_ and _Chlorella_ to kefir, which is consistent with the general knowledge of good-quality protein in these algae. Interactions with milk proteins do not seem to significantly reduce overall bioavailability, and the fermentation process of kefir may even aid in protein digestion. Unfortunately, there are no direct studies on the effect of _Spirulina_ and _Chlorella_ on the bioavailability of iron in kefir. According to the literature, iron absorption may increase  
Figure 6: The amount of released vitamin B12 (**A**) and its bioavailability (**B**) after in vitro digestion of kefir enriched with different doses of algae (X axis; doses from 0 to 5%): on the left (light columns), _Spirulina_; on the right (dark columns), _Chlorella_. The columns represent different doses of microalgae, and columns marked with different letters differ significantly (\(p\leq 0.05\), test LSD). with the addition of microalgae, mainly due to phycocyanin, which forms complexes with iron [45]. On the other hand, calcium and casein in dairy products are known inhibitors of non-heme iron absorption (which is the form of iron found in plants and microalgae) [46]. This could partially explain the decrease in iron bioavailability at higher microalgae concentrations, where the greater amount of algal biomass introduces more iron, but at the same time, other components may impede its absorption in the presence of kefir components. Some studies suggest that algal components like some proteins and phenolic molecules may bind iron, reducing its solubility [47; 48].