## 4 Discussion
### Evaluation of the Compliance of Marked Mineral Content in Infant Formulas  
As previously discussed, the levels of some minerals in IFs can be significantly higher than in breast milk due to the lower bioavailability of macro- and microminerals [21]. The evaluation of the compatibility of the mineral composition of IFs with the manufacturer's claims has previously been assessed by several researchers. Consumers expect the content of individual minerals in IFs to maintain a certain standard and be able to provide the right amount to infants [3]. [PERSON] et al. [21] and [PERSON] [54] determined lower Ca contents relative to label information. In our study, large differences were noted between determined and declared Mg levels. The marked values in all formulas (except S1) were lower than those declared by the manufacturer, and the differences were up to 30%. [PERSON] et al. [3] observed lower Mg contents relative to the label in three initial and three follow-on formulations; however, the differences were less than 20%. The greatest differences in the marked and labeled values of IFs were noted for Fe. In two initial formulas (C-A1, C-B1), the values were about 40 and 70% higher in contrast to the values declared by the manufacturer. In the follow-on IFs, the Fe content was lower than that on the product's labels, and in three of them, C-B2, C-C2, and C-D2, the differences were greater than 20%, including in two formulas reaching about 50%. On the other hand, the Fe content of all IFs tested (except C-B1) was within the range specified by the European Commission (0.3-1.3 mg/100 kcal--initial IFs; 0.6-2 mg/100 kcal--follow-on IFs; 0.45-2 mg/100 kcal--specialist IFs [25]. [PERSON] et al. [3] also indicated differences in Fe content between the determined and declared values; however, they reached a lower range than in our study. Such large differences in Fe content may also have been due to the different test methods for mineral determination used in this study, including the sampling method, affecting measurement uncertainty [55]. In most of the IFs, the determined Mn content was lower than declared by the manufacturer on the label (except for C-A1, C-C2, and S2); however, it was within the range of values specified by European Union regulations. On the other hand, [PERSON] et al. [56] indicated lower than legislated Mn levels in IFs. Only in one formula (C-A1) did the Mn values exceed the concentration difference by more than 20% from the manufacturer's declared value. [PERSON] et al. [3] report that in one of the IFs, the determined Mn content was up to 87% lower than on the label. In two specialist IFs (S4 and S5), experimental Cu values exceeded those declared by the manufacturer by more than 20%. The large deviations in Mn and Cu can be justified by the low densities of these elements in the IFs; such small values may be more susceptible to large variations in the content of these elements in the formulas [57]. Deviations in mineral content from the standards recommended by the European Commission were noted in several specialist IFs. In S1, Ca values, both value labeled and value marked, were slightly higher (by about 5-6%) than the range of standards. This situation was also observed for Zn in S1 and S4 (for value marked). Composing specialist IFs can be difficult due to different values of standards for selected minerals, including higher P or Zn content [35]. It should be noted that differences in mineral content between the value marked and that declared by the manufacturer can also result from the type of water used to prepare a given formula [58]. In the present study, deionized water was used for the preparation of each IF to avoid the presence of minerals found in water and to obtain the most reliable results. The labels of some of the IFs do not specify what type of water should be used for their preparation, and only in a few is it clarified to use cooled boiled water. We also do not know under what conditions the studies of the IFs were performed and which method and reagents were used.