## 4 Discussion
### Mineral Content of Infant Formulas
#### 4.2.2 Comparison of Mineral Content in Human Milk and Infant Formulas  
This study reported differences in mineral content between HM and IFs. The levels of some minerals in IFs may be significantly higher than those in HM due to differences in bioavailability [21]. The higher bioavailability of minerals from HM is due to the presence of enzymes (amylase and lipase) that aid digestion. IFs are enriched with selected inorganic salts during production, which can lead to the precipitation of elements during digestion, and the presence of other anti-nutrients can also lead to reduced solubility of some elements [33]. Calcium content in HM was two to three times lower than in IFs. The highest differences in Ca content were noted between HM and specialist IFs. The significantly higher amount of Ca in IFs is associated with lower Ca absorption by infants compared to HM. [PERSON] et al. [21] report that Ca bioavailability from formulas is about 38%, while from breast milk, it is almost 60%. In contrast, [PERSON] et al. [33] found that the bioavailability of Ca from IFs ranged from 3 to 24%, with the exception of milk intended for premature infants, which showed a value of 43%. Among macrominerals, significant differences in content between HM and IFs were also noted for P and K, and in both cases, the levels of these minerals were higher in IFs. The content of these nutrients in IFs is tightly regulated, as excess levels of these compounds taken in with the diet must be excreted in the urine, which can strain an infant's immature kidneys and increase the risk of dehydration [3].  
It was reported that Zn content in IFs was up to four times higher than in transitional and mature milk. The lower Zn content in IFs is related to the reduced bioavailability of this element in them. While the bioavailability of Zn from HM reaches about 50% [48], in cow's milk, which is the basis for the production of IFs, it reaches about 26% [50]. The higher bioavailability of Zn from HM compared to cow's milk is partly explained by the different chemical composition of the two milks, particularly the different casein-to-whey protein ratios. By adjusting the ratio of these proteins to a value of 40:60 (as in HM), the absorption of Zn from cow's milk can be significantly increased [50]. Whereas Zn deficiencies are almost non-existent in the adult population, the percentage is as high as 50% among infants [51]. Considering the lower bioavailability of Zn from IFs and the impaired ability of the infant's not fully mature gastrointestinal system to absorb the elements, the risk of Zn deficiency among infants is higher [51]. To prevent Zn deficiencies in infants' diets, it is reasonable to add higher amounts of this element to IFs. A similar trend was noted for Fe--its content in breast milk was even more than three times lower than in the IFs analyzed. This is also due to differences in the bioavailability of this element--from breast milk, it is about 50%, while in IFs, it is about 12% or less [52]. Other sources report that Febioavailability from IFs can be as low as 4-7% [5], and Fe bioavailability from breast milk is more than 10 times higher than from IFs [53]. During the first few months, the infant utilizes Fe reserves accumulated in the body; however, in the later months of life, it is reasonable to provide it with solid foods or to include supplementation [5]. [PERSON] et al. [45] report that trace elements, including Fe and Zn, in IFs can be found in concentrations up to 50 times higher than in breast milk. During the production process of IFs, it is important to consider the loss of trace elements during storage. In addition, the bioavailability of minerals in IFs is significantly lower than in HM [49].