## 4 Discussion
Recently, a rather popular research direction has been the determination of the effect of ketosis on selected milk components [10, 29, 31, 35, 36, 37, 38, 41, 42]. Due to the correlation between high biomarkers for diagnosing ketosis and high milk fat content, the study of fatty acid content is considered to be one of the most frequently chosen research areas related to the effect of ketosis on milk composition [42]. The conducted experiment also confirmed the theses of several other research teams [29, 31, 42], pointing out the key role of fatty acids in metabolism, especially those acids that are largely associated with adipose tissue, which animals begin using during NEB; thus, that the acid content increases. During the first week of lactation, PH and MH milk had by far the highest TVA content, while the lowest content was found in MK (Figure 1). The study also confirmed a higher CLA content in healthy animals than sick animals during the early stage of lactation, as also indicated by results obtained by [PERSON] et al. [42] and [PERSON] et al. [10]. However, the studies of the above-mentioned authors did not confirm the influence of the age of cows on the formation of CLA levels with metabolic disorders. Only multiparous cows were included in [PERSON] et al. 's [10] study. In ruminants, CLA9 is formed in two ways. It can be obtained as an intermediate product during the biodegradation of linoleic acid into steearic acid using a bacterial isomerase. During this process, vaccenic acid is first formed, which is then hydrogenated to stearic acid. The second way is the endogenous synthesis of CLA9 from vaccine acid originating from the rumen, which takes place using the enzyme \(\Delta\)-9-desaturase [43]. It is estimated that more than 91% of the CLA cis9, trans11 secreted into milk comes from endogenous synthesis (i.e., cows fed on pasture) [44]. This observation is due to the fact that during lactation in dairy cows, the epithelial cells of the udder have high \(\Delta\)-9-desaturase activity, as well as the fact that it is also found in the small intestine and adipose tissue of the ruminants. Studies have shown that the age of cows was an additional factor in determining the formation of CLA levels during the early stage of lactation (Figures 3 and 4). The CLA9 content in the milk of PH cows in the first week of lactation was 32.75% higher than in to PK milk, while in MH milk, it was 67.7% higher than in MK milk. As a result, several bacterial strains--Megasphaera eldenii, Bifidobacterium, Proprionibacterium, Lactococcus, Lactobacillus--produce CLA trans10,cis12 [45]. In addition, rumen pH has a significant role in keeping a viable rumen environment appropriate for the aforementioned bacteria [46]. BHBA significantly decreased the diversity of microbiota community and increased the abundance of some pathogenic bacteria, implying that ketone bodies might influence the function of mucosal barriers [47]. MH milk had the highest CLA10 content over the course of the experiment; however, a reduction of 15% was observed in the second week, along with another reduction of 19% in the third week of lactation, compared to those of each preceding week. The CLA10 content in these animals, when compared to the healthy groups, was 319% lower for primiparous cows, as well as 1.018% lower for multiparous cows. In the second week of lactation, there was still a significant difference in CLA10 content between healthy cows and those diagnosed with ketosis, though these values were already lower in the healthy animals, amounting to 161% for primiparous cows and 233% for multiparous cows; in contrast, its value increased in animals with ketosis. This study compared primiparous and multiparous cows and shows differences between the groups, which are of great importance. As [PERSON] et al. [48] pointed out, this observation may be influenced by the less adapted nature of rumen primiparous cows to the fed ration. Additionally, [PERSON] et al.