The process of milk FA formation varies: some milk is synthesized by the rumen's micro-organisms (i.e., bacteria and protozoa), while other milk is synthesized in the mammary gland [16]. FAs that have more than 16 carbon atoms are either derived from the cow's food and absorbed during digestion or mobilized from the cow's fat reserves. The acids derived from the fat reserves are transported as non-esterified fatty acids (NEFAs) and captured from plasma [17; 18; 19]. Of all the FAs present in milk, about 14% are considered to be unique to milk [16]. FAs of different structures (i.e., carbon chain length, degree of saturation, and configuration) have very different effects on human health [20; 21; 22]. The best-known FA with properties for promoting health is C18:2 cis9,trans11. The profiles of milk fatty acids are correlated with energy balance in dairy cows, and selected milk fatty acids can be considered as biomarkers for ketosis [10].  
The increase in milk yields has resulted in the emergence of adverse phenomena in dairy herds that cause deterioration in the health and fertility of the animals. During the transition period (the period of late pregnancy and early lactation), there is stress associated with the intense hormonal changes accompanying the birth of a calf and the entry into lactation. Stress is usually associated with a decrease in appetite. At the time of rapidly increasing nutritional needs associated with milk secretion, reduced feed intake leads to a negative energy balance and mineral and vitamin deficiencies [10; 23]. Ketosis affects more than 80% of cows in the herd. Cows are 3 to 8 times more likely to have displacement of the digestive tract. They have a two-fold increased risk of placental retention and threefold increased risk of uterine inflammation. They are as much as 6 times more likely to have cystic ovaries. During the transition period (late pregnancy and early lactation), there is stress associated with the intense hormonal changes accompanying the birth of the calf and the entry into lactation. In the case of some diseases, e.g., post-partum paralysis, there is a risk of cow death, while other diseases, e.g., ketosis or acidosis, cause significant production losses, such as reduced milk yield, deterioration of the chemical composition of milk and higher somatic cell count, deterioration of reproductive indices, the occurrence of hood problems (laminitis), and, as a result of these issues, higher veterinary and herd repair costs [9]. Therefore, it is far better for breeders to take measures to prevent the occurrence of these diseases. An additional difficulty in the control of metabolic diseases is their subclinical states, especially in the case of subclinical ketosis and acidosis. The lack of unambiguous syndromes associated with the diseases means that they often go unnoticed, resulting in measurable economic losses on a herd-wide scale and over a long period of time. Ketone bodies are a group of organic compounds that are intermediate metabolites of fat. They include acetone (formed via spontaneous decarboxylation of acetoacetate), acetoacetic acid (in the form of anion-acetoacetate), and \(\beta\)-hydroxybutyric acid (in the form of anion-\(\beta\)-hydroxybutyric; BHBA). The first step in ketogenesis involves the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA. Acetyl-CoA is also the substrate for CLA9 synthesis [10]. The hypothesis that we want to verify assumes that differences in the formation of CLA9 and CLA10 may be indicative of metabolic disorders in the bodies of cows. However, we hypothesize that the age of the cows will be an additional factor in determining the formation of different CLA isomers levels during the early stage of lactation, because primiparous cows need extra energy for growth [24]. The aim of the experiment was to study the relationship between the ages of cows, blood BHBA content, and CLA (different isomers) content during the first three weeks post-partum.