###### Abstract  
Supplementing the diet of ruminants with agro-industrial by-products is a common practice. In this study, we applied an untargeted lipidomics approach to study the changes in the milk lipid metabolite profiles linked to the addition of different doses of spent coffee grounds (SCG) to the diet of lactating goats. The carryover of caffeine from feed to milk was also studied. Compared to controls, the milk of goats on the SCG diet showed higher levels of cholesteryl esters, sphingomyelins, and phospholipids, while nonesterified fatty acids were downregulated. After 12 h from the last SCG dose, the carry-over of caffeine was, on average, 3%. Collectively, our results establish that SCG supplementation induces changes in the milk levels of complex lipid molecules and causes the transfer of caffeine and caffeine metabolites from feed to milk.  
lipid profile; lipidomics; animal feeding; lactating goats; mass spectrometry; caffeine; coffee by-products 20232002332472023333  
caffeine [15]. Many substances, such as caffeine and its metabolites, ingested by lactating mothers pass through the epithelial cells of the mammary gland to the milk. The concentration of these metabolites in the milk depends on the dose ingested, the duration of the assumption, the amount of milk excreted daily, the mother's health, and genotype [16].  
From a chemical and structural point of view, the lipid profile of milk is quite complex. In milk, lipids exist as an emulsion, organized in fat globules (MFG) of different sizes. They consist of an inner core of triacylglycerlycerols surrounded by a membrane of different layers of proteins and complex lipids, mainly of an amphipathic nature (MFGM). Milk lipids are predominantly composed of triacylglycerlycerols (approximately 98%), followed by phospholipids, sphingolipids, sterols, and analogs, which potentially play an important role in the health of human consumers [17]. The composition of the milk fat globule membrane can be modified by the diet [18].  
Lipidomics is an omics science largely used to explore and characterize lipids in biological specimens. To study milk fat, due to its structural and compositional complexity, highly sophisticated analytical techniques are needed. In hyphenated techniques, to separate and identify complex lipids, chromatography is paired in-line with mass spectrometry. Current MS-based lipidomics can achieve the detection of all lipid classes in one run, helping us to fully characterize most lipids in the target sample [19]. The huge amount of data produced by these analytical tools is submitted to multivariate statistical analysis (MVA) to extract meaningful information and highlight lipid compositional changes due to different factors. We recently reported the use of lipidomics and metabolomics approaches to analyze the influence of ovine milk in different diet regimens [20; 21] and the metabolic effects of supplementing the diet with cocoa husk by-products [22]. The aim of this research was to gain an understanding of the impact of a diet supplemented with SCG on the milk lipid profile in lactating goats using a lipidomics approach based on a liquid chromatography time-of-flight mass spectrometry (UHPLC-QTOF/MS) analytical platform. To this goal, two different SCG doses were administered to lactating goats, and the results were compared with the control group. Caffe carryover in milk was also studied. To the best of our knowledge, research data on caffeine and caffeine metabolites carryover in the milk of ruminants are not available in the literature.