Simple SummaryAnimal nutrition plays an important role in the therapy of many diseases, including heart failure. The aim of the research was to assess whether 6 months of feeding diet enriched in unsaturated fatty acids (from fish meat) in dogs suffering from heart failure due to mitral degeneration impacts the dogs’ metabolic profile and clinical status. Twenty small breed dogs in early stages of heart failure were randomly divided into two groups. One group receiving a standard diet, the second one a diet enriched in fish meat. All dogs continued to receive appropriate cardiac therapy throughout the study. Control examinations were performed at the start of the study, after 3 and 6 months of appropriate feeding. The results showed no differences in clinical, cardiological, haematological and biochemical parameters between the two study groups. The metabolomic changes was more pronounced with time. After 6 months of feeding the diete enriched in fish meat, there was a favorable reduction in glycerophosphocholine and xanthine levels, but an adverse increase in lactate and furvan and a decrease in alanine was not stopped.Animal nutrition plays an important role in the therapy of many diseases, including heart failure. The aim was to assess whether 6 months of feeding an AEP + ADH enriched diet (from fish meat) in dogs suffering from heart failure due to mitral degeneration impacts the dogs’ metabolic profile and clinical status. Twenty small breed dogs were included: 50% were in stage B2 of MMVD and 50%, in stage C according to ACVIM. Dogs were randomly divided into two groups. One group receiving a standard diet, the second one a diet enriched with EPA + DHA (from fish meat). All dogs continued to receive appropriate therapy throughout the study. Control examinations were performed at the start of the study, after 3 and 6 months of appropriate feeding. Examinations included ECG, ECHO, blood hemathology and biochemistry, morphometric measurements, body fat index and subcutaneous fat tissue thickness. Serum samples were analyzed with a high-performance liquid chromatography system. Data were analyzed using the Progenesis QI (PQI, Non-linear Dynamics). The results showed no differences in clinical, cardiological, haematological and biochemical parameters between the two study groups. An effect on the metabolomic profile following a continued diet enriched in DHA + EPA (from fish meat) was more pronounced with time. After 6 months of feeding the diete enriched with DHA + EPA (from fish meat), there was a favorable reduction in glycerophosphocholine and xanthine levels, but an adverse increase in lactate and furvan and a decrease in alanine was not stopped.