Loss of appetite is caused by toxins. Healthy kidneys can discharge toxins out while injured kidneys fail to do this job. In this way, toxins will build up in your body. Toxins can irritate your gastrointestinal tract and make you loss of appetite. Besides, other complications from damaged kidneys can also cause poor appetite including high phosphorus levels, anemia, fluid retention or heart problems, metabolic acidosis, the use of antibiotics and constipation.