Fat-soluble. The kidneys are designed to filter water-soluble waste products along with excess water itself from the blood and excrete them in the form of Urine. This is why when a patient is properly hydrated, the urine will appear to be almost clear, as the water-soluble waste products are diluted with an excess of water. As a patient becomes increasingly dehydrated however, the urine will turn a darker yellow as the ratio of water-soluble waste products to pure water increases. Things that are fat-soluble will need to be eliminated through other metabolic pathways which usually involve the liver, and take much more time.