Thus, these foods can be considered to have a cleansing action. Through conjugation, the liver is able to turn drugs, hormones and various toxins into water soluble excretable substances. Individual xenobiotics and metabolites usually follow one or two distinct pathways. This makes testing of the various pathways possible by challenging with known substances. This is called the conjugation pathway, whereby the liver cells add another substance (eg. cysteine, glycine or a sulphur molecule) to a toxic chemical or drug, to render it less harmful. Detox diets that severely limit protein or that require fasting, for example, can result in fatigue. Long-term fasting can result in vitamin and mineral deficiencies.