A precursor is a substance that is converted into another substance in a metabolic pathway. For example, amino acids are precursors to proteins, and glucose is a precursor to glycogen. In the context of nutrition, a precursor may refer to a nutrient or other substance that is needed for the body to synthesize another nutrient or substance. For example, beta-carotene is a precursor to vitamin A, and tryptophan is a precursor to serotonin.