Metabolic engineering is the directed improvement of cellular properties or products by modifying specific biochemical reactions or introducing new ones using recombinant DNA technology. It is a subset of biochemical engineering, which has industrial and medical applications. Metabolic engineering is used by biotechnology, pharmaceutical, chemical, energy, and food companies to design or optimize microorganisms that overproduce particular chemicals. Products via metabolic engineering include drugs, enzymes, food, commodity chemicals, and fuels. Metabolic engineering has also been used to study human diseases.