Sodium nitrite is used for the curing of meat because it prevents bacterial growth and, as it is a reducing agent (opposite of oxidation agent), in a reaction with the meat's myoglobin, gives the product a desirable pink-red "fresh" color, such as with corned beef. According to Binkerd and Kolari, this use of nitrite goes back to the Middle Ages . Historians and epidemiologists argue that the widespread use of nitrite in meat-curing is closely linked to the development of industrial meat-processing. In the US, nitrite has been formally used since 1925. Because of the relatively high toxicity of nitrite (the lethal dose in humans is about 22 milligrams per kilogram of body weight), the maximum allowed nitrite concentration in meat products is 200 ppm. At these levels, some 80 to 90% of the nitrite in the average U. S. diet is not from cured meat products, but from natural nitrite production from vegetable nitrate intake. Under certain conditions – especially during cooking – nitrites in meat can react with degradation products of amino acids, forming nitrosamines, which are known carcinogens. However, the role of nitrites (and to some extent nitrates) in preventing botulism by preventing C. botulinum endospores from germinating have prevented the complete removal of nitrites from cured meat, and indeed by definition in the U. S. , meat cannot be labeled as "cured" without artificial nitrite addition, although many meats labeled as "uncured" contain nitrites produced by treating nitrate-rich vegetable extracts with a bacterial culture. In some countries, cured-meat products are manufactured without nitrate or nitrite, and without nitrite from vegetable source. In the US, nitrites are considered irreplaceable in the prevention of botulinum poisoning from consumption of cured dry sausages by preventing spore germination. To reduce nitrosamine generation, sodium ascorbate or its stereoisomer sodium erythorbate may be added to cured meat. In mice, food rich in nitrites together with unsaturated fats can prevent hypertension, which is one explanation for the apparent health effect of the Mediterranean diet.