Monosodium Glutamate (MSG) is a food additive used as a flavour enhancer. It is common in fast foods, snacks, canned, and frozen foods where it restores or enhances flavours that processing has damaged or destroyed. It is also used in non-processed food, often in its naturally occurring form, to heighten flavour. MSG is based on an amino acid called glutamate, which is stabilised with salt (the monosodium part of the name). It was first isolated in Japan about a century ago and was introduced into the United States after the Second World War. American soldiers had noticed how much tastier Japanese military rations were than their own.
MSG's enhancement of flavour encourages appetite, an advantage in medical situations where people lack the desire to eat, through age or illness, and need to be encouraged to take food. Another benefit is as a substitute for the extra salt often added to many processed foods. Salt has traditionally been used to enhance the taste of food, but is proved to have health risks.
MSG has many business advantages for the food industry, including its cheapness and easy availability. But its biggest advantage is that it encourages people to buy more of the industry's products.
Disadvantages are all on the consumers' side and relate to health fears. The only way to avoid any risk is to avoid MSG altogether -- and that's hard to do. Laws regulate the listing of ingredients on food labels, but MSG can be legally listed under several other names, such as glutamic acid or L-glutamic acid, depending on the product. More than 40 years after the first health scares, most evidence against MSG remains anecdotal, but public scepticism remains high. There are particular concerns about its direct medical effects on the very young and elderly, and on whether it contributes significantly to obesity and unhealthy eating habits.
Governments and universities in several Western countries, including the United States, have failed to find convincing evidence against MSG. The Food and Drug Administration (FDA) says it is "generally recognised as safe." In 2007, an international committee decided it was "harmless for the whole population." But MSG was removed from baby foods in the U.S. in the 1970s and in 1992; a study by the Federation of American Societies for Experimental Biology recommended it should not be used by pregnant women, infants, children, women of childbearing age, and individuals with affective disorders.
The symptoms which MSG is suspected of causing include headaches, dry mouth, flushing, tightening of facial muscles, numbness, tingling, chest pain, heart palpitations, nausea and general weakness. Most are said to pass after a couple of hours. There are also allegations, again without scientifically supported evidence, of it being a contributory factor in asthma, migraine, Parkinson's, Alzheimer's and depression.