Many processed foods and restaurant dishes contain a food additive called monosodium glutamate or MSG. MSG boosts the taste of food by stimulating nerves on the tongue and in the brain.

MSG is a type of glutamate, which is an amino acid the body uses to help transmit messages within the brain. In addition to the body, glutamate is found in all foods that contain protein.

As an additive used on food, MSG appears white and crystalline, like salt, and is often used in Asian cooking. It is also commonly used in canned vegetables, clear soups and processed meats. MSG is made from fermenting starch and corn sugar (or molasses, sugar cane and sugar beets), and is considered a safe additive by the U.S. Food and Drug Administration (FDA).

However, some people are sensitive to MSG and numerous reports of wide–ranging symptoms (some corroborated and some uncorroborated) have been filed with the FDA. Some critics of MSG believe the additive (or similar additives) causes or contributes to long–term or chronic conditions, such as Alzheimer’s disease.

In response, the FDA partnered with the Federation of American Societies for Experimental Biology (FASEB) in 1992 to investigate the health effects of MSG. The FASEB reaffirmed the safety of MSG when it is consumed at normal levels by the general population.

No evidence was found that connected MSG and serious long–term reactions. Further, the report states that no evidence exists to suggest that MSG contributes to Alzheimer’s disease, Huntington’s disease or any other chronic diseases.   

According to the FDA, people who chronically suffer from adverse reactions when consuming MSG are said to be "sensitive to MSG" or "MSG intolerant." Officially suffering from an adverse MSG reaction is called MSG symptom complex and unofficially it is sometimes referred to as Chinese restaurant syndrome.

Some studies have found that people who suffer from allergies or severe and poorly controlled asthma may be prone to MSG sensitivity. There have also been reports of people with asthma having more severe asthma attacks after consuming MSG. Consuming large amounts of MSG (3 grams or more) on an empty stomach or in a clear soup are also associated with reported intolerance to the additive.

Some people react immediately after eating MSG while others may experience symptoms up to 48 hours later. Reactions to MSG vary from individual to individual and there is no evidence to suggest exactly why certain people are affected by it. For example, it is not known whether adverse reactions to MSG exacerbate underlying health conditions, or if undesirable cumulative effects are created after ingesting it over a period of years.

MSG intolerance is not an allergy because reactions do not involve the body’s immune system. After consuming food with MSG some people experience mild and temporary symptoms such as a headache, and long–term symptoms ranging from depression to insomnia. There may be one or more symptoms experienced at the same time.

Ultimately, the best way to treat MSG sensitivity is to avoid MSG. Processed foods that may contain other questionable additives should be avoided as well.