Exactly why MSG can cause symptoms in some people is not understood. MSG is a glutamate, a type of amino acid found naturally in many foods. Glutamate is also a neurotransmitter in the brain, involved in cognitive functions such as learning and memory.

At the core of the debate is whether the body treats MSG the same as it does natural or free glutamate. Scientists believe MSG is digested the same as free glutamate and therefore poses no health risk. However, some critics of MSG have argued that the difference between natural glutamate and the additive MSG (which has been processed) can be harmful. However, scientific evidence has not established that the body treats MSG any differently from natural glutamate.

Some people do have a sensitivity to MSG, which may arise after eating MSG over a period of time. The sensitivity is not considered an allergy, because it does not involve a reaction by the immune system.

Individuals who have other allergies or asthma may be more susceptible to an adverse MSG reaction. However, scientific studies designed to research the potential connection have not provided evidentiary links.