Monosodium glutamate in the diet does not raise brain glutamate concentrations or disrupt brain functions

Abstract

The non-essential amino acid glutamate participates in numerous metabolic pathways in the body. It also performs important physiologic functions, which include a sensory role as one of the basic tastes (as monosodium glutamate [MSG]), and a role in neuronal function as the dominant excitatory neurotransmitter in the central nervous system. Its pleasant taste (as MSG) has led to its inclusion as a flavoring agent in foods for centuries. Glutamate's neurotransmitter role was discovered only in the last 60 years. Its inclusion in foods has necessitated its safety evaluation, which has raised concerns about its transfer into the blood ultimately increasing brain glutamate levels, thereby causing functional disruptions because it is a neurotransmitter. This concern, originally raised almost 50 years ago, has led to an extensive series of scientific studies to examine this issue, conducted primarily in rodents, non-human primates, and humans. The key findings have been that (a) the ingestion of MSG in the diet does not produce appreciable increases in glutamate concentrations in blood, except when given experimentally in amounts vastly in excess of normal intake levels; and (b) the blood-brain barrier effectively restricts the passage of glutamate from the blood into the brain, such that brain glutamate levels only rise when blood glutamate concentrations are raised experimentally via non-physiologic means. These and related discoveries explain why the ingestion of MSG in the diet does not lead to an increase in brain glutamate concentrations, and thus does not produce functional disruptions in brain. This article briefly summarizes key experimental findings that evaluate whether MSG in the diet poses a threat to brain function.