Percentage contribution to free glutamate
Percentage contribution to free glutamate intake was also calculated at the food item level. A list of the top 30 foods for children and adults are presented separately in Table 2. The top 4 foods included in this list are watermelon, raw; tomato catsup; tomatoes, raw; and roll, white, soft. These account for approximately 15% of total free glutamate intake in both children and adults.
Discussion To our knowledge, this is the first study to estimate free glutamate intake and identify its food sources among US children and adults using data from NHANES 2009-2014. Previous works have assessed total glutamate intake, including both protein-bound and free glutamate rather than just free glutamate intake ,  or have assessed MSG rather than naturally-occurring free glutamate , , , , , . One previous report described the average intake of glutamate in Germany, EU countries, and Asian countries, but there are no detailed study descriptions or references provided . Glutamate is known to have a multifunctional involvement in taste perception, intermediary metabolism, gut-brain axis activation, energy homeostasis, and excitatory neurotransmission in the mammalian central nervous system . Glutamate is one of the major neurotransmitters and the most abundant free amino Octreotide acetate in the brain , which is synthesized in the brain  because the passage of glutamate from the blood into the brain is effectively restricted by the blood-brain barrier . Glutamate is also consumed from many kinds of foods. Dietary glutamate is absorbed from the intestinal lumen and primarily ingested in the intestine to generate energy necessary for intestinal motility , . Consequently, normal dietary consumption of glutamate has no effect on plasma glutamate concentration , . However, excess intake, to the extent which is higher than normal human dietary consumption, results in elevated plasma levels but is normalized within a few hours . For these reasons, a recent review demonstrated that dietary MSG intake neither increases brain glutamate concentrations nor disrupts brain function . Nevertheless, there is still controversy about the safety of ingestion of dietary MSG, and it has been suggested to cause adverse health effects . In addition, a new interest of the academic community in MSG safety has been triggered by the increase of prepackaged food usage and simple flavor enhancers in the world . The human body does not distinguish glutamate added to foods from those naturally occurring in foods, and thus, metabolize them in the same way , . Therefore, when assessing the safety of MSG intake, naturally-occurring free glutamate as well as MSG should be considered. Our study revealed that the population mean intakes of free glutamate occurring naturally in foods was 258 mg/d and 322 mg/d in children and adults, respectively. A previous study using the data from the Nurses' Health Study estimated total glutamate intake, not free glutamate intake, among US adults as 7.27 g/d by gene sequencing method and 14.46 g/d by conventional biochemical methods . These values were 20-40 times higher than our estimation. This substantial gap between the previous and the present studies is due to the difference of the target substances. The previous study assessed total glutamate intake including not only free glutamate but also protein-bound glutamate, while this study focused on free glutamate. In addition, the proportion of free glutamate is much smaller than protein-bond glutamate, and its concentration is not related to its protein content . For example, free glutamate content in beef meat was 4028-5661 μg/g dry weight, while the content of the total amount of proteinogenic glutamate was 46.8-49.9 mg/g dry weight . Considering such a small occurrence of free glutamate in foods, our estimation was probably reasonable.