Central mechanism of umami taste perception and effect of dietary protein on the preference for amino acids and sodium chloride in rats

Abstract

Preference for amino acids can take two basic forms. The first may be associated with dietary deficiencies of specific essential amino acids and would be expected to favor the ingestion of that particular amino acid. This preference would disappear once the deficiency was corrected. The other type may bear some resemblance to changes in preference for sweet, salty, and umami taste solutions during nutritional manipulation of basic macro‐ or micronutrient substitutes such as carbohydrates, proteins, or minerals. These preferences would depend upon the specific nutrient imbalance. Although many studies have examined taste preference changes following manipulations in micronutrient status, very few have examined taste preference changes following alterations in the quality or quantity of dietary protein due to maintenance of amino acids homeostasis and adaptation to various states of protein nutrition. Data from our studies using a bovine circumvallate preparation suggest the following: 1) the biochemical mechanism of umami taste synergism between L‐glutamate and 5'‐ribonucleotides involves specific glutamate binding to the bovine circumvallate preparation and is enhanced by the presence of 5'‐ribonu‐cleotides such as GMP and IMP; 2) rats not suffering from protein deficiency preferred the umami taste. Rats during a state of protein deficiencies preferred the taste of NaCl; 3) recognition of an essential amino acid, (i.e., L‐lysine) deficiency is registered in both the feeding and the satiety centers (lateral hypothalamic area and the ventromedial hypothalamus). A few umami taste‐sensitive brain neurons capable of recognizing the deficiency and its particular taste during ingestion play a very important role in controlling its quantitative ingestion. These data indicate that the umami taste‐sensitive neuron in the brain may serve as an indicator of dietary protein status. A preference for umami taste indicates a normal protein diet and a rejection of umami taste indicates that an essential amino acid is needed to recover from protein malnutrition.