Diurnal Variation of Human Sweet Taste Recognition Thresholds Is Correlated With Plasma Leptin Levels

OBJECTIVE—It has recently been proposed that the peripheral taste organ is one of the targets for leptin. In lean mice, leptin selectively suppresses gustatory neural and behavioral responses to sweet compounds without affecting responses to other taste stimuli, whereas obese diabetic db/db mice with defects in leptin receptor lack this leptin suppression on sweet taste. Here, we further examined potential links between leptin and sweet taste in humans. RESEARCH DESIGN AND METHODS—A total of 91 nonobese subjects were used to determine recognition thresholds using a standard stair-case methodology for various taste stimuli. Plasma leptin levels were determined by an enzyme-linked immunosorbent assay at several timepoints during the day under normal and restricted-meal conditions. RESULTS—The recognition thresholds for sweet compounds exhibited a diurnal variation from 0800 to 2200 h that parallels variation for leptin levels, with the lowest thresholds in the morning and the highest thresholds at night. This diurnal variation is sweet-taste selective—it was not observed in thresholds for other taste stimuli (NaCl, citric acid, quinine, and mono-sodium glutamate). The diurnal variation for sweet thresholds in the normal feeding condition (three meals) was independent of meal timing and thereby blood glucose levels. Furthermore, when leptin levels were phase-shifted following imposition of one or two meals per day, the diurnal variation of thresholds for sweet taste shifted in parallel. CONCLUSIONS—This synchronization of diurnal variation in leptin levels and sweet taste recognition thresholds suggests a mechanistic connection between these two variables in humans.