Diurnal Variation of Sweet Taste Recognition Thresholds Is Absent in Overweight and Obese Humans.

Keisuke Sanematsu,Yuzo Ninomiya,Noriatsu Shigemura,Masatoshi Nomura,Yuki Nakamura

doi:10.3390/nu10030297

Keisuke Sanematsu, Yuzo Ninomiya + Show 3 more

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https://doi.org/10.3390/nu10030297

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Abstract

Sweet taste thresholds are positively related to plasma leptin levels in normal weight humans: both show parallel diurnal variations and associations with postprandial glucose and insulin rises. Here, we tested whether this relationship also exists in overweight and obese (OW/Ob) individuals with hyperleptinemia. We tested 36 Japanese OW/Ob subjects (body mass index (BMI) > 25 kg/m2) for recognition thresholds for various taste stimuli at seven different time points from 8:00 a.m. to 10:00 p.m. using the staircase methodology, and measured plasma leptin, insulin, and blood glucose levels before each taste threshold measurement. We also used the homeostatic model assessment of insulin resistance (HOMA-IR) to evaluate insulin resistance. The results demonstrated that, unlike normal weight subjects, OW/Ob subjects showed no significant diurnal variations in the recognition thresholds for sweet stimuli but exhibited negative associations between the diurnal variations of both leptin and sweet recognition thresholds and the HOMA-IR scores. These findings suggest that in OW/Ob subjects, the basal leptin levels (~20 ng/mL) may already exceed leptin’s effective concentration for the modulation of sweet sensitivity and that this leptin resistance-based attenuation of the diurnal variations of the sweet taste recognition thresholds may also be indirectly linked to insulin resistance in OW/Ob subjects.

Highlights

Leptin is a hormone that regulates food intake, energy expenditure, and body weight mainly through the activation of the hypothalamic functional leptin receptor (Ob-Rb) [1,2]
Consistent with our previous report on normal weight (NW) subjects [5], a significant gender difference was observed in plasma leptin levels (p < 0.01), whereas no such difference was observed in plasma insulin, blood glucose, and taste recognition thresholds for any stimulus in the overweight and obese (OW/Ob) subjects (p > 0.05)
The relative plasma leptin concentrations of the OW/Ob subjects showed diurnal variation, whereby the level started rising before noon and peaked in the night (Figure 1, Table 1), significant diurnal variation was not observed in the absolute values of plasma leptin (Figure S1, Table S1)

Summary

Introduction

Leptin is a hormone that regulates food intake, energy expenditure, and body weight mainly through the activation of the hypothalamic functional leptin receptor (Ob-Rb) [1,2]. Leptin inhibits the responses to sweet substances without affecting the responses to sour, salty, and bitter substances in the chorda timpani (CT) nerve innervating the anterior two-thirds of the tongue in lean mice Such selective sweet response inhibition by leptin was not observed in leptin receptor-deficient obese diabetic db/db. Our subsequent human study with normal weight (NW) subjects demonstrated that the recognition thresholds for sweet compounds exhibit circadian variations which parallel circulating leptin levels. That is, both sweet recognition thresholds and leptin levels begin rising before noon and peak in the night [5]. When leptin level variations were phase-shifted by altering the number of meals per day, the diurnal variation of the thresholds for sweet compounds shifted in parallel

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