(HealthDay News) — High endogenous melatonin, which characterizes late eating, impairs glucose tolerance, especially for MTNR1B G-allele carriers, according to a study published online in Diabetes Care.

Marta Garaulet, PhD, from the University of Murcia in Spain, and colleagues conducted a randomized, crossover study in a Spanish natural late-eating population. A total of 845 participants underwent 2 evening 2-hour 75-g oral glucose tolerance tests following an 8-hour fast: an early condition scheduled 4 hours prior to bedtime and a late condition scheduled one hour prior to bedtime. Differences in postprandial glucose and insulin responses were compared between early and late dinner timing.

The researchers found that melatonin serum levels were 3.5-fold higher in the late versus early condition; insulin area under the curve (AUC) was 6.7% lower and glucose AUC was 8.3 percent higher with late dinner timing. In the late condition, lower glucose tolerance was seen for MTNR1B G-allele carriers than noncarriers. Reductions in β-cell function accounted for the genotype differences in glucose tolerance.

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“These results support the hypothesis that the concurrence of elevated melatonin concentrations (during dinner close to bedtime) and food intake decreases glucose tolerance,” the authors write. “Our results suggest that differences in glucose tolerance are primarily attributed to decreased insulin secretion and β-cell function in the late condition, particularly in risk allele carriers at this locus.”

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