Eating dinner close to bedtime, when melatonin levels are high, disturbs blood sugar control, especially in individuals with a genetic variant in the melatonin receptor MTNR1B, which has been linked to an elevated risk of type 2 diabetes. A recent research published in ‘Diabetes Care’ examined whether late eating that usually occurs with elevated melatonin levels results in disturbed blood sugar control.

A randomized, crossover study was conducted in a Spanish natural late-eating cohort of 845 participants. Each participant underwent two evening 2-h 75-g oral glucose tolerance tests following an 8-h fast: an early condition scheduled 4 h prior to habitual bedtime (“early dinner timing”) and a late condition scheduled 1 h prior to habitual bedtime (“late dinner timing”), simulating an early and a late dinner timing, respectively. Differences in postprandial glucose and insulin responses between early and late dinner timing were determined using incremental area under the curve (AUC) calculated by the trapezoidal method.

The investigators found that melatonin levels in participants' blood were 3.5-fold higher after the late dinner. The late dinner timing also resulted in lower insulin levels and higher blood sugar levels. The team opined that this connection makes sense because insulin acts to decrease blood sugar levels. In the late dinner timing, participants with the MTNR1B G-allele had higher blood sugar levels than those without this genetic variant. According to the researchers, concurrently high endogenous melatonin and carbohydrate intake, as typical for late eating, impairs glucose tolerance, especially in MTNR1B G-risk allele carriers, attributable to insulin secretion defects.