Background & aims: Glucose metabolism is, in part, regulated by the circadian rhythm. Postprandial glucose response is exaggerated and insulin sensitivity is reduced at night compared with the morning. Sustained poor glucose tolerance may be related to the increased risk of type-2 diabetes mellitus and cardiovascular disease experienced by shift workers. Manipulation of meal type may be able to dampen such postprandial excursions. Therefore, the study's aim was to investigate postprandial glucose and insulin responses to a low glycemic index (GI) meal in the morning compared to night in healthy volunteers.
Methods: An oral glucose tolerance test (OGTT), was undertaken to confirm diurnal glucose response. Participants consumed a glucose solution at 0800h (morning) and 2000h (evening). In a separate trial, participants consumed a low GI meal (3.3 MJ, 48% energy (E) from carbohydrate, 40%E from fat and 11%E from protein, 22 g fiber) at 0800h, 2000h and 0000h (midnight). Postprandial glucose and insulin were collected over 3 h. Incremental area under the curve (iAUC) was calculated and significance tested using Wilcoxon-signed rank. A p-value <0.05 was taken as significant.
Results: In the OGTT (n = 10), postprandial glucose iAUC was higher in the evening compared to morning (p = 0.007). In the low GI meal trial (n = 9), postprandial glucose iAUC at evening and midnight were higher than the morning (p = 0.008, p = 0.021) but not significantly different between evening and midnight (p = 0.594). Postprandial insulin iAUC was also higher in the evening and at midnight compared to the morning (p = 0.008 for both).
Conclusions: The current study confirms that meal intake at night, even when comprised of low glycemic ingredients, contributes to higher glucose excursions and concomitantly greater insulin levels, compared with an equivalent meal in the morning. This demonstrates that meal timing has an effect on glucose metabolism, which can be observed from as early as 8pm and persists throughout the night. This identifies meal timing as an important modifiable risk factor for metabolic-related disease, which may have implications for high risk populations such as shift workers but also the general population.
Trial registration: Study ID number: ACTRN12616000164493; Website of trial registry: http://www.anzctr.org.au/.
Keywords: Circadian rhythm; Diurnal; Insulin; Night; OGTT; Shift work.
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