Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans

Abstract

Time-restricted feeding (TRF) is a form of intermittent fasting that involves having a longer daily fasting period. Preliminary studies report that TRF improves cardiometabolic health in rodents and humans. Here, we performed the first study to determine how TRF affects gene expression, circulating hormones, and diurnal patterns in cardiometabolic risk factors in humans. Eleven overweight adults participated in a 4-day randomized crossover study where they ate between 8 am and 2 pm (early TRF (eTRF)) and between 8 am and 8 pm (control schedule). Participants underwent continuous glucose monitoring, and blood was drawn to assess cardiometabolic risk factors, hormones, and gene expression in whole blood cells. Relative to the control schedule, eTRF decreased mean 24-hour glucose levels by 4 ± 1 mg/dl (p = 0.0003) and glycemic excursions by 12 ± 3 mg/dl (p = 0.001). In the morning before breakfast, eTRF increased ketones, cholesterol, and the expression of the stress response and aging gene SIRT1 and the autophagy gene LC3A (all p < 0.04), while in the evening, it tended to increase brain-derived neurotropic factor (BNDF; p = 0.10) and also increased the expression of MTOR (p = 0.007), a major nutrient-sensing protein that regulates cell growth. eTRF also altered the diurnal patterns in cortisol and the expression of several circadian clock genes (p < 0.05). eTRF improves 24-hour glucose levels, alters lipid metabolism and circadian clock gene expression, and may also increase autophagy and have anti-aging effects in humans.

Keywords: circadian rhythms; circadian system; intermittent fasting; meal timing; time-restricted feeding.

Figure 1

Study Protocol. Eleven participants were randomized to eat between 08:00 and 20:00 (control arm) or between 08:00 and 14:00 (early time-restricted feeding (eTRF) arm) for 4 days and then crossed over to the other arm after a 3.5–5-week washout period. On day 4, they consumed 3 identical meals that constituted one-third of their daily energy requirements, while undergoing 24-hour continuous glucose monitoring. In addition, blood was drawn in the evening (PM) on day 3 and in the morning (AM) on day 5 to measure serum analytes and gene expression.

Figure 2

24-Hour Glucose Levels. Relative to the control schedule, early time-restricted feeding (eTRF) (A) changed the temporal profile of 24-hour glucose levels, as measured by continuous glucose monitoring, particularly in the evening, (B) lowered mean glucose levels while asleep and decreased 24-hour mean glucose levels, and (C) lowered glycemic excursions as measured by Mean Amplitude of Glycemic Excursions (MAGE). Error bars on panel (A) are suppressed for visual clarity. * p < 0.05.

Figure 3

Glycemic Markers. Relative to the control schedule, early time-restricted feeding (eTRF) decreased (A) fasting glucose, (B) fasting insulin, and (C) Homeostatic Model Assessment of Insulin Resistance (HOMA-IR) in the morning (AM) and increased (B) fasting insulin and (C) HOMA-IR in the evening (PM). (D) eTRF also increased the expression of the IRS2 and AKT2 genes in the morning and evening, respectively. Data for GLUT1 and GLUT4 are shown excluding a participant whose expression levels were ~10× higher than the sample mean. * p < 0.05.

Figure 4

Lipids. Relative to the control arm, early time-restricted feeding (eTRF) increased fasting levels of (A) total cholesterol, (B) LDL cholesterol, (C) HDL cholesterol, and (F) β-hydroxybutyrate (ketones) in the morning (AM) but did not affect levels of (D) triglycerides or (E) free fatty acids. (A–F). Levels of all lipids were unaffected in the evening (PM). * p < 0.05.

Figure 5

Hormones. Relative to the control arm, early time-restricted feeding (eTRF) tended to increase (F) cortisol levels in the morning (AM). In the evening, it lowered (F) cortisol and tended to increase (E) brain-derived neurotrophic factor (BDNF). The hormones (A) insulin-like growth factor (IGF-1), (B) IGF-binding protein 1 (IGFBP-1), (C) IGF-binding protein 3 (IGFBP-3), and (D) human growth hormone (HGH) were not significantly different between arms. * p < 0.05, ^# p < 0.10.

Figure 6

Gene Expression in Whole Blood. (A) Early time-restricted feeding (eTRF) changed the expression of several circadian clock genes, including BMAL1, PER1, CRY1, CRY2, REV-ERBA, and RORA in the morning (AM) and/or evening (PM). It also increased the expression of (B) the longevity genes SIRT1 in the morning and MTOR in the evening. (C) The autophagy genes LC3A and ATG12 were elevated in the morning and evening, respectively, although the latter was no longer significant after adjustment for multiple comparisons. (D) The expression of oxidative stress genes was unaffected. Genes in panel A were pre-specified outcomes, while genes in panels B–D were exploratory and had their p-values adjusted for multiple comparisons. ^# p < 0.10 (applied only to pre-specified genes in panel A), * p < 0.05, ** p < 0.007 (significant after adjustment for multiple comparisons, applied only to exploratory genes in panels B–D).