Introduction: Altering the temporal distribution of energy intake and incorporating extended fasting periods induces metabolic effects that may improve health (Templeman et al., 2020). Early time-restricted eating (eTRE), involving restricting daily energy intake to an early eating window, may be an optimal due to better alignment of eating with circadian rhythm (Clayton et al., 2020). Adherence to restrictive dieting is low (Rogers et al., 2016), so determining the effects of short-term adherence on health is important. This study investigated the metabolic and behavioural responses to 5-days of eTRE, in lean males.

Methods: Sixteen healthy males (age: 24 ± 3 years, BMI: 23 ± 1 kg/m², Body fat: 15 ± 3 %) completed control (CON) and eTRE trials, in random, crossover order. eTRE required adherence to an 8-hour eating window (0800-1600), and CON a 12-hour eating window (0800-2000), for 5 consecutive days. Standardised diets (2840 ± 185 kcal; 51% carbohydrate; 18% protein; 30% fat) were consumed on day-1 and day-4, and responses to a high carbohydrate breakfast (711 ± 51 kcal; 73% carbohydrate; 12% protein; 15% fat) was assessed during laboratory visits on day-2 and day-5. Blood samples were collected 0-, 1-, 2-, and 4-h post-breakfast, to assess metabolic (glucose, insulin, homeostatic model of insulin resistance (HOMA-IR) and lipids) and appetite-regulatory (acylated ghrelin (AG), glucagon-like peptide-1 (GLP-1), peptide YY (PYY), and leptin) markers. Subjective appetite and substrate oxidation (via indirect calorimetry) were measured 0-, 1-, 2-, 3-, and 4-h post-breakfast, and ad-libitum energy intake was assessed at lunch/snack/dinner meals, taking place at 12:00/13:00-19:00/19:30 during CON, or 12:00/13:00-15:00/15:30 during eTRE. Area-under-the-curve (AUC) was calculated for blood and subjective appetite. Linear mixed models assessed differences between trials and laboratory visits, followed by Bonferroni-adjusted post-hoc pairwise comparisons, where appropriate. Study was approved by the Nottingham Trent University Human Invasive Ethics Committee (REF: 704). 

Results: There were no effects for glucose (P=0.111), but insulin AUC was 844 ± 570 pmol/L lower and HOMA-IR was 0.18 ± 0.14 lower on day-5 during eTRE vs. CON (both P<0.001). Low-density lipoprotein was higher on eTRE vs. CON (P<0.05), with no other blood lipid differences between trials (P>0.05). Across both laboratory visits, total fat oxidation was 35 ± 25 % higher and total carbohydrate oxidation was 11 ± 6 % lower, during eTRE vs. CON (both P<0.001). GLP-1 AUC was 494 ± 759 pmol/L higher in eTRE than CON on day-5 (P<0.05), with no differences in PYY or AG (P>0.05). Leptin was greater overall during CON vs. eTRE (P<0.05). Energy intake was 179 ± 259 kcal lower on day-2 (P=0.05) and 363 ± 390 kcal lower on day-5 (P<0.001), during eTRE vs. CON. Hunger was lower overall during eTRE vs. CON (P<0.001) and reduced by 15 ± 9 % between day-2 and day-5 during eTRE (P<0.001).

Conclusions: Short-term adherence to eTRE can improve insulin sensitivity, increase fat oxidation, and reduce daily energy intake, and may support appetite control over time. These metabolic and behavioural effects of eTRE may be conducive to improved health and weight management long-term.