The Window Matters: A Systematic Review of Time Restricted Eating Strategies in Relation to Cortisol and Melatonin Secretion.

None

• Other human and animal studies have highlighted the benefits of restricting the feeding window for longevity and metabolic health.
• Whether one chooses early or late TRE may help to determine the optimal time of alertness and sleep.
• Early TRE, which involves skipping dinner, may allow for greater alertness in the morning as the lower levels of cortisol at night can improve sleep quality and higher morning cortisol raises alertness, optimising wakefulness and productivity.

Introduction: This systematic review aimed to examine the effects of two patterns of Time Restricted Feeding (TRE): (1) traditional TRE, and (2) Ramadan fasting, on two markers of circadian rhythm: (1) cortisol and (2) melatonin.

Methods

• A search was performed on PubMed, and Web of Science (all databases) up to December 2020 using search terms relating to “time-restricted eating”, “intermittent fasting”, “cortisol”, “melatonin”, and “orexin”.
• 14 randomised controlled trials and observational studies that met the criteria were included.
• Quality assessment of all studies was performed

Results

14 studies met the inclusion criteria. 10 of the studies used TRE in the context of Ramadan (total n=242) whilst 4 followed non-Ramadan TRE protocols (total n=124). Of the 14 studies identified in the literature review, 13 reported results on cortisol, and 4 on melatonin.

• All 4 melatonin studies found a statistically significant reduction of melatonin (p < 0.05) during Ramadan.
• 2 of 3 studies assessing 24-h serum cortisol changes during Ramadan noted an “abolishing” of the circadian rhythm of cortisol (p < 0.05), meaning there was a flattening of cortisol levels during the fasting month compared with the non-fasting month.
• All 13 cortisol studies recorded a cortisol shift in response to time restricted eating, albeit over different time frames and different cortisol recording methods.
• 1 randomised 4-day crossover study (n=11) found skipping dinner resulted in significantly reduced evening cortisol (p=0.03) and non-significantly raised morning cortisol. Skipping breakfast resulted in significantly reduced morning cortisol (p=0.10) .
• 1 observational study of females aged 18-45 (n-65) found that skipping breakfast demonstrated decreased morning (waking) cortisol, elevated midday cortisol, and no significant evening cortisol differences compared to the control group.

Conclusion

The potential blunting of the circadian cortisol rhythm during Ramadan may impact the HPA axis and is associated with poor cardio-metabolic outcomes. A reduction in melatonin could lead to poor sleep duration and quality.

• The mechanisms behind these findings are not well understood, but are likely to include: the complex endocrine interplay with insulin response and glucocorticoid secretion.
• Caution must be taken in terms of generalisability due to the small sample size of the included studies

Early or late TRE can determine the optimal time of alertness and sleep:

• Early TRE (eating 8am-2pm / skipping dinner), may allow for greater alertness in the morning as the lower levels of cortisol at night can improve sleep quality and higher morning cortisol raises alertness, optimising wakefulness and productivity.
• Late TRE (skipping breakfast) may provide greater alertness mid-day, due to an intensified cortisol response mid-day.

In order to better understand the effects of meal timing on health outcomes, further larger studies should be conducted. They should:

• Examine TRE during the inactive and active phases of the circadian rhythm under controlled conditions.
• Exclude studies relating to pregnancy, pathology, and athletic status.
• Ensure sufficient control for confounding factors that can affect serum parameters, including sleep duration and timing, total energy expenditure, and light exposure