Plain language summary
Time-Restricted Eating (TRE) is an eating pattern based on the circadian rhythm that gives the body a daily fasting period. This involves limiting daily food intake to 8–12 hours with an intention to prolong the time spent in a fasting state. In fact, chrono-nutrition is an emerging field of nutritional science that aims to understand how timing of food-intake may impact our health by affecting our circadian rhythm. The aim of this study was to examine the effects of two patterns of TRE, traditional TRE (a predetermined window of eating and fasting) and Ramadan fasting, on two markers of circadian rhythm, cortisol and melatonin. This study was a systematic review and meta-analysis of fourteen studies (13 reported results on cortisol, and four on melatonin). Results showed that: - Ramadan practice of TRE resulted in a reduced waking response of cortisol, and in most cases a statistically significant increase relative to non-Ramadan in the evening. - the Ramadan studies also found significant reductions in levels of nocturnal melatonin during Ramadan compared to baseline. - in the non-Ramadan TRE studies, skipping dinner led to a decreased evening cortisol level, and, potentially, an elevated morning level (however not in all cases waking time was accounted for). - skipping breakfast led to a relative increase in midday cortisol. Authors conclude that their findings indicated a potential blunting of the circadian cortisol rhythm during Ramadan and a reduction in melatonin which could lead to poor sleep duration and quality.
Expert Review
Conflicts of interest:
None
Take Home Message:
- 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.
Evidence Category:
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A: Meta-analyses, position-stands, randomized-controlled trials (RCTs)
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X
B: Systematic reviews including RCTs of limited number
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C: Non-randomized trials, observational studies, narrative reviews
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D: Case-reports, evidence-based clinical findings
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E: Opinion piece, other
Summary Review:
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.
Clinical practice applications:
- 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.
Considerations for future research:
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
Abstract
Time-Restricted Eating is an eating pattern based on the circadian rhythm which limits daily food intake (usually to ≤12 h/day), unique in that no overt restriction is imposed on the quality, nor quantity, of food intake. This paper aimed to examine the effects of two patterns of TRE, traditional TRE, and Ramadan fasting, on two markers of circadian rhythm, cortisol and melatonin. PubMed and Web of Science were searched up to December 2020 for studies examining the effects of time restricted eating on cortisol and melatonin. Fourteen studies met our inclusion criteria. All Ramadan papers found statistically significant decrease in melatonin (p < 0.05) during Ramadan. Two out of the three Ramadan papers noted an abolishing of the circadian rhythm of cortisol (p < 0.05). The non-Ramadan TRE papers did not examine melatonin, and cortisol changes were mixed. In studies comparing TRE to control diets, Stratton et al. found increased cortisol levels in the non-TRE fasting group (p = 0.0018) and McAllister et al. noted no difference. Dinner-skipping resulted in significantly reduced evening cortisol and non-significantly raised morning cortisol. Conversely, breakfast skipping resulted in significantly reduced morning cortisol. This blunting indicates a dysfunctional HPA axis, and may be associated with poor cardio-metabolic outcomes. There is a paucity of research examining the effects of TRE on cortisol and melatonin. The contrasting effect of dinner and breakfast-skipping should be further examined to ascertain whether timing the feeding window indeed has an impact on circadian rhythmicity.
Lifestyle medicine
Fundamental Clinical Imbalances
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Hormonal
Methodological quality
Jadad score
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Not applicable
Allocation concealment
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Not applicable
