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Is when we eat just as important as what we eat?

There is increasing awareness about the impact of when we eat and how this influences disease risk. This is the study of chrononutrition.

What is chrononutrition?

‘Chrono’ comes from the Greek chronos, meaning time. Chrononutrition is the study of how the timing of eating affects health. It has developed from the study of chronobiology, which is how natural ‘chronomes’ or timing processes affect organisms. In humans it is how circadian rhythms and ‘biological clocks’ impact metabolism.


Glossary of terms

Circadian rhythm is the pre-determined behavioural and physiological pathway over a 24-hour cycle that regulates metabolic homeostasis.

Time-Restricted Eating (TRE) limits the daily eating time window.

Intermittent Fasting (IF). An alternating pattern of fasting and normal ad libitum eating.


The bigger picture

Since the industrial revolution and the invention of electric lighting our lifestyles have changed from being awake, active and eating during daylight hours, to longer work schedules, meal skipping, evening screen entertainment and late-night eating. This change has been termed ‘social jetlag’ and it is hypothesised that this has led to disruptions in circadian rhythms and increased metabolic risk. Chrononutrition approaches such as intermittent fasting and time-restricted eating have become a pop-diet phenomenon with numerous books, websites and programs available.

Daily eating time

Research in the USA found the average eating time window was about 12 hours (7am - 7pm, for example), but for some it reached 15 hours (7am - 10pm, for example) (Kant 2018). There is now a suggestion that a longer eating time window may be a factor in the development of metabolic disorders and limiting the eating window may reduce risk. The two main approaches used are Time-Restricted Eating (TRE) and Intermittent Fasting (IF). Both are showing promising beneficial results on metabolic indicators including weight loss.

Time-Restricted Eating (TRE)

Time-Restricted Eating (TRE) is an eating pattern that limits the daily eating window to between 4 - 12 hours; consequently, a fasting window of 20 ­­- 12 hours. One author suggests restricting the eating window to 6-10 hours is best matched with the circadian clock (Charlot 2021). The TRE approach does not require energy restriction (although this may incidentally occur), but rather a time-limited ad libitum approach followed consistently.

Prolonging the overnight fast has been found to reduce breast cancer recurrence and lower systemic inflammation. In animal studies TRE reduces the onset and progression of metabolic disease even with poor diet and existing metabolic disease.

Intermittent Fasting (IF)

Intermittent Fasting (IF) is an intervention that alternates between a period of fasting and a period of normal ad libitum eating. The period of fasting can vary from 1-3 days per week, or it can be fasting on alternate days. This approach was popularised by the ‘5:2 diet’, written by UK media presenter Dr Michael Mosely, although he promoted a 500 calorie limit on the fasting days rather than fasting per se.

What mechanisms are implicated?

Charlot (2021) believes beneficial results of TRE are due to better alignment between food intake and circadian rhythms, exemplified by an early meal timing window (8am->4-6pm). She suggests that mixed results in the literature may be due to inconsistent methodological control of subjects feeding windows because both early meal timing (metabolically ideal) and more socially convenient late meal timing are both used in research.

In a perspective article about TRE, O’Connor (2021) suggests there are multi-level influences to explain why TRE might work better than traditional energy restriction for some people. It may counteract physiological adaptation to weight loss, allow usual preferences to be maintained, preserve executive functioning (managing oneself to achieve a goal) and enable individuals to withstand social pressures to overeat. On the other hand, it may not be good for people with poor diets to begin with, ‘grazers’, or people with lifestyles that make scheduled meals difficult.

What does the evidence say?

Research in chrononutrition (TRE and IF) has flourished in the past few years, and numerous systematic reviews have been published.

A systematic review of 23 studies on Time-Restricted Eating (TRE) by Adafer (2020) found TRE led to an average 3% weight loss and loss of fat mass even without energy restriction, and beneficial metabolic effects independently of weight loss suggesting an intrinsic timing effect.

A systematic review of 19 studies on the effects of TRE on metabolic disease by Moon (2020) found significant decreases in body weight, fat mass, blood pressure, fasting glucose and cholesterol profile.

A systematic review and meta-analysis of intermittent fasting (IF) versus energy-restricted diets (ERD) on lipid profile by Meng (2020) found both IF and ERD resulted in significant reductions in total cholesterol, LDL cholesterol and triglyceride concentrations.

There have been several systematic reviews and meta-analyses on Intermittent fasting that have found:

  • Improved anthropometric outcomes and intermediate disease markers (weight, waist, fat mass, triglycerides, systolic blood pressure), although weight outcomes were similar to standard energy restriction (Schwingshakl 2021).
  • Reduced weight, BMI, fat mass and total cholesterol (Park 2020).
  • A reduction in the inflammatory marker C-reactive protein (CRP), particularly in overweight individuals (Wang 2020)
  • More effective reduction in systolic and diastolic blood pressure than regular energy-restricted diets (Kord-Varkaneh 2020)
  • Diurnal IF of Ramadan may confer a small reduction in body weight in non-athletic healthy people over 16 years, directly associated with fasting time (Jahrami 2020).

Perhaps the final word on IF should go to a Cochrane Database Systematic Review published this year (Allaf 2021). The authors concluded IF was superior to ad libitum feeding in reducing weight but it was not clinically significant. There was no clinically significant benefit from IF in improving cardiovascular risk factors. Further research is needed to identify risk-benefit analysis in specific groups (such as people with diabetes) as well as longer term outcomes.

Limitations to chrononutrition

One study of TRE in people with type 2 diabetes found hunger, daily life stressors and emotions were barriers to adherence. However, overall TRE was neutral in its impact on emotional wellbeing (Park 2020).

Like any change to a normal eating pattern, implementing chrononutrition approaches can be challenging because they are inconsistent with cultural and societal norms. For many families, dinner is the most important meal, practically, nutritionally and socially. Bringing this meal forward can be difficult.


  • There is growing evidence that chrononutrition approaches have metabolic benefits, although their clinical significance is not yet clear.
  • Intermittent fasting or time-restricted eating may suit some people better than others.
  • Implementing chrononutrition approaches can be challenging within modern lifestyles.
  • For best outcomes it may be beneficial to enhance diet quality during ad libitum


  • Adafer R, Messaadi W, Meddahi M, et al. (2020). Food timing, circadian rhythm and chrononutrition: a systematic review of time-restricted eating's effects on human health. Nutrients12(12): 3770.
  • Allaf M, Elghazaly H, Mohamed OG, et al. (2021). Intermittent fasting for the prevention of cardiovascular disease. Cochrane Database Syst Rev, 1(1):CD013496. doi: 10.1002/14651858.CD013496.pub2.
  • Charlot A, Hutt F, Sabatier E, & Zoll J. (2021). Beneficial effects of early time-restricted feeding on metabolic diseases: importance of aligning food habits with the circadian clock. Nutrients13(5): 1405.
  • Jahrami HA, Alsibai J, Clark C, & Faris M. (2020). A systematic review, meta-analysis, and meta-regression of the impact of diurnal intermittent fasting during Ramadan on body weight in healthy subjects aged 16 years and above. European Journal of Nutrition59(6): 2291–2316.
  • Kord-Varkaneh H, Nazary-Vannani A, Mokhtari Z, et al. (2020). The influence of fasting and energy restricting diets on blood pressure in humans: a systematic review and meta-analysis. High Blood Pressure & Cardiovascular Prevention27(4),\: 271–280.
  • Kant AK. (2018). Eating patterns of US adults: meals, snacks, and time of eating. Physiol Behav,193(Pt B): 270-278. doi: 10.1016/j.physbeh.2018.03.022.
  • Meng H, Zhu L, Kord-Varkaneh H, et al. (2020). Effects of intermittent fasting and energy-restricted diets on lipid profile: a systematic review and meta-analysis. Nutrition77: 110801.
  • Moon S, Kang J, Kim SH, et al. (2020). Beneficial effects of time-restricted eating on metabolic diseases: a systemic review and meta-analysis. Nutrients12(5): 1267.
  • O'Connor SG, Boyd P, Bailey CP, et al. (2021). Perspective: time-restricted eating compared with caloric restriction: potential facilitators and barriers of long-term weight loss maintenance. Advances In Nutrition12(2): 325–333.
  • Park J, Seo YG, Paek YJ, et al. (2020). Effect of alternate-day fasting on obesity and cardiometabolic risk: a systematic review and meta-analysis. Metabolism: clinical and experimental111: 154336.
  • Schwingshackl L, Zähringer J, Nitschke K, et al. (2021). Impact of intermittent energy restriction on anthropometric outcomes and intermediate disease markers in patients with overweight and obesity: systematic review and meta-analyses. Crit Rev Food Sci Nutr, 61(8):1293-1304. doi: 10.1080/10408398.2020.1757616.
  • Wang X, Yang Q, Liao Q, et al. (2020). Effects of intermittent fasting diets on plasma concentrations of inflammatory biomarkers: a systematic review and meta-analysis of randomized controlled trials. Nutrition79-80: 110974.


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