19 May 2020
Chrono-nutrition is an emerging field of nutritional science that aims to develop understanding of how when we eat may impact on our health. One of the initial definitions given in the scientific literature was by Asher in 2015: “This concept reflects the basic idea that, in addition to the amount and content of food, the time of ingestion is also critical for the well-being of an organism.”( 1 See references)
In Northern European countries, ~40% of energy intake is consumed at ‘dinner’ compared to ~18% at ‘breakfast’.2 In the UK, the last few decades has seen a shift towards increased energy intake in the afternoon and evening.3
The contribution of these eating patterns to cardiometabolic disease has been increasingly explored in nutritional epidemiology. Observational studies have found that, independent of energy intake and physical activity, the timing of food intake can influence weight gain.4 In particular, consuming a higher percentage of daily energy intake during the evening has been associated with cardiometabolic syndrome and obesity.5 Assessment of the 1946 British Birth Cohort observed a higher prevalence of hypertension in those eating a greater percentage of their energy intake in the evening compared to the morning.6 This finding supports the positive association found between breakfast frequency and reduced cardiometabolic risk.7 However breakfast skipping has been shown to be associated with higher evening energy intake.5 Therefore, future observational studies should take into account energy consumed in the evening to determine the relative benefit of breakfast eating compared to higher evening energy intakes.
Randomised controlled trials (RCT) have expanded on observational findings by testing various hypotheses to understand biological plausibility. The Bath Breakfast Project observed that breakfast eaters demonstrated higher physical activity thermogenesis during the six-week randomised controlled trial and, while no changes in anthropometric measures were reported, more stable afternoon and evening blood glucose concentrations were observed when breakfast was consumed.8 There have been numerous studies that have aimed to investigate differential responses of glucose metabolism to time of feeding protocols. These were summarised in a recent meta-analysis, the results of which concluded that glucose tolerance at night is poorer compared to the daytime.9 Moreover, simulated night shift work experiments have demonstrated reduced glucose tolerance after an acute period of ‘night work’ in healthy volunteers.10 These findings have relevance to not only shift workers who are at increased risk of type 2 diabetes11 and weight gain,12 but also to the general population who tend to eat later in the day.2
Understanding the likely causal mechanisms is complex. The influence of eating patterns on cardiometabolic health is related to circadian rhythms. Humans have evolved endogenous physiological rhythms to mirror a diurnal existence where activities take place during daylight hours and sleep during darkness. These rhythms have been conserved as advantageous as they anticipate regular changes in our environment to maintain homeostasis.
These endogenous molecular rhythms are entrained via ‘zeitgebers’ (cues). The main zeitgeber is daylight that entrains the ‘master clock’ (the suprachiasmatic nucleus, located in the hypothalamus)), which controls various neuroendocrine pathways, including those involved in metabolism.13
Cellular pathways that follow a circadian rhythm are found in almost all tissues in the human body. To maintain homeostasis, these peripheral cellular pathways synchronise themselves to the master clock. Many functions of digestion, absorption and metabolism follow a daily rhythm based on cycles of light and dark.
Nutritional intake has recently been recognised as a ‘zeitgeber’ and has been shown to entrain peripheral ‘clocks’ including gene transcription.14 Eating out of ‘sync’ with our master biological clock (e.g. eating at night – when our master clock is signalling us to sleep) has been hypothesised to contribute to adverse cardiometabolic health.13
A further candidate mechanism is via the microbiome; there are limited studies conducted in humans but studies in rodents have reported functional and compositional changes in line with circadian rhythmicity15 and alerted altered gut microbiota profile with circadian disruption.16 While it is important to acknowledge the importance and limitations of animal models to nutritional research, in circadian research it needs to be remembered that rodents are nocturnal species, and therefore the translation to humans is yet to be determined.
It is clear that further research is required to develop our understanding of how eating patterns can modify health, and subsequently its relevance to dietetic and public health nutrition practice.
In the last few years chrono-nutrition has gained traction in nutritional science. In 2017 the UK Medical Research Council highlighted the importance of research into “the effect of biological rhythms on nutritional response”17 and the American Heart Association published a scientific statement, Meal Timing and Frequency: Implications for Cardiovascular Disease Prevention.18 There are also exciting studies that are underway including The Big Breakfast Study19, a Medical Research Council-funded RCT that aims to investigate the mechanisms linking temporal energy distribution to energy balance. There is also increasing interest around the efficacy and mechanisms of daily fasting duration (time restricted eating) on cardiometabolic health.20
Developments in nutritional research methods will be key to advancing the field. The capturing of eating occasions via smart phone applications, and the wearing of continuous recording devices measuring actigraphy, heart rate and blood glucose will provide valuable information in understanding how temporal eating patterns influence physiological processes and health.
Dietitians are well placed to investigate important research gaps in the role of chrono-nutrition in different patient groups. For example, how do different eating patterns in people with diabetes influence glucose control, and can the timing of enteral nutrition regimes impact on dietetic outcomes?
Is when we eat as important as what we eat? Only time, and further research will tell.