Have you ever wondered whether it is true that it is better to eat carbs in the morning and protein in the evening? Or if eating late at night can make you gain more weight than eating during the day? Most probably, you did. And you are not alone. Many scientists have been wondering about the same issues recently, and they have conducted many studies on this topic. This brand-new research trend was named chrono-nutrition.
Chrono nutrition is the science aiming to make sense of the relationship between dietary intake, biological clocks, and human health. It is gradually revealing that meal timing has an impact on human health, by showing that eating at specific times of day can affect metabolism, hormone levels, and overall well-being.
Chrono-nutrition is gaining momentum as more people become interested in optimizing their health through diet. It offers a different approach to traditional nutrition recommendations, which commonly focus on the types and quantities of foods consumed rather than meal timing. Some proponents of chrono-nutrition argue that it can effectively improve health and prevent chronic diseases. However, while some evidence supports the benefits of chrono-nutrition, more studies are needed to understand its effects on human health.
If you are familiar with this topic, you have undoubtedly heard someone talking about “chronotypes”. Although there is no official scientific classification for “chronotypes”, we know that there is a natural tendency to sleep and wake at certain times of the day, which differs among individuals. While some of us are naturally “morning people” and tend to feel more alert and awake in the morning, others are “evening people” and prefer to stay up late and wake up later in the morning. As a rule of thumb, a quarter of the population tends to stay up at night, a quarter is a “morning person,” whereas half sits between these two phenotypes. Evening chronotypes subjects tend to have more social jetlag problems.
Curiously, your personality is also related to your chronotype. Conscientiousness is more common among morning people, whereas openness is more common among night owls. Various factors, including genetics, age, and lifestyle, can influence your chronotype. Newborns do not have a specific chronotype since they develop it within the first months of life. The chronotype changes during adolescence and again at old age.
Diet timing and chronic disease-epidemiological evidence of chrono-nutrition
Irrespective of what chronotype we identify with, we all have an inner biological clock, so the metabolic processes happen at the right time of the day. Our “central clock” is in our brains and responds to light. Peripheral clocks exist and are located in various organs and tissues. Communication among clocks guarantees they act in synchrony and is also related to maintaining body temperature. Internal clocks and eating patterns play a widespread role in metabolism.
Our biological clocks constitute what scientists call the “circadian system,” which is our inner system that regulates daily sleep-wakefulness timing and other behavioral and physiological processes.
Several metabolic processes that occur in our body follow a circadian rhythm. For instance, there is an evident circadian variation in glucose levels during the day, showing a 24h pattern even if we do not eat. Diet-induced thermogenesis is also significantly higher in the morning due to the higher insulin sensitivity. Late eating increases hunger and decreases energy expenditure in overweight and obese adults (Vujovic et al., 2022).
Disturbances of our biological clock due to shift work, jet lag, and social jetlag (i.e., when your body clock is not coupled with your social program) are connected to higher cortisol, increased resting heart rate, insulin resistance, obesity, metabolic syndrome, and systemic inflammation. Dietary intakes are also affected negatively since people with unregular schedules are more likely to eat sweets and less likely to consume vegetables and other fiber-rich foods (e.g., wholegrain, fruit).
The number of people with irregular lifestyles is growing. Shift work is probably the most common disruptor of circadian rhythm. Notably, 17% of the European workforce works in shifts. More specifically, 10% of the total European workers work at night, while 53% of the EU workforce works during the weekend. Shift workers have a higher risk for chronic disease, suggesting circadian disruption potentially leads to many diseases. Shift workers tend to eat later than workers with more regular schedules. They eat their last meal at night, and their meal timing is often irregular. They are also more likely to have poor diet quality. Early eaters tend to lose more weight despite no difference in calories. In line with the latter findings, studies have never found any difference in calorie intakes compared to non-shift workers.
Meal timing and body weight
Irregular meals are a hallmark of irregular lifestyles. Not surprisingly, meal irregularity negatively impacts BMI (Pot et al., 2014). Irregular breakfast and snacks increase the risk of metabolic syndrome, a higher waist circumference, and BMI. Irregular meal frequency is also associated with increased cholesterol, glucose, and insulin levels, and a lower thermic effect of food. The latter is how much energy from food our body dissipates as heat after a meal. A systematic literature study by Canuto et al. (2017) suggested (but did not prove) the presence of an inverse association between meal frequency and adiposity.
The large American study NHANES showed that the absolute amount of energy is greater among people eating more frequently, despite more calories consumed through snacks mean less consumed at lunch and dinner (Kant and Graubard, 2015; Kant, 2018).
Studies on long-term meal timing (Garaulet et al., 2013; Jacubowitz et al., 2013) suggest that earlier calories work better to lose weight. When comparing morning and evening meals, studies found no difference in energy intake and expenditure. However, individuals having a large breakfast but a small dinner tended to have lower subjective hunger. Dinner carbohydrates appear to be crucial for body weight control. The fewer carbs at dinner, the better it is for your BMI.
A few years ago, Sutton et al. (Cell Metabolism, 2018) performed an interesting study where they demonstrated, for the first time, that early-time restrictive feeding (6-hr feeding period, with dinner before 3 pm) has beneficial health effects (such as increased insulin sensitivity, healthier blood pressure as well as lower oxidative stress and appetite) irrespective of weight loss or food intake.
What’s the best time to eat breakfast for weight loss?
Another topic strictly related to chrono-nutrition that is constantly debated is the “importance” of breakfast for health. Breakfast skipping is usually associated with a higher risk of overweight and having type 2 diabetes. Those who skip breakfast have a higher risk of cardiovascular risk too. However, the problem with skipping breakfast is also that one gets hungry later in the morning when there is a limited access to healthy food (e.g., at work, in the car, etc.). If you tend to get hungry later during the morning and do not feel like eating right after waking up, make sure you bring some healthy food with you and have a healthy mid-morning snack. Some examples of healthy food to keep at hand include fruit, nuts, yogurt and muesli, wholegrain bread, and hummus.
Also, people who skip breakfast are also more likely to smoke and have other unhealthier behaviors. Therefore, the higher weight gain and disease risk we commonly observe among these individuals, could be because skipping breakfast tends to be more common among people with unhealthy lifestyles but is not necessarily a direct cause of disease.
Does caffeine affect our circadian clock?
We know that exposure to bright light before bed can make us stay awake for several hours because it disrupts our master circadian clock and sleep pattern. Caffeine acts similarly and strongly affects the circadian clock, and the human internal clock is affected by night caffeine consumption (Burke et al., 2015; Narishige et al., 2014). Science showed that caffeine can keep us awake and disrupt sleep. Therefore, caffeine acts similarly to other factors that disrupt our circadian clocks such as light, feeding, physical activity, and specific pharmaceutical drugs (Burke et al., 2015).
Drinking coffee or another source of caffeine induces changes in several cells and tissues since it stimulates the release of excitatory neurotransmitters and decreases melatonin levels. The effect on melatonin is crucial for our internal clocks because melatonin signals to our internal biological clock that it is night, initiating a physiological response that results in falling asleep. Melatonin concentration fluctuates across the day, regulated by our central nervous system. Monitoring melatonin levels is one of the most precise measures of circadian timing. However, as for the effect of caffeine consumption of melatonin levels, there are differences among individuals, which explains why certain people are more sensitive to it and have trouble sleeping if they drink coffee after a particular time. Plasma melatonin rhythms differ between lean, overweight, and obese individuals, with more considerable variations among the latter. On a positive note, caffeine consumption can help adapt to a new time zone when traveling long distances, especially towards the west.
What’s the best time to eat? Take home message
Based on what you read in this article, meal irregularity is potentially associated with a greater risk of metabolic syndrome and obesity, which are, in turn, risk factors for many other chronic diseases. Therefore, clock time could be an important determinant of weight loss and chronic disease risk.
It is essential to point out that most studies on chrono-nutrition were observational studies conducted using diet questionnaires. The latter are more prone to measurement errors and their validity is difficult to prove due to a lack of a “gold standard” for measuring human dietary behavior. It will be interesting to see the results of more “experimental” studies like intervention studies, particularly randomized controlled trials.
To sum up, the recommendations for a healthy chrono-nutrition are to avoid snacking and only eat 2 to 3 meals a day, not skip breakfast, have the last meal of the day earlier (i.e., around 3-4 pm) to ensure a longer fasting time, at least 12 hours. It is also a good idea to avoid late night meals, and increase meal protein content.
In case you want to track the timing of your dietary habits, you can try an app called “my circadian clock”, developed by the Salk institute in California. This app allows users log in what they are eating (including food pictures) along with their sleep patterns and information about their physical activity level. By combining the latter information, the app develops a so-called “feedogram”, which is a raster plot of when people eat. Data collected through this app so far, show that over 50% of adults had an eating window larger than 14h. A study by the researchers who developed the app showed that participants could decrease their body weight by 5% by decreasing the eating window. Try it and let me know if you discover something surprising about your meal timing and if it works.
- Burke TM et al. Effects of caffeine on the human circadian clock in vivo and in vitro. Sci. Transl. Med. 2015 September 16; 7(305): 305ra146.
- Canuto R et al. (2017). Eating frequency and weight and body composition: a systematic review of observational studies. Public Health Nutr. 20(12):2079-2095.
- Garaulet M et al. (2013). Timing of food intake predicts weight loss effectiveness. Int. J. Obesity 37(4):604-11.
- Jacubowitz D et al. (2013). High Caloric intake at breakfast vs. dinner differentially influences weight loss of overweight and obese women. Obesity 21(12): 2504-12.
- Kant AK (2018). Eating patterns of US adults: Meals, snacks, and time of eating. Physiology & behavior. 193(Pt B):270-278.
- Kant AK and Graubard BI (2015). 40-year trends in meal and snack eating behaviors of American adults. J Acad Nutr Diet. 115(1): 50–63.
- Pot GK, Hardy R, and Stephen AM (2014). Irregular consumption of energy intake in meals is associated with a higher cardio-metabolic risk in adults of a British birth cohort. Int J Obesity 38(12): 1518–1524.
- Sutton EF et al. (2018). Early Time-Restricted Feeding Improves Insulin. Sensitivity, Blood Pressure, and Oxidative Stress. Even without Weight Loss in Men with Prediabetes. Cell Metabolism 27, 1212–1221.
- Vujovic N. et al. (2022). Late isocaloric eating increases hunger, decreases energy expenditure, and modifies metabolic pathways in adults with overweight and obesity. Cell Metab; 34(10):1486-1498.e7.