Written By: Brett Singer MS,RD,CSSD,LD – Sports Dietitian Memorial Hermann IRONMAN Sports Medicine Institute
- The body is rhythmic in nature, cycling through every 24 hours. Emerging evidence suggests that limiting eating to certain periods of the cycle may be beneficial. This is called time restricted eating.
- It is unclear whether it is the window of time or the total length of the eating window that is responsible for the benefits seen in the research.
- There is limited data on athletes and women, so it may be inappropriate to apply the findings of research to these populations.
Intermittent fasting, an umbrella term for various schedules of meal timing, offers dietary strategies that are very trendy at the moment. The most popular methods are alternate day fasting, periodic fasting and daily time restricted eating. What makes fasting different from other diets is it focuses on when to eat rather than what to eat. Particularly, daily time restricted eating may prove to be a more sustainable option for people trying to reduce energy intake. Because this method may align more closely with the body’s natural rhythm, there is some evidence suggesting that it has metabolic benefits independent of weight loss.
The body’s circadian rhythm, a 24-hour cycle, is a growing topic of interest in a modern world where more and more environmental cues work against it. A central clock in the brain and peripheral clocks throughout the body work together to regulate the body’s sleep-wake cycle. The central clock is primarily influenced by light. When it gets dark, the central clock sends a message to the body to release melatonin, marking the beginning of the “rest phase”. When the sun rises, the body suppresses melatonin initiating the “active phase”.
Eating is a process that is regulated by the body’s internal clock but is greatly influenced by behavior and environmental cues. The “food clock” in the brain regulates the feeding fasting cycle. It tracks changes in food availability and changes behavior in anticipation of food. The “food clock” responds to feeding, along with signals from peripheral clocks in the kidneys, liver, pancreas, fatty tissue and GI tract.
At the start of the active phase, the body prepares to be fed. The rates of gastric emptying and gastrointestinal motility peak in the morning in order to make room for more food to be consumed. Ironically, appetite is also clock-controlled and tends to be lowest in the morning for most foods. This is believed to occur so that the body can sleep despite the depletion of energy stores overnight. The thermic effect of food, or the energy burned to fuel digestive and absorptive functions, also peaks in the morning. This suggests that for those individuals focused on losing weight, it may be more beneficial to consume a higher proportion of energy in the morning to promote a negative energy balance over time. However, overcoming the body’s natural tendency to diminish appetite in the morning proves to be a challenge for this potential recommendation.
The stress hormone, cortisol, also follows a rhythm. Cortisol levels typically peak 30-45 minutes after waking and stimulates the liver to produce glucose and raise blood sugar. The consumption of large (>30% of daily calories) dinners can exacerbate this blood sugar spike which can be problematic for people with poor control of blood sugar. It is suggested that delaying breakfast until after the spike recedes (~10:00 a.m.) could improve blood sugar control after that first meal (Parr et al., 2020). However, skipping or delaying breakfast is known to impair blood sugar control throughout the day for people with Type 2 diabetes, so the benefit may not hold true for everyone (Heilbronn & Regmi, 2020).
Melatonin is an incredibly important hormone in circadian synchronization since its main role is to main the body’s clock and adjust the rhythm accordingly. When melatonin is released in the body, it indicates the start of biological night, or the “rest phase”. Melatonin production and release is stimulated by dim light and darkness and peaks between 11:00 p.m. and 5:00 a.m. Exposure to light at night causes melatonin levels to fall and disrupts the circadian rhythm.
Once the body enters the rest phase, the body stops expending as much energy for digestion and absorption of food. So, eating closer to or after the onset of melatonin may be associated with increased risk of positive energy balance and weight gain over time. In addition, insulin secretion and insulin sensitivity are both heavily regulated by the circadian rhythm. They peak early in the day and drop in the evening. This explains the association between late-night dinners and the body’s impaired ability to clear glucose from the blood. Lastly, leptin release from fat cells also peaks at night. This hormone works to inhibit hunger, which in turn diminishes fat stores in the fat cells overnight. Light exposure or eating during the rest phase may disrupt these rhythms and have metabolic consequences.
Time Restricted Eating
Time restricted eating (TRE) is a category of intermittent fasting. In TRE, the time frame of eating is controlled rather than the type, amount, or frequency of food eaten within that time period. This method may prove to be more sustainable since it does not require specific foods to be restricted. There are many forms of TRE, ranging from three to twelve-hour periods. Early TRE (eTRE) is when a person eats all of their food for the day within 6-8 hours of waking. Eating this way is theoretically in better alignment with a person’s circadian rhythm, also known as the sleep-wake cycle, and may provide metabolic benefits. Though TRE is most commonly utilized for weight loss, one study showed that cardiometabolic health was improved through eTRE independently of weight loss in prediabetic men (Sutton et al., 2018).
There are still discrepancies whether it is the length of the eating window or the initiation of the window that is more important. eTRE is the most common approach in humans, yet there is no standard definition of length or placement of the window. eTRE was first studied as a 6-hour eating window where the last energy intake took place before 3:00 p.m. Compared to completely unrestricted eating, there was an observed improvement in insulin sensitivity, blood pressure, and oxidative stress. Another study defined eTRE as eating between 8:00 a.m. and 5:00 p.m. and compared it to delayed TRE (dTRE) which required eating from noon to 9:00 p.m. While both protocols improved blood sugar levels in response to a test meal, only eTRE improved overnight fasting blood sugar levels (Hutchinson et al., 2019). Considerations to keep in mind with a strict eTRE protocol are the social acceptability and sustainability for the general population.
Studies of “late” or delayed TRE are limited and again there is no standard window of time. Some research suggests that when total energy intake is consumed only in the evening (after 4:00 p.m), it can result in impaired fasting glucose, lowered glucose tolerance, and increased ratings of hunger. More research should be done comparing the two protocols on other measures.
It is still unclear whether it is the timing of the eating window or the duration that is responsible for the potential benefits of TRE. One study found that when individuals who typically ate over the duration of 14 hours or longer reduced their eating window to 10-12 hours, they reduced total and excess body weight along with BMI (Gill & Panda, 2015). Subjectively, participants also reported higher sleep satisfaction, less hunger at bedtime and improved energy levels. At the one-year follow-up, participants had maintained the weight loss. However, it is unclear whether the weight reduction can be attributed to TRE or the reduction in calories eaten. Discretionary foods such as sweets and alcohol are almost always consumed by humans in the evenings. Rather than moving those foods into the new feeding period, it is possible participants simply chose not to consume the food instead of consuming it at the wrong time of day.
It is important to note that there are concerns about the quality of research we have currently. Much of the research is conducted on prediabetic or diabetic men, with limited data and application in females and athletes. Much of the research shows that TRE results in some degree of lean body mass lost. For athletes who require additional energy to fuel their training and sports, TRE may be more harmful than beneficial. Ultimately, consult with a registered dietitian to see if TRE would be right for you.
Much more research needs to be studied on the human circadian rhythm and nutrition. Time restricted eating according to the body’s natural 24-hour cycle seems to be a promising dietary method to improve metabolic health, however no standard recommendations are available at this time due to considerable variability in protocol and individual results.