Intermittent fasting is an umbrella term that refers to repeated periods of fasting that extend beyond the duration of a typical overnight fast. It encompasses three branches: alternate day fasting, periodic fasting, and time-restricted feeding (TRF).
Alternate-day fasting rotates between days of complete fasting and ad libitum (i.e., unrestricted) eating. Alternate-day modified fasting follows the same pattern but “fasting” days feature an energy intake <25% of baseline energy needs.
Periodic fasting can vary anywhere from utilizing complete or modified fasts once per week to once per month. The most popular form is the 5:2 diet, which includes two days of severe energy restriction (400-600 kcal per day) and five days of ad libitum eating.
Lastly, TRF encompasses limiting food intake to a specific window of time. Since the median American eats over a 12-hour period, it has been formally defined as eating within a ≤ 10-hour period and fasting for at least 14 hours per day.
Regardless of the specific version of intermittent fasting in question, it appears to be on par with continuous energy restriction for weight loss, but there may be other benefits to utilizing an extended period of fasting on a consistent basis.
If you haven’t made the connection yet, TRF is colloquially referred to as intermittent fasting, but this is incorrect. TRF is a specific method within the broad category of intermittent fasting.
In popular media, TRF appears to be the most utilized form of intermittent fasting. This is likely due to it being the least extreme of the bunch, and as such, the easiest to adhere to.
The morning time is typically associated with low hunger. As a consequence, breakfast is usually the smallest meal of the day, and many forgo consuming any calories during this period and have their first meal in the early afternoon.
The 16/8 method has emerged as one of the most popular TRF paradigms as many see this as a strategy that corresponds well with their hunger signals and lifestyle, while potentially offering unique benefits.
Due to the relative ease of implementation and the traction it has gained, I will be focusing on TRF for the remainder the article and aim to answer the question: when is the best time to not eat?
Interest in TRF as a dietary pattern originally emerged from rodent research. In these subjects, it has been demonstrated that limiting the feeding window to 8-9 hours, without reducing caloric intake, can mitigate the detrimental metabolic effects of a high-fat Western diet, even when it’s combined with sugary drinking water.
While rodent models have consistently produced remarkable outcomes, the data on humans is a bit more controversial. Up until fairly recently, it was believed that any beneficial impact of TRF on health was simply a byproduct of weight loss. Multiple trials have now showcased that TRF results in a subconscious decrease in energy consumption in humans.
In a pilot study by Antoni and colleagues, subjects were assigned to a control or TRF group for 10 weeks. In the TRF condition, subjects delayed their first energy intake of the day and advanced their last energy intake of the day by 1.5 hours each. In comparison to the control, it was found that TRF resulted in an average body fat reduction of 1.9%.
Similarly, Gill and Panda had eight individuals reduce their eating window from > 14 hours per day to a self-selected window of 10-12 hours per day for 16 weeks. The subjects were also told to consistently follow this dietary pattern both during weekdays and weekends. On average, subjects reduced their eating duration by ~4.5 hours and lost 3.27 kg. Furthermore, it was reported that after 36 weeks, the participants maintained their weight loss and felt more energetic.
In another study, obese subjects were instructed to eat ad libitum from 10 a.m. to 6 p.m. daily. During the 12-week trial, energy intake decreased by 341 ± 53 kcal/day and body weight decreased by ~3%.
As a registered dietitian, I think these results are fascinating. In these trials, there were no calorie counting or meal plans, the subjects were simply told to reduce the time they allotted to eating each day, and as a consequence, they lost weight. The simplicity and effectiveness of this strategy for weight loss is sufficient enough to justify its utility, but more recent research suggests there could be further benefits for cardiometabolic outcomes.
When is the Best Time to Not Eat?
In general, it seems like a good idea to align your eating window with your hunger signals and lifestyle. As suggested in the beginning of the article, if you’re not hungry in the morning, it might make sense to forgo the traditional breakfast mealtime. But for those who don’t have a specific preference, or perhaps are metabolically compromised, it’s worth considering an early time-restricted feeding (eTRF) approach.
In contrast to the above examples, there are a few eTRF studies that have demonstrated improvements in health related biomarkers independent of weight loss.
In this randomized controlled crossover study, participants were assigned to either a control schedule (12-hour eating period) or eTRF (eat between 8 a.m. and 2 p.m.) for 4-days. Subjects then switched to the other arm after a 3.5-5 week washout period. Three daily meals were matched across arms and were designed to meet weight-maintenance energy requirements under sedentary conditions.
It was found that eTRF significantly lowered mean glucose levels while the subjects slept, resulting in an average 24-hour glucose level reduction by 4 ± 1 mg/dL. In addition, eTRF reduced glycemic excursions by 12 ± 3 mg/dL as measured by a continuous glucose monitor.
In another short-term experiment in healthy men, free-living subjects were allocated to either eTRF or a control/caloric restriction condition for two weeks. Since shortening the feeding window can lead to decreases in energy intake, the eTRF study arm was completed first. Macronutrient composition and caloric content of the diet was then matched to the control group. The eTRF group was asked to restrict their daily energy intake window to between 8 a.m. and 4 p.m., while the control group utilized a ~12-hour feeding window.
In the end, similar responses in body, lean, and fat mass were observed between groups. However, improvements in whole-body insulin sensitivity and increases in postprandial skeletal muscle nutrient uptake were documented in eTRF. There were also lower postprandial glycemic and insulinemic responses to a carbohydrate and protein drink in the eTRF group.
Moving on to a longer-term trial, and perhaps the most notable study in this area, Sutton et al. conducted a tightly controlled experiment in pre-diabetic men.
In this one, the eTRF intervention featured a six hour daily eating period with dinner before 3 p.m., while the control schedule utilized a 12-hour eating period. The participants were randomly allocated to each diet for five weeks followed by a washout period of seven weeks to separate each phase.
The diets were isocaloric (enough food to maintain body weight) and matched for meal frequency (three meals per day) and food intake on a meal by meal basis. Here’s the kicker, all food was provided by staff and the participants ate all meals while being monitored. Under these conditions, we can be pretty certain that any observed effects were a product of the timing of meals rather than weight loss.
Compliance to the intervention was outstanding and body weight remained approximately stable, with similar changes between groups. It was found that five weeks of eTRF did not affect fasting glucose or glucose levels at any time point during a three hour oral glucose tolerance test, but it did affect insulin levels. eTRF reduced mean and peak insulin values by 26 ± 9 mU/l and 35 ± 11 mU/l, respectively. Along with increasing insulin sensitivity, eTRF was found to lower morning levels of systolic and diastolic blood pressure by 11 ± 4 mm Hg and 10 ± 4 mm Hg.
Lastly, a newly published study compared the effects of two different TRF protocols in a randomized crossover design. Baseline measurements were taken for the first week, then for seven days each, separated by a two week washout period. Participants followed an eating window between 8 a.m. to 5 p.m. (eTRF) and an eating window of 12 p.m. to 9 p.m.
Between the three treatment periods, similar weight loss was observed. In comparison to baseline, it was found that both TRF protocols resulted in a ~36% reduction in the glycemic response to a test meal. However, only the eTRF intervention resulted in lower mean fasting glucose levels.
These findings help paint a clearer picture. While incorporating an extended period of fasting at any point in the day may pose some unique benefits for cardiometabolic health, the largest magnitude of effect is likely experienced by restricting the eating window from the morning to the late afternoon/early evening. Further evidence for this idea derives from the field of chrono-nutrition.
Chrono-nutrition refers to the interplay between nutrition and circadian biology, and eTRF is a form of TRF that aims to align energy intake with daily oscillations in metabolism. Indeed, specific rhythms have been documented for various components of energy metabolism. For example, it has long been known that there are diurnal variations in oral glucose tolerance, typically peaking in the morning, with impairments in the afternoon and evening.
Correspondingly, a dietary pattern that conflicts with circadian rhythms has been shown to elevate glucose, insulin, and triglyceride levels, and numerous trials have tested the response to a standard meal at different times of the day to showcase this phenomenon.
In a randomized controlled crossover trial, subjects were provided with standardized meals and studied under two lunch-eating conditions. Lunch was consumed at either 1 p.m. or 4:30 p.m. Postprandial glucose area under the curve (AUC) was found to be significantly increased at the later lunch (46%, to be specific).
Similar experiments have reported equivalent outcomes. It has been found that consuming the same meal at 8 a.m., in comparison to 8 p.m., leads to an increase in diet-induced thermogenesis, and lower glucose, insulin, and free fatty acid AUC values.
It seems clear that the human body is primed for energy intake early in the day, but what is the significance of this fact? Does it make a difference in real-world health outcomes and weight loss?
Glucose intolerance and insulin resistance are prominent features of type 2 diabetes, and a more subtle impairment in these markers appears to increase the risk of chronic metabolic diseases and cardiovascular disease. In non-diabetic subjects, both fasted and post-challenge (i.e., two hours after an oral glucose tolerance test) blood glucose levels are significantly and independently related to cardiovascular risk, as well as all-cause mortality.
Furthermore, glycemic excursions can induce oxidative stress, inflammation, and endothelial dysfunction in healthy volunteers. For these reasons, it has been suggested that the prevention of hyperglycemia is not only crucial for diabetics, but for otherwise healthy individuals.
Transitioning to body composition outcomes, observational research tends to report an association between evening energy intake and obesity. In addition, there is evidence to suggest that consuming the bulk of daily calories earlier in the day may improve the success of weight loss therapy.
In a study examining the dietary patterns of more than 50,000 Seventh-day Adventists, with a follow-up time of roughly 7-years, it was found that individuals who had a long overnight fast (≥ 18-hours) experienced a relative decrease in BMI. Related to this, those whose largest meal was breakfast, as opposed to dinner, experienced a significant decrease in BMI.
While these results lend credence to the value of aligning your dietary patterns with your biological clock, observational data should be interpreted with caution due to the variety of potential confounders.
It’s very likely that those who consume a greater proportion of calories in the evening, or breakfast skippers, may simply be less health conscious in general (e.g., smoke, drink alcohol, low consumption of fruits and vegetables, sedentary behavior). As such, while these findings are certainly valuable, they are less so than those from studies with a bit more rigor.
In a randomized trial by Jakubowicz et al., subjects were assigned to two isocaloric (~1400 kal) weight loss groups with identical macronutrient composition, but disparate energy distribution. The breakfast group (BF) featured a 700 kcal breakfast, 500 kcal lunch, and 200 kcal dinner. In comparison, the dinner (D) group consumed 200 kcal at breakfast, 500 kcal at lunch, and 700 kcal at dinner for 12 weeks.
At the end of the intervention, BF displayed a 2.5 fold greater weight loss than D. Though this is an impressive finding, it does imply the food diaries of the participants weren’t completely accurate. For this reason, some might consider this study invalid, but there were other valuable outcomes to consider.
For starters, though BF did lose a greater amount of weight than D, both groups lost a significant amount of weight. Naturally, weight loss leads to improvements in most health related biomarkers, but while serum triglyceride levels decreased by 33.6% in BF, they increased by 14.6% in D. Furthermore, BF resulted in greater decreases in fasting glucose, insulin, and HOMA-IR, and significantly lower glucose excursions in response to an oral glucose tolerance test.
Perhaps explaining the large differences in weight loss, BF reported decreased hunger and higher satiety scores throughout the day. In contrast, D reported consistently high hunger scores throughout the day. These effects have been observed in other research as well.
Together, the above outcomes suggest that shifting a larger percentage of calories to earlier in the day can improve both blood glucose and weight management, which has important implications for the rising rates of obesity and type 2 diabetes across the globe.
TRF is a subcategory of intermittent fasting. At the moment, TRF is a very fashionable dietary strategy, and emerging evidence suggests it can be quite useful for facilitating weight loss and improving cardiometabolic health in some individuals.
As it appears, restricting the feeding window in any capacity can lead to subconscious decreases in energy intake. This effect can improve health related biomarkers as a result of decreases in body weight, but more recent data suggests that eTRF may promote cardiometabolic improvements independent of changes in body weight.
The research on meal timing suggests that consuming breakfast is typically a good idea for most people as it can help manage hunger throughout the day. Moreover, consuming a large proportion of calories in the evening has a strong relationship with obesity, and the same meal consumed later in the day leads to a poorer postprandial response.
For these reasons, in combination with the compelling data on eTRF, it is likely that most people could derive significant benefits from shifting a proportion of their calories to earlier in the day and reducing the length of their feeding window by a few hours.