October 15th, 2021


Various diets and eating styles have emerged over the years as nutrition science continues to progress. Chrononutrition, commonly known as time-restricted eating (TRE), is a relatively new concept that looks at the physiologic consequences of what we eat and when we eat. While we currently have limited large, controlled trials, there’s some evidence to suggest that chrononutrition/time-restricted eating can promote positive cardiovascular, glycemic, and metabolic changes. Let’s look at the differences between TRE and other common diets and the potential benefits of TRE.


What is Chronobiology?

Chronobiology is the study of biological rhythms, like circadian rhythms, that follow a 24-hour cycle. (1)  The sleep-wake cycle is a well-known example of a circadian rhythm in chronobiology wherein under healthy and normal conditions, we sleep when it’s dark and wake when it’s light in response to environmental cues: lightness and darkness.


What is Chrononutrition?

Chrononutrition is a branch of chronobiology to which the name alludes, focuses on the influence of meal and nutrient timing on metabolism and longevity. The study of chrononutrition encompasses two main parts: 1) how meal timing impacts our health and 2) how meal timing entrains our body clock. (2)

There’s a feedback loop between what we eat, when we eat, and circadian rhythms. For example, the signaling molecules that regulate our built-in circadian clocks are significantly influenced by food intake, nutrient timing, and hormones like melatonin, growth hormone, and insulin. On the flip side, our circadian clocks influence our behaviors, like eating and sleeping, which impact nutrient timing, hormone secretion, and disease risk. (3)

Regularly being out-of-sync with natural body rhythms can have a health-compromising domino effect. Studies show that shift work, or consistently working during the night and sleeping during the day, increases the risk of cardiovascular disease, metabolic syndrome, and obesity. It’s thought that eating and sleeping at times that are out of sync with our circadian rhythms increases the risk of such conditions. (4, 5)


Time-Restricted Eating

Chrononutrition is often synonymously referred to time-restricted eating (TRE) or time-restricted feeding (TRF). TRE is defined as a dietary approach that consolidates all calorie intake to 6- to 10-h periods during the active phase of the day, without necessarily altering diet quality and quantity.  In preclinical studies, multiple health benefits have been demonstrated including reduction of body weight, improved glucose tolerance, increased metabolic flexibility, and better gut function and cardiometabolic health.(6)


Differences between TRE, Calorie Restriction, and Intermittent Fasting

There’s a lot of interest in different eating patterns that help support weight loss and metabolic health. Calorie restriction (CR) and intermittent fasting (IF) are two of the most common strategies, but there are notable differences compared to TRE.


Calorie Restriction (CR)

As nutrition professionals, we’re well aware of calorie restriction i.e., simply reducing daily intake to allow for a caloric deficit. Meal timing and fasting windows usually aren’t specified in a standard calorie-restricted diet.


Intermittent Fasting (IF)

Intermittent fasting has gained tremendous popularity over the years. There are various IF strategies, including alternate-day fasting, alternate-day modified fasting, and periodic fasting.

Alternate Day Fasting: water-only fast every other day

Alternate Day Modified Fasting: reduce calorie intake by 25-50% every other day. For example, if you typically 2,000 calories per day, reduce intake to <1,000 calories on fasting days.

Periodic Fasting: complete fasting, usually 1-2 times per week. Consuming a very low-calorie diet for 3-5 consecutive days every 2-3 months is also considered periodic fasting.


Time-Restricted Eating (TRE)

The most significant difference between calorie restriction, intermittent fasting, and time-restricted eating is that TRE does not emphasize calorie restriction. The primary focus is eating within a specified time window, more specifically during the most active part of the day. This typically consolidates calorie intake to a 4 to 12-hour window during daylight hours. TRE is a simple approach that could be highly beneficial in clinical practice, since it does not require extensive nutrition knowledge or a significant time commitment on the part of the patient. (7, 8)

There’s science to support CR, IF, and TRE and each eating style can undeniably work well for different people. For brevity purposes, let’s look at the specific science supporting time-restricted eating.


Evidence to Support Time-Restricted Eating

Nutrient sensing pathways are closely connected to circadian clocks and fed/fasted states. Frequent, irregular eating with the absence of a fasted state can disrupt the “rest and repair” processes achieved during a fasted state. For example, according to a 2017 study in Cellular Metabolism, a fed state activates the insulin-mTOR pathway. This pathway drives specific genes that promote anabolic processes, including lipogenesis. Conversely, during a few hours of fasting, AMPK is activated, which stimulates catabolic processes necessary for cellular recovery and repair. Without a clear division of a fed and fasted state, or a clear division of eating and not eating, these metabolic processes can become interrupted and lead to metabolic issues and disease over time. (9)

The timing of the fed and fasted state also impacts metabolic health, confirming the importance of meal timing. A small randomized control study compared the difference between three meals per day versus six meals per day in 28 people with type 2 diabetes. The three meals per day group ate a large breakfast (700 kcals), medium-sized lunch (600 kcals), and small dinner (200 kcals). The daily eating window was between 9:30 am-8 pm. The six meals per day group consumed the same daily calories but spread out between three meals and three snacks. The daily eating window was between 9:30 am-10 pm.

At the end of the 12-week study, the participants who consumed three meals per day lost more weight, revealed increased clock gene expression, had reduced hunger and cravings, and lower hemoglobin A1C. Beyond that, continuous glucose monitor data showed lower fasting, daily, and overnight glucose values than the six meal per day group. (10)

Another 5-week cross-over feeding trial found that finishing the last meal by 3 pm increased insulin sensitivity and beta-cell activity while lowering oxidative stress compared to the control. (11) One study found that an eating window between 8 am to 2 pm reduces fasting glucose and postprandial glucose spikes while simultaneously increasing daytime energy expenditure and enhancing clock genes involved in regulating circadian rhythm. (12)

However, it’s important to note that many human studies assessing TRE’s risk: reward ratio are small studies with a short time frame, usually from 4 days to 16 weeks. It’s also important to note that most studies looking at the benefits of TRE also reported reduced calorie intake during the prescribed eating window. This very likely contributes to some degree to the positive health outcomes observed with TRE. Regardless, TRE is a strategy that avoids dramatic, restrictive diet changes while simultaneously benefiting metabolic health.


Application of TRE

The realistic implementation of TRE on a day-to-day basis can be challenging for a few reasons. First of all, eating is a social magnet that brings family, friends, and colleagues together. Such dining events typically occur in the evening, making a “no evening eating” objective rather difficult. Secondly, modern-day work-life balance is, well, rather imbalanced and leaves many working adults to eat the largest meal later in the evening. Interestingly, a 2014 study in mice found that practicing TRE during the weekdays and eating as desired during the weekends still resulted in beneficial effects. (13) So, it could be feasible for people to follow a time-restricted eating pattern during the week with more flexibility on the weekend.


CLINICAL PEARL:  If interested in experimenting with TRE either professionally or personally, it’s helpful to start slowly by bumping the evening meal up by 1 hour over a period of time. Once achieved, slowly tighten the eating window to allow for at least 12-14 hours of overnight fasting.



As functional nutrition professionals, we know that a one-size-fits-all approach does not exist, and we must consider the individual as a whole before making nutrition recommendations. It’s worthwhile to consider time-restricted eating as an effective lifestyle intervention. TRE supports circadian rhythms while offering an eating structure that’s not deliberately or rigidly low in calories. Most importantly, TRE has the potential to significantly benefit metabolic health and longevity with minimal risk.

By Tori Eaton, RDN, IFNCP



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