-By now, you're likely aware of fasting — where you go for a prolonged period of time without eating. This has proven beneficial for weight loss and has shown improvements in multiple health indicators, including insulin resistance and reductions in risk factors for cardiovascular disease. There are, however, differences when it comes to fasting. For example, intermittent fasting vs. time restricted eating. You might think these are interchangeable, and in many cases, people likely use them as such, but there are differences to be aware of. In this article, we'll explain how they differ.

If you're looking for an easy way to manage your fast and understand what type of fasting is right for you, be sure to check out the free January app. It not only allows you to look up the impact a food will have on your blood sugar and track calories and macros, but it even has a new fasting feature so you can see how long you have left and get reminders to keep you on schedule.

You can download the January app for free here.

What is intermittent fasting?

Many religious groups have practiced fasting rituals for thousands of years. When people fast, they voluntarily abstain from food for different periods of time — which is known in the medical/science arena as intermittent fasting (IF).

Intermittent fasting eating patterns typically involve extended time periods (e.g., 16-48 hours) with little or no energy (food) intake and intervening periods of normal food intake, on a recurring basis. While some intermittent fasting schedules follow such a fasting/eating split each day, others break up the week, with whole fasting days followed by eating days. 

In most clinical circles, intermittent fasting schedules restrict overall weekly caloric intake. Clinical studies of intermittent fasting often set up a schedule of 60% energy restriction on two days per week, or every other day.

What are the benefits of intermittent fasting?

Most of the intermittent fasting data is based on animal models. A 2019 research review concluded that intermittent fasting-fed animals show improved glucose metabolism, resistance to cardiovascular diseases and cancer, and increased life span.

In humans, most studies of intermittent fasting show reductions in glucose and insulin concentrations, improvement of lipid levels, and reductions in inflammatory factors. 

A 2017 clinical paper theorizes that intermittent fasting regimens work by inducing a metabolic switch that shifts metabolism from lipid/cholesterol synthesis and fat storage to the mobilization of fat through fatty acid oxidation and fatty acid-derived ketones, serving to preserve muscle mass and function. The result is improved body composition in overweight individuals, which promises to slow aging and disease processes.

The beneficial effects of intermittent fasting seem to be particularly explained by weight loss (found in most intermittent fasting studies) and a prolonged fasting time.

Another research review conluded that " growing evidence demonstrates IF’s benefits on glucose and lipid homeostasis in the short-to-medium term.”

What is time-restricted eating?

Time-restricted eating (TRE) is a type of intermittent fasting that limits your food intake to a certain number of hours each day. A common timeframe for eating is an 8-to-12 hour window and fasting for the remaining 12-to-16 hours. While some intermittent fasting regimens also limit eating to a similar time window as time restricted eating, most intermittent fasting regimens have longer fasting periods and typically prescribe a lower intake of total calories over the long term. Time restricted eating, on the other hand, has in recent years become associated with eating the same total calories, just within a shortened time window. 

Before time-restricted eating began to be seriously studied in humans, scientists studied similar dietary regimens in rodents and termed them time-restricted feeding (TRF) regimens. What many of these TRF studies reported was that by limiting food availability and quantity to a time period of no greater than 8 hours, and during a time of day when rodents are usually resting and less hungry, the diets were protective against metabolic disturbances such as obesity, glucose intolerance, leptin resistance, hepatic steatosis, and tissue inflammation.

Time-restricted eating vs intermittent fasting: the circadian rhythms relationship

This “time of day” manipulation, in rodent studies, is an important distinguishing characteristic of most TRF (animal) and some recent TRE, time-restricted eating, (human) studies, which increasingly have been looking at the relationships between circadian rhythms, nutrition, and metabolic rhythms — which some researchers have coined the science of chrononutrition. The idea here is that since many physiological and behavioral aspects follow a natural circadian clock, proper functioning of such circadian clocks is critical. Metabolism is one such physiological set of events that is highly regulated by circadian rhythms, which are affected by factors that include: 

• sleep and wake timing 
• light and darkness 
• amount of food ingested
• timing of food intake
• activity levels
• body temperature 
• aging

Studies have shown that circadian misalignment may have negative effects on body weight, while other researchers have noted that delayed eating during the day can lead to metabolic dysfunction, weight gain, and abnormalities in appetite. Other studies suggest that consuming a larger portion of daily caloric intake during the first half of wakeful hours may be preferred for better blood glucose regulation and weight control. Another study demonstrated greater weight loss if lunch is consumed earlier rather than later, and that eating an earlier dinner correlates with a reduced risk for cancer.

Accordingly, recent time-restricted eating studies have begun to adjust the eating time window to occur at the most advantageous time relative to human circadian rhythm patterns, such as morning-to-mid-day, when blood glucose levels tend to stay lower than the elevation typically experienced with late evening eating. However, this “meal timing,” as they call it, is complicated in the cases of those with type 1 or type 2 diabetes, who can experience the dawn phenomenon — abnormally high blood glucose levels very early in the morning (or upon waking). So, much has still to be learned and determined about meal timing and circadian rhythm diet alignment. Even in the animal studies which do show that TRF (timed to match important circadian rhythms in mice) resulted in very marked health benefits (reduced body weight, total cholesterol, and concentrations of triglycerides, glucose, insulin, interleukin 6, and tumor necrosis factor-α; as well as with improvements in insulin sensitivity), it is not clear whether these benefits are the direct effect of the feeding timing or an indirect effect of sleep loss and lack of physical activity from the altered daytime feeding schedules. 

What are the benefits of time restricted eating (TRE)?

Hearing that animal studies were demonstrating such positive health outcomes from TRF regimens using an 8-hour eating window per day, people have increasingly flocked to the “16:8 diet” — a time-restricted eating regimen in which one eats for 8 hours of the day and then fasts the other 16 hours of a 24-hour day. However, it is unclear how many people attempt to align the 8-hour eating window with their circadian rhythms, or whether overall caloric intake is reduced (or not).

Three major clinical reviews have been published, one from abroad and two from the U.S. In a 2019 Germany/UK clinical review, researchers concluded that “TRF (especially early TRF) represents a promising dietary approach for the prevention and therapy of metabolic disturbances [that] deemphasizes caloric intake, making it an attractive and easily adoptable lifestyle modification.” They went on to state that “in humans, restricting the eating window to the early time of the day will expectedly induce beneficial metabolic effects in contrast to a delayed eating window.” However, they caution that “most of the published human TRF studies did not carefully monitor calorie intake (which led to weight loss, and this makes it difficult to interpret the TRF and weight loss effects separately), macronutrient content, activity levels, and timing of sleep-wake cycles and used small sample sizes.”

What's the difference between time-restricted eating and intermittent fasting?

Let's summarize the difference between time-restricted eating and intermittent fasting. While similar in that they both involve cycling between periods of eating and fasting, they differ in their specific timing and duration.

Time-Restricted Eating (TRE):

• Time-restricted eating involves limiting the hours during which you consume food each day, typically within a specific window of time.
• The fasting period usually ranges from 12 to 16 hours, while the eating window is restricted to 8 to 12 hours.
• For example, a common TRE schedule might involve fasting for 16 hours overnight, from dinner until the next day's lunch, and eating all meals within an 8-hour window, such as from 12:00 PM to 8:00 PM.
• TRE primarily focuses on aligning eating patterns with the body's natural circadian rhythms, aiming to optimize metabolic function and promote weight management.

Intermittent Fasting (IF):

• Intermittent fasting involves alternating between periods of fasting and eating on specific days or throughout the week.
• There are various methods of intermittent fasting, including the 16/8 method (fasting for 16 hours and eating within an 8-hour window each day), the 5:2 method (eating normally for five days and restricting calorie intake on two non-consecutive days), and alternate-day fasting (alternating between fasting days and non-fasting days).
• Intermittent fasting can be more flexible in terms of fasting and eating windows, allowing for greater customization based on individual preferences and goals.
• While weight loss is a common goal of intermittent fasting, it may also offer potential health benefits such as improved insulin sensitivity, reduced inflammation, and enhanced cellular repair processes.

Key takeaways

In summary, time-restricted eating focuses on limiting the daily eating window, whereas intermittent fasting involves alternating between periods of fasting and eating on specific days or throughout the week. Both approaches have been associated with various health benefits and can be effective strategies for achieving weight management and promoting overall health. If you're looking for help managing your fasts, consider using the free January app. You can select from a variety of fasts and keep track of your progress.