In modern societies, food is typically consumed at least three times daily. Along with the more than occasional snacks in between, this pattern of eating may lead to metabolic morbidities (such as insulin resistance and visceral fat accumulation), particularly when associated with a sedentary lifestyle. The result?: a high rate of obesity, diabetes, and cardiovascular disease — among other health disorders. In small clinical trials, certain fasting regimens (like intermittent fasting and time restricted eating) in which eating is restricted to shortened periods of time have demonstrated efficacy for weight loss and improvements in multiple health indicators, including insulin resistance and reductions in risk factors for cardiovascular disease. However, randomized controlled clinical trials of such fasting diets will be required to establish their efficacy in improving general health and preventing and managing major diseases of aging.
But what is intermittent fasting, or time restricted eating? Let’s dig in.
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 which shifts metabolism from lipid/cholesterol synthesis and fat storage to mobilization of fat through fatty acid oxidation and fatty acid-derived ketones, serving to preserve muscle mass and function. The result: improved body composition in overweight individuals, which holds the promise of slowing the 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 concluded similarly that there is “growing evidence demonstrating IF’s benefits on glucose and lipid homeostasis in the short-to-medium term.” However, this same study cautioned that more long-term safety studies are required.
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.
An important side note: TRF/TRE and 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
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 bigger 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 done to date, 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).
In the clinical setting, researchers are divided on the benefits of time restricted eating — especially when the time restricted eating regimen does not also include at least modest caloric restrictions over the long term. A 2014 clinical review paper summarized studies published to date on the effects of TRF on body weight and markers of metabolic disease risk (i.e., lipid, glucoregulatory, and inflammatory factors), concluding that, like in animal studies, health benefits included decreased body weight (though not consistently); lower concentrations of triglycerides, glucose, and low-density lipoprotein cholesterol; and increased concentrations of high-density lipoprotein cholesterol (the “good cholesterol”). However, a 2017 clinical review paper states that “whilst the metabolic benefits of TRF appear quite profound in rodents, findings from the few human studies have been mixed.”
More recently, 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.”
Bottom line: “Further experimental studies in humans are needed to investigate effects of TRF: (i) Without weight loss; (ii) in long-term studies; (iii) in diverse populations (i.e., subjects with overweight, obesity, diabetes, in subjects of different age); and (iv) including analyses of molecular mechanisms underlying the TRF-induced changes.”
In referring to a soon-to-be-released 2021 Endocrine Reviews manuscript, the lead author states in the September 22, 2021 press release that time restricted eating “can improve sleep and a person’s quality of life as well as reduce the risk of obesity, diabetes and heart disease.” While the full review was not available at our publication date, a 2019 review by some of the same researchers summed up much of the research done on time restricted eating to date, concluding with the finding that “confining all caloric intake to within a defined time interval that is a few hours separated from the daily sleep interval has multiple health benefits.”
Interestingly, this same group makes the point that a big difference between laboratory rodents and humans is the production of melatonin and its effect on glucose homeostasis, which leads them to believe that “it may be beneficial to avoid meals for 2–3 hours before going to bed and for up to an hour after waking up, after which melatonin levels decline.”
However, the group does admit that, to date, only a few studies have been published that used time restricted eating as a health intervention, most notably a controlled study on a small cohort that has shown that time restricted eating of 6 hours for 5 weeks did not lead to weight loss, but did increase β-cell function and insulin sensitivity and decreased post-prandial insulin, oxidative stress, blood pressure, and appetite — lending support to the idea that time restricted eating can improve human health even without a reduction in weight.
Fasting in general and time restricted eating in particular are gaining attention because initial, small studies show positive health benefits in the populations studied to date. Adjusting one’s eating schedule to an 8-hour window, adapted to when you plan on being the most active and sleeping, appears to be a feasible lifestyle change that many individuals can successfully adopt. However, more rigorous human studies are needed to assess the mechanisms and efficacy of time restricted eating and other fasting practices (like intermittent fasting) in a wide range of diseases. Many researchers and practicing physicians believe that such regimens might be best combined with some level of caloric restriction; but, once again, more clinical studies are needed to determine the best approaches (time restricted eating vs intermittent fasting, for example). In the meantime, the recommendations to limit your eating hours, refrain from eating a few hours before bedtime, and decrease your overall caloric intake (even if modestly) all seem valid approaches that can help you achieve a healthier metabolic profile.