Time-restricted feeding
Time-restricted feeding is a specific type of intermittent fasting that limits the daily feeding window to a 1- to 12-hour interval.
Three subtypes of time-restricted feeding include 16:8 fasting, 20:4 fasting, and One Meal A Day. TRF is one of the few dietary regimens commonly practiced for the purpose of weight loss without recourse to direct calorie restriction, instead utilizing circadian rhythm mechanisms by limiting food access to a controlled window of time. Especially in the case of OMAD, which is regarded by many dieticians as unnecessarily extreme, most successful practitioners do consciously shape the composition and size of their daily meal in order to achieve caloric balance alongside nutritional sufficiency. In the modern 21st century, TRF is being studied as a preventative practice for metabolic diseases and weight loss.
While preliminary findings show mixed results for the effects of TRF, the long-term effects are still uncertain. Although the US National Institute on Aging found that TRF may increase lifespan, they cannot recommend the practice due to a lack of clinical studies with durations of over 2 years. A news article associated with the US National Institute of Health states that while TRF exhibits promising success in weight loss, it is not recommended for children or pregnant individuals among other populations. Johns Hopkins Medicine concluded that TRF shows positive results in protecting against obesity, insulin resistance, and other metabolic disorders.
TRF has been performed by many religious or spiritual practitioners, namely by individuals participating in Ramadan, the Black Fast, and Buddhist principles.
Types
Three forms of time-restricted feeding include: 16:8 fasting, 20:4 fasting, and OMAD:- 16:8 fasting restricts food access to an 8-hour period each day, which induces a 16-hour fasting period. It is one of the most common forms of TRF and frequently used for weight loss, ranging from a 12% reduction to 28% reduction in body weight for some animal studies.
- 20:4 fasting restricts food access to a 4-hour period each day, which induces a 20-hour fasting period. Some research observes that reducing the feeding window to >8 hours reduces total caloric intake. Other studies noted a 9-18% decrease in body weight along with a >40% reduction in visceral fat following an 18-week study.
- One Meal A Day fasting or OMAD or 23:1 fasting restricts food access to a 1-hour period each day, which induces a 23-hour fasting period. This method is the most demanding form of TRF since an entire's day's calories is consumed in a single hour, increasing the risk of binge eating during the feeding window and hunger pangs and irritability during the fasting window.
Recommendations
A news article in the American Heart Association News mentioned that the interviewee felt "vindicated by his low cholesterol and blood pressure readings" after adopting TRF. Dr. Satchidananda Panda, author of a novel about time-restricted fasting, noted in his studies that TRF induces weight loss, reduces LDL cholesterol, and leads to multiple other positive effects.NIH News in Health mentions that while TRF demonstrated weight loss or improved blood sugar control in early trials, it does not recommend TRF for individuals who are under the age of 25, are pregnant, use insulin or medications prescribed to be consumed with food, are prone to seizures, or work night shifts. Dr. Valter Longo, cited in the article, mentioned that "people who regularly fast more than 16 or 18 hours a day have a higher risk of gallstones" and recommends speaking with a healthcare provider before attempting a new diet.
The NIA mentions that while TRF may increase lifespan in animal models, clinical trials with humans have been too short to observe the long-term effects of TRF; therefore, it does not recommend TRF due to the lack of evidence.
According to an article in NIH MedlinePlus, NIH researchers found success in weight loss in early clinical trials of TRF, along with protection against metabolic diseases and a slowing of the aging process; however, TRF "is not safe for everyone, especially those with health conditions like diabetes, kidney disease, or heart-related issues". The National Eating Disorders Association also mentions that fasting is not appropriate for individuals suffering from or having suffered from eating disorders.
Johns Hopkins Medicine considers that while TRF initially induces hunger pangs, consistent practice of TRF for at least 2 weeks can protect against obesity, diabetes, and neurological disorders. It is based on the idea of metabolic switching, in which glucose and other sugar-based stores of energy are used up, and the body begins utilizing fat stores in a low-level of ketosis. Dr. Pam Taub, a cardiologist at the University of California, San Diego's School of Medicine and researcher of TRF, saw a "3% reduction in their weight and a 4% reduction in abdominal visceral fat" after a 3-month clinical trial.
Research
Weight and Obesity
Some research demonstrates that TRF leads to weight loss, reduces body mass index, and prevents obesity in rodent models as well as in human models. Some studies observed that weight loss produced by TRF may persist for up to 1 year after the diet. Other studies observe that although TRF does not reduce body weight, it reduces adipose tissue mass and increases lean muscle mass, potentially replacing circulating and hepatic triglycerides with energy-demanding proteins and muscle. TRF has similar body composition effects with an 8-, 9-, or 12-hour feeding window.Circadian Rhythm
and its effects on metabolic processes may also be maintained or protected by TRF, which is based on the idea that circadian rhythm processes influence metabolism and vice versa. There is limited evidence that TRF improves oscillations in circadian rhythmic transcripts and reduces the detrimental effects of a high-fat diet on several metabolic pathways, independent of caloric quantity or quality. In addition to the effects of a high-fat diet, TRF may protect individuals from age-associated, chronic metabolic diseases. TRF is also noted to indirectly restore the cyclical expression of key circadian rhythm transcripts such as Bmal1 and Rev-erb, and TRF may alter the timing of the suprachiasmatic nucleus oscillator and circadian clocks in peripheral tissue such the liver, altering the amount of circulating metabolites released by the liver throughout the day. Starting the feeding window in the morning may increase insulin sensitivity and improve gut microbiome profile, likely by aligning the feeding-fasting cycle with the peripheral circadian clock in the gut microbiome.Mood
In some studies, fasting correlated with an improvement in mood, increased self-assessment of well-being, and a sense of euphoria. Observational studies on subjects performing Ramadan - a form of TRF with the feeding window limited to the interval before sunrise and after sunset - found that bipolar subjects practicing Ramadan and treated with lithium experienced less depression and mania; however, it is yet to be determined if this mood improvement persists after practicing TRF.Other biological effects
Research also demonstrates that TRF reduces lipid accumulation in the liver and circulating triglyceride, leptin, and inflammatory cytokine levels in animal models, and TRF reduces free fat mass in human models. 18:6 TRF may also reduce insulin resistance. Some studies mention that TRF improves glucose metabolism by reducing gluconeogensis while increasing activity in the pentose phosphate pathway and TCA cycle. A few insect models observe that TRF improves cardiac function and arrhythmia by improving mitochondrial function and ATP-dependent chaperone function, increases muscle endurance by improving creatine metabolism, and protects against an age-associated reduction in sleep. In a few human models, TRF improves sleep quality and reduces risk of contracting breast cancer by reducing inflammation, mainly by reducing macrophage infiltration of white adipose tissue. TRF may also reduce fibrosis and hepatic fat deposits by improving fatty acid synthesis and increasing beta oxidation and mitochondrion volume in fat deposits, and it may reduce hypercholesterolemia by increasing Cyp7a/b expression, which correlates with reducing cholesterol levels and increasing bile acids. TRF, by itself, does not appear to reduce activity levels, and in some animal studies, TRF is noted to increase endurance of subjects fed a high-fat diet compared to subjects fed an unrestricted, standard diet. Some studies observed that TRF leads to an increase in ketone body production, which aids metabolism and the central nervous system.Tolerance
A 2019 review cited one study that noted that subjects were compliant with an 8-hour feeding window for 6 days per week for a 12-week study, and while the drop-out rate was 26%, none of the reasons for dropping out were because of the diet paradigm.Areas of Future Research
A growing area for future research involves altering the timing of the feeding window, either by starting the feeding window in the daytime or the nighttime. Longer randomized, controlled studies are also necessary to observe the lasting effects of TRF on metabolic and cardiovascular indicators.History and Religious Practice
Throughout human history, fasting has been practiced for physical and spiritual reasons. Prehistoric human ancestors practiced a similar form of 16:8 or 23:1 fasting while scourging for food, tapping into fat stores for energy during periods without food and adjusting to a variety of food sources. In Buddhism, fasting consists of skipping a meal in the evening to promote a healthy body and mental acuity.fasting generally involves fasting for an 18-hour period, from noon to dawn, as noted in this Buddhist Mass festival.
Forms of fasting, including TRF, have been practiced since the time of Ancient Greece, pushed by Hippocrates as a treatment against acute and chronic diseases. Fasting itself has been "thought to favor man's awakening to spirituality," and Greek philosophers noted that fasting awakened the body's "Inner Physician" that cleaned and detoxified wastes in the body. One form of early TRF practiced by the Greeks involved simply skipping dinner, termed a simple fast which Hippocrates noted alleviated fevers and acute illnesses. An example of religious TRF includes the 40-day fasting period of Lent known as the Black Fast. While the food restrictions during the Black Fast vary according to the Christian denomination, a general rule for strict adherence to the practice is that the only meal of the day may only be consumed after 3 p.m. or in the evening. In Christianity, many Lutheran adherents observe that fasting improves mood, promotes physical health, encourages appreciation, and is "good for the soul" among other benefits.
One of the most modern and practiced forms of TRF has been fasting during the time of Ramadan. For the month of Ramadan, adherents do not eat or drink anything from sunrise to sundown, and the sick and elderly are exempt. Pregnant individuals are usually exempt from Ramadan, especially since research observes that fasting during pregnancy is linked to an increased risk of gestational diabetes. This spiritual form of TRF is meant to strengthen self-control and promote empathy with the impoverished suffering from constant hunger. Research has observed that Ramadan fasting promoted anti-oxidative and anti-inflammatory effects along with weight loss. However, a meta-analysis review noted that this weight-loss was temporary, with subjects regaining lost weight "6 weeks from the beginning of Ramadan."