In a recent review published in Frontiers in Nutrition, researchers summarized the current scientific knowledge on how fasting diets (FDs) affect eating behaviors, mood, sleep, and overall well-being. Their findings highlight the potential mechanisms underlying the beneficial effects of FDs, one crucial mechanism being the gut microbiome.
To identify relevant papers, researchers conducted a systematic literature search across scientific databases such as Web of Science and PubMed using keywords related to FD regimens and outcomes associated with physical and mental well-being.
Fasting is known to have several health benefits
FDs have been used since as early as the 5th century BCE when the Greek physician Hippocrates prescribed abstaining from food or drink to treat seizures. Compared to dietary regimens that require calorie restriction (CR), FDs are perceived as being easier and more satisfying to follow.
There are several regimens through which FDs are implemented, but all require at least eight hours of fasting daily. A popular diet, 5:2 intermittent fasting (IF), involves two non-consecutive days of fasting per week while not restricting food timings during the other five.
Followers of periodic fasting follow their regular diets for 5-6 days a week while limiting their food intake during the remainder. Alternate day feeding (ADF) restricts food intake to a specific window every other day. There are also religious and cultural reasons for following FDs, e.g., during the Muslim holy month of Ramadan.
FDs have drawn scientific attention as a potential strategy to reduce serum glucose, deplete hepatic oxygen, and bring about glycolysis to ketogenesis shift in the body. Research suggests that FDs could help control weight and be protective against type 2 diabetes, cancer, cardiovascular disease, multiple sclerosis (MS), Alzheimer’s disease (AD), Parkinson’s disease (PD), stroke, and epilepsy.
They may also promote mental health by reducing anxiety and improving cognition. However, these effects and those on eating behaviors have not been the subject of as much research focus.
Studies found contradictory effects of FDs on eating behaviors
Weight management strategies can moderate eating behaviors that are associated with obesity. In CR diets, subjects may compensate by overeating and regaining lost weight. Some studies suggest that following an FD can lead to lower feelings of hunger, compensatory eating, and weight regain than CRs, while others find that the two strategies are equivalent.
In some studies, the effects were seen only in the short term but not over longer periods, pointing to the difficulty of continuing to follow any dietary regimen. Individuals fasting during Ramadan have markedly different experiences in terms of eating behavior, reporting lower levels of hunger at the end of the month.
Similarly, varying results were obtained in studies examining the effect of FDs on disinhibition and dietary restraint. Some papers found that CR is associated with flexible restraint, which is a more graduated approach, leading to lower adiposity and binge eating, while FDs often follow a more rigid ‘all or nothing’ approach, which can cause overeating. Others have found no difference in emotional eating between ADF regimens and regular diets.
The reviewers suggest that these inclusive findings could be due to the lack of uniformity across the different studies, including differences in sample size, study populations, intervention duration, and study design. Younger subjects appear more likely to overeat during FDs compared to middle-aged individuals.
Several studies did not have control groups, and none explored the effect of FDs on satiation. Further systematic study using comparable study designs and analyses is needed to assess the efficacy of FDs in improving eating behaviors over short and long periods.
FDs modify sleep-wake patterns and significantly improve mood
While FDs cause misalignments in circadian rhythms, some studies have not found them to significantly affect insomnia severity or sleep duration, while others have found them to improve sleep quality.
One study found that individuals fasting during Ramadan exhibited more daytime sleepiness. There are some indications that FDs could be a promising way to alleviate disturbances to circadian rhythms due to sleep-interrupting health conditions or shift work, but this requires further exploration.
In terms of effects on mood, FDs were found to significantly reduce anger, confusion, tension, depression, and overall mood disturbance while increasing energy levels. Similar effects were seen in fasting populations during Ramadan. However, these findings were all seen in the short term; future studies can examine the impact of FDs in improving moods in the long term.
Gut-brain interactions could underlie the effects of FDs
The gut-brain axis may be instrumental in driving the effects of FDs, particularly on mood. Gastrointestinal complaints are common among individuals with depression and anxiety, pointing to the complex connections between brain health and gut functioning.
Intermittent fasting has been shown to improve gut health, which is then associated with higher production of active metabolites and neurotransmitters. Another mechanism by which FDs improve mood could be through the increased production of ketone bodies.
The literature suggests that FDs can be most effective in connection with adequate sleep and by synchronizing mealtimes with the body’s circadian rhythms.
However, further studies are urgently required to provide science-driven recommendations for incorporating FDs into regimens to address obesity and to improve the quality of life for individuals living with cardiovascular diseases and other health conditions.
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