In a recent review published on Frontiers in Nutrition, researchers summarized current scientific knowledge about how fasting diets (FDs) affect eating behaviors, mood, sleep, and overall well-being. Their findings highlight potential mechanisms underlying the beneficial effects of FDs, a critical mechanism being the gut microbiome.
To identify relevant papers, the researchers conducted a systematic literature search in scientific databases, such as Web of Science and PubMed, using keywords related to FD regimens and outcomes related to physical and mental well-being.
Fasting is known to have many health benefits
FDs have been used since the 5thu century BC when the Greek physician Hippocrates prescribed abstinence from food or drink to treat epileptic seizures. Compared to diets that require calorie restriction (CR), FDs are considered easier and more satisfying to follow.
There are several regimens through which FDs are administered, but all require a minimum of eight hours of fasting daily. A popular diet, the 5:2 Intermittent Fasting, involves two non-consecutive fasting days per week while not restricting eating times during the other five.
Intermittent fasting followers follow their normal diet for 5-6 days a week while restricting their food intake the rest of the time. Alternate day feeding (ADF) restricts food intake to a specific window every other day. There are also religious and cultural reasons to follow FD, e.g. during the Muslim holy month of Ramadan.
FDs have attracted scientific attention as a potential strategy to reduce serum glucose, deplete hepatic oxygen, and alter glycolysis to ketogenesis in the body. Research shows 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 can also promote mental health by reducing stress and improving cognitive function. However, these effects and those on eating behaviors have not been the subject of as much research.
Studies have found conflicting effects of FDs on eating behaviors
Weight management strategies can moderate obesity-related eating behaviors. On CR diets, individuals may compensate by overeating and regaining lost weight. Some studies suggest that following a FD may lead to lower feelings of hunger, compensatory eating, and weight regain than CRs, while others find the two strategies to be equivalent.
In some studies, results were only seen in the short term but not over longer periods, indicating the difficulty of continuing to follow any dietary regimen. People who fast during Ramadan have significantly different experiences of eating behavior, reporting lower levels of hunger at the end of the month.
Similarly, different results were obtained in studies examining the effect of FDs on disinhibition and dietary restriction. Some studies have found that CR is associated with flexible restraint, which is a more graded approach, leading to lower obesity and binge eating, while FDs often take a more rigid all-or-nothing approach, which can lead to overeating . Others found no difference in emotional eating between ADF regimens and regular dieting.
The reviewers suggest that these inclusive findings could be due to a lack of uniformity between different studies, including differences in sample size, study populations, duration of intervention and study design. Younger individuals appear more likely to overeat during FDs compared to middle-aged individuals.
Several studies lacked control groups and none investigated the effect of FDs on satiety. Further systematic study using comparable study designs and analyzes is needed to assess the effectiveness of FDs in improving dietary behaviors over short and long periods.
FDs modify sleep-wake patterns and significantly improve mood
While FDs cause misalignment in circadian rhythms, some studies have found that they do not significantly affect insomnia severity or sleep duration, while others have found that they improve sleep quality.
One study found that people who fasted during Ramadan experienced more daytime sleepiness. There is some evidence that FDs could be a promising way to alleviate disruptions in circadian rhythms due to health conditions that disrupt sleep or shift work, but this requires further investigation.
In terms of mood effects, FDs have been found to significantly reduce anger, confusion, tension, depression, and overall mood disturbance while increasing energy levels. Similar results were observed in populations fasting during Ramadan. However, all these findings appeared to be short-lived. future studies can examine the impact of FDs on improving mood 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 problems are common among people with depression and anxiety, pointing to the complex connections between brain health and gut function.
Intermittent fasting has been shown to improve gut health, which is then linked to higher production of active metabolites and neurotransmitters. Another mechanism by which FDs improve mood could be through increased production of ketone bodies.
The literature suggests that FDs may be more effective relative to adequate sleep and timing of meals with the body’s circadian rhythms.
However, further studies are urgently needed to provide science-based recommendations for incorporating FDs into regimens to address obesity and improve the quality of life of people living with CVD and other health conditions.
