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By Juliette Frank

The obesity epidemic and its metabolic consequences are a major public health problem both in the United States, as well as globally. While the underlying causes are multifactorial, dysregulations within the brain–gut–microbiome (BGM) system play a central role.

As obesity rates hit peak levels, causing a major public health crisis, many Americans are looking to popular diet programs for a simple solution. There are countless diet recommendations for losing weight and reducing cardiovascular risk factors associated with being overweight such as heart disease, metabolic syndrome, high blood pressure, high cholesterol, and C reactive protein (inflammation in the body).

Most popular dieting methods aim to either restrict or reduce the total or relative amounts of macronutrients, e.g., fat, protein, and carbohydrates, without much consideration of the important role of the gut’s microbial ecosystem in modulating brain–gut interactions.

Intermittent fasting diets have quickly jumped into the mix of mainstream weight-loss methods in the past few years as many people see them as a less complicated and potentially more sustainable dieting program compared with other popular diet fads. Intermittent fasting (IF) can be used as an umbrella term referring to various fasting methods that require restrictive caloric intake. IF is not a newfound practice; it has deep historical roots and has been used in many religions for spiritual or physical benefits. The types of IF that have received the most research attention include alternate-day fasting (ADF), the 5:2 diet and time-restricted eating (TRE), even though the latter represents a fundamentally different approach.

ADF involves 24-hour periods of either no or minimal calorie consumption, also known as a “fast day”, rotating with 24-hour periods of normal or ad libitum energy consumption, also known as a “feast day”. The second popular IF method is the 5:2 diet which consists of 2 nonconsecutive days of fasting and 5 days of unrestricted eating in a 7-day cycle.

Time-restricted eating (TRE) is another proposed therapeutic approach for obesity and metabolic dysregulation which has gained significant attention in the past few years. TRE differs from ADF and the 5:2 diet in that it restricts daily energy consumption to a narrow window of 6-8 hours during a 24-hour period without limiting calorie intake. This is in vast contrast to the 15-17 hour period of food intake which is common in many Western countries. TRE allows for the combination of a ketogenic state during the 16-hour period of no calorie consumption along with, ideally, a healthy, largely plant-based diet during the 8-hour period of food intake. However, the effectiveness of such a combination of TRE with a specific diet has not been evaluated in controlled studies.

TRE can be a more practical option than IF since it does not require reduction of daily caloric intake, but rather to compress the window of unrestricted energy consumption within a 24-hour time period, with most of the food restriction period taking place during sleep.

Based largely on studies in laboratory mice, time-restricted eating is hypothesized to influence metabolic regulation through its effects on circadian biology, the gastrointestinal microbiota and modifiable lifestyle behaviors . Disruptions in these circadian clock-regulated systems can increase risks of developing obesity, cardiovascular disease, diabetes, and cancer due to a hostile metabolic network (Racz, B.; 2018). In animals, TRE actually reduces whole-body fat accumulation and inflammation while improving glucose tolerance, reducing insulin resistance, improving homeostasis, and restoring cholesterol homeostasis.

Recent research in animals demonstrates the importance of normal daily circadian rhythms for maintaining optimal metabolic function and a number of publications have demonstrated the effectiveness of such diets in animal models of obesity and metabolic diseases.

The Chaix et al. study tested TRF (8–9 hour food access in the active, dark phase) on mice with preexisting conditions of obesity and type II diabetes. The TRE regimen showed stabilizing effects and a reversal of the progression of metabolic diseases. TRE’s beneficial effects were even maintained on weekends when the fasting regimen was briefly paused due to ad libitum eating. Giving the digestive tract distinct time periods without food forces the body to burn fat instead of using energy from a continuous glucose supply. In the absence of food ingestion, a metabolic change occurs which forces the liver to produce ketones from the metabolism of body fat when glucose is inaccessible. As well as acting as fuel, ketone bodies are signaling molecules that have significant effects on major functions of the cells and organs, including the brain. These systemic and cellular responses switched on during fasting are thought to stay activated and fortify mental and physical functioning as well as disease resistance even after resumption of food ingestion.

Another possible factor that might contribute to the benefits of TRE include the unintentional reduction in daily caloric intake due to individuals not being able to consume the same amount of food they previously ate in the 6-8 hour window. As opposed to many other popular diets, time-restricted eating does not require individuals to keep track of their calories consumed, which is often a cause of high dropout rates, making TRE a much more practical and realistic fasting method.

Although these TRE experiments on rodent models have produced impressive results, there are currently few studies with well-designed clinical trials that have investigated the benefits and risks of these diets on humans in the long term. Current findings conclude that ADF, the 5:2 diet and TRE produce mild to moderate weight loss results that are on par with traditional calorie restrictive dieting approaches.

Although most intermittent fasting methods do not seem to be more effective than conventional dieting, TRE, especially combined with a healthy, largely plant-based diet, may be a more practical, realistic, and sustainable approach with long-term benefits. Even with progress having been made, more human studies with in-depth evaluation of the role in gut microbial function are needed to confirm whether TRE is a cost-effective method for weight-loss.

Juliette Frank is a UCLA student majoring in Public Affairs with a minor in Food Studies. Her interests include the interrelation between food systems, digestive health and the environmental impacts of food production.