It’s Not All About WHAT You Eat, It’s About WHEN You Eat

By Juliette Frank

The recent publication of a number of studies on the benefits of time-restricted eating (TRE) on losing weight and improving overall metabolic health has led to an increased interest in the topic. As TRE and intermittent fasting (IF) gets pushed into mainstream discussion, a common question people have is if it is the right diet for them. Even though the terms TRE and IF have been used interchangeably, there is a significant difference between the two: In TRE, the time of unrestricted food intake is limited to 8-10 hours a day, while no food is allowed for the remaining 16-14 hours. Overall, no calorie restriction is required in TRE. In contrast, IF refers to a eating pattern where several days of unrestricted food intake is followed by 1-3 days of fasting, resulting in a significant calorie restriction over time.1 For the average person looking to lose weight there is a growing body of research supporting the efficacy of both eating patterns as an intervention to manage weight and chronic diseases for a period up to one year.2 With nutrition and proper fueling being at the forefront of athletic performance, it is not surprising that there has been increased interest on the impacts of these dietary strategies on the body composition and athletic performance of athletes.

Most research on the impacts of IF and TRE on athletic performance has been done in athletes observing the month long Ramadan fasting tradition. This is the most important month in Islam where observers abstain from all food and liquids during the hours from sunrise to sunset. Depending on where one is located geographically this means fasting for around 11-16 hours and eating only during the nighttime. The prominence of Ramadan-style TRE sparked interest when two major sporting events: the London 2012 Olympic Games and also the FIFA 2014 Soccer World Cup were staged during the month of Ramadan. The high profile of these events increased interest in the effects of this fasting method on top athletic performance. It is important to note that there are various factors to keep in mind when looking at the athletic performance of athletes observing Ramadan that may not be present when following a more standard TRE pattern, including sleep deprivation, altered circadian rhythm, heightened stress levels, low blood sugar, and dehydration.

According to two recent studies, one published in Nutrients and the other in the Journal of Sports Medicine in 2020, training in a fasted state without carbohydrate ingestion and decreased glycogen levels is hypothesized to enhance lipid utilization for energy generation, which in turn has been shown to improve glucose tolerance and increase insulin sensitivity. These effects force the metabolism into a ketotic state, reducing overall blood sugar levels and burning more fat instead. Studies found that time-restricted eating (TRE) is associated with preservation of lean body mass and a reduced fat mass, as well as higher blood levels of fatty acids and ketone bodies. Fasting stimulates various hormone adaptations leading to increased metabolic rate and preserved lean mass which is vital for achieving the prime balance of body composition for improved athletic performance.

Contrary to some Ramadan fasting observations, previous studies have shown TRE to have a positive impact on VO2 max. Research suggests that TRE causes an increase in cardiac output, or the amount of blood the heart pumps through the circulatory system per minute, caused by the stimulation of the sympathetic nervous system during exercise or a larger oxidative capacity in fat adapted muscle. Together, these processes may lead to improvements in VO2 max and overall aerobic performance in properly hydrated conditions.

Although various studies show benefits of TRE and IF on athletic performance, not all training is created equal. For example, some studies on high intensity training while fasting has shown a decrease in performance while others show no change. One explanation for these varying results may be that short term IF depletes muscle energy stores. Another highly probable cause may be peripheral fatigue, which suggests that the correlation between IF and muscle strength might interconnect with the more widely tested circadian variation in athletic performance. These findings may not be generalizable to more standard methods of IF due to the impacts of Ramadan-specific protocols (such as perturbations of sleep schedule, dehydration, and common psychological effects) that exert negative influence on performance.

When looking at the findings of studies done thus far on the relationship between TRE and athletic performance it seems that while the potential benefits or detriments on athletic performance are minimal, the metabolic changes and impact on body composition are more significant. A common consensus amongst researchers is the importance of the final energy balance achieved on a daily basis. It is hypothesized that as long as athletes receive the proper amount of macro and micro nutrients during feeding hours they should be able to fully recover from their training. Overall, further investigations and more controlled studies on elite level female and male athletes are needed in order to make concrete recommendations for improving athletic performance through fasting methods.


  1. de Cabo, R.; Mattson, M.P. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med 2019, 381, 2541-2551, doi:10.1056/NEJMra1905136.
  2. Ge, L, et al. Comparison of dietary macronutrient patterns of 14 popular named dietary programmes for weight and cardiovascular risk factor reduction in adults: systematic review and network meta-analysis of randomised trials. BMJ 2020, 369, m696, doi:10.1136/bmj.m696.

Additional Sources

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.