Nutrition 6 MIN READ

Fasting For Athletes And Its Effect On Metabolism

Fasting is a millennia-old practice that has long been undertaken for religious purposes. Yet, recent studies have revealed the huge impact it has on the body’s metabolism,

Written by Team Ultrahuman

Oct 14, 2022
Effect On Metabolism

Fasting is a millennia-old practice that has long been undertaken for religious purposes. Yet, recent studies have revealed the huge impact it has on the body’s metabolism, ranging from increasing longevity, preventing chronic metabolic diseases and promoting overall better health. Athletes who are endurance-oriented can certainly benefit from fasting in order to improve their performance.

Athletes Effect Metabolism


  • Fasting has been a part of many cultures and traditions for over a millennia and refers to the period of abstention from food or caloric beverages,
  • Intermittent fasting is a pattern of time-restricted feeding in which food is consumed during a particular feeding window,
  • Various kinds of intermittent fasting include the 16/8 method, the Eat-Stop-Eat method, the 5:2 method, time-restricted feeding and early time-restricted feeding.

Today, we will explore the intertwined relationship between fasting and metabolism. 

Fasting 101 and intermittent fasting

Fasting refers to the period of abstention from food or caloric beverages; one usually ingests a minimal amount of food during this period, which can last for up to three weeks. On the other hand, intermittent fasting (IF) has been defined as a pattern of time-restricted feeding in which food is consumed during a particular feeding window. In most cases, IF involves calorie-restriction as well. 

Kinds of intermittent fasting

  1. The 16/8 method: This is one of the most common forms of IFand involves the individual fasting for a period of 16 hours, which usually starts at 8 p.m. the previous evening and ends at noon the next day. Breakfast is typically skipped in this form of IF. 
  2. The Eat-Stop-Eat method: This involves fasting for a whole 24-hour period without any feeding window; however, you can drink non-caloric beverages like teas during this period. This can be done twice a week. 
  3. The 5:2 method: This involves caloric restriction in which the individual eats only 500–600 calories for two days in the week and then eats normally for the rest of the week. 
  4. Time-restricted feeding: This refers to a nutrition intervention in which the feeding period is restricted to a window of 8–10 hours. A study done by Satchin Panda et al, has revealed that this method, rather than IF, can have huge implications on the metabolism of an individual. This idea can also truly aid individuals in attaining their sleep goals as well. 
  5. Early time-restricted feeding (eTRF): This refers to a form of IF in which the fasting period is even longer. This means that the feeding window lasts from 8 a.m. to 2 p.m.

The Science Behind Why We Fast

Time-restricted feeding and its impact on the body’s metabolism:

Humans, like most mammals, follow the movement of the sun. The human circadian rhythm is tuned to the shifts in the amount of sunlight exposure one receives. Recent studies have also found that the timing of food intake is crucial to maintaining the circadian rhythm. This interrelation has been found to have significant impacts on body weight, lipid homeostasis, the gut microbiome, cardiac function, quality of sleep and blood glucose values. 

1. Weight loss

Time-restricted feeding has proven to be highly effective in weight loss. A study showed how over a period of six months, a group of individuals were able to lose 1.6 percent of their body weight through this eating intervention. The reason for this could be on account of the body’s switch from glycolysis (the first phase of glucose utilization) to ketogenesis (the production of ketone bodies). Ketones are produced by the liver from fatty acids, and ketogenesis occurs in the absence of intake of carbohydrates. This alternate energy pathway is highly effective in maintaining the metabolic circadian rhythm, lowering oxidative stress as well as reducing inflammation. The study which brought this aspect of weight loss to light was based on the intervention which used the 14:10 principle (in which the overnight fast begins just after dinner and extends till breakfast the next morning, 14 hours after the previous meal. In addition to this, the volunteers also consumed a high-fat, low-carb, low-protein snack during the fast. It was found that these individuals had lost between 9–12 kg after the six-month experiment phase.  

2. Glucose levels

eTRF has also been seen to influence the levels of glucose in the body. One study has shown that restricting food consumption early in the day can potentially improve cardiometabolic health and several factors tied to glucose metabolism including reduced levels of insulin and greater insulin sensitivity. It was observed that during lunchtime, the blood-glucose peak had decreased; this was happening because the beta cells were still actively secreting insulin on account of the small interval (3 hours) between breakfast and lunch .  

3. Lipid synthesis

In terms of lipid synthesis, the eTRF increased the LDL (low-density lipoprotein, sometimes called “bad” cholesterol) and HDL (high-density lipoprotein, sometimes called “good” cholesterol) levels in the body. This was an expected outcome due to the increased fasting window and the previously mentioned ketogenesis. The values of beta-hydroxybutyrate (a marker for ketone levels) were also slightly elevated.

Fasting Athletes Metabolism

Athletes and fasting—an insight 

Do athletes truly benefit from fasting? 

Research has shown that as a nutrition strategy, IF has aided in improving loss of fat mass as well as jumping performance. One study measured the performance of athletes during the festival of Ramadan. The main focus was high-intensity exercise, especially sprint performance. It was found that the athletes’ sprint times during this Ramadan period were significantly lower, thus concretizing that fasting does impede anaerobic exercise.

With regard to aerobic exercise such as marathon running and endurance cycling, a study found that the running speed for athletes improved under fasting conditions. Research conducted on elite cyclists revealed the same findings. It was observed that the cyclists had increased body composition, weight loss and increased peak power output/body weight (PPO/BW). It was found that the levels of adiponectin (a fat-derived hormone that seemingly plays a critical role in protecting against insulin resistance/diabetes and atherosclerosis) increased after eight weeks of time-restricted eating. The paper also connects the improvement in body composition to the circadian clock, wherein the activation of the metabolic regulators occurs. These include mammalian target-of-rapamycin (mTOR), AMP-activated protein kinase (AMPK), cAMP-response element binding protein (CREB) and Akt. In addition to this, the PPO of the athletes was observed to have increased. 

Traditionally, endurance athletes are seen to have lower leucocyte counts, neutrophils and monocytes as a result of aerobic training. The pro-inflammatory marker IL-6 (interleukin-6) was shown to be lowered. This in turn significantly prevented the decrease in leukocytes.


Fasting has been a part of many cultures and traditions for over a millennia. Its significance has grown in recent years as more awareness spreads about its many benefits. 

In the context of athletes, IF may be one of the best ways to lose fat mass. However, studies have shown that this technique primarily benefits athletes who pursue sports which are related to aerobic exercise rather than anaerobic exercise, so bear this in mind before embarking on this path.

Disclaimer: The contents of this article are for general information and educational purposes only. It neither provides any medical advice nor intends to substitute professional medical opinion on the treatment, diagnosis, prevention or alleviation of any disease, disorder or disability. Always consult with your doctor or qualified healthcare professional about your health condition and/or concerns and before undertaking a new healthcare regimen including making any dietary or lifestyle changes.



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