Intermittent Fasting

Written by Ashley Fitzgerald & Emily Fultz, MS, RDN

Intermittent fasting is alternatively cycling between periods of fasting and eating. Typically people do this overnight and rather than eating breakfast when they first wake up, they wait until late morning or early afternoon to break their fast. This type of intermittent fasting is called time-restricted fasting with the goal being to fast for around 8 to 12 hours per day, with some people fasting for as much as 16 hours (1).

Weight Loss

The initial theory behind intermittent fasting is to restrict the window for eating so less calories are consumed, potentially leading to weight loss. For some people it helps prevent mindless eating at night as well as prevents overeating when they’re not really hungry. For others – it causes increased hunger, leading to over consumption at the time they break the fast. However, the research in this area is limited and more studies are needed to understand the effects of intermittent fasting, especially in the long-term (1).

What we do know is that studies have been pretty consistent with the findings that the combination of physical activity and a well balanced diet are optimal for weight loss. Many of us know that a nutritious meal pattern consists of fruits, vegetables, whole grains, healthy fats, and high-quality lean proteins. People who follow this way of eating may find it easier to control their body weight without skipping meals and/or strictly counting calories (2).

Per science: “Most importantly, the best diet is a diet that people can comply with for a long period of time without significant weight regain, so whatever facilitates this effort is greatly appreciable (2).”

If diet culture is confusing you, then READ THAT AGAIN

Athletic Performance

There is also currently limited research on intermittent fasting for athletic performance. Most of the studies reported have been on fasting during Ramadan, where food is not consumed between dawn and dusk for a month. This study from 2007 found that after fasting during Ramadan, professional soccer players noticed their performance in speed, agility, dribbling speed, and endurance all declined. In 2011 a similar study was carried out, and 24% of the athletes reported that their performance was affected, 29.3% of the athletes reported that the quality of their training was affected, and 66.6% of the athletes complained of fatigue. Another more recent study from 2017 found similar results that the speed of sprinters declined after fasting. Although these studies all found performance was affected in some way, other studies have found no change in athletic performance (at least in the short term).

The American College of Sports Medicine mentions how important the type, amount, and timing of nutrients are to not only maintain adequate health but also maximize training outcomes and recovery time (3). This means that even when fasting, it is important to meet normal energy and nutrient requirements for exercise. If you are able to do so, your performance may not be affected. However, everyone is different.

If you do decide to try intermittent fasting, just remember it does not mean you can eat anything you want during those designated hours. You still need to ensure you are getting all the important nutrients and therefore should consume a variety of nutrient-dense foods like fruits, vegetables, legumes, whole grains, lean meats, and nuts and seeds.       

If Anything – Take Away This

Today’s diet culture might make you feel otherwise, but intermittent fasting is not for everyone. It may not be for you if you:

  • Are getting dizzy or weak during workouts
  • Have decreased energy
  • See a decline in your performance
  • Find yourself so hungry that you overeat during your eating window
  • Struggle to consume enough calories for your body’s needs.
  • Have diabetes, are a woman who is pregnant or breast-feeding, or an individual with a history of eating disorders or disordered eating (1)

If you have ever tried intermittent fasting before, we would love to know how its worked for you. Let us know in the comments!


  1. Gordon B. What is Intermittent Fasting? Academy of Nutrition and Dietetics. Published May 7, 2019. Accessed October 24, 2019.
  2. Koliaki, C., Spinos, T., Spinou, Μ., Brinia, Μ. E., Mitsopoulou, D., & Katsilambros, N. (2018). Defining the Optimal Dietary Approach for Safe, Effective and Sustainable Weight Loss in Overweight and Obese Adults. Healthcare (Basel, Switzerland)6(3), 73. doi:10.3390/healthcare60300.73
  3. Thomas DT, Erdman KA, Burke LM. Nutrition and Athletic Performance. Med Sci Sports Exerc. 2016;48(3):543-68. doi: 10.1249/MSS.0000000000000852.

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