What is Intermittent Fasting?

Written and Reviewed by: Elysium Health

Image of a empty plate with a prescription for Intermittent Fasting

Intermittent fasting is a catch-all term that refers to several eating schedules or patterns with periods of time when you don’t eat or when you reduce the number of calories you’re taking in. It’s a departure from how many of us have been eating most of our lives: three meals per day, with snacks in between. And research suggests that the health benefits might be significant.


Key Takeaways:

  • There are three general categories of intermittent fasting: time-restricted eating, intermittent calorie restriction, and periodic fasting. 

  • All three types have demonstrated health benefits in animal studies, and many of those benefits seem to translate to human studies, too. 

  • Fasting triggers a “metabolic switch” where the body switches from using liver-derived glucose to fat-derived ketones as the primary source of energy. 

  • Preclinical studies have shown that fasting increases NAD+ levels and sirtuin activity, suggesting that these pathways may contribute to some of the benefits of metabolic switching.

Related Product:

  • Signal: Contains NMN, a highly efficient NAD+ precursor and clinically proven to boost NAD+ levels in humans. Signal also contains a SIRT3 Activation Complex, which activates the mitochondrial sirtuin, SIRT3, to support a healthy metabolism. 


You might be thinking, do we really need another diet plan? Fortunately, intermittent fasting doesn’t ask that you give up meat, monitor your lectins, or only eat chocolate mousse under a full moon. In this much-researched dietary intervention, scientists are asking not what to eat, but when to eat and whether to eat at all—at least for certain periods of time. 

What about that conventional wisdom to eat three meals per day, or that breakfast is the most important meal of the day? Those might be the preferences of our modern culture, but fasting may have a precedent that reaches back further. Humans are wired to go for long stretches without food. Until recently, in the long timeline of evolution, we were hunters and gatherers who had to search and work for our food, physically. More recently, but before The Information Age, we could imagine that life, including meal times, was more in sync with the cycles of nature and in particular the hours of lightness and darkness. 

Fast-forward to the present day where we have 24/7 entertainment, and food delivery, and many of our options for earning a living are sedentary, in front of screens.

As a result of our heritage of occasional food scarcity and life in sync with biological rhythms, we may actually be designed to thrive with a bit more time between meals, less time digesting, and more time in recovery. Here’s what scientific research says about intermittent fasting.

What are the types of intermittent fasting?

There are a variety of styles or plans of intermittent fasting that have been studied in animals and, increasingly, in humans. Generally, they fall into three categories: time-restricted eating, intermittent calorie restriction, and periodic fasting. All of these have demonstrated health benefits in animal studies, and many of those benefits seem to translate to human studies, too. Like all areas of research, more conclusive studies are in the works that will shed vital light on fasting and long-term health. 

  • Time-restricted eating is the practice of limiting calorie intake to a certain window of time each day, usually to 10 hours, eight hours, or six hours. You might see that written as 16:8 or 14:10 intermittent fasting, which means (in the first example) 16 hours of fasting and eight hours of eating. The goal is to follow the regimen every day or most days.
  • Intermittent calorie restriction calls for reducing daily caloric intake to 500–1,000 calories, for two consecutive days per week. This is also called 5:2 intermittent fasting or the 5:2 diet (for 5 days of “normal” and 2 days of restricted caloric intake).  
  • Periodic fasting calls for limiting caloric intake for between three and five days, either with no food or with a calorie-restricted diet that keeps the body in a state that mimics fasting. This type of intermittent fasting should always be done under the supervision of a doctor.

What happens to the body during fasting?

There are two ways to think about what’s happening in the body during fasting and why these processes might be good for our health. Let’s start with the most tangible: In studies, people tend to lose weight. For people with obesity, for example, carrying less weight can have downstream health benefits.

The other changes are happening at the cellular level as the body adapts to less food. In all the categories of fasting described above, a so-called “metabolic switch” gets flipped eight to 12 hours after the onset of fasting. During this period, the body switches from using liver-derived glucose to adipose-cell-derived ketones as the primary source of energy.  When people eat and snack often, the body tends to burn glucose for energy and store fat in adipose tissue as triglycerides. In the metabolic switch that happens during fasting, the body breaks down triglycerides into fatty acids and glycerol, which are instead used as energy. 

In the liver, fatty acids turn into ketone bodies. You’ve probably heard of ketosis, which is when the body is burning primarily ketones for energy. Ketones not only serve as fuel for cells during the fasting period but also trigger highly coordinated responses that even influence the fed state. They alert the body to its current metabolic status and influence a variety of cellular pathways that are known to be beneficial. These include pathways that involve PGC-1α, fibroblast growth factor, NAD+, sirtuins, PARP1, and CD38. 

Let’s look at an example of a few of these cellular pathways and how they change during fasting. Preclinical research suggests that NAD+ levels and sirtuin function (especially SIRT1 and SIRT3), both of which decline during aging, actually increase during fasting, contributing to some of the health benefits associated with metabolic switching. This discovery is one reason researchers have pursued compounds that can “mimic” some of the changes to cellular pathways that happen during fasting. 

Signal is one such product. It provides an NAD+ precursor molecule called NMN (nicotinamide nucleotide) which is efficiently converted to NAD+ by the cell, as well as a SIRT3 activation complex consisting of two naturally occurring products: honokiol, isolated from the bark of Magnolia officinalis, and viniferin, derived from grapevine. The ingredients in Signal work synergistically to increase NAD+ and activate SIRT3, helping to maintain overall metabolic health. 

Intermittent fasting is also known to stimulate autophagy, a process where non-functional or unnecessary cells are removed and recycled so healthy new ones can be built. For the majority of us who consume three meals a day plus snacks, intermittent fasting, and metabolic switching do not occur.

What are the benefits of fasting?

Ultimately, you can think of fasting as a way to offer the body a break from the work of digestion and more time for maintenance and repair. According to research, doing this has benefits for health and performance.

The biological changes that happen during fasting happen during the fasting period, during the recovery period, and then coalesce into long-term adaptations over time. Studies suggest that the long-term adaptations include increased insulin sensitivity, increased heart rate variability (an indicator of the body’s ability to tolerate stress), improved lipid metabolism, helpful changes to the gut microbiota, and reduced abdominal fat, inflammation, and blood pressure. Studies have also shown cognitive benefits in humans including improvements in working memory and other aspects of cognitive function. 

Is fasting right for you?

Like any other health intervention, intermittent fasting is best approached under the guidance of a healthcare practitioner—in particular for a fast lasting more than 24 hours, which should always be under the observation of a doctor. Time-restricted eating offers a lower barrier to entry since it only requires adjusting mealtimes, but it may not be right for children, pregnant women, people with diabetes, and anyone with a history of eating disorders. 


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