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What Is Intermittent Fasting and How Does It Affect Your Body?

This post defines what intermittent fasting is, how it can be performed, the benefits, the resulting cellular activity, and the safety considerations.

Defining Intermittent Fasting

You’ve likely heard about it if you’re interested in various diet protocols, but what is intermittent fasting? Simply put, intermittent fasting involves alternating periods of eating with predetermined periods of fasting. The fasting window is generally longer than the time spent eating. You can eat meals as you usually would during your eating window, although some people eat less simply because there isn’t time for three meals and snacks.

How Does Intermittent Fasting Affect Your Body?

Intermittent fasting (IF), an increasingly popular wellness practice, is usually advertised as a weight-loss tool. But the benefits go far beyond caloric reduction. From gut health to metabolic support, the evidence behind IF continues to grow. Keep reading to learn about the benefits of intermittent fasting.

Fasting, or abstaining from foods or beverages with calories for an extended period, is considered a stressor to the body. Unlike chronic stress, short-term nutritional stress is associated with positive adaptations that can lead to cellular support and protection. 

While weight loss may be an added value of intermittent fasting, looking a bit deeper below the surface into the cellular changes can provide the science behind why this is more than just the next fad diet.

Intermittent Fasting Schedules

There are multiple ways to practice IF, but the most popular IF schedules are as follows:

16:8 Fast

This cycle includes a 16-hour fast with eight hours of eating. It can be repeated daily or only on certain days of the week, depending on your lifestyle and individual needs.


Here, you’d eat normally for one to two days and then fast for 24 hours the following day, repeating once or twice a week.

20:4 Fast

Also called the Warrior Diet, this cycle includes a short four-hour eating window with a 20-hour fast for the rest of the day.

5:2 Method

Instead of completely fasting, this method includes one to two non-consecutive days of very low calories (500-600 total) and normal intake the rest of the week.

Benefits of Intermittent Fasting

So why do people fast? Is fasting just the latest health trend? Is fasting good for you? The answer is, intermittent fasting can definitely be valuable. There is certainly no shortage of IF benefits listed in medical journals and on various websites. In our appearance-obsessed American culture, the IF benefit usually discussed is weight loss. Just by virtue of eating less frequently, people typically consume fewer calories when intermittent fasting, which by itself can result in weight loss.

But it’s the hormonal changes that can really make a weight loss difference for many people—the impact to insulin, human growth hormone, and norepinephrine levels all drive augmented fat burning. In addition to weight management, a review article published in the New England Journal of Medicine found that IF is associated with many health benefits, including:

While researchers are still attempting to understand precisely why fasting may be so helpful, many of these advantages are likely related to the cellular adaptations that occur when the body is in a fasted state. 

What Happens to Your Cells When You Fast?

How Intermittent Fasting Affects Your Cells - Image

While nutrition research usually examines diet and how the body responds when you eat certain nutrients, the power of IF happens when there is a lack of nutrients, so the body is forced to adjust.

When you eat, your body is in a cycle of growth, or anabolism. Nutrients from your diet are used to build molecules in the body. While this is an essential physiological process, your body also needs time to balance anabolism with periods of repair.  

In these times of rest, the body can focus on clearing out cellular debris, waste, and free radicals that otherwise lead to oxidative stress and damage in the body. Many of these restorative and protective processes only happen during periods of nutrient scarcity (most commonly when you sleep, as described in Nature Communications) or while fasting.

As research tells us that oxidative stress is intimately connected with chronic health conditions, especially those associated with aging, fasting may be a tool to help. Some of the known cellular adaptations that may protect your body against oxidative damage include:

Repair and Waste Removal Through Autophagy  

A well-studied cellular adaptation to fasting is autophagy. Autophagy is the body’s way of removing damaged cells, cellular debris, and waste products to make room for healthier, stronger cells. It’s like the body’s programmable robotic vacuum that turns on at night when you are sleeping, moving from room to room, cleaning up the dust and mess created during your daily activities. We like to call it cellular hygiene. 

The absence of nutrients during fasting and low insulin levels (also seen in ketogenic diets) drives autophagy. As seen in the journal Cell Death and Differentiation, autophagy has been well studied as a protective measure against oxidative damage.  

A review published in Frontiers in Cell and Developmental Biology also suggests that autophagy is a potential tool to fight back against the aging process.

Inflammation and Immune Response 

Acute inflammation is a normal part of the immune response, but chronic inflammation is associated with significant health concerns. Fasting may help downregulate inflammation by impacting the pro-inflammatory immune cells that communicate messages to turn on the inflammatory process. 

Researchers from Mount Sinai discovered that IF may reduce the number of circulating monocytes (inflammatory immune cells) in your blood. Further, this study found that the monocytes found in blood from the IF group had less inflammatory activity than those found in fed subjects.  

As researchers are interested in the association between monocytes and certain chronic health conditions, reductions through fasting could be a simple, non-invasive approach to drive down inflammation.

Mitochondrial Health  

Mitochondria are often called the powerhouses of the cell because they generate energy throughout the body. When nutrients are converted to energy, they also create reactive oxygen species (free radicals) as a normal byproduct.

Mitochondrial health can also be negatively affected by various lifestyle factors or simply as a regular part of metabolism. Mitochondrial dysfunction is associated with accelerations in aging and many chronic health concerns, as described in the Journal of Integrative Medicine.

Fasting can support mitochondrial function, cleaning up free radical byproducts through autophagy. Research published in Molecular and Cellular Biochemistry found that fasting also increases NAD+, which stimulates sirtuin activity. 

Sirtuins are proteins associated with many healthy-aging benefits, including the support of mitochondrial function and adaptations to stress. They may also play a role in the regulation of autophagy as documented in Experimental and Molecular Medicine.

Downregulation of mTOR Activity  

The mammalian target of rapamycin (mTOR) is a signaling pathway that helps with cellular growth and metabolism. Fasting turns off mTOR activity, which is one form of autophagy. Autophagy can allow for the growth of new, healthy cells. Once nutrients are reintroduced during the feeding window, mTOR can turn back on and regenerate new healthy cells, but only after the body has had a chance to clear out any that are damaged. One caveat about turning off mTOR through fasting—this can also disable muscle protein synthesis. For older adults (50+), deactivating mTOR can pose some potential risks if not monitored properly, as mTOR is important to building and maintaining muscle as you age. 

Activation of Nrf2  

Nrf2 is a protein that turns genes on or off, activated in response to oxidative stress to protect against free radical damage. 

A review article published in Frontiers in Pharmacology found that while Nrf2 levels naturally decline with age, fasting activates Nrf2, contributing to the clean-up of reactive oxygen species in the body.  

It also plays a vital role in healthy detoxification and is associated with neuroprotective benefits through its antioxidant activity, as seen in a review from ASN Neuro.

Is Intermittent Fasting Safe?

Many people enjoy the benefits of intermittent fasting without side effects, but there are some groups that should avoid it:

  • Children and teens

  • Pregnant or breastfeeding women

  • Insulin-dependent type 1 diabetics

  • Anyone who has suffered from an eating disorder

  • Active women who are peri- or postmenopausal should check with their OB/GYN before implementing IF

While IF can be safe and utilized long term, side effects could include headaches, nausea, fatigue, or anxiety. A healthcare professional should be consulted before implementing IF. 

And while the previously discussed fasting schedules are regarded as viable and beneficial, fasting for extended periods may not necessarily result in greater benefit, and may actually be physically dangerous.

Also, when first starting IF, there is an average ramp-up period of roughly two to four weeks, during which people can feel hungry or moody. Once people make it past this hurdle, they typically feel the benefits of IF and continue with it.          

Intermittent Fasting Is a Flexible Wellness Tool

Intermittent fasting may be a way to support your health through the cellular adaptations that happen when the body goes without nutrients. A conversation with your healthcare practitioner is always a good idea before starting, but IF can provide a flexible framework to benefit from these changes without longer-term caloric deprivation. It’s important, also, to continue to meet the appropriate caloric intake requirements for your body during your eating window if you do practice IF. 

While everyone responds differently, and some will benefit from shorter or less frequent fasts, IF can be a simple, effective tool to add to your wellness practice.

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