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This Is What Happens Inside Your Body When You Intermittent Fast

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This Is What Happens Inside Your Body When You Intermittent Fast

The term “intermittent fasting” refers to the practice of regularly not eating during a repeating schedule of time. While some individuals practice intermittent fasting in a deliberate attempt to improve their physiology, there are many other reasons to practice intermittent fasting.

A person’s work schedule may make one meal very inconvenient, so that meal is skipped every day. For other people, the practice may be used as part of religious practices, such as Muslims, who fast from dawn until dusk during Ramadan, and Christians, Jews, Buddhists, and Hindus, who traditionally fast on designated days of the week or calendar year.


Common Intermittent Fasting Methods

Intermittent fasting may be a newer dietary movement, but it has been practiced for thousands of years in other forms. The term “intermittent” means that there are periods during which the fasting individual may consume foods and times when they do not consume any calories.

Much of the intermittent fasting research on humans involves people fasting for religious reasons, such as Ramadan. A “Ramadan Fast” means not eating anything between dawn and dusk. Ramadan is the ninth month of the Islamic calendar year, and the dawn-to-dusk fast is followed the entire month. This type of fasting has provided researchers with a good pool of research subjects.

Time-Restricted Eating: 16/8

Other types of fasting are determined by the time window during which eating is allowed. Typical examples included 16/8, or 16 hours of fasting followed by an 8-hour window where eating is permitted. The most common form of fasting, and the easiest for many, is not eating after supper until lunch the following day. This simple strategy has proved to be transformative for many followers.

The most popular window for 16/8 eating tends to be 12 pm to 8 pm, but it’s entirely up to you to choose a time that fits your schedule.

The 5:2 diet

The 5:2 diet involves eating as you usually do five days a week and restricting your calorie intake to 500–600 on the remaining two days. Unlike a true fast, which involves eating nothing for a set amount of time, the goal of the 5:2 diet is to cut caloric intake on fasting days to 25 percent of a person’s regular calorie consumption on the remaining days.

For example, a person who regularly eats about 2,000 calories daily would eat 500 calories on fasting days.

One Meal A Day (OMAD)

Some advocate for taking fasting one step further and practicing OMAD or One Meal A Day. The most common version is only to eat a large meal at supper. Eating one meal a day is considered more advanced and more challenging, so this might not be a wise entry point to the practice of intermittent fasting.  

Alternate-day fasting (ADF)

ADF is an intermittent fasting approach that has you fast one day and then eat what you want the next day. On fasting days, you can drink as many calorie-free beverages as you like. Examples include water, unsweetened coffee, and tea.

Following a modified ADF approach, you can eat about 500 calories on fasting days, or 20–25% of your energy requirements.

The most popular version of this diet is called "The Every Other Day Diet" by Dr. Krista Varady, who has conducted most of the studies on ADF.

*Note: There are many other types of intermittent fasting not discussed in this article.

7 Evidence Based Health Benefits of Intermittent Fasting

1. Gut Bacteria Health

One of the most basic measures of microbiome health is species diversity. Just like in any ecosystem, the loss of species leads to a functional decline. Species' family trees are documented in a science known as taxonomy, so the microbiome is thought to be healthier when taxonomic diversity is higher. Intermittent fasting has been found to improve taxonomic diversity. It also has been found to increase the bacteria's production of butyrate, a short-chain fatty acid that reduces gut inflammation and promotes healing.  

2. Heart Health

It has been shown that intermittent fasting alters human gut bacteria in a way that significantly reduces the risk of heart disease. The gut bacteria changes resulted in higher production of short-chain fatty acids that reduce inflammation, combined with decreased release of inflammatory lipopolysaccharides.

Improved cardiometabolic health has also been found to result from intermittent fasting. This includes improving blood fats such as triglycerides and cholesterol and improving how well blood vessels can open up to allow blood flow to the heart muscle.

3. Reduced Inflammation

Intermittent fasting (aka "time-restricted feeding") with daily eating restricted to an 8-hour window has been shown in a 2016 study to reduce inflammation. Researchers stated, "Our data demonstrate that during a resistance training program, time-restricted feeding was capable of maintaining muscle mass, reducing body fat, and reducing inflammation markers." Dawn to dusk intermittent fasting can increase the levels of anti-inflammatory IL-10 while reducing the inflammatory cytokines IL-6 and TNF-alpha. These immune system chemicals help steer how the immune system functions, and imbalances in cytokines result in increased inflammation.

4. Cell and Mitochondria Cleanup

Autophagy is a critical process that the body uses to clean up damaged proteins, cells, mitochondria, and even pathogens. This makes autophagy very important for slowing the aging process and preventing diseases resulting from damaged cells. Intermittent fasting initiates and improves autophagy through periods of caloric restriction. Improving autophagy through intermittent fasting may answer the age-old question of how caloric restriction extends lifespan.

5. Metabolic Syndrome

Intermittent fasting has a pronounced effect on insulin resistance and metabolic syndrome since it improves the body's ability to react to insulin levels. The improvement in insulin sensitivity has been found to be due to reduced adipose tissue inflammation and improved body fat distribution.

6. Obesity

Obesity is perhaps today's most challenging health issue, affecting 41.9% of people in the US, with the rate of severe obesity has jumped from 4.7% to 9.2% in the last decade. A review of the research shows that intermittent fasting is a useful tool for controlling obesity.

One significant association supported by high-quality evidence was that a modified "alternate-day fast" for 1 to 2 months was associated with a total reduction in body mass index in healthy adults and adults who were overweight, obese, or with nonalcoholic fatty liver disease.

Compared to caloric restriction, IF resulted in better weight loss and adherence to the protocol. Study participants found IF less challenging than calorie-restricted diets, making the strategy more successful because of better compliance.

7. Memory and Cognition

Mild cognitive impairment affects 12-18% of people age 60 or older, and 10-15% of those develop dementia each year. A 3-Year progressive study showed that older adults with loss of memory function improved cognitive function after being prescribed intermittent fasting as part of their therapy.

Frequently Asked Questions on Fasting: FAQ

Who Shouldn't Fast?

In general, fasting is usually safe and different versions of fasting have been around for centuries. Despite this long history, there are some people who should not practice intermittent fasting without consulting a medical professional first. This includes:

  • Pregnant or breastfeeding women
  • Women who are trying to conceive
  • People who have problems with blood sugar regulation
  • People with low blood pressure
  • Those who are taking prescription medications
  • Those who are underweight
  • People with a history of eating disorders
  • Those with advanced liver or kidney disease

*There are many other medical conditions that can be affected by intermittent fasting. It's important to discuss this type of eating with your provider before changing your eating habits.

Can I Drink Liquids During a Fast?

Without exception, the best beverage to consume during intermittent fasting is water. Other drinks can be consumed without getting out of the fasting state. These drinks should not have artificial sweeteners, as these can alter your insulin sensitivity and cause the problems most are trying to help with intermittent fasting. Tea, coffee, lemon water, and similar drinks (even coconut water!) can be fine as they do not contain enough calories to stop the fasting state. Adding cream or sugar to the coffee adds enough calories to end the fasted state. Fruit juice and soda have sufficient sugar/carbohydrate to end a fasted state.

Can I Exercise During Intermittent Fasting?

It has been found that even during one-meal-a-day (aka OMAD) fasting, exercise ability was not affected after a two-week adaptation period.

Athletes' ability to exercise was not reduced in strength, speed, or endurance. Another study on elite cyclists found that restricting eating to an eight-hour time window, 10 am to 6 pm, resulted in weight loss and no change in power output. For long-distance runners, eating within an eight-hour window for eight weeks was shown to reduce body mass without altering the athlete's performance.

These studies show that intermittent fasting can work well for athletes, but it is still always a good idea to approach significant changes gradually and with a degree of caution. It's always important to listen to your body and do what feels best for you.

Can I Take Medications or Supplements While Fasting?

This is a common and logical question. First, it is essential to note that you need to discuss IF with your provider if you are on a prescriptive medication or supplement. They will work with your individual needs.

Generally speaking, taking medications or nutritional supplements during a fast doesn't end the benefits of the fast. But there may be other problems with taking supplements or medications on an empty stomach.

Many medications are formulated to be absorbed in a steady rate to spread out the dosage absorbed over time. Taking a prescription or supplement on an empty stomach tends to slow the absorption as the pills dissolve in an inactive stomach. When you finally do eat, the stomach will absorb much of that dosage over a short time. In addition, taking some medications or supplements on an empty stomach may leave you nauseous until you eat later.

*Timing medications is essential, so if you aren't sure how to do that, reviewing the dosage schedule with your prescribing physician is a good idea.


Intermittent fasting involves not eating for some periods and can have many health benefits. While many people fast for religious reasons, fasting is also often practiced for health or convenience. If you are interested in trying this way of eating out, discussing the risk and benefits with your provider is important.

The information provided is not intended to be a substitute for professional medical advice. Always consult with your doctor or other qualified healthcare provider before taking any dietary supplement or making any changes to your diet or exercise routine.
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