Benefits of Fasting

Humans are only meant to eat in a short window of time? When we consider our ancestors and the time in which they lived, it can help us have a better understanding of why this is.

We live busy lives and as a result, we find ourselves prioritizing everything but food. Most Americans snack throughout the day and end up eating for 12-14 hours, which is a much longer window measured from the time of your first meal to your last.

Maybe you start your morning off with a coffee and breakfast from Starbucks. Maybe you packed a lunch or some snacks, but you end your night with takeout and dessert right before bed. Unlike us, our ancestors only consumed food when it was available, resulting in long periods of fasting. 

The time at which we eat seems to differ from person to person. Breakfast, lunch, and dinner have been modernized to fit our nutritional needs, activity levels, and schedules. But when it comes to undesired weight gain, a common denominator found among these individuals is that they eat for longer than what is physiologically needed.

This leaves us with the question, what is intermittent fasting and how does eating during awake hours affect our health?

Types of Intermittent Fasting

Intermittent fasting can be defined as periods of eating alternated with periods of not eating [1]. The three types of intermittent fasting are ADF (alternate day fasting), the 5:2 diet, and TRE (time restricted eating) all with variations in their feast/fast days.

ADF includes a feast day alternated with a fast day. On the feast day, participants are allowed to eat ad libitum, as much or as often as necessary; however on the fast day, individuals are only permitted water (zero-calorie ADF) [1].

Fast days during modified ADF involve the consumption of approximately twenty-five percent of an individual’s energy needs and can be eaten at once or spread out during the day [1].

The 5:2 diet includes five feast days and two fast days per week. Participants are allowed to eat ad libitum on the feast days and on fast days, approximately twenty-five percent of energy needs are consumed. It can be noted that the fast days can be placed on consecutive or nonconsecutive days during the week [1].

TRE on the other hand has individuals fast for a short period of time every day. Time restricted eating involves limiting the eating window to a certain amount of hours per day and fasting with zero-calorie beverages for the remaining hours of the day [1]. During the eating window, participants are not required to count calories or track food intake[1].

Weight Loss

Physiologically, it is unhealthy to eat during a majority of your awake hours [2]. Meal timing and frequency work hand in hand to help regulate satiety ( the state of feeling full), digestion and disease prevention.

A review written by the Department of Kinesiology and Nutrition at the University of Illinois at Chicago summarized the effects of intermittent fasting on health and concluded that the three types of intermittent fasting explained above induce mild to moderate weight loss. ADF and the 5:2 diet are the only fasting types that induce clinically significant weight loss which is greater than five percent, excluding TRE [1].

Within the fasting period, we give our bodies time to digest, detoxify and process everything we’ve consumed throughout the day. Therefore, allowing adequate time for these processes is essential [2]. After the evening fast, our bodies switch from using carbohydrates to fats as fuel causing us to tap into our fat reserves and ultimately lose weight [3].

Metabolic Flexibility

Our bodies store carbohydrates throughout the day and when we use up all this energy, we naturally start using fat for energy. The ability to switch between carbohydrate and fat metabolism is known as metabolic flexibility and can occur at about the tenth hour of not eating during the nightly fast [4].

Metabolic flexibility is the ease and speed at which our metabolism transitions between the absorptive and postabsorptive state [4]. This flexibility works to prevent hypoglycemia, or low blood sugar, as it ensures a sufficient amount of blood sugar is available to our brain and tissues during fasting [4].

The more metabolically flexible we are, the more efficiently we are able to switch from carbohydrates to fat and back. When we eat late at night and then early in the morning, we lose our metabolic flexibility and become insulin resistant leading to weight gain. By intermittently fasting and exercising these stores, we can increase our metabolic flexibility [4].

Sugar Burners versus Fat Burners

Fat burners use fat stores for energy. These individuals have clearer cognition, they lose weight easier, they sleep better, and they age more slowly [3]. This comparison goes to show just how crucial meal timing and frequency are for our health.

When we eat carbohydrates and protein, insulin is secreted by the pancreas to transport sugar into the cells. We store most of our sugar in our liver and skeletal muscle so when we exceed these storage sites, we store it as fat. Therefore when we are fasted, insulin levels are low and we can use these fat stores for energy [5].

Cancer Prevention

Obesity is a growing epidemic. Certain cancers have been linked to obesity due to increased total body inflammation and insulin resistance which results in a high insulin-like growth factor, a metabolite known to promote tumor growth [6]. Additionally, obese women have higher levels of estrogen, which are associated with endometrial and postmenopausal breast cancer [7].

In the 1950s, Otto Warburg observed that most cancer cells rely on glucose for energy [5]. Thus with the help of intermittent fasting, we can lower glucose levels in our blood making it more difficult for cancers to survive [4]. This observation gave way to the theory that fat metabolism, which is when ketones are used for energy, can induce oxidative stress to cancer cells leading to their death [5].

More recently in 2021, Dr. Valter Longo demonstrated the benefits of fasting on cancer promoting metabolites and body composition in cancer patients [4]. Now more than ever, cancer patients are considering intermittent fasting during treatment. However, not everyone is a good candidate [9]. Talk to your health care provider to see if fasting is a possibility for you.


Autophagy allows the body to break down and reuse old cell parts so cells can operate more efficiently [10]. Autophagy is similar to cleaning your house from dirt and clutter as it allows us to be in good spirits, grounded and well-balanced. One way autophagy can be activated is through fasting, allowing for better, more efficient cleaning [8].

But when the cleanliness of your house is not maintained, impurities and dirt accumulate. Therefore a lack of autophagy will decrease levels of tumor-suppressing genes causing tumor development [8].

Other Benefits

While intermittent fasting is not for everyone, it can be very beneficial for those who can. Researchers have discovered many benefits of fasting including sleep, disease prevention, and detoxification.

Improved Sleep

Sleep is vital for the function of our mind and body. Our sleep is regulated by circadian rhythm and is defined as physical, mental, and behavioral changes that follow a 24-hour cycle [11]. The natural timing device that regulates the cycle of circadian rhythm is our biological clock. It is composed of molecules, hormones that interact with cells which can be affected by meal timing [11]. The clock in our brain that coordinates all the biological clocks and keeps them in sync is our master clock. It is composed of nerve cells that form a part of the hypothalamus which can also be affected by meal timing [11]. Timed meals play a significant role in synchronizing circadian rhythms and by regulating feeding time, human molecular clocks may be regulated using blood sugar changes [11].

Disease Prevention

Disease prevention is extremely important as intermittent fasting can greatly decrease the probability of developing type two diabetes and obesity [8]. Research has shown that fasting can help individuals lose weight and lower their levels of fasting glucose, insulin, and leptin ( the hormone that tells you to stop eating ). This can lead to a reduction in insulin and leptin resistance, while increasing levels of adiponectin ( health protective hormone ) [2].


Detoxification is a key component of clearing our system of unwanted toxins through this method of fasting. The foods that we eat and the air we breathe all contain chemicals and toxins that can poison our bodies. Intermittent fasting can help us detoxify our bodies by giving our bodies a period to rest and digest. This can allow toxins to be filtered out properly [12].

Is Intermittent Fasting for everyone?

Individuals that should avoid intermittent fasting are type two diabetics who are not on glucose lowering medication, underweight or undernourished individuals, individuals with eating disorders, children and adolescents, or individuals older than 70. Intermittent fasting should be avoided if you are pregnant, have chronic heart, kidney, or renal issues or if you have a history of disordered eating [8].


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