What is Intermittent Fasting?

Fasting is not a new practice,  in one way or another, fasting has been part of human culture for various reasons for thousands of years. For Muslims, one of the largest religions in the world, the culturally intrinsic practice of fasting has been around for centuries. The Muslim community worldwide, including Muslims in Saudi Arabia, fasts for an entire lunar month during Ramadan (1).

Modern humans evolved to survive periods of food shortage and have voluntarily fasted for at least 2000 years. Catholics all around the world participate in fasting during the Lenten season as well. Practitioners of Buddhism, Christianity, Islam, Judaism, Taoism, Jainism, and Hinduism fast as well. Fasting can last for just a few hours or even a few weeks, usually with practitioners eating at night.

Recently there has been a resurgence in the popularity of Fasting as a way to manage a person’s food intake, and as a way to lose weight. In particular, different types of intermittent fasting have  been very popular because they help people eat less throughout the day

Intermittent fasting, also known as intermittent energy restriction, is any of various meal timing schedules that cycle between voluntary fasting and non-fasting over a given period (2). 

Before we get into the benefits of Fasting and of Intermittent Fasting specifically, we should talk about the different ways in which people fast. 

Types of Intermittent Fasting

Intermittent fasting diets have become very popular in the past few years, as they can produce clinically significant weight loss. It is a broad term but the different types of intermittent fasting can be categorized depending on when the fasts start and end and when a person can eat.

Simply put, periods of fasting alternated with periods of eating. The most studied forms of intermittent fasting include alternate-day fasting (0–500 kcal per ‘fast day’ alternating with ad libitum intake on ‘feast days’); the 5:2 diet (two fast days and five feast days per week) and time-restricted eating (only eating within a prescribed window of time each day). 

It is important to note that fasting typically refers to complete abstinence of calorie intake, but some intermittent fasting programs allow small amounts of food consumption (up to approximately 25% of daily caloric needs) during fasting periods, meaning they are actually utilizing modified fasting (3).

The three main forms of intermittent fasting are alternate-day fasting (4),  periodic fasting, and daily time-restricted feeding (5). Time-restricted feeding has the most number of variations in the number of hours where the window to eat is restricted, usually, the fasting periods are at least 12 hours or longer, a popular method is the 16/8 method where there is a 16-hour fasting period and an 8 hour period in which to take 2-3 meals.

Intermittent fasting is a broad term that encompasses a variety of programs that manipulate the timing of eating occasions by utilizing short-term fasts in order to improve body composition and overall health. 

Alternate-day fasting trials of 3 to 12 weeks in duration appear to be effective at reducing body weight, body fat, total cholesterol, and triglycerides in normal-weight, overweight, and obese humans.

Whole-day fasting trials lasting 12 to 24 weeks also reduce body weight and body fat, and favorably improve blood lipids reducing cholesterol and triglycerides.

Weight Loss Benefits of Intermittent Fasting

Weight loss is a primary motivator for people to fast. Though it has recently been dubbed as a fad diet, Intermittent Fasting has gained a lot of traction for how effective it is. Weight loss is the most studied benefit of fasting and is one of the most obvious to notice. Of course, the degree of weight loss achieved with intermittent fasting is on a par with that achieved with traditional dieting approaches.

There are more health benefits other than weight loss. Some studies demonstrate that intermittent fasting improves cardiometabolic risk factors such as blood pressure, levels of LDL cholesterol and triglycerides, insulin resistance, and HbA1c, while others show no benefit on these parameters. This diet limits many risk factors for the development of cardiovascular diseases and therefore the occurrence of these diseases. Fatty acids and ketones become the main energy fuel, thus burning more fat. By affecting the biochemical transformations of lipids, it decreases body mass and has a positive influence on lipid profile parameters — it reduces the concentration of total cholesterol, triglycerides, and LDL cholesterol (6).

Intermittent fasting is generally safe and produces few gastrointestinal, neurological, hormonal, or metabolic adverse effects (7).

A study found that intermittent fasting may produce improvements in body composition by decreasing body mass, body mass index, fat mass, and body fat percentage while preserving fat-free mass in adults performing resistance training (8). It helps retain lean muscle mass even when there is weight loss (9).

Intermittent fasting shows promise for the treatment of obesity. To date, the studies have been small and of short duration. Longer-term research is needed to understand the sustainable role fasting can play in weight loss (10).

Overconsumption of food with such eating patterns often leads to metabolic morbidities such as insulin resistance, and excessive accumulation of visceral fat, which often coincides with a sedentary lifestyle. Because animals, including humans, evolved in environments where food was relatively scarce, they developed numerous adaptations that enabled them to function at a high level, both physically and cognitively, when in a food-deprived/fasted state (11).

Other Benefits

In a study conducted in 2020, the aim was to determine if intermittent fasting enhances weight loss or muscle gains in humans younger than 60 years old. It was found that Intermittent fasting could be beneficial in resistance-trained subjects or in overweight individuals to improve body composition by decreasing fat mass and at least maintaining muscle mass; decreasing GLP-1 levels and; improving health-related biomarkers such as glucose and insulin levels (12). Accumulating evidence shows that short-term fasting has a greatly positive effect in animal studies and contributes favorable benefits in human trials as well (13).

Another study found that persons doing intermittent fasting had profound benefits for diabetes prevention. (14) It also has been found to help with repairing damaged cells (15).

These feeding schedules have favorable metabolic effects by intermittently inducing the metabolism of fatty acids to ketones. The regimens overall lead to a decrease in weight and have been linked to improvements in dyslipidemia and blood pressure. The more that intermittent fasting is studied, the more it should be considered for individuals who have a pattern of unhealthy weight gain using standard eating patterns (16). 

In a study regarding obese individuals with time-restricted eating, the researchers did not find a significant increase in weight loss in over 118 individuals who were overweight. It concluded that Time-restricted eating, in the absence of other interventions, is not more effective in weight loss than eating throughout the day (17).

Intermittent fasting has been shown to be beneficial to brain aging and neurodegeneration, it has been found to protect mitochondrial function in the brain, liver, and skeletal muscle, enhance mitochondrial biogenesis and regulate mitochondrial dynamics (18). In a separate 2019 study regarding protecting the brain against Parkinson’s Disease, Intermittent fasting was found to have promise in protecting the brain against Parkinson’s disease, though further studies are yet to be conducted (19).

More Recent Developments

Helps with Non-Alcoholic Fatty Liver Disease

In a research conducted in 2022, Time‐restricted fasting, alternate‐day fasting, and prolonged fasting may each be beneficial in Nonalcoholic Fatty Liver Disease (20).

May Help with Abdominal Obesity, Visceral Obesity, and Cardiometabolic Risk

A cross-sectional study done in 2022 concluded that Accumulating evidence shows that short-term IF has a greatly positive effect in animal studies and contributes favorable benefits in human trials as well. Nevertheless, as an emerging, diverse, and relatively premature behavior intervention, there are still limited studies considering patients with obesity and type 2 diabetes. (21).

Better Insulin and Metabolic Response

Intermittent Fasting has been found to be helpful in eliciting better insulin and better metabolic response (22).

Helps with Hypertension

Another study was recently found that Intermittent Fasting is helpful for reducing the effects of hypertension (23).

Prevents Type 2 Diabetes

In 2021, time-restricted fasting was studied as to its effect on obesity and diabetes Obese patients are 3–4 times more likely to develop diabetes than non-obese patients.  The researchers found a lot of promise in the use of Time-restricted fasting in the prevention of Type 2 diabetes. 

Patients with diabetes, of both types 1 and 2, who undertake Intermittent Fasting has been shown in a few small human studies to induce weight loss and reduce insulin requirements. 

It also reduces fasting insulin and improves insulin sensitivity in individuals with prediabetes and those with obesity. Moreover, it improves glucose tolerance and decreases serum glucose excursions. The possible mechanisms underlying these benefits include increased autophagic flux, mild elevations in ketone bodies, a reduction in oxidative stress, and the stimulation of β-cell responsiveness (24).

Consequences and Risks of Intermittent Fasting

The drawback of intermittent fasting lies mostly in the difficulty of its execution (for some) and secondly on its impact on a person’s health. Just like with any diet, it takes a while to get used to but unlike any other diet, Intermittent Fasting allows you to eat whatever you want, the only restriction is when you eat.

For most people, eating three square meals of breakfast, lunch and dinner with a snack or two in between is the norm. So much so that anything short of that may be construed as some kind of deprivation. Most of us grew up with the recommendation of having 3 heavy meals a day, and even later recommendations of health professionals and fitness experts to eat multiple smaller meals spaced over the course of the day run counter to the intermittent fasting shorter window within which you can eat. The truth is that for anyone who has kept to what can be considered as standard diets and mealtimes, without fasting before, intermittent fasting can be a very daunting task. For most, if you skip breakfast or any other meals, it's already self-deprivation for extended periods of time.

High Dropout Rate

A study conducted in 2018 shows the difficulty with some forms of intermittent fasting. In this study conducted with 100 participants who underwent different forms of intermittent fasting, the dropout rate was highest in the alternate-day fasting group vs the daily or continuous calorie restriction group and control group while the average weight loss between alternate-day fasting and calorie restriction were about the same. They found that the more rigorous Alternate-day fasting did not produce superior adherence, weight loss, weight maintenance, or cardio protection vs daily calorie restriction (25).

Having tried Intermittent Fasting myself, there is a huge difference and transition to be made from being able to eat whatever I want anytime I want versus only being able to eat during a certain time period in the day. With the sheer number of food delivery services and accessibility to food, there is such a huge temptation to avoid ordering in and satisfying a late-night craving during a fast, especially when it has become a solid habit.

Affect on Quality of Sleep

Most of the previous studies on experimental IF have assessed its effect on metabolic and cardiovascular risks with many studies addressing the impact of fasting on sleep on circadian rhythm being conducted during fasting for Ramadan (26).

Mild Headaches

A 2020 study found that most participants in an intermittent fasting diet experienced at least mild headaches (27).

Long-term Risk of Cardiovascular Mortality

Another study found that taking a regular breakfast appears to be associated with lower overall and cardiovascular mortality, particularly when consuming fiber >25 g/day. Further studies examining specific breakfast foods and the timing of foods would be helpful (28). So this idea of skipping meals may not be as healthy as previously thought.


Those who are transitioning a fast may suffer the following side effects: brain fog or just not having enough energy, feeling fatigued (29).

 or lethargic. It takes some time for us to adjust to these lifelong habits. 


The most immediate risk with intermittent fasting is the potential for hypoglycemia in patients who are on antidiabetic medications that are associated with hypoglycemia, specifically insulin. Low Blood Sugar or being hypoglycemic is a significant risk if you have diabetes and are trying out intermittent fasting (30).

For those of us who are so used to having breakfast, the most important meal of the day, and suddenly adjusting to fasting is very disruptive to the habits that have been ingrained over time, it could lead to people being highly irritable and having strong cravings for food afterward.


People who go through intermittent fasting also experience some level of constipation or some manner of digestive issue. This may be due to a level of dehydration that those fasting may have. As such, it is very important to stay hydrated even in the middle of a fast (31). 


For conditions where dehydration is a risk, such as stroke, encouragement to hydrate well during any fasting regimen is recommended. Drinking water, including replacing fluids that normally would be consumed in foods, is an important consideration for people of all ages who are participating in intermittent fasting (32).

Mood Swings or Irritability

In addition to the mood swings, it has been found in a 2018 research that those who fast in the daytime have been found to have shorter REM sleep (33).


A study found that there was a risk of malnutrition when the intermittent fasting diet was done over a long period of time. Thus stressing the importance of eating healthy foods that still hit your body’s macros.  With long-term intermittent fasting, one needs to also be concerned about protein malnutrition if patients are not cognizant to maintain adequate protein intake during eating window. Vitamin and mineral malnutrition could also occur and, depending on how many days a week the patient is fasting and what they are eating on the days they do eat, might necessitate taking vitamin and/or mineral supplements (34).

With all its known benefits and positive impact on weight loss, it is important to remain eating and keep a well-rounded and healthy diet so as to avoid the risk of malnutrition while on an intermittent fast. 

Vague or Unknown Long-Term Effects

Unfortunately, for those committing to a long-term intermittent fasting diet, there is not yet an abundance of studies on the subject, which means the benefits or drawbacks of long-term intermittent fasting are not well known. 

Before going on and committing to an intermittent fasting diet. Make sure that it is suitable for you. Healthcare professionals offer a general idea of who it may not be for (35).

Some groups of persons may have unique risks and should be dissuaded from engaging in intermittent fasting, especially if they have diabetes. Pregnant and lactating women, young children, adults of advanced age, and older adults who are frail should not try to risk intermittent fasting as it may result in complications. Individuals with immunodeficiencies, including those who have had a solid organ transplant and are on medical immunosuppression, People with existing eating disorders, and those with dementia who have unique challenges that will likely be exacerbated by deliberately engaging in fasting, should avoid fasting. Those who have a history of traumatic brain injury or post-concussive syndrome may also be at higher risk of adverse effects as well. These groups' special needs should first be considered before making large changes in their diets.

Likewise, consult your dietician or your family doctor before making such a substantial change in your diet. A medical professional with insight into your medical history and your family’s medical history can be invaluable in order to avoid any negative or unintended long-term effects.


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