Intermittent fasting (IF) is a dietary approach that has become increasingly popular in recent years. However, this method of eating has actually been practiced by humans for centuries, since the time of hunter-gatherer societies. Fasting was also used historically for medicinal purposes by ancient Roman, Greek, and Chinese civilizations. (1) Today, certain religions, including Buddhism, Islam, and Christianity, incorporate fasting for spiritual reasons.
Fasting involves abstaining from calorie-containing food and beverage for at least twelve consecutive hours. An intermittent fasting diet results in a number of hormonal and metabolic changes in the body. Research demonstrates that intermittent fasting benefits include managing weight, (17)(19) protecting the brain during aging, (10) decreasing inflammation, (13) and improving blood glucose and insulin levels. (13)
Does intermittent fasting work? Read on to learn about the types of intermittent fasting, how it works, the benefits, and safety considerations.
What is intermittent fasting?
Intermittent fasting involves abstaining from calorie-containing food or drink for periods between 16 to 24 hours at regular intervals. (20) While other fasting methods involve abstaining from food for several days or even weeks, intermittent fasting, one of the most common methods, typically involves shorter fasting.
Types of intermittent fasting
There are several types of intermittent fasting, determined by the duration of the “feeding window”, the timeframe in which food is consumed, and the “fasting window”, the timeframe in which food is avoided. During the “feeding window”, the person can consume food and beverages ad libitum, which means without restrictions. The common types of intermittent fasting, outlined below, each have a different intermittent fasting schedule.
Time-restricted feeding (TRF) or time-restricted eating – the feeding window is 4 to 12 hours every day, followed by a fasting window for the remainder of the day when only water is permitted. The most common variation of TRF is 16/8, which involves 16 consecutive hours of fasting per day.
Early time-restricted feeding (eTRF) – a variation of TRF that involves a 6-hour feeding window early in the day (e.g. from 8 am to 2 pm). The remainder of the day makes up the fasting period.
Alternate day fasting (ADF) – involves alternating one day of unrestricted eating with one day of complete fasting.
Periodic fasting (or cyclic fasting) – fasting for one or two days per week with 5 or 6 days of eating as desired. Variations of periodic fasting include 5:2 and 6:1 fasting.
Modified fasting – some methods of intermittent fasting, such as alternate day fasting, can be modified to include very-low-calorie consumption during the fasting window (e.g. 25% of regular caloric intake). (1)(20)
How does intermittent fasting work?
Intermittent fasting results in changes to hormonal patterns and energy metabolism in the body. After you eat, food is broken down into nutrients and absorbed in the digestive tract. Carbohydrates, specifically, are broken down to glucose (sugar), which can then be absorbed into the bloodstream and distributed to body tissues, where glucose is used as the body’s primary source of energy. The hormone insulin regulates blood glucose levels by signaling for your cells to uptake glucose from the blood into cells, where it provides fuel for body functions.
With prolonged fasting after a meal, the supply of glucose is depleted, resulting in fasting blood glucose levels. To meet energy needs, the body will break down glycogen, which is the storage form of glucose found in the liver and skeletal muscles. The body also uses gluconeogenesis, a process in which the liver produces glucose from non-carbohydrate sources. (5)(17)
After approximately 18 hours of fasting, which occurs in some types of intermittent fasting, the level of insulin is low and a process called lipolysis begins. During lipolysis, the body breaks down adipose tissue (fat) into free fatty acids. When there is insufficient glucose available to meet the body’s energy demands, the body will transition to using fatty acids and fatty-acid derived ketone bodies (ketones) for energy, a metabolic state known as ketosis. (1) Liver cells are responsible for ketogenesis, the production of ketone bodies. During ketogenesis, fatty acids are broken down in the mitochondria of cells by a process called beta-oxidation and converted to the ketones acetoacetate and beta-hydroxybutyrate. (5)
These ketones are used by muscle cells and neurons (brain cells) to generate adenosine triphosphate (ATP), the primary carrier of energy in cells. The availability and use of fatty acids and ketone bodies for energy inhibit the use of glucose in other vital body tissues, including the heart, liver, pancreas, and brain. (8)
Intermittent fasting hours
The four metabolic states induced by fasting are referred to as the fed-fast cycle.
- The fed state (approximately three hours post-meal) – glucose levels are high; energy comes from glycolysis (the breakdown of glucose); excess glucose is stored in the body as glycogen and excess fatty acids are stored in adipose tissue
- The post-absorptive state or early fasting state (approximately 18 hours post-meal) – energy comes from glycogenolysis (breaking down glycogen into glucose); lipolysis begins, which increases the level of free fatty acids in the blood
- The fasting state (approximately 36 to 48 hours post-meal) – the metabolic shift occurs and the body begins to use fatty acids and ketones as fuel
- The starvation state (longer than 48 hours post-meal) – glucose oxidation or use is inhibited; ketone levels are increased and the body relies heavily on fatty acids for energy (17)
Physiological effects similar to intermittent fasting results can also be achieved by following a ketogenic diet (keto diet), which is a very high-fat, very low-carbohydrate diet. The purpose of the keto diet is to shift the body’s metabolic state into ketosis. The keto diet restricts the intake of carbohydrates to 50 grams or less per day, which minimizes the amount of glucose available for energy production. A high intake of fat provides fatty acids that are used for ketone and energy production. (11)
Learn more about the ketogenic diet on the Fullscript blog.
Intermittent fasting induces a metabolic shift to burning fatty acids and ketones for energy.
Intermittent fasting benefits
Research has demonstrated a variety of intermittent fasting benefits, including:
- Cellular cleansing (14)
- Decreased inflammation (13)
- Improved cardiometabolic risk factors (18)
- Neuroprotection (10)(12)
- Type 2 diabetes prevention and management (3)(7)
- Weight loss (17)(19)
Intermittent fasting results
Intermittent fasting results in changes in certain factors, including circadian biology, gut microbiome, and lifestyle behaviors. These mechanisms are suggested to contribute to intermittent fasting benefits. (15)
Circadian biology changes
The circadian system is responsible for the body’s daily metabolic, physiologic, and behavioral rhythms. In a controlled study with overweight adults, early time-restricted feeding (eTRF) was shown to alter the expression of several circadian clock genes and diurnal patterns of cortisol, a hormone involved in the sleep-wake cycle. (9) Similarly, another study found that individuals who underwent eTRF with a 6-hour feeding window experienced improvements to insulin sensitivity, oxidative stress, and blood pressure. The authors suggest that the benefits of IF may depend in part on the time of day that food is consumed as glucose, lipid, and energy metabolism are regulated by the circadian rhythm. (18)
Gut microbiota improvements
The gut microbiota is the ecosystem of microbes that resides in the gastrointestinal tract. Intermittent fasting may influence the microbiota by reducing gut permeability and systemic inflammation, therefore exerting a number of health effects. (14) A clinical trial examining the effects of modified alternate day fasting (mADF) in humans with multiple sclerosis (MS) showed beneficial effects on levels of gut flora and inflammatory markers. IF increased Lactobacillus Johnsonii and Lactobacillus reuteri, beneficial species of bacteria that have immunomodulatory effects. (4)
Intermittent fasting can also impact other lifestyle factors including sleep. Studies report that eating late at night results in poor quality sleep and reduced sleep duration. This can lead to poor health outcomes, such as insulin resistance and a higher risk of obesity, cancer, and cardiovascular disease. (15)
Energy intake reduction
Fasting may result in fewer calories consumed on non-fast days. A controlled study found that, following a one-day fast, participants reduced calorie intake by about 30% on the next three consecutive days. (2) Similarly, binge eating decreased in obese patients following an eight-week ADF intervention. The study authors suggested that the diet may help control unrestrained eating behavior. (6)
Intermittent fasting side effects
While IF is generally considered to be safe, potential intermittent fasting side effects have been reported in a small proportion of IF participants, including low energy, hunger, feeling cold, and irritability or other changes to mood and behavior. (16) Studies in obese adults and patients with type 2 diabetes have found an intermittent fasting diet to be safe and tolerable. (3)(6)
However, diabetic patients should take special precautions with intermittent fasting. When there is a lack of insulin or resistance to insulin, a state called diabetic ketoacidosis (DKA) can occur. DKA is a life-threatening condition, characterized by an overproduction of ketone bodies. The symptoms include dehydration, confusion, nausea, and vomiting. The condition is treated by administering glucose and insulin, which downregulates the ketogenic pathway. DKA is more likely to occur in type 1 diabetics but also occurs in advanced type 2 diabetics. (5)
The bottom line
Following an intermittent fasting plan has been associated with a number of health benefits, including weight loss, reduced inflammation, and improved risk factors of cardiometabolic diseases. Intermittent fasting benefits may be a result of effects on the circadian rhythm, microbiota composition, sleep, and caloric intake. If you’re interested in following an intermittent fasting diet, be sure to consult an integrative healthcare practitioner to ensure you follow the diet safely and effectively.
Download a handout on intermittent fasting.
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