What Is Intermittent Fasting and How Does It Work?

Intermittent fasting differs from traditional fasting. Fasting is refraining from consuming food or drinks, except for water, for a set period. Traditional fasting diets, for lengthy periods of time, aren't a healthy means of weight loss and can be extremely dangerous. In fact, long-term fasting starves the body of essential nutrients, causes the body to shut down (metabolism slows dramatically), and can be life threatening.

As the name suggests, intermittent fasting refers to alternating periods of fasting with periods of eating. It’s a broad term, encompassing several specific types of short-term fasting protocols. The common theme among intermittent fasting regimens is that people periodically abstain from eating for periods longer than the typical overnight fast. Individuals either fast during a certain window every day or block out certain days of the week. These short eating rest periods allow the body’s numerous systems to rest and reset without triggering the risk of malnutrition and metabolic slowdown that accompanies severely restrictive long-term fasting regimens.

The basics of intermittent fasting

Here are the key principles of intermittent fasting lifestyle methods:

• All intermittent fasts restrict eating and drinking for set, short periods of time. Every method of intermittent fasting has eating and fasting periods that vary, depending on the regimen.
• The intermittent fasting approach involves alternating periods of eating and fasting. These time periods differ depending on the variation of intermittent fasting, so you choose the method that works best for your lifestyle.
• All intermittent fasting protocols are safe and effective for healthy individuals. Each recognized method is safe and has been shown to improve a person’s health and well-being, if practiced correctly.
• All intermittent fasting protocols have certain rules you must follow during your fasting window. These steps include drinking plentiful amounts of water, black coffee, tea, and any other non-caloric beverage during your fasting window; just no solid foods allowed. Make sure to stay hydrated during your intermittent fasting periods.
• All intermittent fasting protocols prohibit you from eating excessive amounts of junk food during your eating windows. This habit will negate the many benefits of intermittent fasting. The biggest mistake people make is eating too much and eating unhealthy foods during their eating periods.
• Intermittent fasting can be practiced for health and fitness and not necessarily for weight loss. Although weight loss is one of the most common reasons for trying intermittent fasting, many people choose to get leaner and fitter and tap into the numerous health benefits intermittent fasting provides without the goal of losing weight. In fact, some follow an intermittent fasting program with the primary goal of gaining muscle weight and losing body fat.

Although intermittent fasting is a healthy choice for some, for others, it can be dangerous. Several groups of people who absolutely should not fast include the following:

• Pregnant or lactating women
• Individuals who have eating disorders
• Individuals with type 1 or type 2 diabetes, unless working with their health care professional (physicians must be consulted if you have any underlying chronic disease)
• Individuals using medications that they must take with food, unless working with their physician
• High level endurance athletes
• Elderly individuals with balance issues
• Children

How intermittent fasting works

Intermittent fasts cycle between periods of fasting with periods of eating. Whether or not you’re fasting, the body still requires energy to run efficiently. The body’s main source of energy is a sugar called glucose, which typically comes from carbohydrates such as grains, fruits, vegetables, and even sweets. Both your liver and muscles store the sugar and release it into the bloodstream whenever the body needs it.

Look closer at the physiology

To understand how intermittent fasting works, you need a quick adaptive physiology refresher. Because food wasn’t always abundant, and sometimes wasn’t available at all, the human body was forced to adapt to fasting involuntarily — and then, when Stone Age humans found food, they would feast. Because of those evolutionary conditions, human bodies evolved to permit their bodies to thrive by adapting to those cycles of feasting and fasting. In order to survive in such environments where food was scarce, humans had to possess the ability to quickly shift their metabolism from fat storage to fat breakdown for energy.

This metabolic flexibility became built into human’s genetic code, producing a system where energy was stored in the form of body fat when food was available and then easily accessed for energy to enable humans to perform at a high level, physically, during extended periods when food wasn’t available. This pattern enabled human brains and bodies to function optimally in a food deprived/fasted state, giving the human race a survival advantage.

Scientists have hypothesized that the human body’s adaptive benefits of intermittent fasting led to the superior cognitive capabilities (brain power) of humans compared to other mammals. These brain adaptations facilitated human’s ability to invent tools, novel hunting methods, animal domestication, agriculture and food storage, and processing.

Because intermittent fasting patterns can replicate the feast-or-famine diet of human ancestors, many researchers have now recognized the advantages of periodically fasting (such as increased brain power, physical enhancements, and disease prevention) for the multitude of health benefits this lifestyle gives rise to.

Examine the timeline of events

What is the physiology of fasting? Although everybody responds to fasting a little differently (genetics, health, and age all play a role), there is a general timeline of events — a predictable set of metabolic responses as your fast stretches from hours into a day or longer.

After fasting for a mere eight hours, here is the timeline of what happens in your body:

1. You have no food coming in, so you exhaust your supplies. Your body has tapped into your liver reserves of blood sugar to continue to keep your blood sugar level in the normal range. You’re now in what’s termed a catabolic or breakdown state.
2. You enter the fasted state; your liver has run out of its sugar reserves. This triggers the liver to manufacture new sugar from noncarbohydrate sources (scientifically termed gluconeogenesis) to continue to supply energy to the cells. With no carbohydrates consumed, the body creates its own sugar by using mainly fat. This marks the body’s transition into the fasting mode. Studies have shown that gluconeogenesis increases the number of calories the body burns, meaning when your metabolism starts to increase.
3. You flip your metabolic switch. One key mechanism responsible for many of the beneficial health effects of short-term, intermittent fasting is flipping the metabolic switch. The metabolic switch is the body’s preferential shift from utilization of blood sugar to fat and fat-derived ketones for energy. In this step, your body breaks down fat, shuttling it to the liver, which creates ketones from fat to use for energy. The metabolic switch typically occurs between 12 and 36 hours after cessation of eating.
4. Extended fasts (longer than 36 hours) begin to slow metabolism down. That’s why you shouldn’t practice extended fasting with intermittent plans. After about 36 hours, the body stops using these energy sources (sugar and fat). The fasting mode then transitions to the more serious starvation mode.
5. You enter starvation mode. At this point, your metabolism has slowed dramatically, and your body begins to burn your own muscle protein for energy. The lack of essential nutrient intake plus using muscle for energy sets off an alarming cascade of dangerous complications.

During your recommended intermittent fasting periods, your fasting periods shouldn’t extend beyond 36 hours. Although some people choose to fast for up to 48 hours, I recommend your intermittent fasting periods don’t extend 36 hours because of the physiological reasons.