Defining Intermittent Fasting
Intermittent Fasting (IF) is a pattern of eating that cycles between a Fasting Period, in which very few to no calories are consumed, and a Feeding Window, in which eating occurs. Intermittent Fasting is a pattern of eating (rather than a diet) and only declares when food is eaten, not how much food or which types of food are consumed.
Intermittent Fasting challenges the conventional wisdom that several small meals spaced 3 to 6 hours apart are ideal. Instead, 1 to 3 large meals are consumed during a small portion of a day, known as the Feeding Window, rather than even distributed among waking hours.
The Fasting Period of Intermittent Fasting refers to the time in which calories are intentionally restricted. The Fasting Period is a scheduled time slot and can last anywhere between 12 hours and multiple days, depending on the fasting protocol.
In its simplest form, the Fasting Period takes place when a meal(s) are intentionally skipped.
The Feeding Window of Intermittent Fasting refers to the time in which food is once again reintroduced. Like the Fasting Period, the Feeding Window is a scheduled time slot. However, unlike the Fasting Period, eating takes place during the Feeding Window. Any time food is being consumed, an individual would be considered in the Feeding Window.
Common Intermittent Fasting Protocols
The Leangains method, developed by Martin Berkhan, is a popular fasting protocol among the bodybuilding community and features daily cycling between Fasting Periods and Feeding Windows. The Fasting Period begins after dinner, continues through the next morning when breakfast is skipped and ends around lunchtime. The Feeding Window takes place between regular lunch and dinner times and consists of 2 to 3 meals. On training days, post workout meals are the largest. On non-training days, the first meal of the Feeding Window is the largest. For men, the Fasting Period has a duration of 16 hours with an 8-hour Feeding Window (16/8). For women, the Fasting Period has a duration of 14 hours with a 10-hour Feeding Window (14/10).
The Warrior Diet, developed by Ori Hofmekler, is a fasting protocol that features long, daily fasts. The Fasting Period of the Warrior Diet is 20-24 hours in length. The Feeding Window of the Warrior Diet is very brief, typically under 4 hours. Essentially, the Warrior Diet consists of one very large meal (typically dinner) spread over multiple courses, every day. The Fasting Period begins immediately following the meal and ends the next evening when the daily meal begins.
Eat Stop Eat
Eat Stop Eat is an Intermittent Fasting protocol developed by Brad Pilon. Like the Warrior Diet, Eat Stop Eat consists of longer duration fasts. However, 24-hour fasts occur twice a week instead of every day and the fast is broken with a meal of regular size. What makes Eat Stop Eat unique is the prescribed flexibility. The protocol is created as a template, meant to be adapted to fit the specific scheduling needs of the individual. Simply by intentionally abstaining from food for a period of time, one would be considered following Eat Stop Eat, making it an ideal protocol for those who enjoy tinkering and fine tuning their eating pattern.
Advantages of Intermittent Fasting
Limited Feeding Window
There are several observed benefits to limiting the duration of the Feeding Windows, including a spontaneous reduction in calories, potential benefits to longevity, and a simpler lifestyle.
Reduction in Calories
Because of the limited Feeding Window, many Intermittent Fasting practitioners observe a spontaneous reduction in calories. While possible to consume a surplus of calories when fasting regularly, it is more difficult to do so when feeding time is restricted. As such, weight loss is common when following an Intermittent Fasting protocol without the need to consciously count or restrict calories .
The results of some studies suggest that Intermittent Fasting protocols may provide benefits to longevity, similar to that of long-term caloric restriction. Autophagy, the cellular self-clearing of dysfunctional cells, is induced by fasting  and has been linked to longevity . In rat models, Intermittent Fasting has been shown to dramatically increase life span .
Many Intermittent Fasting practitioners enjoy the simpler lifestyle of the limited Feeding Window. Conventional wisdom dictates small meals and snacks at regular intervals, which can be cumbersome for those who are busy. Instead, less frequent and larger meals require less planning and follow a more natural eating pattern.
Managing insulin is a key component in controlling body weight and sustaining health in general. Intermittent Fasting has been shown to dramatically decrease insulin levels in nonobese humans .
Insulin is considered a storage hormone, so high levels of insulin create an environment in which fat is more easily stored. Additionally, insulin inhibits the mobilization of fat stored in cells. Therefore, reducing insulin levels is critical in both preventing fat from accumulating and accessing stored energy in fat cells.
Insulin resistance is an increasingly common ailment in which chronically high levels of insulin result in a desensitization to the hormone, leading to further increased insulin secretion. Insulin is released in response to food, especially meals high in carbohydrates . By restricting all food, as is the case during the Fasting Period, insulin levels plummet and tissues can regain their sensitivity to insulin.
Human Growth Hormone
Intermittent Fasting has been shown to elevate the production of Human Growth Hormone (HGH) . Deficiency in HGH can often result in low quality of life, including higher rates of depression . Increasing Growth Hormone production through diet and exercise has many benefits in terms of performance, aesthetics, and general health.Human Growth Hormone has a profound impact on nutrient partitioning, body composition, brain functioning, and the health of all human tissues . Intermittent Fasting can be a useful tool to raise Human Growth Hormone levels.
Hormone Sensitive Lipase
Hormone Sensitive Lipase (HSL) run antagonistically with Insulin, meaning when Insulin is low, HSP is high and when Insulin is high, HSL is low. The main function of Hormone Sensitive Lipase is to mobilize the fat stored in cells , and HSL is activated when the body needs to tap into its energy stores. As such, Intermittent Fasting can provide a benefit to those looking to mobilize energy stored in their fat cells.
The combination of the reduction in calories due to the limited Feeding Window and the favorable hormonal response of Intermittent Fasting provides a notable benefit to body composition. Those seeking to shed the last few pounds of body fat often find Intermittent Fasting to be an effective tool to do so. Intermittent Fasting does not guarantee fat loss, but it may provide an ideal state for those looking to drop extra weight.
Disadvantages of Intermittent Fasting
Those beginning an Intermittent Fasting protocol may have difficulty adjusting to the modified eating schedule. Low energy levels, hunger pangs, and GI issues are common among those who are too ambitious when switching to Intermittent Fasting. Instead, it is recommended that new fasters ease into their target Fasting Periods, rather than jumping into extended fasts at the start.
Athletes or individuals seeking high-level performance may find some difficulty in reaching their goals when attempting to implement Intermittent Fasting. While a benefit to those seeking weight loss, the limited Feeding Window may be a detriment to athletes because of the aforementioned reduction in calories and can cause sluggishness or fatigue.
Athletes should exhibit caution before attempting an Intermittent Fasting protocol. The priority for individuals with performance goals should always be providing themselves with the appropriate fuel to reach those goals. Whether or not that includes the use of Intermittent Fasting is dependent upon the individual and their unique goals.
Women and Fasting
Women, in particular, may find Intermittent Fasting to be a challenge. Some of the benefits of Intermittent Fasting are less pronounced among women and there are some observed negative side-effects unique to women. One study showed that alternate day fasting actually reduced blood sugar regulation among the female sample . Female rats saw more detrimental effects of Intermittent Fasting than male rats , but the fasting group also included caloric restriction over the course of 6 months.
Anecdotally, women report difficulties with their monthly cycles when attempting to fast for long durations. Women may benefit from slightly shorter duration fasts, as noted in the Leangains protocol of a 14-hour Fasting Protocol with a 10-hour Feeding Window.
Anyone looking to start Intermittent Fasting should ease into it slowly to avoid problems during the Adjustment Period, but women may need to be extra cautious when beginning an Intermittent Fasting protocol.
How to Implement Intermittent Fasting
To begin Intermittent Fasting set a target duration and schedule for Fasting Periods and Feeding Windows. Duration and schedule of Fasting Periods and Feeding Windows are dependent upon goals and activity levels. Higher activity levels may require shorter and less frequent Fasting Periods. Body composition goals may require longer and more frequent Fasting Periods.
The Fasting Period is easy to implement; simply abstain from eating any food during the scheduled Fasting Period.
What to Eat
No food should be consumed during the Fasting Period. Beverages, including water, black coffee, tea, and calorie-free soft drinks may still be consumed during the Fasting Period, but calories should be kept to an absolute minimum. Small amounts of milk and zero calorie sweeteners are acceptable in coffee and tea, so long as total calories are restricted during the Fasting Period. Caffeine is allowed while fasting, and many find the appetite suppression of caffeinated beverages (especially coffee) to be helpful in tolerating longer duration Fasting Periods.
The Feeding Window is essentially the inverse of the Fasting Period. The Feeding Window is the scheduled block of time in which food is reintroduced and consumed normally.
What to Eat
Food is once again consumed during the Feeding Window. As Intermittent Fasting is a pattern of eating, not a diet, neither food quality nor quantity is dictated. With any dietary pattern, including Intermittent Fasting, high-quality, nutrient dense foods should constitute the bulk of the diet. Food quantity should be set to meet the unique needs of the individual.
Getting Started: The Slow Close
When beginning an Intermittent Fasting protocol, going long durations without food can seem like a daunting task. Instead of switching from regular occurring meals to full, 24-hour fasts, new Intermittent Fasting practitioners should ease into the new eating pattern, giving themselves and their bodies an opportunity to adapt.
The recommended method for adapting to Intermittent Fasting is known as The Slow Close. Instead of switching immediately to the target Fasting Period, slowly close the Feeding Window over a period of time. Begin with a 12-hour fast by eating an earlier dinner or later breakfast. Over time, increase the length of the Fasting Period, adding an hour every two to three weeks. Doing so will give the body ample time to adapt, reducing much of the discomfort involved in starting an Intermittent Fasting protocol.
One of the main purposes of following an Intermittent Fasting eating pattern is the simplicity in lifestyle it provides. This simplicity is completely negated when too much focus is placed on the nuances of an eating schedule.
The solution? Be flexible. The difference between an hour or two of fasting on a particular day is negligible. Eat when hungry and don’t feel as though eating at regular intervals is necessary. Simply skipping meals when not hungry is preferable over both regularly starving or force feeding oneself just to align to any of the above fasting protocols.
Intermittent Fasting FAQs
Do I need to track calories?
NO. Intermittent Fasting is a pattern of eating rather than a diet. As such, it does not dictate a set amount of food that should be consumed. Calorie consumption is the biggest determiner of weight gain/loss, so those who are looking to dial-in their body composition may prefer to track caloric intake. Tracking calories isn’t necessary to follow an Intermittent Fasting eating pattern, however, and those who consume high-quality, nutrient dense foods will likely see a reduction in calories due to the condensed Feeding Window.
So I can eat as much as I want during the Feeding Window and still lose weight?
NO. While meals do tend to be larger during the Feeding Window, Intermittent Fasting is not an excuse to overeat. It may be more difficult to overeat in a condensed Feeding Window but is still entirely possible.
Do I need to train during the Fasting Period?
NO. Fasted training is an option while Intermittent Fasting, but not a necessity. Those training in the morning hours may find that fasted training followed by a large meal in the afternoon to break the fast fits their schedule, but those with different training schedules needn’t extend the Fasting Period just to train in a fasted state. Any benefit to fasting training is moderate, if not non-existent, and some may find fasting a detriment to their training progress.
Many who train fasted see marginal benefits from supplementing with Branch Chain Amino Acids (BCAAs) during their training sessions. Once again this is optional, but not necessary. Intermittent Fasting is meant to be pliable, so there are no strict rules. Experiment with different techniques, schedules, and protocols to see what works.
I thought the human body could only absorb 30g of protein per meal?
NO. This is a common myth among the fitness industry, but the human body is far more efficient than that. First, protein absorption rates vary drastically depending on the source . With regard to Intermittent Fasting specifically, one study  showed no difference in lean body mass between the experimental group eating their daily protein allotment in one 4 hour Feeding Window, compared to a conventional eating pattern. Another study  actually showed improvements in body composition for those following the same Intermittent Fasting protocol, with no improvements for those following the three meal a day plan. The bottom line is that there is no evidence that there is a maximum amount of protein that can be absorbed in one meal, nor is there evidence that 30g is an ideal amount that elicits the maximal anabolic response.
Will fasting slow my metabolism?
NO. In fact, there is some evidence to suggest that regular, short duration Fasting Periods may actually boost metabolism. [16, 17] So long as the Fasting Period doesn’t regularly exceed 72 hours, there should be no long-term suppression of the metabolism.
Does the time I’m sleeping count toward my Fasting Period?
YES. Those who implement daily Fasting Periods (such as the Leangain 16/8 protocol) should could sleeping hours toward their Fasting Period. The Fasting Period begins after the last meal of the day is finished and ends the next time food is consumed when the Feeding Window begins. Fasting time is not “paused” during sleep.
Is Intermittent Fasting safe?
MAYBE. Intermittent Fasting is perfectly safe for healthy individuals. Those with pre-existing medical conditions, however, should consult with their doctors before implementing any Intermittent Fasting protocol. In particular, individuals with diabetes, blood sugar regulation issues, or low blood pressure, and those who are taking any medications, underweight, have a history of eating disorders, pregnant, breastfeeding, or trying to conceive should talk with a medical professional before trying Intermittent Fasting. Women, in particular, should exercise caution before beginning an Intermittent Fasting protocol because of the unique, previously mentioned concerns.
Is Intermittent Fasting right for me?
MAYBE. Intermittent Fasting is an effective dietary tool for many, but there is no one size fits all approach to nutrition. Those interested in starting a protocol should do research before starting and consult a medical professional if concerned about the health consequences.
In general, the following populations are most likely to see benefits from Intermittent Fasting:
- Overweight but otherwise healthy individuals.
- Moderately lean individuals looking to shed a little extra body fat.
- Busy people who lack the time and energy to plan and cook six meals a day.
The following populations should be cautious when starting an Intermittent Fasting protocol:
- Athletes or those with performance-specific goals.
- Women, especially those who are pregnant, breastfeeding or trying to conceive.
- Those with pre-existing medical conditions.
The following populations may want to avoid Intermittent Fasting altogether:
- Underweight individuals.
- Anyone with a history of eating disorders.
All individuals interested in practicing Intermittent Fasting should ease into whichever protocol their choose and be willing to adapt the structure to meet their unique needs.
- Intermittent Fasting means going without eating for a period of time. The most common protocol involves a 16 hours Fasting Period and an 8-hour Feeding Window.
- There are several benefits to Intermittent Fasting, but it isn’t magic.
- Many of the disadvantages are related to the adjustment to the eating pattern. These can be alleviated by slowly changing habits.
- The flexibility of not being tied to a strict eating schedule is the real beauty of Intermittent Fasting. Don’t fall back into the trap of neurotically watching the clock to resume eating.
- Intermittent Fasting is generally safe, but consider the details of your specific situation before deciding whether to give it a try. Context, as always, is key.