- What is Intermittent Fasting?
- Benefits & Uses of Intermittent Fasting
- How Does Intermittent Fasting Work?
- Different Types of Intermittent Fasts
- How Does a Ketogenic Diet Affect the Benefits of Intermittent Fasting?
- Common Misconceptions About Intermittent Fasting
- Who Should Not Intermittent Fast
- Top Tips for Intermittent Fasting
The Beginner's Guide to Intermittent Fasting
With obesity and its associated health problems becoming a public health crisis, many people are looking for a way to lose weight and improve their health. There are many popular diets that claim to offer weight loss and other health benefits; however, there's one in particular, called Intermittent Fasting, that stands out from the rest.
Many of these diets use some kind of calorie or food restriction. Intermittent fasting is slightly different in that it focuses on restricting eating times more than restricting particular foods.
Unlike fad diets, intermittent fasting has a significant past and has been used throughout history for physical and spiritual benefits. But does intermittent fasting live up to the hype? And how can you use intermittent fasting to improve your health above and beyond weight loss?
What Is Intermittent Fasting?
Intermittent fasting, also called IF, is the practice of interspersing periods of a strict fast with periods of eating. The ratios and lengths of time can vary, but all types are based on the same science.
When we fast for extended periods of time, even twelve hours, our bodies go into a mild state of ketosis. They begin to burn fat to make fatty acids and ketones to supply our caloric needs. Hence, many people lose more fat when they are in this state.
Many people misunderstand what this kind of eating plan means. It does not mean that people using this method can eat whatever they want when they are not in a fasting period.
Instead, people using intermittent fasting eat a reasonable healthy diet during eating periods, interspersed with regular periods of fasts.[i] They, like most people, enjoy an occasional treat or cheat day.
Intermittent Fasting vs Calorie Restriction
An intermittent fast is not the same as a typical calorie restriction diet. A calorie restriction diet involves eating fewer calories every day, usually by carefully counting them. Calorie restriction allows people to eat throughout the day, so they never enter a fasting state. This is typical of the majority of weight loss diets.
Instead, time restricted eating leads to a reduction in calories based simply on the fact that people are only allowed to eat during certains hours of the day. It does not require strict calorie counting, although people can count if they are strict. Because people are only eating during a few hours of the day, they naturally enter a metabolic state in which their body burns fat preferentially as a source of fuel. The metabolic effects of IF are a more significant contributor to the weight loss that many people experience on this eating plan.
Time-Restricted Eating Throughout History
Various types of short term food restriction have been used throughout history for a variety of health purposes. Ancient writers such as Hippocrates and Plutarch recommended it to both prevent and treat disease. Ancient Greeks believed that periods of fasting were important in maintaining good cognitive function.
In addition, food restriction has also been used for religious purposes throughout history. Popular religious figures such as Jesus, Mohammed, and Buddha all used fasting for spiritual purposes.
The Uses of Fasting Today
Fasts are still used among several major world religions, such as in Buddhism and during the Muslim holiday Ramadan. Some denominations of Christianity also encourage food restriction for spiritual reasons.
In modern times, however, this kind of eating plan is mainly used for health and weight loss purposes. It has been found in several studies to be effective in fighting obesity. Incorporating this habit into your diet can help not only to lose weight, but to keep it off permanently.
There are also several metabolic benefits to an occasional fast.[ii] People who engage in this practice have lower insulin levels, less insulin resistance, and a decrease in visceral fat, which is the "dangerous" fat packed around organs. These effects are especially notable when combined with a healthy diet and routine exercise.
There is a great deal of ongoing research showing the benefits of IF. It is likely that more people will adopt this dietary habit as the advantages are becoming more well known. We also are learning more about the kinds of diets that amplify weight loss and other health effects of a short term fast.
Benefits and Uses of Intermittent Fasting
Any kind of fast will lead to less calorie intake in most people. However, a reduction in calories is not the main source of weight loss for many of the people who use it. The main source of weight loss is that time-restricted eating causes metabolic changes on a molecular and cellular level. We will talk about these below.
In addition, there are several other benefits to intermittent fasting that shouldn't be ignored.
The Benefits of Autophagy
Many of the health benefits of regular, short term food restriction are due to the increase in autophagy. Autophagy is the recycling of cells and cell components that occurs when a person has not eaten in a significant amount of time.
Autophagy is important for human health. First, the cells that are destroyed in a fast are usually fat cells, including the metabolically dangerous visceral fat cells. Second, the body often targets diseased cells for this process, replacing damaged tissues with fresh and new ones.
Intermittent fasting alternates short periods of autophagy with periods of ample nutrients, so tissues can be repeatedly replaced and healthy cells and organs are never targeted for destruction. In stark contrast, long term starvation can lead to autophagy of even healthy cells, which causes organ damage. (Long term starvation is never recommended and that is not what intermittent fasting is about.)
Cellular Regeneration and Anti-Aging Effects
Periodic restrictions in eating appear to reduce the march of time by encouraging cellular regeneration and also changing the activity of certain genes related to aging.[iii] It increases production of natural antioxidants, which in turn prevent much of the daily damage that contributes to aging and disease.
In addition, to stop taking in calories for even short to medium periods of time increases the activity of certain genes known as housekeeping genes. These genes produce proteins that target aged cells and organelles for replacement.
The result is an overall decrease in the aging of tissues along with a corresponding increase in anti-aging activity. As a result, clinical trials have shown that people on a diet with periods of fasting have slightly decreased aging processes.
Short term time restrictions can lead to dramatic changes in the metabolism.[iv] People who use this method go into a state similar to that seen in a ketogenic diet called ketosis. Their insulin levels drop to healthy low levels and the body switches from carbohydrate metabolism to burning fatty acids and ketones.
This process leads to increased fat burning as well as an overall increase in metabolism. Researchers also believe that there are other, less understood effects on metabolism such as changes in circadian rhythm and better metabolic regulation.
Many people look into a diet such as this for its weight loss benefits. However, current research suggests that time-restricted eating leads to positive and beneficial changes in people of all weights, even those who are in a normal BMI[v].
Intermittent Fasting for Weight Loss
Many people using this type of diet are seeking weight loss. Intermittent calorie restriction via a short fast appears to be very useful for this purpose. People who practice this diet tend to lose weight faster than people who do not, even when they eat the same average amount of calories.
The metabolic effects are likely responsible for the additional weight loss. Classic calorie restriction diets often burn muscle as quickly as they burn fat. A short fast induces a mild ketogenic state in which fat cells are burned much more quickly.
In addition, the ketogenic state induced by a fast can be amplified by staying on a high fat, low carbohydrate diet on days when eating is allowed, leading to more weight loss overall. This keeps the metabolism burning strong and keeps the body out of a mode where it seeks to get energy from protein, aka your muscle.
As a result, people using this kind of diet will retain calorie burning muscle and gain it much more quickly. This results in overall faster weight loss specifically from fat.
In addition, a diet incorporated intermittent fasts appears to be more sustainable than calorie restriction diets for many people. Many people struggle to maintain calorie restricted diets after they have lost weight, which is why the majority of diets ultimately fail.
Future Applications in Medicine and Health Care
In the future, time-restricted eating may be used to treat a variety of different health disorders. Most of these are due to the aforementioned effects on metabolism and cellular regeneration. However, several studies have shown more unexpected benefits.
A diet that includes intermittent fasts has been shown to reduce autoimmune activity, which can be effective in treating diseases such as multiple sclerosis. A recent study has shown that restricted eating is very effective in reducing the symptoms of this disease.[vi]
In addition, the cell regeneration effects have been found to reduce trauma and long term damage in people who have had a traumatic brain injury.[vii] In the future, this may improve outcomes for people who have had strokes and other damage to the brain.
Cancer patients also may benefit from IF. This type of diet has been found to slow the progression of cancer and also to improve the effectiveness of other treatments such as chemotherapy and radiation. It appears that autophagy can sometimes target cancer cells, so this may be an important auxiliary treatment for people trying to beat cancer.
How Does IF Work?
There are several different intermittent fasting plans, each with different time periods. However, they all have a few similarities. People undergo periods of eating no calories interspersed with times when they follow their normal diet.
Most people using this method follow a generally healthy diet such as a ketogenic diet so they can get the very best results. Regardless of one's diet, it is important to break your fast with a healthy, low carbohydrate meal.
In general, people have three different metabolic states. The fed state begins when you eat and continues for three to five hours afterward. The next phase is called a post-absorptive state and occurs when you are no longer processing food, but also not yet in fasting. Last, we enter the fasting state about twelve hours after our last meal.
The fed state is when the body is "laziest." Blood glucose is high, so the body makes high levels of insulin as well. This is the easiest means of feeding cells and storing fat, although not necessarily what a well-fed modern person wants.
The body begins to burn fat rapidly in the post-absorptive state.
However, the real benefits occur in the fasting state.
When Do Benefits Begin?
As we stated earlier, the "fasted state" occurs around 12 hours after your last meal. As a result, most people will not see the benefits of time-restricted eating until around twelve hours after eating.
Once your body enters a fast state, it goes into ketosis. Fat is burned quickly to sustain the body. This is when the other physical effects occur as well, such as cellular regeneration and anti-aging effects.
Benefits of a fast increase until around 16 hours in men, when they then plateau. For women, this occurs earlier at around 14-15 hours. Most people find that 24 hours is a good maximum, before they become fatigued or feel unbearably hungry.
As a result, many people choose to make their intermittent fasts 16-24 hours long. This allows them to get maximum benefits without restricting themselves unnecessarily.
Breaking the Fast
When it is time to break a fast, it is preferable to choose a meal that is high in fat and protein. This combination amplifies the effects of ketosis a bit longer. It also lets your body enter a fed state more slowly, without a spike in blood sugar.
Breaking a fast with carbs is a poor idea for many reasons. This practice can cause an immediate spike in blood glucose and thus insulin, which can be unhealthy and uncomfortable. This can begin a pattern of wide blood sugar fluctuations that sabotage weight loss.
High fat meals allow the body to stay in ketosis, technically still perceiving a fast. However, protein brings the body into a fed state and stops some of the benefits of fasting like autophagy. Many people choose bulletproof coffee as a meal at the end of a fast. The standard bulletproof coffee recipe contains fat, like heavy cream, and collagen as a protein source. People that are using bulletproof coffee need to know that the added collagen is technically breaking their fast; however, if they remove the collagen and only use fat like butter, benefits like autophagy will continue.
Breaking the fast is a very important component in IF. People who do so with high amounts of carbohydrates can actually undo many of the benefits of their fast. This can sabotage your success and set your body up for metabolic problems in the future.
Different Types of Intermittent Fasts
There are many different kinds of time-restricted eating. However, the most common are the 5:2, the 16:8, Eat-Stop-Eat, Alternate Day Fasting, and the "Warrior Diet."
24 Hour Fasts
The 5:2 form of fast is very simple. People choose two days during the week to eat a restricted amount of calories, such as 500 calories. These calories can be consumed in one meal, or divided into two small meals. They will then follow their normal diet the other five days eating as normal throughout the day. The fasted days are not in a row, but rather broken up so at least one normal eating day is between the two fasted days. People vary on when to consume the calories: whether at breakfast, dinner or lunch. In reality, most people have a favorite time of day to consume food. For me personally, I always prefer to wake up and eat breakfast, and would rather not consume food at dinner. I know a lot of people have the opposite feeling. It is basically up to the preference of the individual, as long as they keep in mind that 12 hours from the last meal is when fasting benefits begin.
Eat-Stop-Eat is similar to 5:2 except that people take in no calories on fast days. As with a 5:2, people do not usually fast two days in a row. This diet will be difficult for those that mentally find it difficult not to eat for an entire day. Others find it very simple because it eliminates any choice of deciding how they will spend their calories.
Alternate Day Fast
Alternate day fasting is exactly as it sounds. Followers of this plan fast every other day. There are many variations of this plan, some allowing a small calorie intake on fast days, but all follow the same pattern.
One of the chief benefits of these methods is that people who follow them can choose days that work best with their schedule and life plans.
Partial Day Fasts
The 16:8 method is the most popular partial day fast method. In this plan, people go 16 hours every day without taking in calories and then follow their normal eating plan the other two days. One of the main benefits of this method is that you can schedule your fast so that eight of the "fasted" hours are spent sleeping.
For example, people using this method can eat an early dinner at 5 PM and complete a 16 hour fast by 9 AM. Other calories can be spread throughout the day. People who prefer an early breakfast but do not mind skipping dinner can fast from 2 PM until the next morning at 6 AM.
The benefits of partial day fasts is that they do not require as long of a fast period. In addition, they allow the same routine every day, allowing continuity. However, these plans can be stressful for people who have difficulty scheduling meals, as they will have to do so every day.
Can a Customized Plan Work?
Most people who use time-restricted eating begin with one or several of the aforementioned methods. However, many go on to develop their own customized plan based on what works best for them. This can be very effective as long as the plan includes intermittent fasts, adequate calories, and a generally healthy diet.
There are many more types of time-restricted diet than can be discussed here. If a person finds that the most popular plans do not work for them, they may be able to find a less popular one that is a better fit. They also can develop a custom program that suits their needs, although this requires in-depth knowledge of human health.
Tips for Fasting Plans
Timing is very important with this diet. Choosing the timing for fasts and eating also is important. In general, fasts should be timed to coincide with sleep as much as possible. This makes it easier to follow the plan.
Many people time 24 hour fasts so they stop eating after dinner and then resume the next night after 24 hours. This is beneficial for people who struggle to fall asleep when they have not eaten recently.
While a keto diet is not a necessary part of intermittent fasting, cutting out sugar and other carbohydrates can have real benefits. This reduces blood sugar fluctuations that can cause hunger, dizziness, and low blood sugar during a fast.
How Does a Ketogenic Diet Affect the Benefits of Intermittent Fasting?
Many people who are interested in IF are already on some sort of special eating plan. Some diets work very well with a fast and can increase its benefits.
These people are often on the popular ketogenic diet due to its many benefits. Adding a ketogenic diet can allow the benefits of intermittent fasting to flourish, and vice versa.
Metabolic Benefits of Ketosis Combined with Fasts
Blood sugar fluctuations are one of the main challenges for people when they are first trying to fast. Alternating between high and low blood sugar makes people feel hungrier. In addition, these fluctuations can lead to dizziness, dry mouth, fatigue, and many of the other effects that we associate with skipping a meal.
People who are on a ketogenic eating plan have more stable blood sugar. As a result, they are not as prone to the symptoms caused by fluctuations. This is one of the reasons that people do not feel as hungry on low carb diets.
This often makes an intermittent fast easier for people who are already eating keto. They do not experience as much hunger or other negative effects.
Benefits of Ketosis
People who are eating a low carb diet will get the same rewards from a fast as those who eat a standard Western diet. In fact, they will usually see more results because they are already in a state of ketosis. Their metabolisms are already using fatty acids and ketones for metabolism even in a fed state.
Ketogenic diets are effective for weight loss because they reduce fat while increasing metabolism. These are the same purported benefits of a plan involving an intermittent fast. As a result, these high fat and low carb diets work very well with diets that include time-restricted eating.
Even if you do not follow a ketogenic diet, it is important to keep your first meal breaking the fast as keto as possible. This will ensure that your blood sugar does not rise sharply and that your body is able to enter a fed state more gently. Choosing a low carb meal with fats and proteins to break a fast is ideal. Like we said earlier, keto coffee is a particularly good choice because it includes collagen, a rich source of protein, as well as butter. It is light yet filling meal to break a fast.
Is a Ketogenic Diet Worth It?
Many people using time-restricted eating believe that a keto eating plan is worth the additional effort. It leads to faster results and fewer negative effects for most people using this combination. That is enough to keep many people motivated to continue making positive lifestyle changes.
In addition, there are other health benefits to a low carb diet. This kind of diet is ideal for people who want to make a lifelong change in their health rather than simply losing a few pounds. Both ketosis and time-restricted eating can be parts of a long term healthy lifestyle.
Common Misconceptions about Intermittent Fasting
Many people associate a fast with starvation and thus with poor health. As a result, there are many misconceptions about a diet that restrict eating times. Let's clear some of these up!
First, many people believe that a diet that includes a fast will lead to a lower metabolism. This is actually not true. As long as there are ample fatty acids and ketones from stored fat, the metabolism will continue at its normal rate.
Long term calorie restriction can slow metabolism if it is severe enough. However, most people have enough fatty tissue to fuel ketosis for an extended period of time. There is a reason that people who live with low access to food are slender; the metabolic effects are much smaller than most people realize.
Second, people wonder if this sort of diet will negatively affect their blood sugar. This also is unlikely in healthy people. The body stops using blood glucose and switches metabolic strategy long before it is depleted. However, those with diabetes should still talk to their doctor.
Some people may have blood sugar fluctuations when they break their fast with a high carb meal, but this is easily prevented. Choosing a meal or beverage with both protein and fat should prevent this.
Another common question is about issues with hunger. People wonder whether they will be constantly hungry, to the point where it is uncomfortable or makes it hard to live their life normally. Some people indeed experience a great deal of hunger initially.
Some people believe that they will not be able to function as well on fast days. However, just the opposite is true. Researchers have found that people (as well as other animals) actually have better cognitive function when in a short term fast.
However, feelings of hunger often resolve with time. The human body can become accustomed to a variety of diets and eating patterns. Our ancestors routinely went through times of reduced food supply yet were healthy and happy.
A last question about this diet is whether it is healthy. Not only is it healthy, but there are numerous health benefits that we have already discussed for most people.
However, we will discuss below that some people with health conditions may not be able to fast safely. People who have concerns should talk to their health care provider before attempting any new eating plan.
Who Should Not Use Intermittent Fasting? Are There Any Dangers?
This type of diet is a healthy choice for most people. However, there are a few conditions in which people should talk to a doctor before starting this or any eating plan.
Type 1 diabetics should talk to their physician before attempting any kind of fast. Type 2 diabetics can usually safely incorporate fasts into their diet. In fact, the metabolic effects of this eating plan can be very beneficial for people with type 2 diabetes and metabolic syndrome. But speaking with a doctor on the best way to implement is still recommended.
Pregnant women also should talk to their physician before any kind of fast. Growing infants need to put on weight, which requires ample blood glucose and a steady supply of nutrients from the mother.
Breastfeeding while using time-restricted eating can be complicated. Many women see a drop in breastmilk supply when they are in ketosis. As always, listen to your body and talk to your doctor.
Top Tips for Intermittent Fasting
- Start with no more than two fasts a week
- Handle hunger with water, coffee, and tea (bulletproof coffee breaks the fast)
- Do not break your fast due to hunger, as hunger is normal the first few times
- Break your fast with a light, moderate protein and high fat meal
- Avoid bingeing the day after a fast
- Plan fasts so they start after dinner, allowing you to sleep through the time of most intense hunger
- Plan meals so you are getting enough calories and vitamins to maintain long-term health
The Final Verdict on Intermittent Fasting
Introducing an intermittent fast can be difficult. The two biggest challenges in the beginning are choosing a type of fast plan and dealing with initial hunger pangs. With time, however, these "growing pains" are easily overcome.
However, the rewards are more than worth the effort. The metabolic changes that this diet induces can help many to achieve their weight and health goals. People who practice this diet usually enjoy weight loss, muscle gain, and better all-around health.
There are several different types of fast that can be successfully incorporated into a healthy lifestyle. In addition, a ketogenic eating plan can be combined with intermittent fasts with excellent results. Ultimately, many people will find that this diet can contribute to the lifelong health and wellness we all seek.
If you are interested in fasting for weight loss, you may want to read how the ketogenic diet can help with weight loss.
[i] Roger Collier. Intermittent fasting: the science of going without. CMAJ. 2013 Jun 11; 185(9):
[iii] Martin P. Wegman,1,* Michael H. Guo, et al. Practicality of Intermittent Fasting in Humans and its Effect on Oxidative Stress and Genes Related to Aging and Metabolism. Rejuvenation Res. 2015 Apr 1; 18(2): 162–172
[v] Michelle Harvie and Anthony Howell. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects—A Narrative Review of Human and Animal Evidence. Behav Sci (Basel). 2017 Mar; 7(1): 4.
[vi] In Young Choi, Laura Piccio. Diet mimicking fasting promotes regeneration and reduces autoimmunity and multiple sclerosis symptoms. Cell Rep. 2016 Jun 7; 15(10): 2136–2146.
[vii] Davis LM1, Pauly JR, Readnower RD, Rho JM, Sullivan PG. Fasting is neuroprotective following traumatic brain injury. J Neurosci Res. 2008 Jun;86(8):1812-22. doi: 10.1002/jnr.21628.