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What Is Intermittent Fasting – Benefits & Downsides, How It Works

Intermittent fasting (IF) has become one of the hottest health trends. Some use it to lose weight, others to combat diseases like diabetes and cancer. Regardless of the reason, people considering IF usually have two big questions: Is it safe and does it work?

You should always consult with your doctor before starting this or any other dietary program. But whether you fast to lose weight or for health benefits, research shows several positive health effects and little danger from occasional fasting for healthy adults.

But with so many diets to choose from, one question remains: Is intermittent fasting right for you?

What Is Intermittent Fasting?

Intermittent Fasting Two Plates Fasted Fed Food

Intermittent fasting is an eating pattern that switches between periods of fasting and periods of eating. If you skip breakfast or forgo snacks, you’re probably already doing some form of intermittent fasting.

IF differs from traditional diet plans because IF doesn’t specify what foods you should eat, only when you should eat. Thus, it’s more accurately described as an eating pattern than a diet, although many people use it to lose weight.

Common methods of intermittent fasting include daily 16-hour fasts, which involve giving up nighttime snacking and foregoing breakfast, or fasting for 24-hour periods two to three times per week.

Dietary experts like Stanley Ulijaszek, a nutritional anthropologist at the University of Oxford, say IF is a more natural way of eating. Our ancient ancestors didn’t have such a readily available food supply. They didn’t have access to refrigerators or 24-hour supermarkets and round-the-clock restaurants. On occasion, they likely had no food to eat for days at a time.

These experts say the human body evolved for survival, meaning we’re fat-storing machines. Feasting during good times to store up fat to burn for fuel during lean times is, therefore, a natural part of how our physiology works.

But for many people in modern society, there are rarely ever lean times. IF works by allowing us to self-impose these lean times so that our bodies use stored extra energy by burning off excess body fat.


How Intermittent Fasting Works

When we eat, carbohydrates are broken down into simple sugars called glucose. Insulin rises and carries these glucose molecules into our cells, which burn them as fuel. Typically, we eat more than we can use immediately, so excess glucose is linked into longer chains to form glycogen. Glycogen is then stored in the liver so that it’s readily accessible when it’s needed.

The liver has limited storage. Once it reaches capacity, it turns excess glucose into fat. Some of this new fat is stored in the liver, but most of it is distributed throughout our bodies – to our hips, thighs, buttocks, stomachs, and even faces. And there is no limit to the amount of fat the body can create and store.

This process reverses when we don’t eat. Insulin and blood glucose levels fall, signaling the body to turn to its own fuel stores.

Fat is more difficult for the body to process into fuel, so the body always reaches for glucose first, followed by glycogen. It only turns to burning fat for fuel after it uses up glycogen stores. And while the liver has limited capacity for storing glycogen, it stores enough to fuel the body for 24 to 36 hours.

That means our bodies are always either in a state of storing food for energy (what IF experts refer to as the “fed” state) or burning stored fuel for energy (the “fasted” state). If you balance eating and fasting, your body weight won’t change. But if you start eating right when you wake up, consume food all day long, and don’t stop until it’s time to return to sleep, you’ll always be in the fed state. That can lead to weight gain over time because you never give your body the chance to burn off any stored fuel.

So if your goal is to lose weight, you need to increase the amount of time you’re in the fasted state. That’s what intermittent fasting is all about.


Benefits of Intermittent Fasting

Detox Liver Stethoscope

You may have discovered IF after hearing about it from a friend or seeing it discussed in the media. Maybe you scrolled across it on your Facebook feed. But IF is more than just a fad. Scientists have been studying this eating pattern for several years, and they’ve found many positive health benefits of IF.

1. It Helps Detoxify the Body & Induces Cellular Repair

It’s a toxic time to be alive. Even those who are excessively health-conscious are exposed to hundreds of chemicals in the air and water daily. And many health professionals believe the toxins we’re exposed to contribute to several modern disease epidemics, including diabetes.

When you fast, several biological processes take place. Your body induces an important cellular repair process to remove waste material and defective cells, including those weighed down with environmental toxins. This process, called “autophagy,” is a way to tap into the body’s innate healing ability. To allow us to thrive during a period of famine, the body works to repair itself by essentially eating defective cells.

It’s why IF has gotten so much attention in recent years. In 2016, Dr. Yoshinori Ohsumi won the Nobel Prize in physiology or medicine for his research into autophagy. Ohsumi found that when you fast for 13 to 15 hours, your cells go looking for food, and they find that food by feasting on the toxins already inside the cell.

Simply put, our bodies have a built-in detoxification system. There’s no need to buy expensive supplements, go on juice cleanses, or eat strange foods. All we have to do is stop eating for a period, and our bodies naturally detoxify.

2. It’s Good for the Brain

IF stimulates certain metabolic factors that are excellent for brain health. One of these is the production of ketones, acids your liver makes when it runs out of glycogen and turns to stored fat for fuel. Research published by the journal Epilepsy Research in 2014 shows ketones are a highly efficient fuel source for the brain. They also increase mental alertness and clarity.

But the benefits don’t stop there. According to a 2000 study published in the Journal of Molecular Neuroscience, IF may increase the growth of new neural stem cells. That increases resistance to age-related degeneration, including loss of memory and learning ability.

Moreover, the Journal of Psychiatry Investigation published a study in 2010 that shows IF increases levels of a brain hormone called brain-derived neurotrophic factor (BDNF). BDNF deficiency has been linked to depression and other brain disorders.

Finally, an animal study published in the journal Annals of Neurology shows IF can protect against damage due to strokes.

3. It Aids Weight Loss

A 2018 study by the German Cancer Research Center found IF is no more effective than any other weight-loss method. But it did conclude it’s a simple and easy way to restrict calories.

Generally, IF leads to eating fewer total calories. As long as you don’t binge after a fast, skipping certain meals or snacks automatically results in consuming fewer calories, even if you otherwise continue to eat as usual. Further, research published in the journal Cell Metabolism found that IF can decrease appetite, which also results in eating fewer calories.

According to a 2015 review of IF research by the Journal of the Academy of Nutrition and Dietetics, IF can reduce body mass by 3% to 8% in as little as 3 to 24 weeks. Participants also lost 4% to 7% of their waist circumference. Abdominal fat has been shown to be particularly harmful to health.

Additionally, a study from Johns Hopkins University found that those following an alternate-day fasting style lost more body fat and preserved more muscle mass than those following a low-calorie diet. That may be because IF increases human growth hormone, which plays a role in fat burning and muscle gain. Another study found that HGH increases as much as fivefold during fasting. IF also enhances insulin’s function and lowers overall insulin levels.

Some studies even show that fasting increases your metabolic rate by 3.6% to 14%. That’s in sharp contrast to traditional low-calorie diets, which studies show decrease your metabolic rate. Plus, your metabolic rate doesn’t return to normal once you stop dieting. So to keep the weight off, you must restrict calories forever. Because that’s difficult for people to do, low-calorie diets may only result in short-term weight loss. Plus, extended calorie restriction puts the body into starvation mode, signaling it to conserve energy in case food isn’t coming. But brief fasts like IF don’t give the body a chance to “believe” it’s starving.

4. It’s Easier Than Dieting

Sticking to your eating plan is crucial. According to a study from the American Heart Association, yo-yo dieting is more harmful than carrying excess weight. But traditional diets can be a pain to follow.

Dieting often requires counting calories, tracking what you eat, weighing and measuring portions, and even restricting certain foods. IF only requires you not to eat for specified periods. It doesn’t get much simpler than that.

Also, many people have trouble sticking to traditional diets because they have to plan for and cook healthy meals. But with IF, there’s no planning, no prep, no cooking, and no need for cleaning up dirty dishes – at least during your fasting period.

5. It May Slow Down Aging

One of the more exciting benefits of IF is its potential to extend lifespan. More than one study, such as a study of mice by the University of Kyushu in Japan, has demonstrated IF’s anti-aging effects are at least as beneficial as that of calorie restriction. And in some of these studies, the results were significant. In a study by the National Institute on Aging, rats that fasted every other day lived 83% longer than rats who didn’t fast.

Our bodies have over 72 trillion cells, and over 200 billion of these replicate every day. When cells are sick and worn out, they replicate more damaged and defective cells – one of the primary contributors to the aging process. Over time, as defective cells build up, you age more rapidly. If you induce autophagy through fasting, old cells become young again.

Another potential anti-aging factor is the reduction of oxidative stress during fasting. Oxidative stress contributes to aging by allowing unstable molecules called “free radicals” to interact with and damage essential molecules like DNA. But when the process of autophagy is turned on by fasting, unhealthy cells and cell parts are replaced with healthy ones over time. Research from the University of Louisiana Medical Center shows IF may enhance the body’s resistance to oxidative stress.

Finally, according to the National Institute on Aging, chronic inflammation plays a role in the aging process by damaging cells. But many studies, including one from the University of Hail in Saudi Arabia analyzing the effects of fasting during Ramadan, show IF is anti-inflammatory.

6. It May Reverse Diabetes

Type 2 diabetes has reached epidemic levels. According to 2017 statistics from the CDC, more than 100 million Americans have either diabetes or pre-diabetes, which, if left untreated, typically turns into Type 2 diabetes within five years.

People with Type 2 diabetes either don’t produce enough insulin to regulate their blood sugar or are resistant to insulin. Since insulin delivers glucose to the cells, when the body is resistant to insulin or doesn’t make enough, blood glucose levels remain elevated. Over time, blood sugar buildup damages the blood vessels, nerves, kidneys, and heart. In fact, heart disease is the No. 1 killer of people with diabetes.

A 2018 case study published in the journal BMJ Case Reports found that IF had a significant positive impact on insulin resistance. It found that insulin-dependent diabetics cells became less resistant to insulin as a result of IF, effectively reversing their diabetes. They were even able to stop taking insulin, even during periods of eating, after following an IF regimen.

Granted, this was merely a case study. But other research investigating the link between IF and insulin resistance also found positive results. That includes 2018 research from the University of Alabama that found fasting for 16 hours a day led to dramatically lower insulin levels and improved insulin sensitivity.

This is why I do IF. I am a Type 2 diabetic, and when my condition started to progress, despite following all the standard health advice, I started the keto diet. After following keto for several months, I experimented with IF, and the combination of the two significantly reduced my need for diabetes medication, including insulin.

7. It May Protect Against Neurological Disorders

Intermittent fasting may have the potential to prevent and treat neurological disorders including Alzheimer’s, Parkinson’s, and Huntington’s.

Alzheimer’s is the world’s most common neurological disorder, and several research studies have found positive benefits for both the prevention and treatment of the disease. A study of rats by the National Institute on Aging, found that IF may delay the onset or reduce the severity of Alzheimer’s. And a series of case studies published in the journal Aging found significant symptom improvement in 9 out of 10 Alzheimer’s patients who underwent lifestyle interventions, including a fasting regimen.

Dr. David Rubinsztein, a professor of molecular neurogenetics at the University of Cambridge and U.K. Dementia Research Institute, discovered that proteins form in clumps in the cells of those with neurological disorders like Alzheimer’s and Parkinson’s. He tells BBC News that autophagy removes these proteins, thereby protecting individuals against neurodegenerative diseases.

Other studies also had positive findings, suggesting IF can prevent or improve Alzheimer’s, Parkinson’s, and stroke. Research on mice by the University of British Columbia shows fasting can help clear away the buildup of the protein responsible for causing Huntington’s. Although we still need more research on humans, the findings are promising. Currently, there is no cure for these conditions, so preventing them before they happen is crucial.

8. It May Help Treat & Prevent Cancer

Autophagy may help prevent cancer, according to a 2009 study published in Cell. Cancer is a disease characterized by the uncontrolled overgrowth of defective cells – the same cells autophagy works to eliminate.

No one has done extensive research on humans yet, but research on animals looks promising. Case studies on humans from the University of Southern California found that fasting before chemotherapy can reduce the side effects of treatment. And a study from Louisiana State University found that fasting before chemotherapy could result in better cure rates.

9. It May Help Treat Autoimmune Disorders

Studies from the University of Southern California (USC) have uncovered IF’s remarkable effects on the immune system for patients receiving chemotherapy. Chemo damages the immune system through immunosuppression. But USC found that turning on autophagy by fasting counteracts this effect in two important ways

First, it kills off old and defective immune cells. Then, it generates brand-new immune stem cells. Dr. Valter Longo, one of the studies’ authors, concludes that fasting “can generate, literally, a whole new immune system.” That means IF could significantly benefit not only those undergoing chemo, but also those suffering from autoimmune disorders.

10. It Will Save You Money

Diets are typically expensive. Buying specialized items like gluten-free foods and organic fruits and vegetables can make your grocery budget skyrocket, even when your diet focuses on calorie restriction – which should mean buying less food.

It’s hard to stretch a grocery budget far enough to cover organic foods when you’re trying to feed a family. Healthy, wholesome food tends to be more expensive than junk food, thanks in part to government subsidies for crops like wheat and corn. Where I live in Ohio, an entire loaf of nutrient-deficient white bread is about $1.99, whereas a pound of organic strawberries might set you back as much as $5.99.

But that’s one of the great things about IF: Fasting is free. There’s no cooking, no meal prep, and no grocery shopping during fasting periods. You reap all the health benefits of IF without having to spend a dime.

In fact, IF can save you money. If you’re doing a fast that includes skipping breakfast, you’ll no longer have to include items like breakfast cereal in your grocery cart. And because IF discourages snacking, snack foods will also be cut from your grocery budget.

Plus, anything that positively benefits your health saves you money in medical bills, prescriptions, and potentially long-term care as you age.

IF can also save you money on medication. I used to spend nearly $500 per month on insulin alone. Thanks to keto and IF, I no longer have to buy any insulin at all. I have a close friend who was able to cut her obsessive-compulsive disorder and attention deficit disorder medications in half after going on keto and IF. These examples are anecdotal, and you should always check with your doctor before making any changes to your diet or medication. However, they do demonstrate the power of lifestyle interventions to improve your health and, consequently, your budget dramatically.


Drawbacks of Intermittent Fasting

Take Out Burgers Pizza Nuggets Chinese Milkshake Binge Eating

Although research has demonstrated many benefits of IF, it isn’t without its drawbacks. It can be difficult for some people to go without eating for that long, and certain health conditions make intermittent fasting impractical or impossible. To determine if IF is right for you, you must take into account its potential drawbacks.

1. Increased Hunger & Deprivation Can Lead to Binging

It may take time to get used to extended periods of fasting, especially if you’re used to frequently eating throughout the day.

For some, skipping meals might result in eating more calories during the eating period. One study from the Rowett Research Institute in the U.K. found that people who fasted for a whole day ended up eating 500 extra calories the next day. Even with this, participants still maintained an overall calorie deficit. But a continuous cycle of bingeing brought on by the prolonged hunger of fasting could be uncomfortable and discouraging for those who attempt it.

2. Prolonged Fasting Can Cause Hypoglycemia in Diabetics

If you have diabetes – especially if you’re insulin-dependent – proceed with caution. Even if you’re able to do a 16/8 fast with little difficulty, fasting longer than that will cause problems with your blood sugar levels if you take insulin. When I was insulin-dependent, if a doctor asked me to fast for longer than 16 hours before a medical procedure, they asked me not to take insulin during the fast due to the risk of hypoglycemia.

Low blood sugar is no joke. It can cause a whole host of symptoms, from the merely uncomfortable to the life-threatening. Research has demonstrated positive effects of IF on diabetes, but talk to your doctor before undergoing an IF regimen.

3. It Could Come With Some Uncomfortable Side Effects

Hunger is the main side effect of IF. Potential side effects of increased hunger include weakness, fatigue, headaches, dizziness, heartburn, and leg cramps.

Many of these symptoms dissipate as your body adjusts to fasting. In the meantime, mitigate the effects by paying attention to what you consume.

  • Drink plenty of water.
  • Use a sodium supplement to treat dizziness caused by an electrolyte imbalance.
  • Eat potassium-rich foods when you’re not fasting to combat leg cramps.

Despite the potential side effects, there’s nothing especially dangerous about IF for healthy adults. An IF regimen can be as simple as skipping breakfast, which many people do every day. During a longer fast, many even report positive side effects, like increased energy and a sense of euphoria.

4. There Isn’t Much Long-Term Human Research

Although scientists have conducted many studies on animals, there are currently few studies on humans, particularly long-term ones. IF’s potential to treat and prevent some diseases is promising. But doctors can’t prescribe it as a therapeutic regimen without more research.


Common Questions About Intermittent Fasting

Question Mark Thought Bubble

Despite the wealth of promising research on the benefits of IF, controversy surrounds the practice. General misconceptions, coupled with a lack of information about nutritional science, lead to the following common questions.

1. Will Fasting Cause Muscle Loss?

All weight-loss methods can cause muscle loss, but a University of Illinois study found IF may lead to less muscle loss than other methods. That may be due to IF’s effect on fat-burning hormones – specifically, the increase in human growth hormone, which plays a crucial role in muscle gain.

If you’re concerned about losing muscle mass, IF expert Mindy Pelz recommends periodic “protein easting,” meaning consuming larger-than-normal amounts of protein during eating hours. It will help conserve – and may even build – more lean muscle mass.

2. Won’t Skipping Breakfast Make Me Gain the Weight Back?

We’ve all heard that breakfast is the most important meal of the day. The National Weight-Loss Registry found the majority of those who were able to keep from regaining their lost weight after two or more years ate breakfast.

However, there’s nothing inherently special about breakfast. A 2014 randomized, controlled trial – the gold standard of scientific testing – published in the American Journal of Clinical Nutrition demonstrated no difference in weight loss between those who ate breakfast and those who skipped it.

3. Isn’t Eating Frequently Good for Health & Weight Loss?

Traditional dieting advice often espouses the virtues of eating six times a day to keep insulin levels steady, especially if you have diabetes.

But insulin remaining steady isn’t necessarily a good thing if it remains steadily high. If you keep eating food, your body continues to release more and more insulin. And since insulin is responsible for fat storage, your body remains in fat-storage mode, not fat-burning mode.

Some studies suggest frequent snacking can have adverse effects on your health. A 2014 study published in the journal Hepatology found that a diet with more frequent meals led to an increase in liver fat, which is a risk factor for fatty liver disease. And a study from the American Association for Cancer Research found that people who eat more often have a higher risk of colon cancer.

On the other hand, some studies show no difference in weight loss between those who eat frequently and those who eat less often. So, how often you eat could be a matter of what’s most effective for you when it comes to weight loss.

5. Is It OK for Everyone to Fast?

Intermittent fasting is a safe and arguably natural way for perfectly healthy adults to eat, especially if you’re only fasting for a few hours per day. But despite all the potential benefits, fasting isn’t for everyone. Certain people should avoid fasting altogether, and others should proceed with caution.

Anyone With an Eating Disorder or At Risk for an Eating Disorder

The National Eating Disorder Association lists a history of dieting as a risk factor for eating disorders. Although IF isn’t a traditional diet, it is a form of dietary restriction.

Thus, IF could amplify eating disorders such as bulimia, anorexia, or binge-eating disorder. If you have an eating disorder or are at risk of one, avoid fasting unless otherwise instructed by your physician.

Some Women, Including Those Who Are Pregnant, Breastfeeding, or Trying to Conceive

There is limited evidence that IF isn’t as beneficial for women as it is for men. One study from the Pennington Biomedical Research Center found that IF improved insulin sensitivity in men but worsened it in women.

Additionally, a study by the National Institute on Aging found that IF can make female rats emaciated, masculinized, and infertile and cause them to miss menstrual cycles. However, Pelz notes that IF is excellent at balancing hormones in perimenopausal, menopausal, and postmenopausal women and decreasing menopause symptoms. The Polish Academy of Science study found IF can help overweight women become more fertile by lowering body fat.

IF appears to be safe for women in general. But if you have low or average body fat and are experiencing fertility issues, table trying IF for now. The same goes if you’re pregnant or breastfeeding. Mothers-to-be and breastfeeding mothers have higher nutritional requirements.

Others Who Should Not Fast

Other people who should not fast include:

  • Children. Kids have higher nutritional needs than adults. Because their bodies are still growing, they need to eat a variety of foods frequently throughout the day.
  • Anyone Healing From an Injury or Surgery. Healing requires consuming large amounts of protein.
  • Anyone Who Is Underweight. Fasting could make you lose even more weight.

Those Who Should Fast With Caution

Additionally, some people might be able to fast but should only do so under a doctor’s direct supervision:

  • Anyone with Type 1 or Type 2 diabetes
  • Anyone who takes prescription medications that require them to eat
  • Anyone with high blood pressure

How to Get Started With Intermittent Fasting

Intermittent Fasting Challenge Are You Ready

There are several popular patterns of intermittent fasting. All of them involve splitting either the day or week into cycles of fasting and feeding. During fasting periods, you generally eat nothing at all, but you can drink water, coffee, tea, or other zero-calorie beverages.

By far, the easiest and most comfortable IF pattern is the 16/8 fast. You divide your day into a period of 16 hours of fasting – about half of which you’ll spend sleeping – and eight hours of eating. For the eating period, you choose your preferred eight-hour window. Most 16/8 fasters decide to forgo breakfast. So, for example, they might stop eating at 8pm and not eat again until noon the next day.

Pelz recommends starting slow. Your body needs time to become adapted to burning fat for fuel during lean times. If you’ve never gone without food before, it often feels uncomfortable at first. Over time, the body becomes better able to switch between periods of fasting and feeding.

So start by skipping your usual evening snacks. Push breakfast back by an hour each day until you’re eating within an eight-hour window and fasting for 16. It’s not as scary as it sounds. At least half the fasting hours take place while you’re sleeping.

You can also follow a pattern of intermittent fasting. In the eat-stop-eat pattern, you fast for 24 hours once or twice a week. In fast-mimicking, you consume only 500 to 600 calories two days of the week and eat normally the rest of the week. IF can even involve longer fasts of three to five days at certain times of the month or year.

Try experimenting with the different approaches to find something you enjoy that suits your lifestyle.


Final Word

IF can be a powerful tool in your overall health arsenal. That arsenal should also include general healthy habits like eating whole foods, exercising regularly, and getting enough sleep.

Although the research on IF is still in its early stages, much of it looks promising. If you have health issues, consult a doctor to find out if there are any potentially positive benefits of IF for you, such as reversing diabetes. But there appears to be little harm in trying it as long as you’re a healthy adult. It might be something that helps you feel healthier, happier, and more energized.

Have you tried intermittent fasting? What has been your experience?

Sarah Graves
Sarah Graves, Ph.D. is a freelance writer specializing in personal finance, parenting, education, and creative entrepreneurship. She's also a college instructor of English and humanities. When not busy writing or teaching her students the proper use of a semicolon, you can find her hanging out with her awesome husband and adorable son watching way too many superhero movies.

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