Intermittent fasting: What is it, and is it safe?
Intermittent fasting may seem like a recent trend, but actually, humans have periodically refrained from eating for millennia for a host of reasons: Believe it or not, physicians of yesteryear used to prescribe fasting to treat a host of ailments, while ancient philosophers pushed their plates away in attempts to boost their mental clarity. Fasting has also been a form of political protest—think: the hunger strikes of Mahatma Gandhi, Irish Republican prisoners in the 1980s, or British and American suffragettes in the early 1900s.
To this day, avoiding food is a regular part of many religions. Traditionally, Muslims forgo food and beverage during daylight throughout Ramadan; Jews skip eating and drinking on Yom Kippur; and Hindus and Buddhists fast weekly.
Now fasting is trickling into the secular spotlight due to media coverage that claims intermittent fasting not only leads to weight loss and boosts metabolism but that it may also curtail cancer risk and even extend lifespan.
But is intermittent fasting, a practice that’s both ancient and newly trendy, as effective as some attest? Below, we take a closer look.
What is intermittent fasting? The ultimate guide
Intermittent fasting—nicknamed “IF”—is an approach to eating that requires you to partially or fully abstain from food and caloric beverages for controlled periods. Some people practice IF because they feel that going one or more days without food better approximates how our caveman ancestors ate—and thus how our bodies are “designed” to eat. Meanwhile, others see the approach to eating as a way to allow the digestive system to “rest and repair.” Some proponents even believe that IF can slow aging and tumor growth due to changes in cellular processes triggered by short periods of starvation (although this is not scientifically proven). But often, intermittent fasters are seeking weight loss.
"By triggering a stress response in the body that prompts the mobilization of resources needed to survive, fasting (for long periods) is thought to increase production of hormones known to boost metabolism and promote muscle growth,” says Dr. Steven Gundry, M.D., author of The Longevity Paradox: How to Die Young at a Ripe Old Age and medical director at The International Heart and Lung Institute Center for Restorative Medicine in Palm Springs, California.
The theory is that this process may strengthen some cells’ ability to withstand additional stressors, which explains the potential link between IF and delayed aging as well as a reduced risk of cancer risk. But the benefits have yet to be proven.
Intermittent fasting is also thought to affect metabolic health by regulating blood sugar and insulin levels. “In the absence of incoming nutrients, our blood sugar and insulin levels drop,” says Vancouver, Canada-based dietitian Megan Wong, RD. Research suggests that fasting for 30 days from dawn to sunset (using the Islamic spiritual practice of Ramadan) could trigger a rise in proteins called tropomyosins that help our cells make better use of available blood sugar (aka improve insulin sensitivity). However, more research is needed to understand the potential benefits and consequences of both this IF approach and alternatives.
Different types of intermittent fasting
IF is a parent category for several fasting protocols. Here are the most popular ones:
Also known as the Leangains Method (coined by nutritional consultant Martin Berkhan), 16/8 (or 16:8) fasting confines eating to an eight-hour window each day with a subsequent 16-hour fast. Typically, this means skipping breakfast. But it can also mean opting out of dinner, which may appeal to breakfast lovers.
In 2018, researchers tested this method in a small pilot study by instructing 23 participants with obesity to eat whatever they wanted between 10:00am and 6:00pm, then forgo food and caloric beverages for 16 hours. On average, participants lost 3 percent of their bodyweight and significantly reduced their blood pressure within 12 weeks. That said, this approach to IF has only been explored in a handful of human studies, so the jury is out on whether it will benefit you.
Popularized by journalists Michael Mosley and Kate Harrison, the 5:2 diet involves eating normally for five days each week and limiting caloric intake to 500 calories for women or 600 calories for men for the remaining two days. Adherents usually eat two larger meals or three smaller meals on “fasting” days.
The 5:2 approach has also been tested in some studies: When more than 100 women with excess weight fasted two out of five days per week for six months, they lost an average of 13 pounds and kept it off for a year, according to a 2011 study. That said, this study also looked at dieters who continuously limited their calorie intake: Researchers found that they lost about the same amount of weight as the intermittent fasters, meaning the most effective approach to weight loss involves behaviors that work for you, not one tried-and-true method. Just keep in mind that there’s not quite enough research to deem the 5:2 method a 100-percent safe way to lose weight and keep it off in the long term.
Alternate-day fasting (ADF):
A standard ADF approach entails eating whatever you want one day (although following healthy eating guidelines is recommended), then fasting completely (no caloric beverages or even snacks) on the next. A modified version allows up to 500 calories on “fasting” days.
A small 2019 pilot study found that ADF reduced belly fat among 57 adults by an average of 14.5 percent within 4 weeks and lowered markers of inflammation. But other studies question whether ADF is sustainable: When alternate day fasting was compared to everyday calorie restriction in a 2017 study, participants’ weight loss was similar regardless of their strategy. However, 10 percent more alternate-day fasters gave up their diets within a year compared to the everyday calorie restrictors.
Popularized by body-builder-turned-researcher Brad Pilon, the Eat-Stop-Eat diet advocates eating what you want for five days, then fasting completely (no small meals even) for two non-consecutive days per week.
When a small group of Malaysian men with excess weight gave this approach a shot in a 2011 pilot study, they saw an average 3-percent reduction in body fat and reported mood improvements. That said, the men also tended to restrict their calories on non-fast days too, so the results can't be attributed solely to fasting.
Among all of these approaches, there’s clearly a common thread: Each is designed to lead to energy restriction. If a dieter follows a given program long-term, this could lead to weight loss over time. But sticking it out can be so difficult that sustainable weight loss isn’t a sure thing.
Foods to eat and avoid when intermittent fasting
While fasting, adherents are advised to avoid food altogether unless their plan allows for limited calories. Water, unsweetened tea or coffee, seltzer, and other non-caloric drinks are allowed—but no milk or milk substitutes, Gundry says. Most IF approaches don’t forbid any foods on non-fast days, though many advocate eating fiber-rich foods, unsaturated fats, vegetables, and lean protein, according to Wong.
For those IF approaches that have unrestricted days, it is not clear if this could lead to overeating, consuming less healthy foods between fasts, or lead to lower energy intake on non-fast days (and potential nutrient deficiencies).
Intermittent fasting for weight loss and other potential health benefits
While intermittent fasting can lead to weight loss, research suggests this is largely due to a reduction in calorie intake. A 2015 review of 40 intermittent fasting studies found that on average, IFers lost an average of 7 to 11 pounds over 10 weeks—similar to the average pounds shed by non-fasting daily calorie-restrictors.
Intermittent fasting is being researched for other health benefits like reducing the risk of cardiovascular disease, dementia, and diabetes being some of them. Some theories also suggest that IF could also help boost cognitive functioning, like thinking about multiple concepts simultaneously or transitioning fluidly between tasks or thoughts.
While there’s little research on IF’s effects on mental health, researchers are interested in examining effects on mood. “[It’s] thought to alter levels of hormones and neurotransmitters that regulate mood and motivation,” says Washington, D.C.-based dietitian Danielle Schaub, MSPH, RD, likening the controlled eating approach to periods of starvation.
All that said, claims about many of IF's benefits tend to stem from studies conducted on mice, rats, worms, bacteria, and yeast rather than humans. So while some animal research has suggested that intermittent fasting may extend longevity and reduce tumor growth, it’s unclear whether IF triggers these processes in humans, who are far more complex.
Potential side effects and health risks of intermittent fasting
IF’s side effects may be too uncomfortable for some to bear—particularly if you have an underlying medical or psychological condition that accentuates your symptoms. (It’s why you should always talk to your physician if you are considering IF.) Here are the main downsides:
- Lightheadedness. “Lightheadedness can be caused by low blood sugar levels or from low sodium or potassium,” says Allison Childress, Ph.D., RDN, chief clinical dietitian at Texas Tech’s Nutrition and Metabolic Health Institute.
- Difficulty concentrating. “Mental fogginess, is also a common short-term side effect of fasting,” Childress says.
- Headaches. Headaches are a commonly reported side effect of fasting. Dehydration and caffeine withdrawal could play a role.
- Constipation. Without adequate fiber (or food to even digest) some people experience constipation during or after fasting, Wong says.
- Fainting. Some intermittent fasters report fainting. This is likely due to a lack of adequate hydration, lower blood pressure, or low blood sugar levels, Schaub says, or an underlying medical condition.
- Insomnia. “Many people notice they don’t need as much sleep when fasting,” Gundry says.
- Fatigue and weakness. “Fewer calories means less incoming fuel, which can lead people—especially those unaccustomed to fasting—to feel less energetic,” Childress says.
- Dehydration. “People who aren’t careful about increasing their fluid intake when fasting can suffer from dehydration,” Childress says. Especially since they aren’t consuming water-rich foods.
- Irritability and anger. Because low blood sugar can make you feel more aggressive, and stress hormones released while fasting can put you on edge, fasting may make you angrier.
- Increased sensitivity to alcohol. Not eating can increase your sensitivity to alcohol if you break a fast by drinking, Childress says.
Is intermittent fasting safe?
At present, there isn’t enough human data on intermittent fasting to definitively deem it safe (or not). People with type 1 or type 2 diabetes or who are on medications for diabetes should not attempt intermittent fasting unless recommended by a physician. The same goes for people with a history of disordered eating, depression or alcoholism, and for women who are pregnant or breastfeeding, according to Childress. Children and teens under the age of 18 should avoid IF, along with people on medications that must be taken with food at the same time each day. Due to the frequency of headaches fasters can experience while forgoing food, people with a history of migraines may not be the best candidates either, Schaub says, along with those who take medications that lower their blood pressure.
Other disadvantages of intermittent fasting
Constraining eating to specific windows of time can throw a wrench in your social life and strain your ability to participate in important events and work functions. The headaches and increased irritability you many experience when fasting—plus the mental fog, dizziness, and lightheadedness some report—could also hamper relationships and impair school or work performance.
Can I exercise while intermittent fasting?
"Yes," Childress says. But exerting yourself on an empty stomach can make you feel light-headed, shaky, or weak, which is why endurance exercise and fasting don’t pair well. "Eating before longer duration aerobic exercise has been proven to increase performance,” Childress says.
The upshot: Does intermittent fasting work?
Intermittent fasting may promote weight loss. But the human studies backing these claims are limited, and other potential benefits—extended lifespan, reduced risk of cancer—haven’t been proven in humans.
As the National Institutes of Health point out, researchers have only just begun understanding how different eating habits influence how we function. Plus: Age, weight, genetics, hormonal balance, lifestyle, and your environments all mediate the effects of any dietary changes you make—so no single eating pattern is going to affect everyone the same way.
Of course, weight loss isn’t the only measure of whether a diet works. What and how you eat should contribute to your overall wellness—not just your appearance, but your physical and mental health. Intermittent fasting may be too extreme for some people, and it may pose potential health risks.
That said, with your doctor’s OK, some of the pillars of IF could be useful when applied to other weight loss programs such as WW: Reduced snacking, eating less at night, and being mindful of caloric intake on certain days are important components of a healthy lifestyle that you can sustain for more than just a few days or weeks.
It’s why applying some tenets of IF on WW could be useful when working toward a weight loss or health goal. "We don't recommend any approach to eating that is overly-restrictive, but for some members, limiting eating to certain times of the day can be a helpful behavioral strategy," says WW global director of nutrition Zoe Griffiths, a UK-based registered dietitian. "But there is no ‘one size fits all’ approach when it comes to adopting a healthier pattern of eating. What’s important is finding foods you enjoy and establishing an eating pattern you can live on for the long term—one that doesn't leave you feeling overly hungry, which can lead to overeating and feelings of restriction that are unsustainable," she adds. WW offers the flexibility to indulge in the foods you love; it’s your choice how and when to eat them.
Katherine Schreiber, MFA, MSW, co-author of The Truth About Exercise Addiction: Understanding the Dark Side of Thinspiration, is a social worker and freelance writer based in New York City. She specializes in working with adults with severe and persistent mental illnesses, like schizophrenia spectrum disorders. Her work has been featured by Psychology Today, Cosmopolitan, Shape, and TIME.
This article was reviewed for accuracy in June 2021 by Angela Goscilo, MS, RD, CDN, Nutrition Manager. The WW Science Team is a dedicated group of experts who ensure all our solutions are rooted in the best possible research.