Searches for #intermittentfasting on TikTok yield nearly 200,000 posts and over two billion views. On Instagram, the #intermittentfasting hashtag has over five million posts. Celebrities like Jennifer Aniston, Jimmy Kimmel, and Nicole Kidman do it to stay camera-ready, and Chris Pratt reportedly lost 60 pounds using this weight loss strategy. Plenty of nonfamous people are also choosing intermittent fasting (IF), from the busy mom with three kids to the guy at the climbing gym to the busy executive, and it’s not just a fad—IF is well-researched and rooted in science. (The Harvard School of Public Health reviewed 40 studies and found that intermittent fasting was effective for weight loss, with a typical loss of 7-11 pounds over 10 weeks.)
Fasting is nothing new. Fasting is intentionally not eating—which sets it apart from starvation—and people fast for a variety of reasons that may be health-related but can also be cultural or religious. Intermittent fasting (IF) as a health strategy differs from religious fasting and has gained traction over the past few years, whether people are seeking to lose weight, increase longevity, or improve a variety of health conditions.
IF is a form of diet, but it’s less focused on what you eat and more on when you eat. Instead of eating around the clock, people choose a time window for eating and drinking. Historically, people didn’t eat the way we eat now, and our hunting and gathering ancestors went long periods—sometimes days at a time—without food. Now, with full refrigerators and pantries, people are always eating. Then there’s DoorDash to fulfill your cravings, and you don’t even have to shop, cook, clean, or leave the house.
The Centers for Disease Control and Prevention reports that obesity is on the rise. “Just 10 years ago, no state had an adult obesity prevalence at or above 35 percent,” the report says, but now there are 22 states with an adult obesity prevalence at or above 35 percent. The data suggests that most of us could ingest fewer calories. One way to do this is to limit the number of hours in a day that we eat.
Research also shows that stabilizing blood sugar before bed leads to less disrupted sleep, but the proverbial scale can tip in the other direction when high-calorie meals with large amounts of fat or carbohydrates are eaten too close to bedtime. IF is beneficial for more than just reducing calories and losing weight.
How It’s Done
The way you incorporate IF into your life depends on a few factors, but all experts suggest figuring out what works best for your schedule, which will set you up for the most success. IF is similar to time-restricted eating (TRE), and the terms are sometimes used interchangeably, although there are some differences. According to the National Library of Medicine, TRE “focuses on the timing of meals and their relation to circadian rhythm, hormonal, and metabolite profile within 24-hour period,” while IF only focuses on eating during certain hours of the day or days of the week.
A popular IF option is the 16:8 schedule, which means you fast for 16 hours and choose an eight-hour eating window, though many people limit their eating to a four-hour window. “The 16:8 schedule ends up being the easier to incorporate into daily life, and so would be easier to stick to,” says Sequoia Kristal, CNC, of Tannersville, founder of Purslane Nutrition and Pathome Meal Delivery. “One of the biggest struggles clients have in our work together is turning ideas into habits,” Kristal says. “You can still go about a fast-paced life, as long as you make sure you’re getting nutrient-dense foods in during the eating window.”
Elizabeth W. Boham, MD, MS, RD, Physician and Medical Director of the UltraWellness Center in Lenox, Massachusetts, recommends a 12-hour fast for everyone. “Everyone can handle this, and this is good for everyone,” she says. “Many people like the 16:8, and that can be helpful, but we focus on not having people eat too late at night,” Dr. Boham says, adding that it’s better to eat between 8 am and 4 pm or 10 am and 6 pm, because people need fuel for their day and eating too close to bedtime can disrupt sleep.
Another option is the 5:2, where people eat normally five days a week and restrict calories to 500 to 600 on two non-consecutive days. “The 5:2 method requires a bit more time to plan—because you are restricting calories, you may feel an energy zap on those days—and it could be harder to take care of kids, work, run errands, rigorous exercise or do whatever you need to do,” Dr. Boham explains. “People might want to schedule less on the fasting days,” she says.
It Isn’t for Everyone
Intermittent fasting is beneficial for many people, but it’s not for everyone, and for people with a history of disordered eating, restricted eating may be a slippery slope. Ilyse Simon, RDN, CDN, is a Kingston-based nutrition therapist specializing in eating disorders, and she says that intermittent fasting isn’t intuitive, easy, or sustainable and that if we want to address issues like obesity, fatigue, or insulin resistance, we need to address the root cause and the behaviors that feed into it. “Intermittent fasting is just another rule or restriction and is extremely harmful,” Simon says, adding that IF has been linked to higher incidences of eating disorders and eating disorder behaviors, including compulsive exercise, vomiting, fasting, laxative use, and binging.
Sustainable Weight Loss?
“IF can be a sustainable way to lose weight and help keep weight off,” Kristal says. “The more manageable the task, the more sustainable, and TRE seems more sustainable than calorie-restricted eating.” Kristal thinks TRE’s long-term success is because people stop grazing or overeating foods they don’t need since they’re not eating the traditional three mega-portioned meals daily with many additional trips to the fridge or the deli for snacks.
Kristal had a client who got a bacon, egg, and cheese every morning, grabbed some sort of deli meat sandwich with chips and a soda for lunch, and had pizza or a hamburger and fries for dinner. “Before we discussed TRE, we discussed the lack of vegetables and fiber in his diet and the need to incorporate way more servings of vegetables, needing some fruits, nuts, seeds, whole grains, where to find better quality protein sources, and the lack of quality, anti-inflammatory fats,” she says. “When this client moved to a TRE window of about four hours in the late afternoon, he stopped eating a few hours before bed, and also started eating more nutrient-dense foods in that window, and lost a ton of weight.” People are more likely to stick with IF when they see sustainable results and feel better when doing it.
“Some people have lower ghrelin and insulin, and IF helps some of our patients lose weight and become more metabolically flexible,” Dr. Boham says. Ghrelin is a hormone produced in the stomach that signals to the brain that the stomach is empty and it’s time to eat. People with higher ghrelin generally eat more whether they’re hungry or not, and tend to eat because it’s mealtime, not because their bodies are seeking fuel. Boham likes the 5:2 plan for people who are stuck and not losing weight, because longer fasting periods have been shown to improve metabolic function and shift how the body uses glucose.
Other Health Benefits
In addition to weight loss, Dr. Boham says that IF may help with type 2 diabetes and metabolic syndrome, plus it may improve blood sugar control, lower insulin, lower HgA1C, and decrease the risk of cardiovascular disease. “Increased autophagy [cleaning up damaged old cells], decreased inflammation, lowered oxidative stress, lowered insulin, activating adaptive cellular stress response, improving mitochondria and energy production, and supporting DNA repair are additional benefits,” Dr. Boham says.
Studies show that IF may also be beneficial for regulating reproductive hormones like testosterone and estrogen in both males and females.
Barriers to Entry
Can people just start fasting or do they need towork with a doctor? The short answer is yes. “I think basic IF or TRE, such as the 16:8 plan, is fine without a doctor or nutritionist on board,” Dr. Boham says. But I think people are more successful when they work with someone who has experience in this area.”
Anyone with a medical condition or a history of disordered eating should consult a medical professional before starting a fast to investigate how fasting may affect them, especially because fasting may affect medications. “Working with a nutritional professional can be extremely helpful in any sustainable weight loss program, including fasting, to make sure you are still getting all the nutrients you need to sustain the body’s basic functioning,” Kristal says.
Fasting is not a quick fix, and it’s not for everyone. In addition to people who have a history of disordered eating, Dr. Boham says fasting is contraindicated for people who are pregnant, breastfeeding, or under 18 years old. “People with type 1 diabetes should work with a nutritionist or doctor with experience in this arena,” Dr. Boham advises.
Last Word on Intermittent Fasting
“Fasts have been used across cultures for centuries, and not just for the physical benefits,” Kristal says, and for many people, IF is a game changer. While it’s undisputed that we need food for our bodies to have fuel to burn, most of us are eating too much too often. If you’ve ever felt an energy slump in the afternoon after eating a big lunch, you’ve experienced the amount of energy it takes to digest that burger or bowl of pasta. By limiting our eating hours—while also being mindful of what we eat—research shows we’ll have more energy, not less, and we’ll also be less prone to chronic disease.
This article appears in April 2024.
