Weight loss trends come and go as quickly as the DoorDash delivery guy, but there is one exceptional diet that doesn’t seem to be going anywhere, and that’s intermittent fasting. Its benefits go far beyond the simple weight loss other diets promise, and current research has just scratched the surface, leaving much more to learn. Intermittent fasting has been linked to better immunity and lower disease rates because foregoing food for a short period has been shown to help the body's immune system fight off infections. Studies also show it may help stop the clock on aging, fend off obesity, and lead to longer lifespans.

The most popular type of intermittent fasting is time-restricted eating, or only eating within an 8-to-10 hour window each day. Another well-publicized alternative method is known as alternate-day fasting, which involves fasting for longer windows of time, such as 24 hours or even 36 hours, followed by periods of unrestricted eating.

What research shows about alternate-day fasting

It appears to work for weight loss and disease prevention, according to some scientific studies. But is alternate-day fasting practical and effective for the long term compared to typical calorie-restricted diets?

We consulted the research and nutrition experts for a holistic answer. Here, you’ll find an explanation of what clinical trials have shown, plus practical advice from Skylar Griggs, MS, RD, LDN, a registered dietitian and owner of Newbury Street Nutrition as well as lead dietitian for the preventive cardiology division at Children’s Hospital Boston; and Jill Edwards, MS, CEP, Director of Education for the T. Colin Campbell Center for Nutrition Studies.

What Is Alternate-Day Fasting?

The alternate-day method of intermittent fasting allows for unrestricted eating one day, or “feasting,” followed by a full day of fasting. On fasting days, advocates for this style of intermittent fasting suggest consuming only 25 percent of your normal caloric intake, rather than foregoing food altogether. So if your normal food intake amounts to 2,000 calories a day, you would eat 500 calories on fasting days, according to this plan.

Comparing alternate-day fasting with calorie-restricting diets, studies have suggested that they work equally well for weight loss since at the end of the week you've had approximately the same number of calories on each. People on an alternate-day fasting diet eat about one-third fewer calories overall for the week, which is comparable to a calorie-restricted diet.

On an alternate-day fasting diet, you would eat about 9,500 calories over the course of a week, or about 1,000 less than if you ate 1,500 calories per day on a calorie-restricted diet. This is significantly less than the standard 2,000 calorie-a-day intake – which amounts to 14,000 calories over the course of a week – and would result in losing between 1 and 2 pounds a week (since a pound is equal to 3,500 calories).

Proponents of this diet method tout the freedom it allows on feasting days: “Diet only half the time!” proclaims the subtitle on one popular book on the subject. “Delay, don’t deny,” declares another. Covers of these books feature images of processed and animal foods, like donuts and burgers.

There are no widely accepted guidelines on what to eat on feasting days, and this is certainly a big part of the appeal for many who try it: The reward for fasting is feasting! Still, if weight loss is a goal, it stands to reason that choosing plant-based whole foods like nutritious fruits and vegetables, whole grains and legumes, nuts, and seeds would be much more beneficial than processed junk, and lead to better health as well.

The Alternate-Day Fasting Schedule

Many people will begin their feasting day by eating soon after waking up, then continuing to eat as they wish through bedtime. Others might fast from after dinner one day — say, from 7 p.m. on Sunday — and then break their fast for a 7 p.m. with dinner the following day, in this example, Monday evening. They would then have a feasting day on Tuesday, through 7 p.m., and then fast on Wednesday.

People can plan their alternate-day fasting schedule in advance and decide which days of the week they choose as fasting or feasting days. A fasting day might include nothing more than a light lunch and an early modest dinner, so it’s better to plan to feast on days that include social engagements.

If this sounds like something you would like to try, there is science behind why and how it works. (If not, you're not the only one who finds the schedule unappealing, studies show.)

Alternate-Day Fasting Results

Research has shown that animals placed on a variety of fasting regimens, including alternate-day fasting, experience extraordinary health benefits, including longer life spans, and slowed or reversed signs of aging. They also benefit from a reduced incidence of disease and other negative health outcomes, including diabetes, cancer, high cholesterol, cardiovascular disease, neurodegenerative diseases, and obesity.

While human research is still emerging, a 2017 randomized controlled trial study on obese adults was less optimistic than the animal studies, showing the subjects who tried alternate-day fasting had the same amount of weight loss as the group assigned to a traditional calorie-restricted diet. One note: The people in the alternate-day fasting group did not adhere to the prescribed calorie count, eating more than the recommended amount on fasting days, and also less than they were supposed to eat on feasting days. The dropout rate was also higher in the alternate-day fasting group (38 percent) than the calorie-restricted group (29 percent), indicating it's harder to sustain this type of dieting.

A more recent randomized controlled trial study on alternate-day fasting in healthy humans showed more positive results. The 2019 report showed a number of beneficial outcomes for the alternate-day fasting group compared to the control group.

The benefits of alternate-day fasting included: Body fat loss (particularly around the belly); improved cardiovascular health; reduced levels of an age-associated inflammatory marker; and lower levels of LDL (or so-called "bad") cholesterol.

Barriers to Intermittent Fasting Diets

So, with promising initial findings, why isn’t everyone fasting? One perspective is quite simply that it’s hard to go without food for large chunks of time. Unlike a lab animal, most humans aren’t fed restricted rations on a controlled schedule. We also don’t live in a carefully monitored biodome, and people on a Western diet are used to eating three meals a day plus snacks. There's little support or precedence for eating differently.

But for people trying to lose weight, more traditional calorie-restricted diets pose their own challenges. Our bodies haven’t caught up to the evolution of our food systems — we’re still hard-wired to prefer calorie-dense foods such as sweets and high-fat foods, which would have been a rare and valuable find for our foraging ancestors but are now readily available at every drive-through, food court, vending machine and grocery store.

Why People Choose Alternate-Day Fasting

This biologically-ingrained urge to indulge is what drives many dieters to try alternate-day fasting, since it allows them to eat with abandon several days a week. This kind of freedom surely comes as a relief for anyone who is tired of constantly counting carbs or calories.

Plus, the potential benefits of slowing the aging process, improving cardiovascular health, and slashing LDL cholesterol while helping the immune system fight off potential infections likely attract a few takers too.

What Experts' Say About Alternate-Day Fasting in Practice

The Registered Dietitian’s Take

Skylar Griggs, MS, RD, has counseled thousands of clients on their nutrition and weight loss goals over the course of her career, but she has not recommended alternate-day — or any style of fasting — to any of them, for a few reasons.

First, the majority of the studies conducted on alternate-day fasting so far have been on animals. And for those studies done on humans, weight loss has not appeared to be significantly different than for people following standard calorie-restricted diets. “Weight loss and weight management are the primary reasons people come to me,” she said, “And the research on weight loss is fairly limited.” Second, people show poor long-term adherence to “extreme diets,” Griggs’ has found, and she puts alternate-day fasting in this category.

“Any diet that is extreme and not approachable is hard to follow,” said Griggs. “Any diet you can’t do 80/20 [80 percent healthy, 20 percent wiggle room] is not sustainable. When the plan is a bit malleable – when it can bend a little bit – people are more likely to stay with it.”

Alternating between feast and famine days could also have negative effects on the metabolism, Griggs says, ultimately inhibiting weight loss and causing other health issues.

“I think any time you restrict yourself down to 500 calories, you’re just creating a disaster," she warns.  "The body is so hungry, it’s likely to cause a decrease in metabolism and your body to go into starvation mode,” Griggs said. “You’ll be more likely to overdo it the next day, and then with binge-type days, over time, triglycerides and blood pressure can become elevated.”

The “black-and-white” thinking that alternate-day fasting encourages can be worrisome for people prone to eating disorders, says Griggs, who served as the lead outpatient dietician for Renfrew Center of New Jersey, an outpatient center for women with eating disorders, “It follows a pretty typical eating disorder pattern: Restrict. Binge. Restrict again."

Overall, Griggs advised that people seeking to lose weight should stick to a more traditional plan and avoid going more than 4-5 hours without food during the day to keep the metabolism chugging along and avoid late-night eating since that's when people are most likely to overeat.

“I think a lot of people are looking for the shiny new thing they think is going to be the answer," Griggs said. "But the things that are good for your health are not usually super sexy.” Instead, she advises: “Eat fruits. Eat vegetables. Increase your fiber intake. Include healthy fats. Eat consistently during the day. These are not going to be on the cover of a magazine, but they’re definitely good for your health.”

For those still bent on giving alternate-day fasting a try, Griggs has this advice: “Whatever [diet] you decide to do, getting a dietician looped in is super important. Get the support of someone who is based in science and has gone through a lot of school, training, accredited programs, and hospital internships. Don’t just go to, say, your gym for advice.”

The Plant-Based Nutrition Educator’s Take

Alternate-day fasting style is not the number-one choice of another respected expert,Jill Edwards, MS, CEP of the T. Colin Campbell Center for Nutrition. For Edwards, the main objection is primarily for practical reasons.

“Research has shown that it is not at all sustainable. Life happens. You have a luncheon, your kids have a birthday party. If you’re only eating 500 calories a day, it makes it hard to sit down for dinner with your family,” Edwards said. She has a personal friend who'd tried the diet style.

“I’d ask her to get lunch, and she’d say, ‘I can’t, this is not my eating day,’” Edwards said. “I think she lasted two weeks.” The same friend then switched over to the time-restricted version of intermittent fasting, which requires eating — every day — within an 8- or 10-, or even 12-hour window. She had “great success” with this approach, which is the same type of intermittent fasting that Edwards herself practices and advocates.

More people would be medically eligible to practice this eating pattern as opposed to alternate-day fasting, Edwards notes, because of the comparatively shorter periods of fasting. “With time-restricted eating, you get all the benefits of intermittent fasting, but it’s much more manageable. You’re eating according to your body’s internal clock, your circadian rhythms, and keeping the same cycle every day,” she said.

“Your body utilizes more fat stores in the morning as its way of planning to get through the day without the need to eat through the night. And then on the other end of that, you automatically cut out late-night eating, which is what sabotages your circadian rhythms, and according to research, leads to weight gain.”

In other words, because people practicing the time-restricted version of intermittent fasting will eat, say, only between 11 a.m. and 7 p.m., they strategically use the body’s tendency to burn fat stores in the morning, and also shut down snacking at night, which is the time people generally go off the rails with their calorie intake and make poor food choices.

“There is a magic number. We should not consume any calories — not one piece of fruit or a cracker — after 7 p.m. That way you have less gastrointestinal distress, higher sleep quality, and better weight management,” Edwards said (though she personally makes exceptions on the weekends).

For people who want to make a go of the alternate-day fasting diet anyway, Edwards advised that they choose their “feast day” foods with care. “You want to make the most of the calories and choose nutrient-dense foods on the days you’re eating a lot. Don’t stuff your face with calorically dense food that isn’t nutrient-dense,” she said.

As far as whether the health benefits of fasting would be compromised if one binged on junk food during their “feast day,” Edwards said the jury was out. “It’s hard to say because fasting is very powerful. But to me, it’s like smoking a cigarette and having an orange afterward. The orange is going to help scavenge some of the free radicals, but not all of them. So my thing is, don’t smoke the cigarette,” she said.

For the fasting-curious, Edwards pointed to the TrueNorth Health Center, a facility that specializes in medically supervised water fasting, as a resource for many well-documented case studies of the powerful effects of fasting on health. And, like Griggs, Edwards advised anyone considering trying alternate-day fasting to consult a medical professional first.

The Expert's Take on Alternate-Day Fasting

While experts don’t all agree on whether any type of intermittent fasting is to be advised, recent studies and the two above-featured nutrition experts all align on at least five points when it comes to alternate-day fasting.

The 5 main things to know about alternate-day fasting, from experts

  • Adherence: Alternate-day fasting is hard to comply with long-term because fasting days may conflict with social obligations and norms (birthday parties, family dinners, etc.) that are common in the western world.
  • Circadian Rhythms: Drastically altering calorie intake and meal timing from one day to the next runs counter to the body’s natural rhythms. Time-restricted eating or more traditional calorie-restricted eating plans might align better with the body’s internal clock.
  • Nutrition: What you eat — not just when — is important. Binge eating unhealthy foods every other day would likely have negative health implications, even when combined with the potential benefits of intermittent fasting.
  • Research: While data from animal studies is more plentiful, research on the effects of intermittent fasting, including alternate-day fasting, on humans is still emerging.
  • Safety: Because it requires extreme calorie restriction on fasting days, alternate-day fasting is a diet plan you should discuss with a health professional before beginning it. People with a history of eating disorders should avoid alternate-day fasting and other fasting diets.

Bottom Line: Alternate-day fasting is hard to sustain and may not work better for weight loss than restricting calories or less extreme intermittent fasting methods.

Check with your dietician or medical expert before you embark on any new diet. If you decide to try alternate-day fasting (and you’re medically cleared to do so), choosing foods that are mostly plant-based and healthy on “feast days,” rather than processed foods high in unhealthy saturated fat and added sugar, is likely to enhance your results.

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