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Alternate-Day Fasting Means Avoiding Food for 36 Hours. Is That Healthy?


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Researchers are learning more about how periodic fasting can affect your health. Getty Images
  • Intermittent fasting is on the rise, but researchers are still trying to understand how it can affect your health.
  • A new study focused on alternate-day fasting found it can help decrease belly fat and inflammation-related biomarkers.
  • But experts point out this is a more extreme diet. More research is needed before it can be recommended.

While we typically eat three meals every day, a recent clinical trial finds that for some people, completely skipping a day may have some health benefits.

Intermittent fasting is a general term for cycling between periods of not eating and eating over a set time period.

Now, new research has examined the health impact of one type of intermittent fasting called alternate-day fasting (ADF).

“Alternate-day fasting tends to include both regular food intake alternating with full fasting, meaning no food intake at all, or a significantly reduced intake of about 500 calories,” Dr. Elizabeth Lowden, a bariatric endocrinologist at the Northwestern Medicine Metabolic Health and Surgical Weight Loss Center at Delnor Hospital in Illinois, told Healthline.

This largest study of its kind looked at the effects of strict ADF in healthy people. Participants alternated not eating for 36 hours with 12 hours of eating as much as they wanted.

The findings were published Tuesday in the journal Cell Metabolism.

“Strict ADF is one of the most extreme diet interventions, and it has not been sufficiently investigated within randomized controlled trials,” said Frank Madeo, study author and professor of the Institute of Molecular Biosciences at Karl-Franzens University of Graz in Austria, in a statement.

Madeo explains they looked at a wide range of markers to see the effects of the diet.

“We aimed to explore a broad range of parameters, from physiological to molecular measures,” he explained. “If ADF and other dietary interventions differ in their physiological and molecular effects, complex studies are needed in humans that compare different diets.”

This was a randomized controlled trial with 60 participants enrolled for 4 weeks. They were randomized to either ADF or a control group that didn’t do ADF. Participants of both groups were healthy and of normal weight.

The ADF group was carefully monitored by glucose testing to ensure they weren’t eating at all on fasting days.

Participants also kept food diaries to document fasting days. They regularly went to a research facility, where they were instructed to either follow ADF or their usual diet; otherwise, they followed their normal, everyday routines.

Researchers also studied 30 people who already did 6 months of strict ADF before this study’s enrollment. They were compared to healthy people with no prior fasting experience. For this group, the main focus was examining the long-term safety of this intervention.

By the end of the study period, the ADF group did experience numerous benefits, some of which are related to longer life span.

They included:

  • reduced belly fat and overall weight
  • increased ketone bodies (produced by burning fat), even on non-fasting days, which are known to promote health
  • reduced levels of a biological marker associated with inflammation and age-associated disease
  • lowered cholesterol levels

An all-day fast may sound intimidating, but Lowden explains you may not need to be so strict to see results, since calorie reduction is the key to weight loss.

“Fasting can mean different things to different people,” Lowden said. “Some drink only water and black coffee. Some include bone broth. And some even take in up to 500 calories per ‘fasting’ day.”

She points out that weight loss depends on being energy deficit. As a result, “All of these methods have the potential to reduce overall weekly intake and lead to weight loss success,” she said.

Although researchers continue to evaluate the benefits of intermittent fasting, Lowden says both calorie restriction and intermittent fasting have been found to be equally effective in terms of improvement in weight and metabolic risk factors.

“Different people may find it easier to do one or the other,” she said. “Because we know weight loss success is at least in part dependent on ability to be compliant with lifestyle changes, it’s important to find the change each person finds most sustainable.”

The current study involved fasting for a total of 36 hours, a pretty long stretch to go without eating. But earlier evidence shows a less extreme version of intermittent fasting can help.

A previous study published in 2013 examined the results of a shorter fasting period: 24 hours.

These researchers looked at 16 people who fasted compared to another 16 who didn’t over a 3-month period. Interestingly, the participants who fasted were allowed to eat 25 percent of their daily calorie requirement on “fasting” days.

Similar to the most recent clinical trial, at the end of the study, the fasting group showed reduced weight, body fat, and improved markers for cardiovascular disease risk, like lower cholesterol levels.

But Tammy Beasley, RDN, CEDRD, CSSD, LD, vice president of Clinical Nutrition Services at Alsana: An Eating Recovery Community, warns that physicians need to be careful when looking at intermittent fasting–type eating plans, since they can mimic symptoms of disordered eating.

“When the guidelines for intermittent fasting include several of the same symptoms of disordered eating, such as eating within short, restricted time frames and consuming limited food groups within a certain calorie range, it’s difficult to support IF as a worthwhile or ultimately safe eating pattern,” Beasley said.

While this evidence is promising, researchers say they don’t recommend ADF as a general nutrition scheme for everybody.

“We feel that it is a good regime for some months for obese people to cut weight, or it might even be a useful clinical intervention in diseases driven by inflammation,” Madeo said in the statement.

Madeo further says research is needed before physicians start to widely recommend such a strict diet.

“Additionally, we advise people not to fast if they have a viral infection, because the immune system probably requires immediate energy to fight viruses. Hence, it is important to consult a doctor before any harsh dietary regime is undertaken,” Madeo cautioned.

Beasley agrees.

“Although intermittent fasting may be a hot trend and doesn’t appear to be losing its fascination, living ‘life in the fasting lane’ is not without risk,” she said.

Beasley explains that trying an extreme diet can affect metabolic systems in people differently. Some people may not be bothered by periods of fasting. Others may not react well to such a long time of fasting and may not stay on the diet.

“The human body is complex, and the metabolic functions that sustain life at its best depend on consistent fuel during both waking and sleeping hours,” she said.

The latest research into alternate-day fasting finds significant health benefits, including reduced belly fat, body weight, and cholesterol levels.

According to experts, it’s not necessary to stop eating completely on fast days or fast for a long period to see benefits.

Both researchers and experts agree that alternate-day fasting is an extreme intervention. It may not be the best diet method for long-term health.



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