'Fasting diets' are all the rage for weight loss. But do they actually work?

Fasting diets are rising in popularity, but while early research shows fasting might afford some health benefits beyond weight loss, the science on the topic is still in its infancy, Julia Belluz reports for Vox.

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What counts as 'fasting'?

Fasting diets have some famous supporters, from musician Moby to actor Chris Pratt to New York Times economics columnist Paul Krugman. Fasting even made U.S. New & World Report's list of the 10 most googled diets of 2018.

But, Belluz explains, there are many types of fasting diets. Each fasting diet entails some form of calorie restriction for periods ranging from 12 hours to a few weeks—or even a few months. Some of the more popular fasting diets include intermittent fasting, which involves significantly reducing or eliminating calories at on certain days of the week, or periodic fasting, which involves restricting calories entirely for "long stretches" of a few days or weeks, according to Belluz.

Some fasting proponents argue that the diets bring humans more in touch with their ancestral roots, noting that humans were designed to run on very few or no calories for weeks or months, Belluz writes. In fact, some fasting diet supporters, such as Valter Longo, director of the Longevity Institute at the University of Southern California, say that "fasting can reset and rejuvenate the human body," by allowing the body to eliminate damaged cells.

But, according to Belluz, most studies on the benefits and effects of fasting diets are still "nascent and exploratory"—and little research has been conducted on humans.

Fasting may improve disease prevention (at least in the short term)

The majority of studies to date focus on how fasting might increase longevity and aide in disease prevention, Belluz reports.

Longo, who co-authored the first human trial to study whether fasting can reduce the risk factors for diseases like cancer, cardiovascular disease, and diabetes, said his research suggests fasting can help humans reverse disease. Belluz notes that Longo is "pretty dug in" on the topic: He authored a book about fasting and works at a company that sells fasting products, though he says he donates all of the proceeds from those businesses to health charities.

For the study, one group fasted for five days per week each month while the control group ate whatever they wanted. According to Belluz, "[t]he group that fasted lost weight (about seven pounds on average), lost some body fat, lowered their blood pressure, and decreased their IGF—1," a biomarker for cancer. 

But, there's a "catch," Belluz writes. Since the research was short-term, researchers still don't know what effects long-term fasting might have on disease. And other studies on the relationship between fasting diets and disease, like one that researched the effect of fasting on Type 2 diabetes, revealed "contradictory results."

In conclusion, when it comes to fasting diets and disease risk "the literature in humans is promising—though far from conclusive," Belluz writes.

Fasting can lead to weight loss, but not all the time

While research may focus on disease prevention and longevity, many fasting diet enthusiasts take part for the weight loss benefits, Belluz reports. Researchers believe fasting may result in weight loss because it requires a person to restrict their calorie intake for long periods of time, and a person's body begins to burn fat when they don't get enough calories for a prolonged period of time.

But research also shows that some people never lose weight on fasting diets, according to Belluz, and the people who do lose weight don't lose any more than they would on any other diet that requires calorie-restriction.

A systematic review published in 2018 observed randomized control trials of intermittent fasting and found that people who fasted lost between 4% and 8% of their bodyweight. However, the researchers also found participants likely would have lost the same amount of weight on any traditional diet that requires calorie-restriction, Belluz writes.

And a different 2018 review of fasting studies revealed another reason that people struggle to lose weight on fasting diets: The diets are hard to maintain.

The researchers wrote, "Dropout rates have been as high as 40%. Thus, despite the statistical significance of weight loss results, the clinical significance and practicality of sustaining an [intermittent fasting] regimen are questionable."

Debra Safer, a professor of psychiatry and behavioral sciences at Stanford University, also noted that some people, such as those with eating disorders, should avoid fasting diets because they might pose health risks.

"The research evidence generally shows that patients with eating disorders do best when they eat regular meals and snacks. Intermittent restriction of intake is often one of the behaviors that people with eating disorders engage in as part of their eating disorder," Safer said.

Similarly, Longo said that people who are "diabetic and tak[e] insulin," or people with "metabolic disorders" should avoid fasting as well.

There might be more questions than answers

Overall, researchers have yet to figure out "the best timing and frequency of eating for health," Belluz writes. While it's possible that fasting diets could be everything that its proponents claim, "[j]ust know that there are lots of question marks about the long-term health consequences" and potential benefits of fasting diets, Belluz writes (Belluz, Vox, 1/14).

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