Popular intermittent fasting diets may not deliver the health benefits many expect

Time-restricted eating has gained popularity in recent years, but a recent study suggests that intermittent fasting — while effective for weight loss — might not live up to the hype in terms of wider benefits.

The small German study found that participants who were placed on two different time-restricted eating schedules lost weight, but experienced no improvement in blood glucose, blood pressure, cholesterol or other key cardiometabolic markers.

The participants included 31 overweight or obese women. One group ate between 8 a.m. and 4 p.m. and the other group ate between 1 p.m. and 9 p.m. for a two-week period, while maintaining their typical caloric intake, according to a press release.

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The findings, which were published in the journal Science Translational Medicine, suggest that the widely touted cardiometabolic benefits of intermittent fasting may be a result of eating fewer calories rather than meal timing, the researchers say.

The participants also showed a shift in their circadian rhythms (sleep/wake cycles) when they were placed on the time-restricted eating schedules, but the associated health impacts are not known.

The study did have some limitations. Some researchers have cast doubt on the significance of the study due to its small size.

"It is severely underpowered to detect any difference, considering how gentle the intervention is," Dr. Dr. Jason Fung, a Canadian physician, author and researcher, told Fox News Digital. He also noted that the participants were fasting for 16 hours a day instead of the normal 12 to 14 hours.

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Lauren Harris-Pincus, a registered dietitian nutritionist in New Jersey, agreed that the findings could be due to the fact that there was no intentional caloric restriction, and reiterated that the sample size is "quite small." 

"As a registered dietitian, I only recommend time-restricted eating when it is carefully planned and shifted earlier within the day," Harris-Pincus, who was not involved in the study, told Fox News Digital.

"Only one in 10 Americans consumes the recommended number of fruits and veggies, and 93% miss the mark on fiber goals. Restricting an eating window necessitates more careful meal planning to ensure adequate intake of macro- and micronutrients."

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The expert also cautioned that skipping breakfast to enable a later eating window may result in lower intake of the "nutrients of concern" in the American diet, including calcium, potassium, fiber and vitamin D. 

Looking ahead, the researchers said more studies are needed to explore the effects of time-restricted eating over longer time periods. It also remains to be seen how the combination of caloric restriction and time-restricted eating may affect outcomes. Future research could also explore how different populations may respond.

Dr. Daryl Gioffre, a gut health specialist and celebrity nutritionist in New York, noted that the study didn’t account for critical factors like chronic stress, sleep quality, medications, hormone status and baseline metabolic health.

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"All of these can significantly blunt fat loss and cardiometabolic improvements," Gioffre, who also was not involved in the research, told Fox News Digital.

"Cortisol, the body’s primary stress hormone, is naturally highest in the morning, which overlaps with one of the fasting windows studied," he went on. "If stress is elevated, cortisol alone can block fat burning, disrupt blood sugar regulation, and mask cardiovascular improvements, regardless of calorie intake or eating window."

Gioffre did agree, however, that growing research shows intermittent fasting — when done correctly and sustained over time — can improve insulin regulation, reduce inflammation, support fat loss and contribute to better cardiovascular health.

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"These are outcomes that simply cannot be captured in a short, stress-blind study like this," he added.

Fox News Digital reached out to the researchers for comment.