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Debunking the myths of interval fasting
Last reviewed: 02.07.2025

In a new paper, researchers from the University of Illinois at Chicago debunk four common myths about the safety of intermittent fasting.
Intermittent fasting is becoming an increasingly popular method for losing weight without having to count calories. A large number of studies have shown it to be safe. However, several myths about fasting have spread among clinicians, journalists, and the general public: that it can lead to poor nutrition or muscle loss, cause eating disorders, or reduce sex hormone levels.
In a new commentary in the journal Nature Reviews Endocrinology, UIC researchers debunk each of these myths. They base their findings on clinical studies, some of which they conducted themselves and some of which were done by other scientists.
“I’ve been studying intermittent fasting for 20 years, and I’m constantly asked if these diets are safe,” said lead author Krista Varady, a professor of kinesiology and nutrition at UIC. “There’s a lot of misinformation out there. But these ideas aren’t based on science; they’re just personal opinions.”
There are two main types of intermittent fasting. In alternate-day fasting, people alternate between very low-calorie days and days when they eat whatever they want. In time-restricted fasting, people eat during a four- to 10-hour window each day and then fast for the rest of the day. Researchers conclude that both types of fasting are safe, despite popular myths.
Here are their findings:
Intermittent fasting does not lead to poor nutrition
The researchers point to studies showing that intake of sugar, saturated fat, cholesterol, fiber, sodium, and caffeine does not change during fasting compared to before fasting. The percentage of energy consumed as carbohydrates, protein, and fat also does not change.
Intermittent fasting does not cause eating disorders
None of the studies found that fasting caused participants to develop an eating disorder. However, all of the studies excluded participants with a history of eating disorders, and the researchers recommend that those with a history of such disorders not attempt intermittent fasting. They also urge pediatricians to be cautious when monitoring obese teens if they begin fasting, as this group is at high risk for developing eating disorders.
Intermittent fasting does not cause excessive muscle loss
Research shows that people lose the same amount of muscle mass whether they lose weight through fasting or another diet. In both cases, strength training and increased protein intake can offset the loss of muscle mass.
Intermittent fasting does not affect sex hormones
Despite concerns about fertility and libido, neither estrogen, testosterone, nor other related hormones were affected by fasting, the researchers noted.
The paper's co-authors are Vanessa Oddo and Sofia Cienfuegos of UIC, and Shuhao Lin, formerly of UIC and now at the Mayo Clinic.