Intermittent fasting (IF) — a dietary approach that restricts daily eating to specific time windows—has gained widespread popularity as a weight-loss strategy. However, health experts consistently stress that its effectiveness ultimately depends on maintaining a calorie deficit, much like conventional weight-loss plans. Several variations of IF exist, including the widely practiced 16:8 method, in which all meals are consumed within an eight-hour window followed by a 16-hour fast. Another common approach is the 5:2 diet, which involves two days of significantly reduced calorie intake and five days of normal eating. Time-restricted eating (TRE), a more specific form of IF, limits food consumption to a daily window of no more than 10 hours, ensuring a fasting period of at least 14 hours.

Despite its growing appeal, new research challenges the assumption that time-restricted eating automatically improves metabolic or cardiovascular health. A recent study, known as ChronoFast, conducted by the German Institute of Human Nutrition Potsdam-Rehbruecke in collaboration with Charité, suggests that individuals must still actively monitor calorie intake to achieve meaningful health benefits, rather than relying solely on shortened eating windows.

The ChronoFast study followed 31 overweight or obese women, each of whom completed two different TRE schedules over two-week periods. One phase involved early time-restricted eating, with meals consumed between 8 a.m. and 4 p.m., while the other used a later schedule, from 1 p.m. to 9 p.m. Importantly, all meals were carefully standardized to ensure nearly identical calorie and nutrient content across both schedules. Researchers collected blood samples during four clinical visits and examined changes in the body’s internal clock—the circadian rhythm—using isolated cells.

The findings, published in Science Translational Medicine, revealed that while eating times can alter circadian rhythms, these shifts had minimal physiological impact. The study found no clinically significant changes in insulin sensitivity, blood glucose levels, blood lipids, or inflammatory markers. Prof. Olga Ramich, the study’s lead researcher, explained that although circadian rhythms influence sleep-wake cycles and metabolic processes, weight loss ultimately comes down to energy balance. “Those who want to lose weight or improve their metabolism should pay attention not only to their clock, but also to their energy balance,” she said. Ramich added that the health benefits reported in earlier TRE studies were likely the result of reduced calorie intake rather than the restricted eating window itself.

The research team also called for further studies to explore how individual factors—such as chronotype, which reflects a person’s natural pattern of alertness, and genetic differences—may affect responses to different eating schedules. Intermittent fasting has received considerable public attention since rising to prominence in the early 2010s, aided by endorsements from celebrities including Jennifer Aniston, Halle Berry, Chris Pratt, and Kourtney Kardashian.

Despite numerous studies supporting its potential benefits, experts remain divided on the long-term effectiveness and safety of intermittent fasting. Advocates cite advantages such as weight loss, a reduced risk of type 2 diabetes, and improvements in gut health. Critics, however, argue that many people compensate for fasting periods by consuming large quantities of food in a short time, undermining the calorie deficit required for weight loss. Some researchers have even warned that intermittent fasting could increase the risk of strokes, heart attacks, or premature death. These concerns stand in contrast to earlier animal studies, which suggested that limiting daily eating periods could improve heart health and reduce obesity.