Calories count more than timing in intermittent fasting, research finds
Study shows total calories matter more than eating schedule in intermittent fasting
Intermittent fasting has become very popular in recent years. People limit their eating to a certain number of hours each day, hoping it will improve metabolism, help with weight management, and support long-term health. Many people say it works for them, but scientific evidence has been mixed.
A new study now suggests that what really matters is not the timing of meals, but the total number of calories consumed. Researchers from the German Institute of Human Nutrition Potsdam-Rehbruecke and Charité Universitätsmedizin Berlin found that when calorie intake remains the same, eating within a specific time window does not improve metabolic or heart health. The study was published in Science Translational Medicine.
Time-restricted eating, or TRE, is the form of intermittent fasting examined in this study. TRE involves eating all meals within a set window of 8 to 10 hours, followed by a fasting period of at least 14 hours. Previous studies, especially in animals, suggested that TRE could protect against obesity and metabolic problems. Early human research also showed benefits such as better insulin sensitivity, improved blood sugar control, healthier cholesterol levels, and modest weight loss. However, researchers wanted to know whether these benefits were due to meal timing itself or because people naturally ended up eating fewer calories during these time windows.
How the study was conducted and what it found
The study, called the ChronoFast trial, was led by Prof Olga Ramich, Head of Molecular Metabolism and Precision Nutrition at DIfE and Professor at Charité. It included 31 women who were overweight or obese.
Each participant followed two different eating schedules for two weeks at a time. One schedule involved eating early, from 8 a.m. to 4 p.m. The second schedule involved eating later, from 1 p.m. to 9 p.m. The researchers made sure that calorie intake and nutrient content remained almost identical in both schedules. This allowed them to focus solely on the impact of meal timing.
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Participants underwent several detailed tests during the study. Blood samples were collected to check glucose and fat levels. Oral glucose tolerance tests were used to see how the body handled sugar. Continuous glucose monitoring tracked blood sugar over 24 hours. Physical activity was recorded with motion sensors, and every meal was carefully logged.
The results showed that calorie intake was more important than meal timing. Prof Ramich explained, “Our results suggest that the health benefits observed in earlier studies were likely due to unintended calorie reduction, rather than the shortened eating period itself.” In other words, the timing of meals alone did not improve metabolism or heart health.
Analysis showed no significant differences in insulin sensitivity, blood sugar levels, blood fats, or markers of inflammation between the early and late eating schedules. Cardiometabolic health indicators remained largely unchanged, meaning that the risk of conditions like diabetes or heart disease was not affected by when participants ate their meals.
However, meal timing did have a small effect on the body’s internal clock, known as circadian rhythm. Using the BodyTime assay developed by Prof Achim Kramer, researchers found that the late eating schedule delayed participants’ circadian rhythms by about 40 minutes compared to early eating. This means that while meal timing may influence biological clocks, it does not appear to change metabolic health directly.
The study’s findings suggest that people who practice intermittent fasting may benefit more from reducing overall calories rather than focusing strictly on eating within certain hours. For weight loss and metabolic health, energy balance—calories in versus calories out—remains the key factor.
Experts note that intermittent fasting can still be useful if it naturally leads to fewer calories consumed. For some people, having a restricted eating window helps control portion sizes or prevents late-night snacking, which can indirectly reduce calorie intake. But if someone eats the same number of calories, the timing alone does not provide extra health benefits.
The study also highlights that meal timing can act as a cue for the body’s biological rhythm. Eating earlier or later can shift circadian patterns slightly, which might affect sleep-wake cycles and hormone release, but it does not directly improve metabolic health.
In conclusion, while intermittent fasting remains popular, this study suggests that the main factor for metabolic and cardiovascular health is total calorie intake rather than the timing of meals. People looking to lose weight or improve metabolism should focus on balanced diets and controlling calories, and they may choose an eating schedule that fits their lifestyle rather than strictly following specific time windows.
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