A review of 22 studies involving around 2,000 people suggests that intermittent fasting is no more effective for weight loss than other approaches. Researchers say the key factor remains the total number of calories consumed.
Intermittent fasting has become one of the most popular strategies for losing weight, with approaches ranging from alternate-day fasting and the 5:2 diet to eating within a limited daily window.
But a new systematic review by Cochrane researchers suggests that, when it comes specifically to weight loss, none of these methods has a clear advantage over more conventional approaches.
The review examined 22 studies involving around 2,000 adults who were overweight or obese. Participants followed different forms of intermittent fasting, received dietary advice or, in some cases, made no significant dietary changes.
The researchers said the available evidence was not sufficient to determine the effects of intermittent fasting on adverse events, quality of life or some other health outcomes, including diabetes control and cholesterol levels.
On weight loss, however, the findings were more consistent.
The evidence suggests that simply changing when people eat does not lead to greater weight loss if their overall calorie intake remains unchanged.
Calories remain the key factor
Nutrition experts say intermittent fasting can work for some people, but mainly because it may make it easier to consume fewer calories overall.
The problem arises when people treat fasting as a way to compensate for unrestricted eating during their permitted hours. Restricting food for part of the day does not necessarily lead to weight loss if the calories are simply consumed later.
There may also be a risk of turning to less healthy foods after a prolonged period without eating.
Nutritionist and psychiatrist Stefano Erzegovesi said fasting can trigger powerful biological responses because the body interprets a lack of food as a potential threat to survival.
A relatively mild fasting period may allow the body to draw on stored energy, but prolonged or poorly managed restriction can have the opposite effect, particularly for people vulnerable to disordered eating.
Constantly thinking about food, worsening mood and episodes of overeating after fasting can make it harder to maintain a healthy relationship with eating and may negatively affect metabolic health.
For this reason, experts stress that intermittent fasting should not be viewed as a shortcut around calorie reduction.
For someone trying to lose weight, the overall objective remains the same: reducing total energy intake.
For some people, it may be easier to skip food for a day or restrict eating to a particular time window than to reduce portion sizes every day. But the mechanism behind weight loss remains the overall reduction in calories.
Eating earlier may be more important than fasting longer
While the evidence does not support intermittent fasting as a superior weight-loss method, the timing of meals may still matter for metabolic health.
Chronobiologist Roberto Manfredini, director of the Medical Clinic at the University of Ferrara, recommends aligning eating patterns with the body’s natural biological clock.
In practice, that means avoiding food late at night and, where possible, finishing dinner before around 8pm.
The body is less prepared to process energy from food during the night. Metabolism slows, body temperature falls as melatonin rises and the body’s hormonal signals change in preparation for sleep.
Insulin sensitivity also tends to decrease at night, while the liver becomes less efficient at processing fats. Eating sugary foods late in the evening can therefore produce a greater rise in blood glucose.
Over time, regularly eating late may contribute to weight gain and increase the risk of metabolic disorders, including diabetes.
According to Manfredini, at the same calorie intake, people may lose more weight when they consume those calories earlier in the day rather than late at night.
The natural overnight fast
This suggests that the most practical form of fasting may simply be the natural period between dinner and breakfast.
Keeping food within the body’s active hours could have benefits beyond weight management, including better blood glucose control, improved insulin sensitivity, lower blood lipid levels and reduced blood pressure.
A recent study from Northwestern University involving middle-aged people at moderate or high risk of cardiovascular and metabolic disease also examined the relationship between meal timing, sleep and cardiometabolic health.
The researchers found that aligning fasting with the sleep-wake cycle was feasible for most participants and was associated with improvements in several measures of cardiovascular health.
The approach was linked to reductions in heart rate and blood pressure, as well as better control of blood glucose during the day.
The findings point to a broader principle: rather than simply asking how long a person should go without eating, it may be more useful to consider when food is consumed in relation to the body’s biological clock.
For weight loss, however, the central message remains simpler. Intermittent fasting is not a magic solution. What matters most is how much energy is consumed overall — and, potentially, when those calories are eaten.
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