Intermittent Fasting: Does the Fad Really Deliver Weight Loss?
Once hailed as a game-changer for shedding kilos and a favourite among Hollywood elites, intermittent fasting (IF) is now facing scrutiny. The latest comprehensive review of the evidence suggests that this popular diet trend might not be the magic bullet for weight loss it’s often made out to be.
Scientists involved in a major review, considered a benchmark for reliable medical evidence, have analysed data from 22 separate studies. These studies collectively involved nearly 2,000 adults, the majority of whom were either overweight or obese. The research pitted intermittent fasting against more traditional dieting methods, such as restricting calorie intake.
The findings paint a less-than-glowing picture for IF enthusiasts. The review concluded that restricting eating to specific windows each day did not result in significantly more weight loss compared to conventional diets, and surprisingly, even showed little difference to those who made no specific dietary changes.
On average, individuals following intermittent fasting plans lost approximately three percent of their body weight within a 12-month period. This figure falls below the five percent threshold that medical professionals generally consider to be clinically significant for meaningful weight loss.
The reasons behind IF’s apparent lack of superior weight loss efficacy remain somewhat unclear. However, previous research has proposed that individuals on IF diets might inadvertently compensate by consuming a similar number of calories to those on other diets, or they may reduce their physical activity levels.
While proponents of intermittent fasting tout benefits ranging from increased fat burning and accelerated weight loss to boosted energy levels and even extended lifespan, recent years have seen a rise in medical concerns. Some studies have begun to flag potential links between IF and an increased risk of conditions like colon cancer and type 2 diabetes.
Dr Luis Garegnani, a researcher at the Universidad Hospital Italiano de Buenos Aires and lead author of the recent review, stated, “Intermittent fasting just doesn’t seem to work for overweight or obese adults trying to lose weight.”
Understanding Intermittent Fasting
At its core, intermittent fasting involves strategically limiting caloric intake. This can be achieved by designating specific hours of the day for eating or by fasting on certain days of the week, with the primary aim of promoting weight loss and better managing eating habits.
Several popular variations of IF have gained traction:
- The 14:10 Plan: This involves eating only within a 10-hour window each day, with a 14-hour fasting period.
- The 5:2 Diet: This approach sees individuals eating normally for five days of the week and significantly restricting their calorie intake on the remaining two days.
The health halo surrounding intermittent fasting began to form in the 2010s, boosted by celebrities like Jennifer Aniston and Mark Wahlberg openly endorsing the plan. Today, it’s estimated that around one in ten Americans follow some form of intermittent fasting.
Jennifer Aniston, for instance, revealed her adherence to the diet in 2019, noting her practice of consuming only water in the mornings and delaying her first meal until midday. Mark Wahlberg, known for his disciplined lifestyle, has publicly stated that he confines his eating to the hours between 12 pm and 6 pm.
These findings emerge at a time when obesity remains a significant public health concern. Federal data indicates that approximately 40 percent of Americans are classified as obese or overweight. Globally, the World Health Organization reports that adult obesity has more than tripled since 1975, with 2.5 billion adults being overweight in 2022, including 890 million living with obesity.
Deeper Dive into the Cochrane Review
The extensive review conducted by scientists at the Cochrane Institute meticulously sifted through various databases, specifically seeking studies that compared participants undertaking intermittent fasting with those adhering to other dietary interventions.
The analysed trial encompassed 1,995 adults from diverse geographical regions, including Europe, North America, China, Australia, and South America. The review examined a spectrum of intermittent fasting approaches, including the well-known 5:2 diet.
Other dietary strategies included in the comparison were:
- Calorie Deficit Diets: Consuming fewer calories than the body expends.
- Low-Carbohydrate Diets: Reducing the intake of carbohydrates.
Researchers noted that only 22 studies met their stringent criteria for inclusion. This relatively small number was attributed to the fact that many studies on intermittent fasting are short-term and often suffer from methodological limitations, making it challenging to draw definitive conclusions.
Weight Loss Outcomes: What the Data Shows
In terms of weight loss, the review found that individuals following intermittent fasting lost, on average, about 0.33 percent more of their body weight than those on standard dieting methods. When compared to individuals who did not actively try to reduce their weight, those on intermittent fasting lost an average of 3.42 percent more of their body weight.
However, these differences were not statistically significant, leading researchers to conclude that intermittent fasting does not offer a substantial advantage for weight loss over standard dieting approaches.
The certainty of these results was rated as low by the researchers. This assessment was primarily due to the limitations inherent in the data, including a relatively small sample size spread across a vast geographical area. The scientists emphasised the need for more extensive and larger-scale studies to validate these findings.
Dr Eva Madrid, a researcher at Cochrane Evidence Synthesis Unit Iberoamerica and a senior author of the study, commented, “With the current evidence available, it’s hard to make a general recommendation. Doctors will need to take a case-by-case approach when advising an overweight adult on losing weight.”
Corroborating Evidence from Other Research
This latest Cochrane review aligns with findings from a separate study led by Harvard scientists, published in June 2025. That earlier research, which reviewed 99 studies, also concluded that intermittent fasting showed no significant superiority over regular dieting for weight loss.
The international research team behind the Harvard study described the benefits derived from intermittent fasting as “trivial” when compared to conventional diet plans. Their analysis indicated that neither time-restricted eating (such as an eight-hour eating window) nor the 5:2 fasting model offered a discernible advantage in weight loss.
Interestingly, the Harvard study did find that alternate-day fasting – where individuals fast for a full 24 hours on alternating days – did lead to more significant weight loss, with participants shedding an average of 1.29kg (2.84lbs) more. However, even this difference did not meet the clinically significant threshold of 2kg (4.4lbs) identified by the researchers.
This Harvard study analysed data from 99 randomised clinical trials involving 6,582 adults, with an average age of 45, comparing intermittent fasting with calorie-restriction diets.
Published in the British Medical Journal, the scientists noted that while their data suggested intermittent fasting was more effective than not dieting at all, it did not stand out when compared to basic calorie counting.
The researchers acknowledged that “minor differences were noted between some intermittent fasting diets and continuous energy restriction, with some benefit for an alternate day fasting strategy with weight loss in shorter duration trials.” They further stated that “All intermittent fasting strategies and continuous energy restriction diets showed a reduction in body weight when compared with an ad-libitum diet (allowing yourself to eat at any time). Of three intermittent fasting diets, alternate day fasting showed benefit in body weight reduction compared with continuous energy restriction.”
