A comprehensive new review has found that intermittent fasting (IF) delivers similar weight loss results as traditional calorie restriction (CR), offering individuals greater flexibility in choosing a diet strategy that suits their lifestyle.
Published on June 18 in The BMJ, the systematic review evaluated nearly 100 randomized clinical trials comparing IF and CR. The analysis concluded that both dietary approaches were equally effective not only in promoting weight loss but also in improving cardiometabolic health markers.
One form of intermittent fasting, known as whole-day fasting or the 5:2 diet, showed marginally greater weight loss compared to traditional calorie restriction. However, researchers emphasized that further long-term studies are needed to confirm this finding.
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Comparing Approaches: When You Eat vs. How Much You Eat
Intermittent fasting focuses on the timing of food intake, often cycling between periods of eating and fasting, rather than strictly limiting calories. In contrast, calorie restriction is centered around maintaining a specific daily caloric intake, with no limitations on when meals are consumed.
The review analyzed several popular IF protocols:
- Alternate-Day Fasting (ADF): Alternates between days of normal eating and days of fasting or significantly reduced calorie intake.
- Time-Restricted Eating (TRE): Limits eating to specific windows of time each day, typically 8 to 12 hours.
- Whole-Day Fasting (5:2 Diet): Involves two or three non-consecutive days of fasting per week, with normal eating on the remaining days.
Each of these IF methods, along with traditional CR, was shown to be more effective in promoting weight loss than an unrestricted, ad libitum diet.
Lifestyle Fit is Key to Long-Term Success
While alternate-day fasting was the only IF protocol to slightly outperform calorie restriction in terms of weight loss, the overall similarities across all methods underscore an important message: flexibility matters.
“This analysis suggests that all three approaches, for the most part, lead to similar weight losses,” said Dr. David B. Sarwer, director of the Center for Obesity Research and Education at Temple University and spokesperson for The Obesity Society. “This is why many of us believe that the best approach to losing weight is finding one that matches your lifestyle and is sustainable in the long term.”
Dr. Sarwer was not involved in the study.
Expanding the Toolbox for Weight Management
Intermittent fasting has surged in popularity over the past decade, yet evidence of its benefits—especially when compared to traditional calorie-controlled diets—has remained mixed. This latest analysis provides reassurance that individuals can choose from a wider array of effective options based on personal preference and adherence.
While the findings may temper enthusiasm among some proponents of IF, experts agree that the broader takeaway is a positive one: multiple diet strategies can be effective, and personalization is key to lasting success.
No One-Size-Fits-All Solution in Dieting, Study Finds
A new comprehensive review of 99 randomized clinical trials involving over 6,500 adults highlights a key takeaway in the ongoing debate over the most effective weight loss strategy: there is no universally “best” diet. Whether through intermittent fasting (IF) or traditional calorie restriction (CR), the outcomes were largely similar, suggesting that long-term success hinges on personal adherence and lifestyle compatibility.
Key Insights from the Research
The analysis, published in The BMJ, included trials ranging from 3 to 52 weeks, with an average duration of 12 weeks. The average age of participants was 45, and two-thirds were women. While a small fraction of subjects were considered metabolically healthy, the vast majority—around 90%—were living with conditions such as overweight, obesity, or type 1 or type 2 diabetes. The mean body mass index (BMI) was 31, falling within the clinical definition of obesity.
Both IF and CR were associated with modest weight loss when compared to unrestricted, ad libitum diets. Of the various IF protocols examined, alternate-day fasting (ADF) led to slightly more weight loss—around three additional pounds—than CR. However, experts noted that while this difference was statistically significant, it likely has minimal clinical relevance.
No Clear Winner Among Fasting Protocols
ADF marginally outperformed other IF methods, such as time-restricted eating (TRE) and whole-day fasting, though the differences in outcomes were not statistically significant. ADF also demonstrated small improvements in total and LDL cholesterol compared to TRE.
“There is no compelling evidence that there is a ‘best’ diet for weight loss,” said Dr. David B. Sarwer, Director of the Center for Obesity Research and Education at Temple University. “The best approach is one where the person can make small, yet impactful changes to their food choices, eating behaviors, and level of physical activity that cause minimal disruption to their lifestyle and that they can maintain for the long term.”
Who May Benefit Most from Intermittent Fasting?
Despite its popularity, the evidence supporting IF’s superiority remains inconclusive. However, studies have associated IF with a number of potential health benefits, including:
- Weight loss
- Improved insulin sensitivity
- Reduced inflammation
- Enhanced brain health
What remains uncertain is whether these benefits surpass those achieved through CR. A 2024 review concluded that both IF and CR resulted in similar outcomes across weight loss, cardiometabolic health, cancer markers, and neurocognitive measures. Notably, participants in IF trials tended to demonstrate better adherence during the study period.
A 2022 NEJM study echoed this, finding no significant advantage of TRE over CR in individuals with obesity.
However, more recent findings add nuance. In a National Institutes of Health (NIH)-funded clinical trial published in April 2025, participants on a 4:3 whole-day fasting schedule (four fasting days per week) lost 50% more weight than those on CR—7.6% vs. 5% of body weight after one year. This additional weight loss correlated with measurable improvements in blood pressure, cholesterol levels, and A1C.
Another 2025 study reported that participants maintained weight loss even after discontinuing a three-month TRE protocol—suggesting potential for long-term benefits.
The Downsides of Calorie Restriction and Fasting
While CR is a longstanding dietary intervention, it isn’t without drawbacks. Reported side effects include fatigue, nutritional deficiencies, and mood-related issues such as depression. Similarly, IF isn’t suitable for everyone.
Dr. Sun Kim, Associate Professor of Endocrinology, Gerontology, and Metabolism at Stanford Medicine, who was not involved in the review, warns that IF may require insulin adjustments for people with diabetes and could be risky for:
- Individuals over age 65
- Those with low blood pressure
- Pregnant or nursing women
- Anyone with a history of disordered eating
“Fasting diets are easier to follow because the rules are fairly simple compared to starting something like a Mediterranean diet,” Dr. Kim noted. “For example, individuals who tend to snack heavily at night may benefit from time-restricted eating.”
Adherence Determines Long-Term Success
Perhaps the most crucial insight from the study was related to adherence. Trials lasting under 24 weeks maintained participant adherence rates above 80%. In contrast, trials extending beyond a year saw a sharp decline. In one whole-day fasting trial, adherence dropped from 74% at six weeks to just 22% at 52 weeks.
These findings reinforce the idea that effective weight loss strategies must be individualized and sustainable.
“I wish there were an easy solution to weight loss,” said Dr. Kim. “In our society, there are many forces that push individuals toward weight gain. I always tell my patients to focus on lifestyle changes they can maintain for the long haul.”
Dr. Sarwer echoed that sentiment: “I’m more likely to suggest that people reduce the number of days they eat ice cream, or reduce portion sizes, rather than cut it out entirely.”
Frequently Asked Questions
What does “No One-Size-Fits-All” mean in the context of dieting?
It means there’s no single diet plan that works universally for everyone. Different people respond differently to various diets like intermittent fasting or calorie restriction.
What was the main finding of the study?
The study found that intermittent fasting (IF) and calorie restriction (CR) resulted in similar weight loss and health benefits, suggesting the most effective diet is the one a person can stick to.
How many trials and participants were included in the review?
The systematic review analyzed 99 randomized clinical trials involving over 6,500 adults.
Was any diet shown to be more effective than others?
Alternate-day fasting (a type of IF) showed slightly more weight loss than CR, but the difference was not considered clinically significant.
Who should be cautious about intermittent fasting?
People over 65, pregnant or nursing women, those with diabetes (especially on insulin), and individuals with a history of disordered eating should consult a doctor before starting IF.
What’s more important: diet type or adherence?
Adherence is more important. The best diet is the one you can follow long term without major disruptions to your lifestyle.
Are there side effects to calorie restriction?
Yes, long-term CR can lead to fatigue, nutritional deficiencies, and even mood issues like depression.
Conclusion
The latest research reinforces a critical truth in weight management: there is no universally “best” diet. Both intermittent fasting (IF) and calorie restriction (CR) are effective strategies for weight loss and improving cardiometabolic health, but their success largely depends on individual preferences and long-term adherence. While some protocols like alternate-day fasting may show slightly greater results, the differences are minor.
Ultimately, the best diet is the one that fits your lifestyle, is sustainable over time, and promotes overall well-being without causing harm. Personalization, consistency, and small, manageable changes remain the keys to lasting success.