Intermittent Fasting May Be as Effective as Calorie Restriction for Weight Loss
One form of intermittent fasting, known as whole-day fasting, did yield slightly more weight loss than caloric restriction.
Experts say that the similar results across different diet protocols mean individuals trying to lose weight have more options available to suit their lifestyle.
New research suggests that intermittent fasting is as effective as calorie-restricted diets for weight loss and cardiometabolic risk factors.
A systematic review of nearly 100 randomized clinical trials involving intermittent fasting (IF) and traditional calorie restriction (CR) found that both approaches yielded similar health benefits. The study, published on June 18 in The BMJ, found that IF and CR were equally effective.
IF involves designated periods of eating and fasting and places less emphasis on what you eat than when you eat. By contrast, CR targets a specific daily calorie goal and allows any eating schedule.
The analysis included several of the most common IF protocols, including:
Alternate day fasting (ADF) alternates standard eating days with fasting days or very low calorie days.
Time-restricted eating (TRE) restricts food intake to a daily window, typically 8 or 12 hours.
Whole day fasting, also known as the 5:2 diet, involves two or three weekly fasting days.
All forms of IF and traditional CR produced weight loss compared with no intervention at all — known as an ad libitum, or unrestricted, diet. Of the three primary IF methods, only alternate day fasting resulted in greater weight loss than CR. However, the authors note that longer trials are needed to substantiate their findings.
'This analysis suggests that all three approaches, for the most part, lead to similar weight losses. This is why many of us believe that the best approach to losing weight is finding an approach that matches your lifestyle and is something you can practice for the long term,' said David B. Sarwer, PhD, director of the Center for Obesity Research and Education at Temple University and a spokesperson for the Obesity Society. Sarwer wasn't involved in the research.
IF has grown rapidly in popularity over the past decade, but evidence of its health benefits — namely, weight loss and cardiometabolic measures, such as blood glucose — has been inconsistent compared with more traditional, calorie-restricted diets.
Although proponents of IF may be disappointed by the latest findings, experts say that similar results across diets mean that people have more options to find a method that works best for them.
No clear 'best' when it comes to diet protocols
Researchers included 99 randomized clinical trials involving more than 6,500 adults in the systematic review. The average age of participants was 45, and two-thirds were women.
A small percentage was considered healthy, but the vast majority (about 90%) had existing health conditions, including overweight, obesity, and both type 1 and type 2 diabetes. Participants' average BMI was 31, meeting the standard clinical definition of obesity, which is a body mass index (BMI of 30 or higher.
Trials ranged between 3 and 52 weeks — with an average of 12 weeks — and varied in quality.
The review found that both IF and CR led to small reductions in body weight compared to an unrestricted diet. Among the different IF protocols, only ADF resulted in slightly more weight loss (nearly three pounds) than CR.
Sarwer pointed out that while this may be statistically significant, such additional weight loss would be trivial in improving weight-related health issues.
ADF also slightly outperformed both TRE and whole-day fasting in terms of weight loss, but these findings were not deemed significant. ADF also resulted in slight improvements to total and LDL cholesterol compared to time-restricted eating.
'There is no compelling evidence that there is a 'best' diet for weight loss,' said Sarwer.
'Rather, 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,' he said.
Who benefits from intermittent fasting?
Though IF is touted as the diet du jour, there isn't enough compelling evidence to support it yet. Nonetheless, it has been linked to an array of health benefits, including:
weight loss
improved insulin resistance
reduced inflammation
brain health
The key question, though, is not whether IF is beneficial compared to an unrestricted diet, but whether it's better than CR. That's where things are a little unclear.
A 2024 review found that IF and CR were both associated with weight loss and equally effective across cardiometabolic, cancer, and neurocognitive outcomes. However, they noted that IF studies reported greater adherence, meaning people were better able to stick to them for the trial duration.
A slightly older study published in NEJM in 2022 also concluded that among people with obesity, TRE wasn't more beneficial than CR.
On the other hand, in a clinical trial funded by the NIH, the results of which were published in April 2025, participants who did a variation of whole-day fasting (a 4:3 schedule rather than the more common 5:2) lost 50% more weight than CR (7.6% vs 5% body weight) after one year.
The additional weight loss also translated into cardiometabolic improvements, such as blood pressure, total cholesterol, and A1C.
Another trial, published in May 2025, found that just three months of TRE led to long-term weight loss, even after participants stopped the protocol.
CR isn't perfect either. Though it's long been a standard dietary intervention, it can be harmful, leading to fatigue, nutrient deficiencies, and even depression.
For now, experts agree that the right diet solution comes down to the individual. The best diet for you is the one you can stick to.
'Fasting diets are easier to follow because the rules are fairly simple vs trying to start a Mediterranean diet, for example. Some patients who may particularly benefit from a time-restricted diet, for example, are individuals who tend to snack a lot at night,' said Sun Kim, MD, an associate professor of Endocrinology, Gerontology, and Metabolism at Stanford Medicine who wasn't involved in the research.
She also cautions that individuals with diabetes who take insulin should be careful with fasting diets, as they may necessitate adjustments to insulin dosage and scheduling.
You should also be cautious with intermittent fasting if you:
are over age 65
have a history of disordered eating
have low blood pressure
are pregnant or nursing
Adherence is key to long-term weight loss
One final notable finding of the study was that adherence rates dropped precipitously the longer trials went on.
Studies shorter than 24 weeks had high adherence, above 80%, while trials longer than 52 weeks typically reported poor adherence. In one trial involving whole-day fasting, adherence crashed from 74% at six weeks to just 22% at 52 weeks.
These findings reinforce the idea that diet protocols should not be viewed as 'one size fits all' but rather as tailored to each individual's lifestyle.
'I wish there was an easy solution to weight loss. In our society, there are many forces that push individuals toward weight gain. I always discuss with patients about finding a lifestyle change that they can sustain for the long haul,' said Kim.
According to Sarwer, small, sustainable changes are key.
'I'm more likely to suggest that people reduce the number of days a week that they eat ice cream, or reduce the portion size they eat, than to recommend that they stop eating ice cream all together,' he said.
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This transcript has been edited for clarity. Is red meat bad for you? On the one hand, meat makes you strong, and it's every American's God-given right to grill a steak on his barbecue during the summer. I believe this came up in a church synod at some point… But on the other hand, the WHO (World Health Organization) has declared red meat a carcinogen, with a hot dog being as bad as cigarette. Yes, that was headline when the report came out. So, how do we reconcile these opposing ideas? Part of the solution is realizing the WHO organization in question is based in France. Maybe they're still angry about the "freedom fries" thing, but actually examining the nuances of the French language will help us understand what's going on. If you don't speak French, don't worry I got you covered. Ce n'est pas si difficile de tout n'inquiétez vous pas. Vous allez voir . Sit back, grab a baguette, and let's find out how dangerous red meat really is. I'm Christopher Labos, and this is Medscape's On Second Thought . Bonjour, tout le monde! Now, meat doesn't seem like it should be a complex topic to study, but it is. Many people around the world eat animals, but we don't all eat the same animals. For example, this is a cow, often used to make hamburger and steak. And this is Tobi, God's perfect angel who gets a more elaborate birthday party than I do each year. He is my son, and I would throw myself in front of a moving car for him. By necessity, when we do medical research on meat, we are lumping together a whole lot of a different human behavior, with people eating different types of animals based on where they live. There's no real alternative, and frankly, you can't let the perfect become the enemy of the good. Most credible research will at least separate out red meat from white meat. But most people don't really know what the difference is. If you thought pork was white meat, you're wrong. You think that because of a marketing slogan. In 1987, the National Pork Board paid for the marketing campaign "Pork. The Other White Meat." They were basically trying to position pork as an alternative to chicken. People also usually think veal or deer is white meat. They think the difference between white and red meat has something to do the age of the animal, whether its free range, or the color of the meat. But it doesn't. Chefs and restaurants say all kinds of things, but the real definition is simple: Mammals are red meat, and birds are white meat. Now, there's another thing we need to explain. We have red meat, but we also have processed red meat. Processed red meat is when red meat is transformed in some way — and that doesn't mean cooking. If you just take a piece of steak and cook it on your barbecue or in the oven, that's not processed meat. Processing is doing things like salting the meat, smoking it, or curing it. Processed meat includes items like bacon, sausages, hot dogs, salami, corn beef, and smoked meat. So, when we talk about red meat and health risks, we are primarily talking about processed red meat. And the people talking about this are the International Agency for Research in Cancer (IARC). IARC is a WHO organization, and their mandate is to promote international research on cancer — particularly its cause. One of their programs is a monograph program that evaluates the evidence of the carcinogenicity of specific exposures. Here's where a knowledge of French is going to come in handy. IARC likes to look at something called the hazard, rather than the risk. In fact, every time they have a press conference, they spend about 5 minutes explaining the difference to people, which begs the question: Why not just study risk and be done with it? In English, those words seem pretty much like synonyms. And with the way most people use them, they essentially are. But in French, they are slightly different. Le risque et le hasard don't quite mean the same thing in French. To be fair, their definitions are technically different in English, as well — as those of you who read the dictionary for fun already know. A risk is the probability that something harmful will happen. A hazard is a potential source of harm. For example, a grenade is a hazardous thing to have on your desk, but the risk of it exploding is quite low… unless you pull the pin. IARC is researching hazard. They are evaluating whether something is associated with cancer, not how risky that something is. IARC categorizes everything into groups: carcinogenic to humans, probably carcinogenic, possibly carcinogenic, or not classifiable. There is technically a "not carcinogenic" group, but there's nothing in there. Well, there was one substance in there for a bit, but they removed it. Comment below if you know what that substance is. Here's a hint: You find it in yoga pants. 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It's basically zero and a heck of a lot lower than the breast cancer risk. Clearly, you should take the drug if you have ER-positive breast cancer. So, this is the problem. IARC is saying how certain they are that something is dangerous, but not how dangerous something is. Conclusive data will land a substance into Group 1: carcinogenic. Strong but not conclusive data goes into Group 2a: probably carcinogenic. If there's only some evidence, contradictory evidence, or maybe just animal data, you get sorted into Group 2b: possibly carcinogenic. And Group 3 is used when there's not much data to work off of. Generally, their system works okay. They put tobacco, asbestos, and gamma radiation in Group 1, which makes sense. But then also put stuff like birth control pills, estrogen, and tamoxifen in Group 1. 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Their analysis was motivated by two things: 1) the funding they received from the beef industry (this is why we can't have nice things), and 2) they dismissed much of the research because it comes from observational cohorts, not randomized controlled trials. In food science, randomized controlled trials are hard to conduct, because telling people what to eat is often met with "make me." Regardless, the NutriRECS Consortium conclusion was, 'Keep eating meat, as the data is uncertain because most of it is observational.' This conclusion is a bit reductionist to me, because we have a lot of observational data pointing toward health risks associated with processed red meat, and I have a hard time believing all the stuff added to processed red meat is doing us any favors. But let's take the IARC assessment at face value. They are convinced by the hazard or the hasard. But what's the risk? The cancer risk is most clear cut for colon cancer, which is pretty logical. Your lifetime risk of colon cancer is about 4%, assuming you're of general risk with no family history or genetic risk factors. It's actually 4.2% for males and 4.0% for females, according to the 2022 Cancer Statistics from the American Cancer Society. But let's say 4% for everyone — just for simplicity. The IARC report estimated that eating an extra 50 g of processed meat per day, every day, increased your risk of colorectal cancer by 18%. Take 4%, multiply it by 1.18, and you get 4.72%. So, let's say 5% if we're rounding. All this to say, if you eat hot dogs every day of your life, your risk of getting colon cancer goes up by 1 percentage point on the absolute scale. Now, on first instinct you might say, "Pfff, that's nothing. Pass the bratwurst." But 1% on the absolute scale is not trivial. That's thousands of cases per year. Millions of cases over the course of your lifetime in a country of 300 million people. It has some important public health implications. 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