Intermittent fasting ties ordinary dieting in trials, which can be good news

Key takeaways

• A 2025 BMJ analysis of 99 randomized trials with 6,582 adults found that eating windows, 5:2-style fasting and alternate-day fasting all led to weight loss similar to daily calorie cutting.
• A February 2026 Cochrane review of 22 trials found that fasting may offer little or no extra weight loss over standard diet advice.
• In a year-long Chicago trial of alternate-day fasting, 38% of the fasters dropped out, compared with 29% of people who cut calories every day.
• The 2024 headline linking 8-hour eating windows to a 91% higher risk of heart death came from an unpublished meeting summary based on two days of food recalls, and a 2025 paper using the same survey still could not rule out other causes.
• If you take insulin or a sulfonylurea, take blood pressure medicines, are pregnant, are under 18 or have had an eating disorder, talk with a clinician before you fast; the National Alliance for Eating Disorders helpline is 1-866-662-1235.

Between 2011 and 2015, 100 adults in Chicago tested a diet that sounded almost too easy. On one day, they ate a single lunch with about a quarter of their usual calories. The next day, they could eat a quarter more than usual. Then they switched back⁶.

Researchers at the University of Illinois at Chicago compared this alternate-day fasting with plain daily calorie cutting and with no diet at all. For the first three months, they even supplied the food⁶.

After a year, the fasters had lost about 6% of their weight compared with the no-diet group. The daily calorie cutters had lost 5.3%. The gap was too small to rule out chance⁶.

Two other details stand out. Thirty-eight percent of the fasters quit, compared with 29% of the calorie cutters. And the senior author, Krista Varady, had received an advance for a book called The Every-Other-Day Diet, which the paper disclosed⁶.

Publishing a tie for a diet you wrote a book about is good science. That tie has since shown up across dozens of trials. It’s welcome news if fasting feels easier than counting, and a reality check if someone promised it would rewire your metabolism.

Five ways to fast, and the one idea they share

Intermittent fasting means going without food, or with very little, for set stretches, then eating normally. The 2026 Cochrane review sorts the plans into three families: daily eating windows, fasting on certain days of the week, and alternate-day fasting⁴.

People choose fasting for reasons beyond weight. Many like one clear rule better than a calorie app. Others see fasting as an old discipline, kept by many faiths, that modern snacking crowded out.

Some fans also believe timing itself matters: that long breaks from food, or eating in step with the body’s daily clock, change how the body burns fuel. Those are fair questions. Here is what happens when researchers test them.

Across 99 trials, fasting matched ordinary calorie cutting

In June 2025, a team led by Zhila Semnani-Azad, with senior author John Sievenpiper, published a sweeping comparison in The BMJ, pooling 99 randomized trials with 6,582 adults¹. A randomized trial assigns people to diets by chance, so the groups start out alike.

The average participant was 45, and two-thirds were women. Almost 90% had an existing health condition. Trials ran from 3 to 52 weeks, 12 weeks on average².

Every approach, fasting or daily calorie cutting, led to more weight loss than eating freely¹. But the fasting plans barely differed from daily calorie cutting.

Alternate-day fasting came out ahead by about 1.3 kg, roughly 3 pounds¹. The researchers had set 2 kg as the smallest gap that would matter for people with obesity, and no difference reached it². No plan beat the others on HbA1c, a three-month average of blood sugar¹.

Cochrane, an independent network known for strict reviews, reached a similar verdict in February 2026. Its team, led by researchers in Buenos Aires, reviewed 22 trials with 1,995 adults with overweight or obesity⁴⁵.

Compared with standard diet advice, fasting may offer little or no extra weight loss, the reviewers found⁴. Compared with no program at all, fasting groups lost about 5% of their weight and control groups about 2%. The reviewers did not judge that gap clinically meaningful⁴¹⁵.

Long trials of eating windows tell the same story. A 2023 University of Illinois Chicago trial followed 90 adults with obesity, from a racially diverse group, for a year. Each was assigned to eat only from noon to 8 p.m., to trim calories by 25%, or to carry on as usual⁷.

Without counting a single calorie, the window group ate about 425 fewer a day than the controls and lost about 4.6 kg (10 pounds) more⁷. The calorie cutters came out about 5.4 kg (12 pounds) ahead of the controls. Statistically, the two diets tied⁷.

A year-long trial in China that added an 8 a.m. to 4 p.m. window to a calorie limit also found no significant extra loss⁸. Our article on extreme fasting describes it in detail.

So why does fasting work at all? Naveed Sattar, a University of Glasgow researcher, said the BMJ results show fasting has no special weight-loss mechanism. It works by helping people keep their calories lower¹².

The schedule that wins on paper is the hardest to keep

If one plan has an edge, it is a tough one. In the BMJ analysis, alternate-day fasting beat eating windows by about 1.7 kg and whole-day fasting such as 5:2 by about 1 kg, gaps the authors called trivial¹.

A 2025 trial in Colorado found a bigger gap. Researchers led by Victoria Catenacci and Danielle Ostendorf assigned 165 adults with overweight or obesity to 4:3 fasting or to a daily 34% calorie cut, which added up to the same weekly deficit⁹.

Both groups got an intensive group program, plus a goal of 300 minutes of moderate activity a week. After a year, the 4:3 group had lost about 2.9 kg (6.4 pounds) more⁹. Notice the support: this was a year of coaching, not a fasting app.

Sticking with a plan is the hard part. In trials shorter than 24 weeks, more than 80% of people kept to their plan, a BMJ editorial noted³. In the longest trials of whole-day fasting, adherence fell below 22% after a year³.

The Chicago trial shows how fasting drifts. People ate more than their target on fasting days and less than allowed on feast days, edging toward a steady daily diet⁶.

Cholesterol results are mixed. In Chicago, LDL, the “bad” cholesterol, was 11.5 mg/dL higher after a year on alternate-day fasting than on daily calorie cutting⁶. In the pooled analysis, alternate-day fasting did slightly better on cholesterol than eating windows¹².

The 91% heart-death headline rested on two days of food recalls

In March 2024, headlines warned that eating within 8 hours a day was linked to a 91% higher risk of dying from heart disease. The finding came from a research summary presented at an American Heart Association meeting in Chicago, not from a published paper¹⁰.

The team, led by Victor Zhong of Shanghai Jiao Tong University, used national survey data on 20,078 U.S. adults. Each person’s eating window came from two days of food recalls, which were then linked to deaths over a median of eight years¹⁰.

The short-window group was tiny: 414 people, about 2% of the total, with 31 heart deaths among them¹⁰.

Donald Lloyd-Jones of Northwestern University said the short-window group had more smokers, a higher body mass index and more Black participants, who have higher heart risk at the start. Some may have eaten in a narrow window because of shift work or two jobs, not by choice¹⁰. Pam Taub of UC San Diego and 34 other researchers wrote to the AHA’s president objecting to press releases about unpublished findings¹⁰.

In August 2025, a paper in the journal Diabetes & Metabolic Syndrome analyzed the same survey with the same 8-hour cutoff, in 19,831 adults¹¹. Against a narrower comparison group, people who ate over 12 to 14 hours, the heart-death risk ran about 2.35 times as high¹¹. The 2024 abstract had compared people with those eating over 12 to 16 hours¹⁰.

Yet there was still no link with death from all causes. The authors wrote that more research must sort out whether the short window itself, or other unmeasured factors, explains the result¹¹.

Studies like this can’t show that fasting causes heart deaths. But no trial has shown that fasting prevents them, either; the Cochrane trials lasted a year at most⁴.

Any diet that takes off weight takes some muscle with it

Some of the weight you lose on any diet is lean mass, which includes muscle. Fasting draws extra worry because long gaps mean fewer meals with protein.

In the Chicago trial, scans at six months put the fasters about 1.5 kg of lean mass below the no-diet group and the daily calorie cutters about 2.1 kg below. The two diets didn’t differ significantly⁶.

The BMJ analysis could not settle the question, because it did not assess muscle, noted Maik Pietzner of Queen Mary University of London¹². Our article on extreme fasting covers a shorter eating-window trial that raised a lean-mass flag.

If you fast, build each meal in your window around protein, and keep up strength work. Our protein guide explains how much you need, including the target in the 2025–2030 Dietary Guidelines.

Pick a schedule that fits your life

Use this chooser to match a schedule to your days. Each row cites its trial evidence.

Schedule	What it involves	What trials show	May suit you if	Ask a clinician first, or skip, if
16:8 window	Eat between, say, noon and 8 p.m.	Matched a 25% calorie cut over a year7	You dislike counting and already skip breakfast	Insulin or a sulfonylurea; eating-disorder history
Early window	Eat between, say, 8 a.m. and 4 p.m.	With a calorie limit, no significant extra loss8	You’re hungriest in the morning	Work or family meals run late
5:2	Two very low-intake days a week	Similar to daily cuts; few kept it up a year13	You’d rather diet hard twice a week	Diabetes medicines, pregnancy, eating-disorder history
4:3	Three nonconsecutive days at about 20% of usual intake	About 2.9 kg more than daily cuts, with coaching9	You can join a structured program	Same as 5:2
Alternate-day	A low-intake day every other day	Edge of about 1.3 kg; 38% quit and LDL rose in one trial16	You like firm on-off rules	High LDL, plus the 5:2 list
Daily calorie cutting	Eat somewhat less every day	Same results as most fasting plans14	Skipping meals leaves you shaky or fixated on food	Diabetes medicines or pregnancy

Then give your choice a fair, four-week test:

1. Clear it first. Read the next section and talk with a clinician if any of it applies to you.
2. Pick one schedule that fits your work hours and family meals, not someone else’s.
3. Ease in. For example, move your last meal an hour earlier each week until you reach your window.
4. Drink freely outside the window. The noon-to-8 trial allowed calorie-free drinks such as water, plain tea and black coffee⁷.
5. Eat real meals inside it: protein, vegetables, beans and whole grains.
6. Check in at week four. Are you eating less without feeling deprived? Sleeping well? If food thoughts are taking over your day, stop and talk with someone.
7. Get checked. Ask about cholesterol after a few months, and about blood sugar if you have diabetes⁶¹³.

Who should check with a clinician first, or skip fasting

Diabetes medicines. Insulin and pills that push the pancreas to release insulin, such as sulfonylureas and meglitinides, can cause low blood sugar. Skipping or delaying meals, or fasting while taking them, raises the risk, according to the National Institute of Diabetes and Digestive and Kidney Diseases¹³.

For many people with diabetes, low means below 70 mg/dL. Warning signs include shakiness, hunger, dizziness, confusion and a fast or uneven heartbeat¹³. Our article on medicine clashes explains how doses are planned around fasting.

Blood pressure medicines. Talk with your clinician before a big change in how you eat.

Pregnancy. The Chicago trial screened out pregnant women⁶. Without trial data, ask your prenatal provider before you fast.

Teens. The Cochrane review covered adults, and the Colorado trial enrolled people 18 to 60⁴⁹. These trials offer little guidance for growing bodies.

An eating-disorder history. Fasting rules can blend into restriction. Talk with your treatment team first. The National Alliance for Eating Disorders runs a free helpline at 1-866-662-1235, weekdays from 9 a.m. to 7 p.m. Eastern, that connects callers with licensed clinicians¹⁴. It is not a crisis line. If you’re in distress, call or text 988.

Religious fasts. Ramadan, Yom Kippur, Lent and other fasts carry meaning far beyond weight. If you have diabetes, plan your medicines with a clinician beforehand. Our article on faith and health explains the International Diabetes Federation’s Ramadan risk groups.

How strong is the evidence?

Claim	Evidence level	What it rests on
Fasting schedules produce weight loss similar to daily calorie cutting, for up to a year	Strong evidence	A 99-trial analysis, a 22-trial Cochrane review and year-long trials agree147
A 16:8 eating window beats ordinary calorie cutting	Evidence says no, or unsafe	Year-long trials and pooled data show a tie178
Alternate-day or 4:3 fasting beats daily calorie cutting	Promising but early	Small pooled edge; one coached trial found 2.9 kg more; one trial found a tie169
Fasting improves blood sugar more than other diets	Evidence says no, or unsafe	No HbA1c differences across 99 trials1
Short eating windows cause heart deaths	Not enough evidence	One survey-based study; confounding not ruled out1011
Fasting prevents disease over the long term	Not enough evidence	Trials lasted a year or less4
Fasting on insulin or a sulfonylurea is safe without a plan	Evidence says no, or unsafe	Fasting on these drugs raises the risk of lows13

Fasting is old wisdom, and the science leaves the choice to you

Fasting asks for nothing you have to buy. No powder, app or program is required. The trials say the right schedule is the one you can keep, and only you know your days, your appetite and your table.

That fits values many natural-health readers hold: simplicity, self-reliance and respect for the body’s rhythms. It also asks for the honesty you’d want from any health claim. Fasting is a useful tool for eating less, not a cure for metabolism.

Questions readers ask

The bottom line

Intermittent fasting works about as well as cutting calories every day. Across 99 trials and a 2026 Cochrane review, no schedule pulled far ahead, and the plans with a small edge were the hardest to keep. That is good news if a window or fasting days make eating less easier for you, and a reason to tune out bigger promises.

Choose a schedule that fits your life, eat real food inside it and give it an honest trial. If you take diabetes or blood pressure medicines, are pregnant or under 18, or have had an eating disorder, talk with a clinician before you start.

Related reading

• Fasting windows help some people eat less, while long fasts add real risks
• Some healthy habits clash with your medicines, so check these goals first
• Faith shapes how people eat, fast and decide, and your clinician should know
• Who needs the new protein goal of up to twice the RDA
• Black-and-white thinking predicts regained weight, so learn to think in “mostly”

Sources

1. Semnani-Azad Z, et al. (senior author Sievenpiper JL). “Intermittent Fasting Strategies and Their Effects on Body Weight and Other Cardiometabolic Risk Factors: Systematic Review and Network Meta-Analysis of Randomised Clinical Trials.” BMJ, 2025;389:e082007 (University of Glasgow repository record). eprints.gla.ac.uk
2. BMJ Group. News release on Semnani-Azad et al., via ScienceDaily, June 20, 2025. sciencedaily.com
3. Medina Rodríguez M, Muñoz AM, Franco Hoyos K. “Intermittent Fasting as a Nutritional Tool.” BMJ, 2025;389:r1156 (editorial). pubmed.ncbi.nlm.nih.gov
4. Garegnani LI, Oltra G, Ivaldi D, et al. “Intermittent Fasting for Adults With Overweight or Obesity.” Cochrane Database of Systematic Reviews, 2026, Issue 2, CD015610.pub2 (plain-language summary). cochrane.org
5. Scimex, Australian Science Media Centre. “Expert Reaction: Intermittent Fasting No Better Than Traditional Diets or Doing Nothing.” February 2026. scimex.org
6. Trepanowski JF, Kroeger CM, Barnosky A, et al., Varady KA. “Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial.” JAMA Internal Medicine, published online May 1, 2017. doi.org
7. American College of Physicians. “Time-Restricted Eating Comparable to Traditional Calorie Restriction for Weight Loss, Study Finds” (on Lin S, Cienfuegos S, Ezpeleta M, et al., Annals of Internal Medicine, 2023). ACP Internist Weekly, June 27, 2023. immattersacp.org
8. Liu D, Huang Y, Huang C, et al. “Calorie Restriction With or Without Time-Restricted Eating in Weight Loss.” New England Journal of Medicine, 2022. pubmed.ncbi.nlm.nih.gov
9. Catenacci VA, Ostendorf DM, Pan Z, et al. “The Effect of 4:3 Intermittent Fasting on Weight Loss at 12 Months: A Randomized Clinical Trial.” Annals of Internal Medicine, 2025 (Epistemonikos record). epistemonikos.org
10. TCTMD, Cardiovascular Research Foundation. “Intermittent Fasting Study Sparks Debate Over Increases in CV Mortality.” March 22, 2024. tctmd.com
11. “Association of Eating Duration Less Than 8 h With All-Cause, Cardiovascular, and Cancer Mortality.” Diabetes & Metabolic Syndrome: Clinical Research & Reviews, 2025;19(7):103278. pubmed.ncbi.nlm.nih.gov
12. Science Media Centre Spain. Expert reactions to the BMJ network meta-analysis of intermittent fasting. June 2025. sciencemediacentre.es
13. National Institute of Diabetes and Digestive and Kidney Diseases. “Low Blood Glucose (Hypoglycemia).” NIDDK, reviewed July 2021. niddk.nih.gov
14. National Alliance for Eating Disorders. “What Is an Eating Disorder Helpline?” Updated July 30, 2024. allianceforeatingdisorders.com
15. Parr E. “Intermittent Fasting No More Effective Than Dieting, Major Review Finds.” The Conversation, republished by Daily Maverick, February 19, 2026. dailymaverick.co.za