Is Intermittent Fasting Actually Dangerous? The 91% Heart-Risk Study, the Longevity Claims, and the Real Answer

Why this fight is louder than ever

Intermittent fasting is the most popular diet on the internet, and the most argued-about. About one in ten American adults say they use it, according to the International Food Information Council's survey, and it was the most Googled diet term in the world back in 2019. The main Reddit community, r/intermittentfasting, has grown past 1.8 million members.

That study is why this question needs answering properly. On 18 March 2024, the American Heart Association issued a news release about research presented at its Epidemiology and Prevention conference in Chicago: people who limited their eating to less than 8 hours a day had a 91% higher risk of death from cardiovascular disease than people who ate across 12 to 16 hours. Within hours, the number was everywhere. "Intermittent fasting kills" was the message most people took away, and it has been repeated ever since.

The reaction split instantly into two camps that have not reconciled since. On one side, the fasting faithful — millions of people who lost weight on 16:8, the influencers who built businesses on it, and doctors like Jason Fung who wrote bestselling books about fasting — dismissed the study as worthless: a non-peer-reviewed poster built on two days of self-reported eating that never studied a single real faster. On the other side, cardiologists said the signal was too big to ignore. Both sides are partly right, and both are arguing past each other.

The stakes are real in both directions. If short eating windows genuinely raise the risk of dying from heart disease, tens of millions of people are following a habit that could harm them — many with heart disease already. If the study is wrong, a scary headline has frightened people away from a tool that helps many of them eat less and feel better. The only way through is to look at what the study actually did, what the trials actually found, and what the longevity claims are built on.

The 91% study: what actually happened

First, the basics, because most coverage skipped them. The research was led by Victor Wenze Zhong, professor and chair of epidemiology and biostatistics at Shanghai Jiao Tong University School of Medicine, and presented at the American Heart Association's Epidemiology and Prevention conference in Chicago in March 2024. It was a conference abstract — a 250-word summary and a poster — not a full paper, and the AHA itself labelled it preliminary: abstracts presented at its meetings are not peer-reviewed.

The data came from the National Health and Nutrition Examination Survey (NHANES), a long-running US government survey. The researchers took about 20,000 American adults, average age 49, enrolled between 2003 and 2018, who had completed two 24-hour dietary recall questionnaires — they were asked what they had eaten on two specific days. Deaths were then checked against the CDC's National Death Index through December 2019: over a median follow-up of about 8 years, 2,797 deaths, including 840 from cardiovascular disease.

The headline finding: people whose eating was squeezed into less than 8 hours a day had a 91% higher risk of cardiovascular death than people who ate across 12 to 16 hours, with no reduction in the overall risk of death from any cause. For people who already had cardiovascular disease, the signal was worse: an 8-to-10-hour window was associated with a 66% higher risk of cardiovascular death.

Zhong himself was surprised. He told reporters he had expected that long-term adoption of an 8-hour eating window would be associated with a lower risk of cardiovascular death, not a higher one. He also stressed, from the start, that the study identified an association, not a cause: "Although the study identified an association between an 8-hour eating window and cardiovascular death, this does not mean that time-restricted eating caused cardiovascular death." That caution is the key to everything that follows. The study never tested intermittent fasting as a diet. It tested whether people who happened to report short eating windows on two survey days died of heart disease more often, years later — a very different question.

What the peer-reviewed paper said — and what it did not

The critics' strongest argument in 2024 was that the finding was only an abstract, and peer review might shrink it or kill it. In August 2025, that test arrived: the full paper, by Meng Chen, Victor Zhong and colleagues, was published in Diabetes & Metabolic Syndrome: Clinical Research & Reviews, analysing 19,831 NHANES adults with a median follow-up of 8.1 years.

The signal did not disappear. Compared with people eating across 12 to 14 hours a day, those eating for less than 8 hours had a hazard ratio of 2.35 for cardiovascular mortality — more than double the risk. The association held across demographic and lifestyle subgroups and survived 14 different sensitivity analyses. The association with overall mortality, by contrast, was weak and inconsistent, and there was no link with cancer mortality.

But the authors were careful where the headlines were not: the findings do not prove causation, and residual confounding could explain the result. Zhong put it bluntly: "Our study provides the first evidence that individuals adhering to a less than 8-hour eating window were more likely to die from cardiovascular disease compared to people who followed a typical eating window of 12–14 hours. Although our observational results are subject to residual confounding, people should be extremely cautious to adopt a short eating window for a long time (e.g., over years) to pursue cardiovascular health or longevity, which has no evidence support from human studies to date."

Read that last clause again, because it is the most important sentence in the whole debate: pursuing cardiovascular health or longevity with a short eating window "has no evidence support from human studies to date." That is the senior author of the scary study telling the fasting community that the longevity case is unproven — while also warning that the risk signal is real enough to take seriously.

Why the critics say it proves nothing

The backlash was immediate and furious, and it was not only coming from influencers with books to sell. Serious nutrition researchers lined up to pick the study apart, and several of their objections are strong.

It did not study fasters. This is the objection that comes up in every thread, and it is the most important one. Nobody in the NHANES data was on an intermittent fasting plan. The researchers never asked anyone whether they fasted. They took two days of dietary recall — "what did you eat yesterday?" — and calculated the gap between the first and last calorie of the day. A person who skipped breakfast because they were ill, worked a night shift, or could not afford three meals would land in the "under 8 hours" group. Christopher Rhodes, a nutritional biochemist at UC Davis, made this point publicly: the study was not looking at people following time-restricted eating plans, but at people who happened to report eating within less than 8 total hours on a specific day.

Reverse causation is a real possibility. This is the problem that haunts every observational diet study: do people die more often because they eat in short windows, or do people who are already dying eat in shorter windows? Someone with heart failure, cancer, depression, or a serious illness often loses their appetite, eats less, and eats across fewer hours. Someone working rotating shifts eats at odd hours. Someone in poverty may eat one meal a day. In all of these cases, the short window is a symptom of the thing that eventually kills them, not the cause.

The diet data is thin. Two 24-hour recalls, taken within a single year, were used to characterise a person's eating pattern over a follow-up of up to 17 years. Nobody's eating pattern is that stable — people change jobs, get ill, move house, and change their diets — and the study has no way of knowing whether the two surveyed days resembled the other 2,900 days in the follow-up. It also has no information about what people ate, only when.

Dariush Mozaffarian, a cardiologist and professor at Tufts University, called the study "very problematic" and questioned whether the 8-hour group might have included many people whose short windows were caused by illness or circumstance rather than choice. Others pointed out that the AHA should not have promoted a conference abstract with a press release at all, given how predictably the nuance would be lost.

None of these objections makes the study worthless. A hazard ratio of 2.35 that survives 14 sensitivity analyses in nearly 20,000 people is not nothing. But the objections do mean the study cannot answer the question most people think it answered. It cannot tell you whether choosing to eat within 8 hours a day, as a healthy person, for months or years, raises your risk of dying from heart disease. No observational study built on two days of diet recall can tell you that. Only a randomised trial could — and no trial has ever measured heart attacks, strokes or deaths from time-restricted eating.

The trials that actually tested the window

If the observational study cannot tell us whether fasting itself is dangerous, the randomised trials can tell us something almost as important: whether the window does anything at all. Here is the twist neither camp likes to advertise — the strongest trials find that time-restricted eating, on its own, does very little.

The year-long test: Liu and colleagues, New England Journal of Medicine, 2022. This is the largest and longest trial of time-restricted eating ever run. Deying Liu and colleagues at Nanfang Hospital in Guangzhou randomised 139 adults with obesity to the same calorie-restricted diet — 1,200 to 1,500 calories a day for women, 1,500 to 1,800 for men — with one difference: half ate all their food between 8 a.m. and 4 p.m., and half ate whenever they liked. After 12 months, the window group had lost 8.0 kg and the calorie-only group 6.3 kg. The 1.8 kg difference was not statistically significant, and no other measure differed either: not waist circumference, not body fat, not blood pressure, not blood sugar, not cholesterol. When calories were held equal, the clock added nothing measurable over a full year.

The pure test: Lowe and colleagues, JAMA Internal Medicine, 2020 (the TREAT trial). The Liu trial tested the window on top of a calorie cut. The TREAT trial tested the window on its own: Ethan Weiss's team at UC San Francisco randomised 116 adults with overweight or obesity to eat only between noon and 8 p.m., with no other dietary instructions, or to eat three structured meals a day. After 12 weeks, the fasting group had lost 0.94 kg and the control group 0.68 kg — a difference of 0.26 kg that was not statistically significant. Time-restricted eating, in the absence of other interventions, was not more effective than eating throughout the day. That is the trial's own conclusion, in its own words.

But the weight result is not the worrying part of TREAT. The worrying part is what the weight was made of. In the 50 participants who had body-composition scans, the fasting group lost 1.70 kg, of which about 1.10 kg — roughly 65% — was lean mass, and only about 0.51 kg was fat. During ordinary dieting, lean mass usually accounts for 20 to 30% of the weight lost. The trial did not measure protein intake, a real limitation, but a 2026 meta-analysis of 30 studies found the same signal: lean mass fell by an average of 0.41 kg even when calories were matched.

The alternate-day test: Trepanowski and Varady, JAMA Internal Medicine, 2017. Krista Varady's team at the University of Illinois at Chicago ran the longest trial of alternate-day fasting: 100 adults with obesity, randomised for a year to alternate-day fasting (25% of calorie needs on fast days, 125% on feast days), daily calorie restriction, or no intervention. After a year, the fasting group had lost 6.0% of their body weight and the daily-restriction group 5.3% — no significant difference. The fasting group also dropped out at the highest rate, 38% against 29% for daily restriction, and their LDL cholesterol rose by 11.5 mg/dL more than the daily-restriction group's. Alternate-day fasting worked — people lost weight — but it did not work better, it was harder to stick to, and it carried a worse cholesterol signal.

What the pooled evidence says. A 2024 analysis in JAMA Network Open pooled 29 randomised trials with 2,485 people and found time-restricted eating produced an average of 1.37 kg more weight loss than control groups — a small effect whose clinical importance the authors said was uncertain, in a pool where 22 of the 29 studies had a high risk of bias. A 2025 network meta-analysis in The BMJ, pooling 99 trials with 6,582 adults, found that every fasting and calorie-restriction diet beat unrestricted eating, but only alternate-day fasting had any edge over plain daily calorie restriction — about 1.29 kg. A Cochrane review published in February 2026, pooling 22 randomised trials with 1,995 adults, found little to no difference in weight change between intermittent fasting and standard dietary advice.

Put together, the trial evidence says something simple and deflating: intermittent fasting works mostly because it makes people eat fewer calories, not because the clock does something special. When calories are matched, the window adds little or nothing. The 2019 NEJM review by de Cabo and Mattson popularised the idea of a "metabolic switch" — the body shifting from glucose to ketones after 12 to 36 hours without food — and the mechanism is real biology. But a mechanism is not a result, and in the trials run since, the mechanism has not delivered extra weight loss.

That matters for the danger question, because it changes the risk-benefit calculation. If fasting delivered unique, powerful benefits, a worrying observational signal might be worth accepting. If fasting delivers roughly the same weight loss as ordinary calorie restriction, with a lean-mass cost, then even a possible risk deserves more respect — because there is less benefit to trade against it.

At a glance: what fasting does, and what it does not

Claim	What the evidence says	Verdict
An 8-hour eating window raises the risk of cardiovascular death by 91%	AHA 2024 abstract (~20,000 adults): 91% higher risk vs 12–16 hours; 2025 paper: hazard ratio 2.35. Observational; residual confounding possible	Association found, causation unproven
Intermittent fasting is proven dangerous	No randomised trial has measured heart attacks, strokes or deaths from fasting; the risk signal comes from one observational dataset	Not proven
Intermittent fasting is proven safe	The signal has not been refuted, and no long-term trial has tested hard outcomes	Not proven either
The eating window adds extra weight loss beyond calorie cutting	NEJM 2022 (139 adults, 12 months): 8.0 vs 6.3 kg, not significant; BMJ 2025 (99 trials): only alternate-day fasting beat daily restriction, by 1.29 kg	Mostly false
Fasting melts fat while sparing muscle	TREAT 2020: ~65% of weight lost on 16:8 was lean mass; 2026 meta-analysis: lean mass fell 0.41 kg on average	False — the opposite is the risk
Fasting extends lifespan	No human trial has shown longer life. Mice on intermittent fasting lived ~28 months vs 25 for free-eating mice; calorie restriction did better at 30–34 months	Unproven in humans
Fasting improves blood sugar and blood pressure	Early-window trials (Sutton 2018, 5 weeks) improved insulin sensitivity, blood pressure and oxidative stress without weight loss	True, modestly
Alternate-day fasting beats daily dieting	Varady 2017 (100 adults, 1 year): 6.0% vs 5.3% weight loss, not significant; dropout 38% vs 29%; LDL rose more	Not superior
Anyone can safely try intermittent fasting	Not advised without medical guidance for pregnant people, under-18s, underweight people, anyone with an eating-disorder history, or people with type 1 diabetes	False

Claim vs evidence: the arguments, scored

The argument you will hear	Evidence for	Evidence against	Score
"The 91% study proves fasting kills"	Hazard ratio 2.35 in the peer-reviewed paper, surviving 14 sensitivity analyses	Observational; two days of self-reported diet; no actual fasters studied; authors say it does not prove causation	Overstated — a real signal, not proof
"The study is garbage — it never studied fasters"	True: participants were classified by two days of recall, not by following a plan	Dismissing it entirely ignores a persistent signal in ~20,000 people that peer review did not erase	Partly true, over-dismissed
"Fasting is a metabolic shortcut, not just eating less"	The "metabolic switch" to ketones after 12–36 hours is real biology	NEJM 2022: same calories, same weight loss with or without the window; Cochrane 2026: little to no difference vs standard advice	Mechanism real, benefit unproven
"I lost 30 kg on 16:8 — the trials must be wrong"	Millions of people have genuinely lost weight this way; the r/intermittentfasting community documents it daily	Trials agree fasting works — through eating fewer calories. The window is a structure for eating less, not a separate fat-burning trick	Real result, wrong mechanism
"Fasting will make you live longer"	Mice on intermittent fasting lived ~28 months vs 25 for free-eating mice (Nature 2024)	Mice are not people; calorie restriction beat fasting in the same study; no human trial has shown longer life	Unproven in humans
"The muscle loss is a myth"	Some trials with resistance training and adequate protein preserved muscle	TREAT 2020: 65% of weight lost was lean mass; 2026 meta-analysis: lean mass fell 0.41 kg on average	False — a real, measurable risk
"Doctors just hate fasting because it is free"	The diet industry and parts of medicine do profit from complicated solutions	The strongest critics are the trialists themselves, who expected fasting to win	Conspiracy, not evidence
"If it were dangerous, we'd know by now"	Short-term trials report mostly mild side effects: hunger, headaches, tiredness	No trial has run long enough to measure heart attacks or deaths	False reassurance

The longevity claims: what the mice actually showed

If the weight-loss story is modest, the longevity story is where the gap between the marketing and the evidence becomes a canyon. Scroll through fasting content for five minutes and you will meet the claim that fasting "reverses ageing", "triggers autophagy" and "adds years to your life". The first two phrases describe real biology. The third is, as of today, unproven in humans.

The strongest animal study ever run on this question was published in Nature in October 2024. Researchers at the Jackson Laboratory tracked 960 genetically diverse female mice — deliberately bred to be as genetically varied as a human population — across five diets for their whole lives: eating freely, fasting one day a week, fasting two consecutive days a week, or eating 80% or 60% of their usual calories.

The results are a problem for the fasting pitch. The free-eating mice lived an average of 25 months. The intermittent fasters lived about 28 months. But the mice eating 80% of their usual calories lived 30 months, and the mice eating 60% lived 34 months. Eating less beat fasting, by a wide margin, in the same study. The 40% calorie-restriction group gained a median lifespan about 36% longer than the free-eating group — but it also lost lean mass and showed immune changes. And genetics mattered more than any diet: some mice on restricted diets actually lived shorter lives.

The human evidence is thinner still. The best human trial of eating less and ageing is CALERIE, a two-year randomised trial in 218 healthy adults aged 21 to 50 who were asked to cut their calories by 25%. They managed about a 12% cut in practice, kept off about 7.5 kg, and one measure of biological ageing slowed slightly. Other ageing measures did not change at all. And CALERIE tested a daily calorie cut, not intermittent fasting: it says nothing about eating windows. No trial of fasting or calorie restriction has ever run long enough to count how many people actually live longer.

Then there is autophagy, the cell's internal recycling programme, which fasting activates strongly in mice. In humans, the evidence is a handful of small studies. One study of eight healthy men on a 72-hour fast found changes in the mTOR growth-signalling pathway, but autophagy itself was only a side question. Nobody knows when autophagy starts in a fasting human, or how much it rises — because measuring it directly in living people is extremely difficult.

There is one genuinely encouraging human finding, and it deserves its place here. In 2018, Courtney Peterson's team at the University of Alabama ran a five-week trial in men with prediabetes, feeding them enough food to keep their weight stable while compressing all eating into a six-hour window ending before 3 p.m. Insulin sensitivity improved, blood pressure fell, and oxidative stress dropped — with no weight loss involved. That suggests meal timing can matter for metabolic health independently of calories, at least in the short term. But five weeks of better markers is not a longer life.

The honest summary of the longevity case is this: fasting extends life in laboratory animals, less effectively than eating less does, with genetics mattering more than either. In people, the trials show better risk markers, most of which track the weight lost. Nobody has shown that people who fast live longer.

The real risks: what can actually go wrong

So far this article has been about what fasting does not do. This section is about what it can do — the risks that are documented and worth knowing before anyone considers changing how they eat.

Muscle loss. This is the best-documented risk, and the one the fasting community talks about least. The TREAT trial found that about two-thirds of the weight lost on 16:8 was lean mass, and a 2026 meta-analysis of 30 studies found lean mass fell by an average of 0.41 kg on time-restricted eating even when calories were matched. Muscle is what keeps older people mobile, independent and out of hospital, and losing it is particularly costly after 40, when adults already lose muscle at roughly 1% a year. The risk is not unavoidable — trials that paired eating windows with resistance training and adequate protein preserved muscle far better — but fasting without a plan for protein and strength training is a reliable way to lose the tissue you most want to keep.

Gallstones. Rapid weight loss by any means increases the risk of gallstones, because long gaps between meals leave the gallbladder idle, bile sits and thickens, and stones are more likely to form. This is a weight-loss-rate problem rather than a fasting-specific one, but fasting that produces fast weight loss carries it, and people with a history of gallstones should discuss any big change in eating pattern with a doctor first. Warning signs include pain in the upper right abdomen, nausea, or yellowing of the skin.

Low blood sugar on diabetes medication. This is the sharpest, most immediate danger, and the one that can put someone in an ambulance. Insulin and sulfonylureas — drugs such as gliclazide, glipizide, glimepiride and glyburide — lower blood glucose whether or not you have eaten. Keep the dose the same while removing the meal, and blood sugar can fall dangerously low. Trials in people with insulin-treated type 2 diabetes have been done safely, but only under supervision, with medication doses adjusted as part of the protocol. Anyone on glucose-lowering medication should not change their eating pattern without their prescriber adjusting the regimen first. SGLT2 inhibitors carry a separate risk of ketoacidosis during fasting.

Eating disorders. This is the risk least often taken seriously in fasting content, and it may be the most serious of all. Structured restriction, rules about permitted hours, and the moral framing that grows around them are close to the mechanics of restrictive eating disorders. The main fasting community on Reddit states plainly that it is not a pro-eating-disorder space and that fasting is not recommended for people with an eating disorder or in recovery from one. The Academy for Eating Disorders has raised concern that restricting diets for young people risks triggering eating disorders.

Who should not fast. The lists from clinical guidance are consistent. Do not fast while pregnant or breastfeeding, when calorie and nutrient demands are elevated. Do not fast under 18, when growth and development need consistent nutrition. Do not fast if you are underweight, with a body mass index under 18.5. Do not fast with a history of an eating disorder. Do not fast with type 1 diabetes except under close specialist supervision, because of the risk of severe low blood sugar and diabetic ketoacidosis. And do not fast without medical advice if you take medication that must be taken with food, or if you have advanced kidney or liver disease. For everyone else, the common short-term side effects in trials are milder — hunger, headaches, tiredness, irritability — and they usually fade within a couple of weeks.

Popular opinions: what people actually say about intermittent fasting

This is the question that splits comment sections down the middle. Here are the positions you will actually meet online — paraphrased from real threads and debates across Reddit, nutrition forums and the comment sections under the 91% study's coverage, not invented — and how each one holds up.

"I lost 30 kg on 16:8 and my blood sugar is normal for the first time in years. Don't tell me it doesn't work." Verdict: nobody should tell you that, because it worked — for you. The trials agree: fasting helps people lose weight, and for many people the structure of an eating window is what finally makes eating less sustainable. The dispute is not about whether it works. It is about why it works, and the trials say the answer is the calories, not the clock. That does not make your result less real. It makes the mechanism less magical than the marketing claims.

"The 91% study is a joke. Two days of 'what did you eat' and they call it science? It never studied a single faster." Verdict: the criticism of the methods is largely fair — the study classified people by two days of recall, and nobody in it was following a fasting plan. Where this argument overreaches is in treating a weak method as proof of a false result. A flawed study can still be pointing at something real, and the peer-reviewed version of this one, with 14 sensitivity analyses, did not make the signal go away. "Bad study" and "wrong conclusion" are different claims, and this argument proves only the first.

"I tried 16:8 and felt awful — dizzy, cold, obsessed with the clock. Never again." Verdict: this is the experience the success stories drown out, and it is common. Hunger, headaches, dizziness and an unhealthy fixation on eating times are among the most reported side effects in trials, and for some people the structure that liberates one person imprisons another. If a diet makes you feel worse and think about food constantly, that is data, not failure — and it is a good reason to try a different approach rather than pushing through on willpower.

"I'm not fasting to lose weight. I'm fasting to live longer. The mice lived longer — that's enough for me." Verdict: this is the most honest version of the longevity argument, and it deserves a straight answer. The mice on intermittent fasting lived about 28 months against 25 for free-eating mice — a real gain, in mice. But the mice eating fewer calories lived 30 to 34 months, genetics mattered more than diet, and no human study has shown that fasting adds a single day to a human life. If the mouse data is enough for you, that is your choice to make with your eyes open. But it is a bet on animal data, not a proven intervention, and it should be described that way.

The pattern is worth noticing: the angriest voices are arguing about different things. One side is defending a tool that finally worked for them, the other is defending a number that frightened them. Strip away the identity, and the evidence leaves a much quieter answer: fasting helps many people eat less, the window itself adds little, the longevity claims are mouse data, and a worrying heart-risk signal exists that nobody has yet explained away or confirmed.

What the argument costs

It is easy to treat this as an abstract debate about statistics. It is not abstract, and the cost runs in both directions.

On one side, if the 91% signal is real — if years of short eating windows genuinely raise the risk of dying from heart disease — then the people most at risk are the ones least likely to hear the warning. Millions of people with heart disease, diabetes and high blood pressure are fasting on the advice of influencers, many of them following windows far shorter than 8 hours: one meal a day, 20-hour fasts, multi-day fasts. For people with existing cardiovascular disease, the study found a 66% higher risk of cardiovascular death even with an 8-to-10-hour window.

On the other side, if the signal is an artefact — sick people eating less, shift workers and the poor swept into the short-window group — then the cost is the harm done by the scare itself. People who had finally found a sustainable way to eat less abandoned it, and people with type 2 diabetes who were improving their blood sugar stopped. A preliminary abstract, promoted with a press release, produced a global panic that the full paper could not undo.

None of this means fasting should be banned, feared, or abandoned. It means the decision deserves better than a headline in either direction. A healthy adult who fasts, eats enough protein, trains with weights, and feels well is in a very different position from a person with heart disease fasting on an influencer's say-so, or a person on insulin skipping meals without telling their doctor. The evidence, taken honestly, says: not proven dangerous, not proven safe, modestly useful for weight loss, unproven for longevity, with real risks for specific groups — and one unexplained heart-risk signal that deserves a proper trial.

Frequently asked questions

Is intermittent fasting actually dangerous?

It has not been proven dangerous, and it has not been proven safe. A 2024 study of about 20,000 US adults linked eating within less than 8 hours a day to a 91% higher risk of cardiovascular death, and the peer-reviewed paper in 2025 still found a hazard ratio of 2.35 — but the study was observational, based on two days of self-reported eating, and its authors say it does not prove causation. No randomised trial has ever measured heart attacks, strokes or deaths from fasting.

What did the 91% heart-risk study actually find?

Researchers led by Victor Wenze Zhong analysed NHANES survey data from about 20,000 American adults enrolled between 2003 and 2018, with deaths tracked through 2019. People who reported eating all their food within less than 8 hours a day had a 91% higher risk of death from cardiovascular disease than people eating across 12 to 16 hours, over a median follow-up of about 8 years. There was no reduction in overall mortality. The finding was first presented as a conference abstract in March 2024 and published as a peer-reviewed paper in August 2025, which reported a hazard ratio of 2.35. The study did not follow people on fasting plans — it classified them from two days of diet recall.

Does intermittent fasting help you lose more weight than normal dieting?

Not meaningfully. The largest year-long trial, published in the New England Journal of Medicine in 2022, randomised 139 adults to the same calorie-restricted diet with or without an 8-hour eating window: after 12 months the window group had lost 8.0 kg and the calorie-only group 6.3 kg, a difference that was not statistically significant. A 2025 network meta-analysis of 99 trials found only alternate-day fasting had any edge over daily calorie restriction, about 1.29 kg. Fasting works mainly by helping people eat fewer calories, not by a separate metabolic effect.

Does intermittent fasting make you live longer?

There is no human evidence that it does. The claim rests on animal studies: in a 2024 Nature study of 960 genetically diverse mice, intermittent fasters lived about 28 months against 25 for free-eating mice — but mice eating fewer calories lived 30 to 34 months, and genetics mattered more than any diet. The best human trial of eating less and ageing, CALERIE, tested a daily calorie cut rather than fasting. No study has followed people long enough to show that fasting extends human life.

Who should not try intermittent fasting?

Do not fast if you are pregnant or breastfeeding, under 18, underweight (BMI under 18.5), or have a history of an eating disorder. People with type 1 diabetes should only fast under close specialist supervision, and people with type 2 diabetes on insulin or sulfonylureas must have their medication adjusted by their prescriber first, because of the risk of dangerously low blood sugar. People with a history of gallstones, advanced kidney or liver disease, or medication that must be taken with food should also talk to a doctor first. This is general information, not medical advice.

What is the biggest risk of intermittent fasting that nobody talks about?

Muscle loss. In the TREAT trial, about two-thirds of the weight lost on 16:8 was lean mass rather than fat, and a 2026 meta-analysis found lean mass fell by an average of 0.41 kg even when calories were matched. Muscle keeps people mobile and independent as they age, and losing it is a real cost of fasting without enough protein and resistance training.

Sources

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3. Liu, D. et al. (2022). Calorie restriction with or without time-restricted eating in weight loss. New England Journal of Medicine, 386(16), 1495–1504. doi:10.1056/NEJMoa2114833
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14. Mozaffarian, D. — called the AHA 2024 abstract "very problematic" in coverage of the study. — nmn.com
15. International Food Information Council (IFIC) Food & Health Survey — about 10% of American adults report using intermittent fasting; it was the most Googled diet term in 2019.
16. Academy for Eating Disorders — position statement on restricting diets and eating-disorder risk in young people.

Medical note: this article summarises published research for general information. It is not medical advice. Intermittent fasting is not suitable for everyone — including people who are pregnant or breastfeeding, under 18, underweight, living with an eating disorder or a history of one, or taking glucose-lowering medication — and changing how you eat while on medication can be dangerous without medical supervision. If you are considering fasting, talk to your doctor first.

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