Benefits of Intermittent Fasting: What the Evidence Shows
Published 22 October 2026 · Last reviewed 22 October 2026 · Figures checked against official sources (how we check)
Short answer: The best-supported benefit of intermittent fasting is weight loss, and in trials it works about as well as cutting calories every day. Small trials also found better insulin sensitivity and lower blood pressure. Claims about autophagy, the brain and a longer life rest mostly on animal studies. Fasting is not safe for everyone.
The benefits of intermittent fasting with the best human evidence are weight loss and, in a few small trials, lower blood sugar and blood pressure. The weight loss comes mainly from eating less, and trials found it about equal to cutting calories every day. Autophagy, brain health and a longer life are popular reasons to fast, yet most of that evidence comes from rats, mice and cells. Below, each benefit is rated by how strong the human evidence is.
How intermittent fasting works in your body
Intermittent fasting means going without food for a set number of hours, then eating within a shorter window. As the hours pass, your body changes the fuel it runs on. The hours below are approximate and vary from person to person.
| Hours since your last meal | What happens |
|---|---|
| 0 to 4 | Fed. Your gut is still digesting the meal. Glucose from that food is the main fuel, and blood sugar and insulin are raised. |
| 4 to 12 | Early fasting. Insulin drops. To hold blood sugar steady, the liver breaks down its glycogen (stored sugar). |
| 12 to 24 | Metabolic switch. Once the liver’s glycogen is running low, the body turns more to fat and begins producing ketones. Studies put this switch anywhere from 12 to 36 hours. |
| 24 and over | Longer fast. Fat use and ketone levels keep climbing, and growth hormone rises. Get a doctor’s advice before fasting past 24 hours. |
Most of the weight lost on intermittent fasting comes from eating fewer calories. A 2026 Cochrane review links it to eating less, more fat burning and better insulin sensitivity. In a 12-month trial of 139 adults, 8-hour eating plus calorie cutting lost 8.0 kg, against 6.3 kg for calorie cutting alone. That difference was not significant.
Benefits of intermittent fasting, rated by evidence
Each row uses one of four grades. Strong means several human trials agree. Moderate means some human trials, early means small or short human studies, and the last grade is only animal or lab studies.
| Benefit | What the research found | Evidence |
|---|---|---|
| Weight loss | In a 2025 review of 99 trials, every fasting and calorie-cutting diet lost more weight than eating freely. Fasting was about equal to daily calorie cutting. | Strong against eating freely; moderate for the match with daily calorie cutting |
| Insulin sensitivity | Improved in 8 men with prediabetes over 5 weeks, with their weight held steady. In a 6-month trial of 107 women, falls were modest with both 2-day fasting and daily cutting. | Early |
| Blood sugar | Average 24-hour blood sugar fell by about 0.2 mmol/L (4 mg/dL) in a 4-day study of 11 adults. The 2025 review found no difference in HbA1c (long-term blood sugar). | Early |
| Blood pressure | Fell by about 11 mm Hg (systolic) and 10 mm Hg (diastolic) in the same 8 men. It fell by similar amounts with fasting and daily cutting in the 107-woman trial. | Early |
| Cholesterol and inflammation | LDL cholesterol and CRP, a marker of inflammation, fell about equally with fasting and daily cutting in the 107-woman trial. | Early |
| Autophagy (cell recycling) | In reviews of mostly animal and cell studies, food deprivation raised autophagy in many tissues. One 4-day human study found an autophagy gene marker rose in blood. | Early in humans; mostly animal |
| Growth hormone | Rose in a small study of six men during a 5-day fast. | Early, and only for long fasts |
| Brain, ageing and longer life | Benefits were seen in rats and mice, including in models of Alzheimer’s disease, Parkinson’s disease and stroke. | Only animal or lab studies |
| Energy and mood | In the 107-woman trial, fewer people on 2-day fasting (32%) than on daily cutting (46%) reported better mood and self-confidence. | Early |
| Gut health | None of the trials checked for this guide tested it. “Gut rest” is one of this site’s own four fasting goals, used to sort drinks, and is not a proven benefit. | Not tested |
The 8 men in the blood pressure trial ate within 6 hours and finished dinner before 3 pm. The 4-day study used an 8 am to 2 pm eating window. Neither tested a later window, such as noon to 8 pm.
Autophagy has its own guide, autophagy and intermittent fasting, which explains why no human study shows the hour it starts.
Does intermittent fasting really work?
For weight loss, yes, about as well as other ways of eating less. The best research on intermittent fasting comes from randomised trials that compare it with daily calorie cutting. Three sources stand out:
- A 12-month trial in the New England Journal of Medicine (2022) added an 8-hour eating window to calorie cutting. It did not improve weight, body fat or metabolic risk factors.
- A 2025 BMJ review of 99 trials (6,582 adults) found fasting and daily calorie cutting gave similar results. Alternate-day fasting lost about 1.3 kg more, mainly in trials shorter than 24 weeks.
- A 2026 Cochrane review of 22 trials (1,995 adults with overweight or obesity) compared fasting with regular dietary advice. It found little to no difference in weight loss or quality of life.
So intermittent fasting is one way to eat less, and its advantage is practical. You follow a clock instead of counting every calorie. Whether it works for you depends on whether you can keep it up. For a plan built around weight, read the guide to using fasting to lose weight.
Is intermittent fasting good for you?
In short trials, most reported side effects were minor. A 2024 analysis pooled 15 trials of 1,365 adults with overweight or obesity. It found no more fatigue, headaches or dropping out than on control diets. Hunger, headaches and low energy still happen to some people, and the intermittent fasting side effects guide covers them. The Cochrane trials lasted up to 12 months, so long-term effects are unclear.
Check with a doctor before you start if you:
- are pregnant or breastfeeding, or trying to conceive
- are under 18
- have type 1 diabetes, or take insulin or sulfonylureas
- have had an eating disorder, or are underweight (in which case we advise against fasting)
- are frail, elderly or recovering from illness or surgery
- take medicine that must be taken with food
Speak to a doctor before you fast for more than 24 hours. Chest pain, fainting or confusion during a fast are reasons to stop at once. Get emergency help on 999 in the UK, 112 in the EU or 911 in the US and Canada, or your local number elsewhere.
When do you see the benefits?
No trial sets a fixed timeline. Much of the weight lost in the first days is water, because stored glycogen holds about 3 to 4 g of water per gram. After that, body weight responds slowly to eating less. In modelling, the change takes about a year to get halfway to its new level.
So judge whether intermittent fasting is working by the trend over several weeks rather than a single weigh-in. Blood sugar changed within 4 days in the small study above, measured with a glucose monitor.
On metabolism, a study of people who restricted calories for years found lower levels of the active thyroid hormone T3. Their TSH and T4 levels were unchanged.
To see which stage your body is in right now, enter the time of your last meal in the fasting calculator. It shows your fasting window and names your current stage.
Frequently asked
What are the main benefits of intermittent fasting?
Weight loss has the strongest evidence, and in trials it is about equal to ordinary calorie cutting. Small human trials found better insulin sensitivity, blood pressure and blood sugar, so that evidence is early. Brain and longevity benefits have only been shown in animals.
Is intermittent fasting healthy?
In trials of adults with overweight, side effects were mostly mild and no more common than on control diets, though the evidence is uncertain. The trials in a 2026 Cochrane review lasted up to 12 months, so long-term effects are unclear. Pregnant women, under-18s and people who take insulin need a doctor's advice before they fast.
Does intermittent fasting improve brain function?
Brain benefits have been shown in rats and mice, including in models of Alzheimer's disease, Parkinson's disease and stroke. In a 4-day human study, a brain growth factor called BDNF tended to rise, but the change was not statistically significant. For people, the evidence is only animal studies so far.
Can intermittent fasting increase height?
We found no study showing that fasting adds height. Guidance for children and teenagers puts healthy habits ahead of weight and dieting, so anyone under 18 should only fast on a doctor's advice.
When do you feel the benefits of fasting?
No trial sets a fixed timeline. Much of the weight lost in the first days is water, and body weight then changes slowly over months. In one small study, blood sugar levels changed within 4 days, measured with a glucose monitor.
Related guides
Intermittent Fasting Side Effects and Who Should Avoid It
Intermittent fasting side effects in trials, from hunger and headaches to bloating, plus serious risks, when to see a doctor and who should not fast.
Autophagy and Intermittent Fasting: How Many Hours?
Does intermittent fasting trigger autophagy, and after how many hours? What human studies found, why the 16-hour figure is unproven, and what may stop it.
Intermittent Fasting for Weight Loss: What the Trials Show
Does intermittent fasting work for weight loss? What the trials found, how much to expect, and how to set your window, calories and protein.
How to Start Intermittent Fasting: A Step-by-Step Plan
To start intermittent fasting, check it is safe for you and choose a gentle first schedule. Set your times, then add an hour or two a week.
This article is general information, not medical advice. See our medical disclaimer for who should not fast.
Talk to a doctor before fasting longer than 24 hours. If you feel faint, confused or unwell, or have chest pain, stop the fast and get help: call your local emergency number (for example 999 in the UK, 911 in the US and Canada, 112 in the EU). If food, weight or eating is causing you distress, the support services in the footer can help.