Marek and Alek at Eiko’s table in Ogimi, Okinawa, October 2023.
My son Alek and I met fasting the hard way. At our first breakfast in Okinawa we looked at the small, colorful plates and asked if there was more. On our quest we met more than a hundred longevity experts, and eating less, or eating within fewer hours of the day, came up again and again — in a lab in Seattle, at a conference in Dublin, on a beach in Okinawa and in a village kitchen in Ogimi. This page explains what we learned, in plain words. Every number has its source. Where something has only been seen in animals, we tell you.
This page shares what research and the experts we met say. It is not medical advice, and it does not recommend any medicine, supplement or treatment. It knows nothing about you. Before you start, stop or change anything — especially if you are pregnant, live with a health condition or take any medication — please talk to your doctor. In an emergency, call your local emergency number.
In short
Intermittent fasting can be a good way to eat, but it is not magic. In fair tests in people, eating within a shorter part of the day, or eating very little on some days, helps weight, blood pressure and blood fats — about as much as simply eating a little less. Whether it helps people live longer is not known yet: the longer lives have been seen in mice and monkeys, and mostly from eating less overall. What the studies and the long-lived places we visited agree on is simpler: don’t eat until you are stuffed, and enjoy your food with the people you love. If you are pregnant, take medicine for diabetes, have ever had an eating disorder or are still growing, talk to your doctor first. [1][2][3][4][5]
Longevity, as we mean it: more healthy, joyful years. What we mean →
99fair tests with 6,582 adults: fasting-style eating and simply eating a little less helped about equally.[1]
2–3%slower pace of aging, measured in the blood, in healthy adults who ate a little less for two years.[6]
~35%longer life in mice that ate less and only in their active hours — against ~10% for eating less alone.[4]
Fasting is as old as humanity. Almost every religion has it, and every family knows the feeling of a long, hungry morning. What is new is the science asking a simple question: does the clock of your meals change how long and how well you live? Here is the honest answer, as far as science has got.
What intermittent fasting is
Illustration: the kitchen closes for the night.
Intermittent fasting means you don’t eat for some hours, or on some days, and then you eat normally again. It is about when you eat more than what you eat. People do it in a few different ways. We describe them here so you know the words — not as a plan to follow.
A shorter eating window
People eat all their meals within part of the day — for example 8 hours — and nothing in the evening and night. Scientists call it time-restricted eating. You may have heard “16:8”: 16 hours without food, 8 hours with.
No fasting at all — simply a bit less food, every day. Scientists call it calorie restriction. It is what most of the famous longevity studies in animals actually tested.
In plain words: Intermittent fasting is a clock for your meals. It doesn’t tell you what to eat — and what you eat still matters.
What studies in people show
Science answers this question in two ways, and it helps to know the difference. In a fair test, people are picked at random to try something or not, so we can see what it does by itself. In a long-term study, scientists watch large groups of people for years and see who stays healthy. A fair test shows a cause. A long-term study shows a link. For fasting, there are many fair tests — but almost all of them last months, not years.
As good as eating a little less — not better
When scientists combined 99 fair tests with 6,582 adults, every kind of intermittent fasting helped weight a little compared with eating freely — and about as much as simply eating a little less every day. Eating very little every other day did slightly better, but by less than doctors count as a real difference. Most of the tests lasted about three months.
Based on: Many fair tests combined (BMJ, 2025) · shows cause [1]
A year-long test: the clock added nothing extra
In China, 139 adults with obesity ate a little less for a year. Half of them also ate only between 8 in the morning and 4 in the afternoon. Both groups improved about the same — in weight, body fat and other health markers.
In America, 116 adults ate only between noon and 8 in the evening for 12 weeks, while others ate three meals a day. The difference in weight between the groups was tiny and could be chance. The window group lost a little more muscle.
Eight men with high blood sugar ate the same food either within six hours ending in the early afternoon, or spread over twelve hours, for five weeks each. With the early window, their bodies handled sugar better and their blood pressure dropped by about 10 points — without losing weight.
In plain words: A shorter eating window seems to help mostly because people end up eating a little less. When the window sits early in the day, there may be something extra — but that has been tested in very few people.
Eating a little less, for two years
Healthy young and middle-aged adults in the United States were asked to eat less for two years; on average they managed about 12% less. Compared with people who ate as usual, their blood pressure, cholesterol, blood sugar control and inflammation all improved. Measured in their blood, their pace of aging slowed by 2–3%.
Based on: One fair test (CALERIE) · shows cause for health markers, not for a longer life [8][6]
In plain words: This was eating less, not fasting. And a slower “pace of aging” in the blood is a promising sign, not proof of a longer life — the study was not long enough to count years.
What has only been seen in animals
This is where the big promises about fasting come from. It is important to know that they come from animals — and that most of them tested eating less, not the clock alone.
Mice: when they eat matters too
Mice that ate less lived about 10% longer. Mice that ate the same smaller amount, but only during the hours when they are awake and active, lived about 35% longer.
Two long studies of rhesus monkeys, run for almost 30 years, first seemed to disagree. Looked at together, they show that monkeys who ate less lived longer and stayed healthier — depending on how old they were when they started and on what they ate.
Among more than 24,000 pet dogs, those fed once a day had fewer problems in five of nine groups of health conditions, and better scores on thinking. The scientists say clearly that this does not prove the feeding caused it.
Based on: One big study in dogs · a link, not proof [10]
How fasting might work
Scientists who study it think fasting flips a switch. When the body runs out of quick fuel from the last meal, it starts using stored fat and turns on programs that repair and recycle parts of our cells. In animals, this switch protects against many diseases of aging. In people, it has mostly been studied for weeks and months, not for a lifetime. [11]
What we don’t know yet
No fair test has shown that intermittent fasting makes people live longer. All the longer lives on this page belong to animals. The scientists say so themselves.
More years of life? Not tested in people. The longer lives were seen in mice and monkeys, and mostly from eating less, not from the clock.
Better than eating a little less? In most fair tests, no. For some people it may simply be an easier way to eat a little less.
How many hours? Nobody knows the best number for people. Most fair tests used 6 to 10 hours of eating a day, for a few months.
A warning sign that still needs checking
In 2024, a short summary presented at a meeting of heart doctors linked eating within less than 8 hours a day to more deaths from heart disease. It was based on what people remembered eating on just two days, and it was not a full, checked paper. It proves nothing — but it reminds us that answers about doing this for many years are still missing.
Based on: One long-term study, not yet fully published · a link, not proof [12]
What our experts told us
We are deeply grateful to every expert who took the time to teach us. Here is what they said about fasting and eating less — and what it means for you.
“One of the things that surprised me was dogs that are fed once a day compared to dogs that are fed more than once a day are less likely to have developed a whole bunch of different age-related diseases […] Now, is that an intermittent fasting effect? Is it a time restricted feeding effect? Is it an anti-obesity effect? We don’t know the mechanism yet, but there’s clearly this very, very strong correlation.”
In plain words: Dogs that ate once a day had fewer diseases of aging. But even one of the scientists behind that study says he doesn’t know why: it could be the fasting, the timing, or simply that those dogs were less overweight. A link is a question, not an answer. [10]
“So we developed a mouse epigenetic clock. And when we done it, we could quantify very accurately how old the mice are, and when we subject them to calorie restriction or some other interventions, we could see that they are younger than other mice and control mice with the same chronological age.”
In plain words: With a clock that reads age from the body’s cells, his team could see that mice which ate less were biologically younger than other mice of the same age. In mice. Measuring the same in people is the next step.
“So we set up experiments in our laboratory to show that autophagy was part of this self-destruction process that leads to the death of cells, finding just the contrary, that autophagy was a way, at least within a certain threshold, to make cells more resistant against other forms of cell death. […] And the discovery was at the end that autophagy turned out to be an anti-aging mechanism that protects cells against stress.”
In plain words: Autophagy is your cells recycling their own old and broken parts. His team found that it protects cells and works against aging. Fasting is one of the things that switches it on — in animals that is clear; in people, nobody yet knows how many hours it takes. [11]
“You also have the fasting people saying it’s better to only have one meal. […] There’s still a lot of debate in the nutrition world, okay? But I think that that shouldn’t stop people from making healthy choices about what they eat. I mean, there’s a lot of commonality in all of these different diets, and I tend to prefer ones that are balanced.”
In plain words: Scientists still argue about the best way to eat — one meal, three meals, small meals. But all the good diets share a lot. Don’t wait for the final answer to eat well.
“A lot of people link fasting to losing weight. It’s not a very effective way to lose weight because if you don’t change the inherent habits behind your eating, fasting’s not gonna solve it. But it’s a great way for me to promote longevity. […] You’ve seen a lot of mouse studies. There’s still a lot of studies that have to be done on humans.”
In plain words: Gary fasts, and he believes in it for longevity. He also says honestly that the proof in people isn’t there yet — and that fasting won’t change your weight unless your everyday eating changes.
Where the joy is
Lunch together in Ogimi, Okinawa.
In the Live Beyond family we have one rule for everything we share: if it brings no joy, nobody keeps doing it. And food is one of the great joys of life. In Okinawa our first breakfasts surprised us: beautiful, colorful little plates — and then we looked around for more. For days we were simply hungry. Slowly we learned that you can leave the table light and happy instead of stuffed.
“You can see that they eat a lot of fresh food, a lot of fresh vegetables, a lot of fermented foods, which of course for the microbiome is really important. […] And of course the portion sizes are much smaller as well. […] In fact, in Okinawa, apparently there’s a rule that most people follow where you only eat till you’re 80% full.”
In plain words: Fresh food, lots of vegetables, smaller plates, and stopping before you are full. No clock and no counting — just a way of eating.
Then our host in Nanjo told Alek something that nobody writes in the books.
“About 10 years ago, Okinawa is famous for longevity. […] But recently, ranking has fallen. ’Cause they eat too much. Maybe.”
In plain words: Even in Okinawa, traditions change. The famous rule is not a law of nature. It is a habit that people keep alive — or lose.
Okinawa’s oldest generation ate little — a link
Scientists who studied the oldest people in Okinawa found that, when they were younger, they ate a little less than their bodies used, and later in life they had low rates of the diseases of aging.
Based on: Long-term study of one population · a link, not proof [13]
And then Eiko, our guide in Ogimi, told us why she came home to the village after many years in America. It was not about fasting at all.
“And my brother and my sister-in-law and we eat three meals a day here. We eat together. So I don’t eat by myself.”
In plain words: In Ogimi, the village famous for its centenarians, what mattered to Eiko was not how many hours she went without food, but who sat at the table with her.
“Usually I tell my patient, ‘The important thing is what you eat, when you eat, how you eat.’”
In plain words: What you eat, when you eat and how you eat all matter. For us, “how” means slowly, with joy, and with people around the table.
So if a shorter eating window feels good to you, enjoy it — let the kitchen close after dinner, and have breakfast with the people you love. If it makes you anxious, tired or alone at the table, it is not your way, and that is completely fine. You are in control of your own game.
Who should be careful
Talk to your doctor first if any of these is true for you. They come from a review written for doctors by researchers who study fasting.
Pregnant or breastfeeding
Fasting has not been shown to be safe for you and your baby. Talk to your doctor first.
In one fair test, people eating within a short window lost a little more muscle. Keep your muscles working with strength training, and don’t let a short window turn into too little food.
“When you fast, you’re sacrificing muscle, so every decision you make to fast is impacting muscle, so you have to be really careful with fasting.”
In plain words: When there is no food, your body takes some of the building blocks it needs from your muscles. Muscle is one of the best friends of a long life, so protect it.
Listen to yourself
If fasting ever makes you dizzy or weak, or starts to fill your head with food and rules, stop — and talk to your doctor or someone you trust. Longevity means more healthy, joyful years. Nothing that takes the joy out of eating belongs in your game.
We wrote this page from Live Beyond’s own recorded interviews and the published studies listed below, with gratitude to everyone who taught us — and to the people of Okinawa who shared their food and their time with us.
Sources
Every number on this page, by the number in square brackets. Quotations come from Live Beyond’s own recorded interviews, cleaned only of “um” and “uh”. Not medical advice. Talk to your doctor before you change anything.
Semnani-Azad Z, Khan TA, Chiavaroli L, et al. 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. doi:10.1136/bmj-2024-082007
Liu D, Huang Y, Huang C, et al. Calorie restriction with or without time-restricted eating in weight loss. N Engl J Med. 2022;386(16):1495-1504. doi:10.1056/NEJMoa2114833
Mattison JA, Colman RJ, Beasley TM, et al. Caloric restriction improves health and survival of rhesus monkeys. Nat Commun. 2017;8:14063. doi:10.1038/ncomms14063
Acosta-Rodríguez V, Rijo-Ferreira F, Izumo M, et al. Circadian alignment of early onset caloric restriction promotes longevity in male C57BL/6J mice. Science. 2022;376(6598):1192-1202. doi:10.1126/science.abk0297
Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Clinical application of intermittent fasting for weight loss: progress and future directions. Nat Rev Endocrinol. 2022;18(5):309-321. doi:10.1038/s41574-022-00638-x
Waziry R, Ryan CP, Corcoran DL, et al. Effect of long-term caloric restriction on DNA methylation measures of biological aging in healthy adults from the CALERIE trial. Nat Aging. 2023;3(3):248-257. doi:10.1038/s43587-022-00357-y
Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of time-restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: the TREAT randomized clinical trial. JAMA Intern Med. 2020;180(11):1491-1499. doi:10.1001/jamainternmed.2020.4153
Kraus WE, Bhapkar M, Huffman KM, et al. 2 years of calorie restriction and cardiometabolic risk (CALERIE): exploratory outcomes of a multicentre, phase 2, randomised controlled trial. Lancet Diabetes Endocrinol. 2019;7(9):673-683. doi:10.1016/S2213-8587(19)30151-2
Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab. 2018;27(6):1212-1221.e3. doi:10.1016/j.cmet.2018.04.010
Bray EE, Zheng Z, Tolbert MK, et al.; Dog Aging Project Consortium; Kaeberlein M, Kerr KF. Once-daily feeding is associated with better health in companion dogs: results from the Dog Aging Project. GeroScience. 2022;44(3):1779-1790. doi:10.1007/s11357-022-00575-7
de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541-2551. doi:10.1056/NEJMra1905136
Intermittent fasting study sparks debate over increases in CV mortality. TCTMD, 22 March 2024 — report on Zhong VW et al., abstract presented at the American Heart Association EPI|Lifestyle Scientific Sessions, Chicago, 18 March 2024 (NHANES 2003–2018, 20,078 adults; not peer-reviewed).
Willcox BJ, Willcox DC, Todoriki H, et al. Caloric restriction, the traditional Okinawan diet, and healthy aging: the diet of the world’s longest-lived people and its potential impact on morbidity and life span. Ann N Y Acad Sci. 2007;1114:434-455. doi:10.1196/annals.1396.037
