Intermittent fasting 101

Intermittent fasting isn't a diet that tells you what to eat. It's a rhythm that tells you when: you alternate periods of eating with periods of not eating. The most popular version, time-restricted eating, fits every meal into a daily window, such as 8 hours, and fasts for the rest. Other versions fast on whole days instead: every other day, or two days a week in the 5:2 plan ¹.

This guide explains what happens inside you when you stop eating, what well-designed studies have found, where the evidence is still thin, and how to start in a way you can keep up.

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Your first 24 hours of fasting

What happens when you stop eating

After a meal, your body runs mainly on the glucose from your food. Insulin rises and tells your cells to take that glucose in and to store what isn't needed: as glycogen in your liver and muscles, and as fat ¹.

A few hours later, the meal is used up and insulin falls. Your brain still needs a steady supply of glucose, so your liver takes over. It does this in two ways: it breaks down the glycogen it stored, and it builds new glucose from other materials, a process called gluconeogenesis. A classic study measured both in volunteers fasting for almost three days. Even in the first 22 hours, about two thirds of the glucose came from new glucose rather than from stores ³.

Figure

Where your blood sugar comes from as a fast goes on

% of glucose produced

Hours fasted

• New glucose
• Liver glycogen

Source: Rothman et al., Science 1991

As glycogen runs low, your fat cells release more fatty acids, and your muscles and organs burn them directly. Your liver turns some of those fatty acids into ketones, which your brain and muscles can use as fuel. Researchers call this shift from glucose to fat and ketones the metabolic switch. It typically happens somewhere between 12 and 36 hours after your last meal, earlier if you're active and later if your liver started the fast full of glycogen ².

What the research really shows

Part of fasting's appeal is that it fixes a modern habit. When researchers asked healthy adults to log everything they ate with a phone app, half of them ate across a span of more than 14 hours and 45 minutes a day, and more than a third of their calories came after 6 p.m. When people with overweight who usually ate over 14 hours shortened that to 10 or 11 hours for 16 weeks, they lost weight, said they had more energy and slept better ⁴. It was a small pilot study, but it shows how much room most of us have.

So does fasting work? An umbrella review pooled 11 meta-analyses covering 130 randomized trials. It found real benefits for body weight, body fat, cholesterol, triglycerides, blood sugar, insulin and blood pressure. It also found that most of those results rest on low or very low quality evidence, from studies that were small and short, typically three months ⁵.

Another trial asked people to follow a 16:8 schedule, eating between noon and 8 p.m., without any other advice. After 12 weeks they had lost no more weight than people eating three meals a day at their usual times ⁶. Shorter windows can do more: adults with obesity who ate only within 4 or 6 hours lost about 3% of their weight in 8 weeks compared with a control group, because they ended up eating around 550 fewer calories a day without counting anything ⁸.

Put together, the honest summary is this: a fasting window helps mainly by making it easier to eat less, without counting calories. It isn't magic, and it isn't better than other ways of eating less. But for many people it's simpler to follow, and the plan you can follow is the one that works.

Timing may matter for health beyond weight, too. In people with metabolic syndrome, most of whom already took medication for cholesterol or blood pressure, eating within a consistent 10-hour window for 12 weeks reduced weight, blood pressure and harmful blood fats ¹⁰. It was a small study without a control group, so larger trials are needed.

Choosing your plan

The number in each plan is the hours you fast, then the hours you eat. Most of a 13- or 14-hour fast happens while you sleep, which is why they're good places to start.

Your first two weeks

MYTH

Fasting makes you eat twice as much afterwards.

FACT

In trials, people with a shorter eating window usually end up eating less overall, not more. In the 4- and 6-hour trial, they ate around 550 fewer calories a day without counting anything ⁸.

Check what you learned

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What does intermittent fasting mainly change?

What you eatWhen you eatHow much water you drink

In the first day of a fast, where does most of your blood sugar come from?

Only from stored glycogenMostly from new glucose your liver makesFrom your muscles breaking down

When does the metabolic switch to fat and ketones usually happen?

Within the first hourSomewhere between 12 and 36 hoursOnly after a week

What did the year-long trial of an 8-hour window find?

It doubled weight lossIt added little on top of eating fewer caloriesPeople gained weight

Sources

1. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. doi.org/10.1056/NEJMra1905136
2. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254-268. doi.org/10.1002/oby.22065
3. Rothman DL, Magnusson I, Katz LD, et al. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR. Science. 1991;254(5031):573-6. doi.org/10.1126/science.1948033
4. Gill S, Panda S. A Smartphone App Reveals Erratic Diurnal Eating Patterns in Humans that Can Be Modulated for Health Benefits. Cell Metab. 2015;22(5):789-98. doi.org/10.1016/j.cmet.2015.09.005
5. Patikorn C, Roubal K, Veettil SK, et al. Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials. JAMA Netw Open. 2021;4(12):e2139558. doi.org/10.1001/jamanetworkopen.2021.39558
6. 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.org/10.1001/jamainternmed.2020.4153
7. 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.org/10.1056/NEJMoa2114833
8. Cienfuegos S, Gabel K, Kalam F, et al. Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity. Cell Metab. 2020;32(3):366-378.e3. doi.org/10.1016/j.cmet.2020.06.018
9. Sutton EF, Beyl R, Early KS, et al. 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.org/10.1016/j.cmet.2018.04.010
10. Wilkinson MJ, Manoogian ENC, Zadourian A, et al. Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome. Cell Metab. 2020;31(1):92-104.e5. doi.org/10.1016/j.cmet.2019.11.004
11. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873
12. Poggiogalle E, Jamshed H, Peterson CM. Circadian regulation of glucose, lipid, and energy metabolism in humans. Metabolism. 2018;84:11-27. doi.org/10.1016/j.metabol.2017.11.017