Types of intermittent fasting compared: 16:8, 5:2 and alternate-day fasting, what a week looks like on each, what the trials and the 2025 BMJ review found, and who should not fast.
Fact-checked against FDA labels and official sources; not yet reviewed by a licensed clinician. This page is education, not medical advice. Talk to your doctor before starting, stopping or changing any treatment.
Key takeaways
- Across 99 trials, every fasting method reduced weight compared with unrestricted eating; only alternate-day fasting beat daily calorie restriction, by 1.29 kg, a difference the authors rated trivial.[1]
- In trials of 24 weeks or more, no fasting method beat a daily calorie cut.[1]
- Alternate-day fasting matched daily calorie restriction at one year but had more dropouts (38% vs 29%) and higher LDL cholesterol.[2]
- A 5:2-style plan and a daily cut of the same weekly size gave similar weight loss at 6 months (6.4 vs 5.6 kg).[3]
- Low blood sugar is the top fasting risk for people on insulin, sulfonylureas or meglitinides; plan any fast with your care team.[9]
In short: The three main types of intermittent fasting are 16:8 (eating within an 8-hour window every day), 5:2 (two low-calorie days a week) and alternate-day fasting (a low-calorie day every other day). In trials, each one led to about the same weight loss as simply eating a bit less every day; a 2025 review of 99 trials found a small extra loss only for alternate-day fasting, mostly in shorter studies. The method that fits your week, and that you can keep up safely, matters more than the label.
This page sets the three methods side by side: what a week looks like on each, what the trials found and who should not fast. It does not name a winner. For the full background on fasting, see our intermittent fasting guide, and if you want a timer to keep track, see our researched picks of intermittent fasting apps. This is general education, not a personal plan. If you take medicines for diabetes or blood pressure, are pregnant or have had an eating disorder, talk with your clinician before you change when you eat.
The three methods in one minute
A 2022 review in Nature Reviews Endocrinology names these three as the most studied forms of intermittent fasting. All of them are ways of eating less over a week; they differ in how the fasting is spread out.
| 16:8 (time-restricted eating) | 5:2 | Alternate-day fasting | |
|---|---|---|---|
| The rule | Eat only within a set window each day, usually 8 hours, and fast for the other 16 | Two “fast days” a week with very low intake, five days of usual eating | Fast days alternate with days of unrestricted eating |
| Fast days per week | None; every day has an eating window | 2 | 3 or 4 (every other day) |
| What a fast day looks like | Not a fast day; an overnight and morning (or evening) fast | About 600 to 650 calories in the main UK trials | 0 to about 500 calories, or 25% of energy needs in the longest trial |
| Calorie counting | Not required in TREAT and Lin 2023; added in Liu 2022 | A low calorie limit on the 2 low days | A calorie limit on fast days |
| In trials vs eating less every day | About the same at 12 months | About the same at 6 months | About the same at 12 months, with more dropouts |
What a week looks like on each method
The easiest way to see the difference is to lay out a week. Here “F” is a fast day (very low intake), “E” is a usual eating day and “8h” is a day with an 8-hour eating window. Which days you fast on 5:2 is up to you; these are examples.
| Method | Mon | Tue | Wed | Thu | Fri | Sat | Sun |
|---|---|---|---|---|---|---|---|
| 16:8 | 8h | 8h | 8h | 8h | 8h | 8h | 8h |
| 5:2 | F | E | E | F | E | E | E |
| Alternate-day fasting | F | E | F | E | F | E | F |
| Daily calorie cut (for comparison) | -25% | -25% | -25% | -25% | -25% | -25% | -25% |
For 16:8, the trials used two different windows. The TREAT trial and a 2023 Chicago trial asked people to eat from noon to 8 p.m.; a 2022 trial in China used 8 a.m. to 4 p.m. Either way, the day splits into 8 eating hours and 16 fasting hours:
| Item | Eating window | Fasting |
|---|---|---|
| TREAT 2020 (noon to 8 p.m.) | 8 hours | 16 hours |
| Lin 2023 (noon to 8 p.m.) | 8 hours | 16 hours |
| Liu 2022 (8 a.m. to 4 p.m.) | 8 hours | 16 hours |
Liu 2022 also set a daily calorie limit; TREAT and Lin 2023 did not ask people to count calories.
Source: Lowe DA et al., JAMA Intern Med 2020; Lin S et al., Ann Intern Med 2023; Liu D et al., NEJM 2022 (checked on October 7, 2026)
For alternate-day fasting and 5:2, the difference is in calories rather than hours. In the longest alternate-day fasting trial, people ate 25% of their energy needs on fast days and 125% on the “feast” days in between, while the comparison group ate 75% every day. Both add up to about the same weekly cut, just spread differently:
| Item | Value |
|---|---|
| Alternate-day fasting, fast day | 25% |
| Alternate-day fasting, feast day | 125% |
| Daily calorie restriction, every day | 75% |
Prescribed amounts. In practice the fasting group ate more than prescribed on fast days and less on feast days.
Source: Trepanowski JF et al., JAMA Internal Medicine 2017 (100 adults with obesity) (checked on October 7, 2026)
The 5:2 trials by Michelle Harvie’s team in Manchester used two days of about 2,500 to 2,700 kilojoules (roughly 600 to 650 calories) a week, set so the overall weekly cut was 25%, the same as the daily-restriction group. The NHS describes the popular 5:2 diet the same way: eat less on 2 days and eat a healthy, balanced diet on the other 5.
What the trials found for each method
16:8: TREAT (2020), Liu (2022) and Lin (2023)
16:8 on its own, short term. In TREAT (JAMA Internal Medicine, 2020), 116 U.S. adults with overweight or obesity either ate only from noon to 8 p.m. or ate three structured meals a day, with no other advice. After 12 weeks the 16:8 group had lost 0.94 kg (about 2 pounds) and the three-meals group 0.68 kg; the difference was not significant. The authors concluded that time-restricted eating alone was not more effective than eating throughout the day. In the subgroup measured in person, the 16:8 group lost a little more lean mass in the arms and legs.
16:8 plus a calorie limit, one year. In a New England Journal of Medicine trial (2022), 139 adults with obesity all followed the same reduced-calorie diet (1,500 to 1,800 calories a day for men, 1,200 to 1,500 for women), and half also ate only between 8 a.m. and 4 p.m. At 12 months, the time-restricted group had lost 8.0 kg and the calorie-only group 6.3 kg; the 1.8 kg difference was not significant. Waist, body fat and blood pressure followed the same pattern.
16:8 without counting, one year. In a trial of 90 adults with obesity in Chicago (Annals of Internal Medicine, 2023), eating from noon to 8 p.m. without counting calories was compared with a 25% daily calorie cut and with a control group. Both groups cut about 400 calories a day on average. Compared with the control group, the 16:8 group lost 4.61 kg and the calorie-cut group 5.42 kg by 12 months, a difference that was not significant. Our comparison of intermittent fasting vs calorie counting looks at these two trials in more detail.
5:2: Harvie trials (2011 and 2013)
In the first trial (International Journal of Obesity, 2011), 107 premenopausal women with overweight or obesity followed either two low days a week or a daily cut of the same weekly size for 6 months. Weight loss was similar: 6.4 kg with 5:2 and 5.6 kg with daily restriction. Cholesterol, triglycerides and blood pressure fell similarly in both groups; fasting insulin and insulin resistance fell slightly more with 5:2.
The follow-up trial (British Journal of Nutrition, 2013) tested a version with under 40 grams of carbohydrate on the two low days in 115 women. Over 3 months, both 5:2-style groups lost more body fat (3.7 kg) than the daily-restriction group (2.0 kg) and improved insulin resistance more. The authors called for longer studies, because 3 months is short.
Alternate-day fasting: Trepanowski (2017)
This one-year trial in Chicago randomly assigned 100 adults with obesity (86 women, average age 44) to alternate-day fasting, a 25% daily calorie cut or no diet change. Compared with the control group, weight loss was the same at 6 months (6.8% in both diet groups) and close at 12 months (6.0% with fasting and 5.3% with daily restriction). More people dropped out of the fasting group (38%) than the daily-restriction group (29%). LDL (“bad”) cholesterol was higher in the fasting group at 12 months, by 11.5 mg/dL compared with daily restriction. The authors concluded that alternate-day fasting was not better than daily restriction for sticking with it, weight loss, keeping weight off or heart risk markers.
| Item | Alternate-day fasting | Daily calorie restriction |
|---|---|---|
| 6 months | 6.8% | 6.8% |
| 12 months | 6% | 5.3% |
Percent of body weight lost relative to the control group. Dropouts: 38% (fasting), 29% (daily restriction), 26% (control).
Source: Trepanowski JF et al., JAMA Internal Medicine 2017 (100 adults with obesity) (checked on October 7, 2026)
The trials side by side (not head-to-head)
Each trial below compared one fasting method with a non-fasting group. They were run in different countries, with different people, lengths and rules, so the bars show how each fasting group did against its own comparison group. They are not head-to-head results between the fasting methods, and the heights should not be compared across trials.
| Item | Fasting group | Comparison group |
|---|---|---|
| TREAT, 16:8 vs three meals (12 weeks) | 0.94 kg | 0.68 kg |
| Harvie 2011, 5:2 vs daily cut (6 months) | 6.40 kg | 5.60 kg |
| Liu 2022, 16:8 plus calorie limit vs calorie limit (12 months) | 8 kg | 6.30 kg |
Not head-to-head: separate trials with different people and lengths. No difference within any trial was statistically significant.
Source: Lowe DA et al., JAMA Intern Med 2020; Harvie MN et al., Int J Obes 2011; Liu D et al., NEJM 2022 (checked on October 7, 2026)
Within each trial, the fasting group and the comparison group ended up close. That is the main lesson of a decade of fasting research: the timing changes how a week feels, but weight loss tracks how much less people eat overall. A calorie deficit is still what drives the change.
What the 2025 BMJ network meta-analysis adds
Single trials can’t compare all three methods at once, but a network meta-analysis can, by linking trials through their shared comparison groups. The largest so far, published in The BMJ in June 2025, pooled 99 randomized trials with 6,582 adults. It grouped plans as time-restricted eating (such as 16:8), alternate-day fasting and “whole-day fasting”, which the authors describe as cycles of 24-hour fasts, for example the 5:2 diet.
The BMJ 2025 network meta-analysis at a glance
- 99 trialsrandomized trials, 6,582 adults; every fasting method and daily calorie restriction reduced weight compared with unrestricted eatingSemnani-Azad Z et al., BMJ 2025
- 1.29 kgextra loss with alternate-day fasting compared with daily calorie restriction (moderate certainty), the only fasting method that beat itSemnani-Azad Z et al., BMJ 2025
- 1.69 kgextra loss with alternate-day fasting compared with 16:8-style eating; 1.05 kg compared with whole-day fasting such as 5:2Semnani-Azad Z et al., BMJ 2025
- 2.0 kgthe smallest difference the authors counted as meaningful, so these gaps were rated trivialSemnani-Azad Z et al., BMJ 2025
Checked on October 7, 2026
Two details matter. First, the authors set 2.0 kg (about 4.4 pounds) as the smallest meaningful difference in body weight, and the extra loss with alternate-day fasting fell below that, so they described it as trivial. Second, in the 17 trials that lasted 24 weeks or more, diets beat unrestricted eating but no fasting method beat daily calorie restriction. The authors’ overall conclusion was that intermittent fasting has similar benefits to daily calorie restriction for weight and heart risk factors, and that longer trials are needed.
On blood fats, alternate-day fasting lowered total cholesterol, triglycerides and non-HDL cholesterol compared with time-restricted eating, while time-restricted eating raised total and LDL cholesterol slightly compared with whole-day fasting. No method changed HbA1c (long-term blood sugar) or HDL cholesterol differently. Most trials reported only mild side effects, such as constipation, nausea, hunger, diarrhea and dizziness.
Sticking with it: where the methods differ most
If weight loss is similar, the real differences are in daily life. The trial data hint at how hard each method is to keep up:
- Alternate-day fasting had the highest dropout rate in its one-year trial (38%, against 29% for daily restriction), and people ate more than planned on fast days. It is the most demanding of the three.
- 5:2 asks for only two hard days a week, but the BMJ review notes one year-long 5:2 trial in which adherence fell from 74% at six weeks to 22% at a year.
- 16:8 has no fast days, which many people find easier, and 85% of people completed the 12-month Liu trial. On its own, without eating less overall, it changed weight little in TREAT.
- Social life and work. Family dinners, shift work and weekend meals can clash with a strict window or a fixed fast day. Many people adjust the window or move a fast day rather than drop the plan.
Whatever the method, what you eat in the eating window still counts. Building meals around protein, vegetables and fiber helps with hunger; our lists of filling foods and the high-protein diet guide have ideas, and the protein calculator gives a daily range. Keeping up strength training helps protect muscle, which matters because TREAT saw a small extra lean-mass loss with 16:8.
Who should not fast, or should ask first
Fasting is not for everyone, and the cautions apply to all three methods. NHS guidance says people with a history of eating disorders such as anorexia or bulimia, and people who are pregnant or breastfeeding, should not try intermittent fasting. It quotes the British Dietetic Association’s advice never to delay or skip meals if you are pregnant, have had or are prone to eating disorders, or have diabetes, and it asks anyone on medication to tell their care team before changing their diet.
For people with diabetes, the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) calls low blood sugar (hypoglycemia) the number one risk of fasting. Insulin, sulfonylureas and the short-acting meglitinides are the medicines most linked with it, and cutting doses too far can push blood sugar too high instead. People with type 1 diabetes face greater risk than people with type 2, because they take insulin, and also face a higher risk of dehydration and diabetic ketoacidosis. Fasting with diabetes should be planned with your health care team; never change a diabetes medicine on your own. Our guide to type 2 diabetes and weight has more on weight loss with diabetes.
Some diabetes medicines carry their own warning. The current prescribing information for Jardiance (empagliflozin), an SGLT2 inhibitor, lists reduced calorie intake among the conditions that can bring on ketoacidosis, a serious condition, and its patient information mentions skipping meals. If you take an SGLT2 inhibitor or a GLP-1 medicine, ask your prescriber before adding a fasting schedule; our guide to eating on a GLP-1 covers meals and protein on those medicines.
What people weigh up
There is no best method for everyone. These are the trade-offs people tend to weigh with their clinician or dietitian:
- Fixed daily routine: a daily window (16:8, or a gentler 12- or 14-hour overnight fast) needs no fast days and fits people who like the same rhythm every day.
- Flexible weeks: 5:2 leaves five days of ordinary eating, which some people find easier around social plans; the two low days are hard for others.
- A bigger short-term push: alternate-day fasting showed a small extra loss in the BMJ review, mostly in shorter trials, but it had the most dropouts and higher LDL cholesterol at a year in its main trial.
- No fasting at all: a steady daily calorie cut did as well as fasting in every trial on this page. The Mediterranean diet is one flexible way to do it, and our ranking of diets for weight loss compares the main patterns by evidence.
Trying a fasting method sensibly
- Check with your clinicianEspecially if you take medicines, are pregnant or have a health condition.
- Start gentlyA 12-hour overnight fast is a simple first step; narrow it only if you feel well.
- Plan the eating windowProtein, vegetables, fruit and whole grains; water and calorie-free drinks while fasting.
- Keep movingStrength training two days a week helps protect muscle.
- Review after a few weeksIf you are hungrier, more irritable or eating more overall, try another structure.
To estimate how many calories a day your body uses, try the calorie calculator. For the wider picture of every eating approach, start at our diets hub, and for keeping weight off once you stop fasting, see maintaining weight loss. More side-by-side pages are in our comparisons hub.
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- Is any form of fasting safe with my medicines and health conditions?
- If I take diabetes medicine, how would my doses or blood sugar checks change on fast days?
- Would a daily eating window, 5:2 or a steady calorie cut fit my routine best?
- Should my cholesterol be checked if I try alternate-day fasting?
- How do I get enough protein and nutrients in a shorter eating window?
Frequently asked questions
What are the types of intermittent fasting?
The three most studied are time-restricted eating such as 16:8 (eating within a daily window), the 5:2 diet (two very-low-calorie days a week) and alternate-day fasting (a fast day every other day, 0 to about 500 calories).
Which type of intermittent fasting works best for weight loss?
None clearly. A 2025 BMJ review of 99 trials found a small extra loss only for alternate-day fasting compared with daily calorie restriction, mostly in shorter trials, and rated it trivial. In longer trials no method beat a daily calorie cut.
Is 16:8 or 5:2 easier to stick with?
It depends on your routine. 16:8 has no fast days and most people finished the year-long Liu trial; 5:2 limits the hard days to two a week, but in one year-long trial adherence fell sharply. Decide with your clinician or dietitian based on what fits your week.
Does alternate-day fasting raise cholesterol?
In its one-year trial, LDL cholesterol was 11.5 mg/dL higher with alternate-day fasting than with daily calorie restriction at 12 months. In the 2025 BMJ review, alternate-day fasting lowered some blood fats compared with time-restricted eating. Ask your clinician to check your lipids if you try it.
Who should not try intermittent fasting?
NHS guidance says people with a history of eating disorders and people who are pregnant or breastfeeding should not. People with diabetes, especially on insulin, sulfonylureas or meglitinides, should only fast with a plan from their care team.
References
- 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. doi:10.1136/bmj-2024-082007 · PMID 40533200 (accessed October 7, 2026) Meta-analysis
- Trepanowski JF, Kroeger CM, Barnosky A, et al.. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine, 2017. doi:10.1001/jamainternmed.2017.0936 · PMID 28459931 · NCT00960505 (accessed October 7, 2026) Randomized trial
- Harvie MN, Pegington M, Mattson MP, et al.. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity, 2011. doi:10.1038/ijo.2010.171 · PMID 20921964 (accessed October 7, 2026) Randomized trial
- Harvie M, Wright C, Pegington M, et al.. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. British Journal of Nutrition, 2013. doi:10.1017/S0007114513000792 · PMID 23591120 (accessed October 7, 2026) Randomized trial
- 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 Internal Medicine, 2020. doi:10.1001/jamainternmed.2020.4153 · PMID 32986097 · NCT03393195 (accessed October 7, 2026) Randomized trial
- 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. doi:10.1056/NEJMoa2114833 · PMID 35443107 · NCT03745612 (accessed October 7, 2026) Randomized trial
- Lin S, Cienfuegos S, Ezpeleta M, et al.. Time-Restricted Eating Without Calorie Counting for Weight Loss in a Racially Diverse Population: A Randomized Controlled Trial. Annals of Internal Medicine, 2023. doi:10.7326/M23-0052 · PMID 37364268 · NCT04692532 (accessed October 7, 2026) Randomized trial
- Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Clinical application of intermittent fasting for weight loss: progress and future directions. Nature Reviews Endocrinology, 2022. doi:10.1038/s41574-022-00638-x · PMID 35194176 (accessed October 7, 2026) Review
- Fasting Safely with Diabetes. National Institute of Diabetes and Digestive and Kidney Diseases (NIH). (accessed October 7, 2026) Government page
- Intermittent fasting. NHS (MyHealth London). (accessed October 7, 2026) Government page
- JARDIANCE (empagliflozin) tablets: prescribing information and patient information. Boehringer Ingelheim Pharmaceuticals via DailyMed (U.S. National Library of Medicine). (accessed October 7, 2026) Drug label
Facts checked on October 7, 2026
Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.