By FoodVersus Editorial Team | Dubai, United Arab Emirates
Intermittent fasting is one of the most searched weight-loss approaches in the world, and two versions dominate: 16:8, where you eat only within an eight-hour window each day, and 5:2, where you eat normally five days a week and sharply cut calories on two. Social media tends to treat one or the other as the winner. The research gives a more careful answer.
This article explains how each method works, what randomized trials and a major 2025 network meta-analysis found, and how to decide which one might fit your life, while being clear about what the evidence can and cannot tell us.
The Two Methods Explained
| Feature | 16:8 (Time-Restricted Eating) | 5:2 (Whole-Day Fasting) |
|---|---|---|
| How it works | Fast for 16 hours and eat within an 8-hour window every day, for example noon to 8 p.m. | Eat normally 5 days a week; on 2 non-consecutive days, eat very little (commonly around 500-600 calories) |
| What is restricted | When you eat; no specific foods or calorie targets required | How much you eat on the fasting days |
| Frequency of restriction | Every day | Two days per week |
| Main appeal | Simple daily routine; much of the fast happens during sleep | Five "normal" days; restriction is concentrated |
| Common challenge | Skipping breakfast or late meals, and social meals outside the window | Hunger, low energy, or irritability on fasting days |
In research terminology, 16:8 is a form of time-restricted eating (TRE), while 5:2 belongs to the group called whole-day fasting or intermittent energy restriction. This distinction matters when reading studies, because they often test one category against standard dieting rather than testing 16:8 and 5:2 against each other.
The Biggest Evidence Review: A 2025 BMJ Network Meta-Analysis
The most comprehensive comparison to date is a systematic review and network meta-analysis published in The BMJ in 2025 (Semnani-Azad et al.). It pooled 99 randomized trials with 6,582 adults (average age 45, two-thirds women, average BMI of 31), with trials lasting 3 to 52 weeks and an average of 12 weeks. It compared several intermittent fasting strategies with each other and with continuous energy restriction, meaning ordinary daily calorie reduction. The key findings were:
First, intermittent fasting diets had weight-loss benefits similar to continuous energy restriction. Second, alternate day fasting (eating normally one day and fasting the next) was the only approach showing a small advantage over continuous calorie restriction, about 1.3 kg more weight loss, and it also outperformed time-restricted eating by about 1.7 kg and whole-day fasting such as 5:2 by about 1.1 kg. Third, none of these differences reached the 2 kg threshold the authors set as the minimum meaningful difference for people with obesity. Fourth, the authors rated the certainty of the evidence as low to moderate for most outcomes, citing variation between trials and small sample sizes, and called for longer trials.
For readers choosing between 16:8 and 5:2, the practical reading of this review is that neither stands out as clearly better for weight loss, and both land in the same range as simply eating less each day.
What Individual Trials Show
16:8: the TREAT trial. In a 12-week randomized trial of 116 adults with overweight or obesity (Lowe et al., JAMA Internal Medicine, 2020), one group ate only between noon and 8 p.m. while a control group ate three structured meals a day. Weight fell by 0.94 kg in the time-restricted group and 0.68 kg in the control group, and the difference between the groups (0.26 kg) was not statistically significant. The authors concluded that time-restricted eating, when not combined with other interventions, was no more successful than consistent meal timing. The study was short, and the participants were not instructed to cut calories.
16:8 plus calorie counting: the NEJM trial. A 12-month randomized trial in China (Liu et al., New England Journal of Medicine, 2022) enrolled 139 adults with obesity and put everyone on a calorie-restricted diet. One group was also limited to an 8 a.m. to 4 p.m. eating window. At 12 months, the time-restricted group had lost 8.0 kg (9.0% of body weight) and the calorie-restriction-only group 6.3 kg (7.2%), a difference of 1.8 kg that was not statistically significant. Blood pressure, waist circumference, and metabolic markers also did not differ between groups. The authors noted the small sample and a shorter baseline eating window than in some other studies.
5:2: a six-month trial in women. In a randomized trial of 107 premenopausal women with overweight or obesity (Harvie et al., International Journal of Obesity, 2011), one group cut calories by 25% on two days a week (using a very low calorie approach on those days) and ate without restriction on the other five, while another group cut calories by 25% every day. After six months, the two-day group had lost an average of 6.4 kg and the daily-restriction group 5.6 kg, a difference that was not statistically significant. In other words, 5:2 matched ordinary daily dieting in this trial, not clearly better or worse.
Side-by-Side Evidence Summary
| Study | Method Tested | Result vs. Comparison Group |
|---|---|---|
| BMJ 2025 network meta-analysis (99 trials, 6,582 adults) | Several types, including TRE and whole-day fasting | Similar to continuous calorie restriction; no difference reached 2 kg |
| TREAT, JAMA Internal Medicine 2020 (116 adults, 12 weeks) | 16:8 style (noon to 8 p.m.) | -0.94 kg vs. -0.68 kg; not significantly different |
| Liu, NEJM 2022 (139 adults, 12 months) | 8-hour window plus calorie restriction | -8.0 kg vs. -6.3 kg; difference not significant |
| Harvie, Int J Obes 2011 (107 women, 6 months) | 2 restricted days per week | -6.4 kg vs. -5.6 kg; difference not significant |
Why the Results Look So Similar
The consistent pattern across these studies is that weight loss tracks total calories eaten over time. Both 16:8 and 5:2 are simply different ways of creating a calorie deficit: 16:8 does so by shrinking the eating window, which often makes people eat less without counting, while 5:2 does so by concentrating a large deficit into two days. If a method does not reduce the weekly calorie total, it does not produce weight loss, and if two methods produce the same deficit, they tend to produce similar results. This matches what our comparison of intermittent fasting and calorie deficit found, and it explains why differences between the methods are small.
How to Choose Between 16:8 and 5:2
Since neither wins clearly on weight loss, the more useful question is which pattern you can actually keep up for months. 16:8 may suit you if you prefer a daily routine, do not mind skipping breakfast or finishing dinner early, and dislike having very low-calorie days. 5:2 may suit you if you prefer eating normally most days and can tolerate two harder days, particularly if your schedule has predictable busy days when skipping meals is easier. Either approach tends to work better when the foods you eat are nutrient-dense and rich in protein and fiber, which helps with hunger and muscle preservation, as covered in our guides on daily protein and fiber targets. Whichever you try, give it several weeks and track how you feel, how well you sleep, and how consistently you can follow it.
Who Should Not Try Intermittent Fasting Without Medical Guidance
Fasting is not appropriate for everyone. The following groups should speak with a doctor first, and some should avoid fasting altogether: people with diabetes, especially those using insulin or medications that can cause low blood sugar; people who are pregnant or breastfeeding; children and teenagers; people with a history of an eating disorder or disordered eating; people who are underweight or malnourished; people taking medications that must be taken with food; older adults at risk of muscle loss; and people with certain medical conditions such as gallbladder disease or advanced kidney, liver, or heart disease. Common short-term side effects reported with fasting include hunger, headaches, fatigue, difficulty concentrating, constipation, and irritability, and these can be more pronounced on 5:2 fasting days. Stop and seek medical advice if you experience fainting, severe weakness, palpitations, or signs of low blood sugar.
What the Research Cannot Tell Us Yet
Several limitations are worth knowing. Most trials last three months or less, so long-term results beyond a year are limited. Few trials compare 16:8 and 5:2 directly against each other; most compare each method with standard dieting. Participants are often volunteers who are motivated and monitored, which may not reflect real-world results. The BMJ review rated the certainty of the evidence as low to moderate. And individual responses vary widely, so an average result in a study does not predict yours.
A Note on Using This Information
This article summarizes findings from published clinical research for general educational purposes and does not constitute medical or nutritional advice, a diagnosis, or a treatment recommendation. The studies cited involved specific populations, such as premenopausal women in the 5:2 trial and adults in China in the NEJM trial, and their group averages do not predict any individual's results. Health outcomes depend on age, medical conditions, medications, sleep, activity, and the quality of the food you eat, which a general article cannot assess.
Talk to a doctor or registered dietitian before starting any fasting regimen, especially if you have diabetes, heart, kidney or liver disease, take regular medications, are pregnant or breastfeeding, or have ever had an eating disorder. Do not change the timing or dosing of prescribed medications because of a diet plan without your prescriber's approval.
Primary sources referenced in this article:
1. 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: https://doi.org/10.1136/bmj-2024-082007
2. 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;180(11):1491-1499: https://pubmed.ncbi.nlm.nih.gov/32986097/
3. 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;386:1495-1504: https://doi.org/10.1056/NEJMoa2114833
4. 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;35(5):714-727: https://doi.org/10.1038/ijo.2010.171
About the Author
This article was written by the FoodVersus Editorial Team, based in Dubai, United Arab Emirates. FoodVersus publishes research-based comparisons and guides on nutrition, diet, and weight loss, with every article citing its primary sources. Our content is for general education and is not a substitute for professional medical advice.
How this article was prepared: We are a team that researched and wrote this article together. We reviewed the primary studies and official sources listed above, checked the figures against the published study summaries, noted where evidence is limited, and organized the findings so the comparison is clear and fair. If you spot an error or an outdated claim, please contact us through the Contact page so we can review and correct it.
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