Intermittent Fasting: 16:8, 5:2 and what the evidence shows

Key takeaways

• In a 12-month trial, 16:8 eating plus calorie limits did not lead to significantly more weight loss than calorie limits alone.^[3]
• Alternate-day fasting matched daily calorie restriction at one year, but more people dropped out.^[4]
• Low blood sugar is the top fasting risk for people on insulin, sulfonylureas or meglitinides: plan any fast with your care team.^[7]
• People who are pregnant or breastfeeding, or have a history of eating disorders, should not try intermittent fasting.^[8]

In short: Intermittent fasting means alternating set periods of eating with periods of fasting or very low eating, for example eating only within an 8-hour window (16:8) or eating very little on two days a week (5:2). In randomized trials it leads to about the same weight loss as simply eating a bit less every day, not more. It can be a useful structure if it suits your routine, but it is not right for everyone, especially people who take insulin or certain diabetes pills, are pregnant or have a history of eating disorders.

This page explains the main methods, what the research shows compared with ordinary calorie cutting, who should not fast, how fasting interacts with medicines, and how to try it sensibly. It is general information, not personal medical advice. To compare it with other eating approaches, see our diets hub.

The main methods: 16:8, 5:2 and alternate-day fasting

A 2022 review in Nature Reviews Endocrinology describes three forms that have been studied most:

• Time-restricted eating (such as 16:8). You eat only within a set window each day, often 8 hours (for example noon to 8 p.m.), and fast for the rest. Many people find this the easiest form because it mostly means skipping late-night eating or breakfast.
• The 5:2 diet. Two “fast days” a week with very low intake, and five days of usual eating.
• Alternate-day fasting. Fast days (0 to about 500 calories) alternate with days of unrestricted eating. It is the most demanding version.

All three work the same way in the end: they make it easier for some people to eat less over a week. None of them changes the basic rule that weight loss needs a calorie deficit.

What the evidence shows compared with daily calorie restriction

16:8 on its own. In the TREAT trial (JAMA Internal Medicine, 2020), 116 adults with overweight or obesity were asked either to eat only between noon and 8 p.m. or to eat three structured meals a day, with no other advice. After 12 weeks, the time-restricted group had lost a little weight (about 0.9 kg, or 2 pounds), but the difference from the three-meals group was not significant. The authors concluded that time-restricted eating alone was not more effective.

16:8 plus calorie limits. In a 12-month trial in the New England Journal of Medicine (2022), 139 adults with obesity all followed the same reduced-calorie diet, and half also ate only between 8 a.m. and 4 p.m. Both groups lost a meaningful amount, about 8.0 kg in the time-restricted group and 6.3 kg in the calorie-only group, but the difference between them was not significant. Waist size, body fat and blood pressure followed the same pattern.

8 kgCalorie limits plus eating 8 a.m. to 4 p.m.

6.3 kgCalorie limits only

Weight loss at 12 months, both groups on the same reduced-calorie diet

Item	Value
Calorie limits plus eating 8 a.m. to 4 p.m.	8 kg
Calorie limits only	6.3 kg

139 adults with obesity. The difference between the groups was not statistically significant.

Source: Liu D et al., New England Journal of Medicine 2022 (checked on October 6, 2026)

Alternate-day fasting. In a one-year trial of 100 adults with obesity (JAMA Internal Medicine, 2017), alternate-day fasting and daily calorie restriction produced similar weight loss at 6 and 12 months. More people dropped out of the fasting group (38%) than the daily-restriction group (29%), and LDL cholesterol was higher in the fasting group at 12 months.

38%Alternate-day fasting

29%Daily calorie restriction

Share of people who dropped out within one year

Item	Value
Alternate-day fasting	38%
Daily calorie restriction	29%

100 adults with obesity. Weight loss was similar in both groups.

Source: Trepanowski JF et al., JAMA Internal Medicine 2017 (checked on October 6, 2026)

5:2. In a 6-month trial of 107 women with overweight or obesity, two very-low-calorie days a week and daily calorie restriction gave similar weight loss (about 6.4 kg and 5.6 kg), with similar changes in cholesterol and blood pressure.

Across studies. A 2024 umbrella review in eClinicalMedicine, which pooled 23 meta-analyses of randomized trials, found that intermittent fasting reduced weight and fat mass and improved some blood fats and fasting insulin. Taken together, the trials suggest that fasting is one valid way to eat less, about as effective as eating less every day. Pick it if the structure helps you, not because it burns fat in a special way.

Note on names: “time-restricted eating” and “16:8” describe one form of intermittent fasting, and alternate-day fasting is another. This page covers all of them; we will add separate pages only if a method has enough distinct substance.

Who should not fast, or should ask first

Fasting is not a good fit for everyone. NHS guidance advises that people with a history of eating disorders, and people who are pregnant or breastfeeding, should not try intermittent fasting, and the British Dietetic Association advises never to delay or skip meals if you are pregnant, have had or are prone to eating disorders, or have diabetes. If you are under 18, are underweight or have been told to eat at regular times for a medical reason, ask your clinician before trying it. If fasting leads to bingeing, constant thoughts about food or feeling out of control around eating, stop and talk to a professional; our guide to emotional eating can help you find support.

Medicines and fasting

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. The medicines that raise this risk most are insulin, sulfonylureas and the short-acting meglitinides. Cutting medicine too much can go the other way and raise blood sugar. The NIDDK’s advice is to plan any fast with your health care team, who can adjust doses and monitoring. Do not change a diabetes medicine on your own.

Other medicines can matter too, for example those that need to be taken with food or those that affect blood pressure or fluids. If you take any regular medicine, ask your pharmacist or clinician before you start a fasting plan. If you take a GLP-1 medicine, talk to your prescriber before adding a fasting schedule; our guide to eating on a GLP-1 covers protein and meals while on these medicines.

How to try it sensibly

Difficulty, sustainability and cost

Intermittent fasting costs nothing extra and can simplify a busy day. Difficulty depends on the method: time-restricted eating is moderate for most people, while alternate-day fasting is hard, which shows up in the higher dropout rate in the 2017 trial. Social meals, shift work and family routines can make strict windows hard to keep for years. As with any approach, the best version is the one you can keep while eating well.

Alternatives

If fasting does not suit you, a steady daily approach works just as well in trials. The Mediterranean diet and the DASH diet are flexible patterns with strong health evidence, and a high-protein diet can help with fullness. Our calorie deficit calculator shows what a daily deficit looks like, and our how to lose weight guide brings all the options together.

The approach

Set periods of eating alternate with fasting or very low intake: daily time-restricted eating (such as 16:8), two low-intake days a week (5:2), or alternate-day fasting. It works by helping some people eat less overall.

What the evidence shows

Randomized trials show weight loss similar to daily calorie restriction, not greater: 16:8 alone was not better than three meals a day over 12 weeks (TREAT, 116 adults); 16:8 plus calorie limits gave -8.0 vs -6.3 kg at 12 months, not significantly different (139 adults); alternate-day fasting matched daily restriction at 1 year with more dropouts (100 adults); 5:2 matched daily restriction at 6 months (107 women).

Foods

Emphasized

• Protein foods at each meal in the eating window
• Vegetables, fruit and whole grains
• Healthy fats such as olive oil and nuts
• Water and other calorie-free drinks during the fast

Limited

• Large portions of ultra-processed food in the eating window
• Sugary drinks
• Late-night snacking outside the window

Protein

Fewer meals can mean less protein. Include a protein food at every meal in the eating window and keep strength training to protect muscle; the protein calculator gives a daily range.

Scorecard

Evidence

ModerateNot supportedHigh

Invasiveness

NoneNoneSurgical

Medical supervision

RecommendedNoneRequired

Convenience

ModerateSimpleIntensive

Cost

$$$$$$

Maintenance

MediumLowOngoing

No overall score: each dimension stands on its own. Hover a value for its definition.

Advantages and limitations

Advantages

• Costs nothing and needs no special foods
• Simple rule that some people find easier than counting
• Weight loss similar to daily calorie restriction in trials
• Flexible: 12-hour overnight fasts are an easy start

Limitations

• Not more effective than eating a little less every day
• Alternate-day fasting had more dropouts in a one-year trial
• Can clash with social meals, shift work and family routines
• Risk of low blood sugar with insulin or sulfonylureas
• Not suitable with a history of eating disorders, or during pregnancy or breastfeeding

Check with a clinician first if

• People who take insulin, sulfonylureas or meglitinides, or have type 1 diabetes
• People who are pregnant or breastfeeding
• People with a current or past eating disorder
• Children and teenagers, and people who are underweight
• Anyone taking medicines that must be taken with food

A starter week

Day	Meals
Monday	12-hour overnight fast (for example 7 p.m. to 7 a.m.). Breakfast: Greek yogurt with berries. Lunch: chicken and vegetable salad. Dinner: salmon, vegetables and brown rice.
Tuesday	12-hour fast. Oatmeal with fruit; lentil soup with whole-grain bread; tofu and vegetable stir-fry.
Wednesday	13-hour fast. Eggs and whole-grain toast; tuna and bean salad; turkey chili with a side salad.
Thursday	13-hour fast. Cottage cheese and fruit; leftover chili; grilled fish, roasted vegetables and potatoes.
Friday	14-hour fast if you feel well (for example 7 p.m. to 9 a.m.). Yogurt and nuts; chickpea and vegetable bowl; chicken, vegetables and quinoa.
Saturday	14-hour fast. Vegetable omelet; hummus wrap with vegetables; bean and vegetable soup.
Sunday	Review the week: were you hungrier or eating more overall? Keep the window that felt comfortable, or return to regular meals.

Questions to ask a professional

• Is intermittent fasting safe with my medicines and health conditions?
• If I take diabetes medicine, how should my doses or monitoring change on fasting days?
• Which eating window or method would suit my schedule and health best?
• How do I make sure I get enough protein and nutrients in a shorter eating window?
• What signs should make me stop fasting and check in with you?

Frequently asked questions

Does intermittent fasting work better than a regular diet?

Not in most trials. Time-restricted eating, 5:2 and alternate-day fasting gave weight loss similar to eating a little less every day. It can still be a good choice if the structure makes eating less easier for you.

What is the best intermittent fasting schedule for beginners?

Many people start with a 12-hour overnight fast and narrow the window gradually if they feel well. There is no single best schedule; the right one is the one you can keep while eating balanced meals.

Can I drink coffee while fasting?

Water and other calorie-free drinks are generally part of fasting periods. Adding milk, sugar or cream adds calories. If you have reflux or take medicines with food, ask your clinician what works for you.

Will intermittent fasting make me lose muscle?

Any weight loss includes some lean tissue. Eating enough protein in your eating window and doing strength training a few times a week helps you keep more muscle.

Is intermittent fasting safe with diabetes?

It can be risky, especially with insulin, sulfonylureas or meglitinides, because of low blood sugar. The NIDDK advises planning any fast with your health care team so medicines and monitoring can be adjusted.

References

1. 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
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. doi:10.1001/jamainternmed.2020.4153 · PMID 32986097 · NCT03393195 (accessed October 7, 2026) Randomized trial
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. doi:10.1056/NEJMoa2114833 · PMID 35443107 · NCT03745612 (accessed October 7, 2026) Randomized trial
4. 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
5. 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
6. Sun ML, Yao W, Wang XY, et al.. Intermittent fasting and health outcomes: an umbrella review of systematic reviews and meta-analyses of randomised controlled trials. eClinicalMedicine, 2024. doi:10.1016/j.eclinm.2024.102519 · PMID 38500840 (accessed October 6, 2026) Meta-analysis
7. Fasting Safely with Diabetes. National Institute of Diabetes and Digestive and Kidney Diseases (NIH). (accessed October 7, 2026) Government page
8. Intermittent fasting. NHS (MyHealth London). (accessed October 7, 2026) Government page

Facts checked on October 6, 2026

Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.