memios
← Longevity Letter

Fasting · September 29, 2026 · Memios · 14 min read

5:2 diet

5:2 leads to weight loss, but randomised trials and a Cochrane review found no clear advantage over ordinary daily calorie restriction or standard dietary advice.

5:2 diet (two low-calorie days a week)5:2 fastintermittent energy restriction (IER)intermittent calorie restriction (ICR)fasting research
Photograph for 5:2 diet: a calm table with an empty plate and a glass of water in morning light.

TLDR

  • Well established. 5:2 leads to weight loss, but randomised trials and a Cochrane review found no clear advantage over ordinary daily calorie restriction or standard dietary advice.
  • What it is: The 5:2 diet restricts energy on 2 days of the week and allows usual eating on the other 5.
  • Main use, supported: In the same trial, fasting insulin and insulin resistance fell modestly in both groups, and slightly more with 2 restricted days a week.
  • Other use, supported: Over 3 months in women with a family history of breast cancer, a 2-day low-carbohydrate restriction gave larger falls in insulin resistance and body fat than daily energy restriction.
  • Claim NOT supported by research: A Cochrane review of 22 RCTs covering several intermittent fasting types, periodic fasting among them, found little to no difference in weight loss compared with regular dietary advice. (low certainty)
  • Another claim NOT supported: Among overweight or obese premenopausal women, 2 restricted days a week gave the same weight loss over 6 months as daily 25% energy restriction.
  • What goes wrong: 5 findings on harm. Among people with type 2 diabetes taking sulfonylureas and/or insulin, 35% had hypoglycaemic or hyperglycaemic events in the first 2 weeks, even though these medicines had been reduced at the start.
  • Be careful if you are: People with type 2 diabetes taking insulin or sulfonylureas; People with type 1 diabetes who take insulin; People with diabetes, pregnant or breastfeeding women, and people with a history of eating disorders or disordered eating; People with a history of eating disorders.
  • Common myth: 5:2 burns more fat or produces more weight loss than an ordinary diet.

What it is

The 5:2 diet restricts energy on 2 days of the week and allows usual eating on the other 5. In the type 2 diabetes trial by Carter and colleagues, the 2 restricted days were non-consecutive at about 500 to 600 kcal. In the HELENA trial they were 2 self-selected non-consecutive days at 25% of energy needs.

What the research says

5:2 leads to weight loss, but randomised trials and a Cochrane review found no clear advantage over ordinary daily calorie restriction or standard dietary advice. Two short trials from one UK group reported slightly larger improvements in insulin resistance than daily restriction. A type 2 diabetes trial found HbA1c equivalent to daily restriction. Adherence fell steeply over a year, and fast days brought hunger, low energy, headaches and bad temper for a minority of people.

Evidence grade: Well established.

The schedule

  • Pattern: weekly
  • Fast days per week: 2
  • Calorie limit on fast days: about 500 to 600 kcal a day (Carter 2018) or about 25% of energy needs (HELENA trial), on 2 non-consecutive days; usual eating on the other 5 days

Why people choose it

People may choose 5:2 because it restricts eating on only 2 days a week rather than every day. In trials it produced about the same weight loss as daily calorie restriction, including over 1 and 2 years, so it was not shown to work better. In one type 2 diabetes trial, blood sugar control (HbA1c) was comparable with daily restriction. Sticking with it was hard for many: in a UK trial only 22% were still following it at one year, and fast days brought hunger and discomfort.

Cautions

  • People with type 2 diabetes taking insulin or sulfonylureas: In the 5:2 diabetes trial, medicines that can cause hypoglycaemia were reduced at the start under a medication protocol. Even so, about a third of those on sulfonylureas and/or insulin had glucose events in the first 2 weeks. (Source 1)
  • People with type 1 diabetes who take insulin: A fasting researcher at Johns Hopkins notes there have been no studies in type 1 diabetes, and there is concern about unsafe hypoglycaemia during fasting. (Source 2)
  • People with diabetes, pregnant or breastfeeding women, and people with a history of eating disorders or disordered eating: The Academy of Nutrition and Dietetics does not recommend intermittent fasting for these groups. (Source 3)
  • People with a history of eating disorders: The Johns Hopkins page says strict fasting windows can trigger or worsen restrictive or obsessive eating. In the Harvie trial, 15% on the 2-day pattern reported preoccupation with food, bad temper or poor concentration. (Source 2)
  • Older adults and anyone at risk of muscle loss: Across meta-analyses of RCTs, intermittent fasting was associated with loss of fat-free mass. Trial participants were mostly under 65. (Source 4)
  • Anyone taking medicines: The Academy of Nutrition and Dietetics advises discussing fasting with a doctor first, especially when medicines may need adjusting. (Source 3)

What goes wrong

Among people with type 2 diabetes taking sulfonylureas and/or insulin, 35% had hypoglycaemic or hyperglycaemic events in the first 2 weeks, even though these medicines had been reduced at the start. (Source 1)

  • Randomized trial, Certainty not rated.
  • Size: 46 participants using sulfonylureas and/or insulin.
  • Who: Adults with type 2 diabetes.
  • How long: first 2 weeks of a 12-month trial.
  • Result: Mean events 4.9 (intermittent) vs 3.2 (continuous), P = .28; 8 of 46 (17%) had hypoglycaemia, 6 of them in the intermittent group.
  • Funding: independent: University of South Australia postgraduate award (Carter) and National Health and Medical Research Council principal research fellowship (Clifton)

Limit of this finding: The authors note that only finger-prick testing was used, so some hypo- and hyperglycaemic events probably went undetected.

Eight of 46 participants (17%) (2 in the continuous energy restriction group and 6 in the intermittent energy restriction group) experienced hypoglycemia

Adherence to 5:2 fell from 74% at 6 weeks to 22% at one year; the authors attribute this to hunger and discomfort on fast days. (Source 5)

  • Randomized trial, Certainty not rated.
  • Size: 100 in the 5:2 self-help arm.
  • Who: Adults with obesity.
  • How long: 1 year.
  • Result: Adherence 74% at 6 weeks, 31% at 6 months, 22% at 1 year.
  • Funding: independent (Medical Research Council)

Adherence to 5:2SH was initially high (74% at 6 weeks), but it declined over time (31% at 6 months and 22% at one year).

Over 6 months, 21% of women on 2 restricted days a week withdrew, compared with 13% on daily restriction, and 15% of the intermittent group complained of hunger. (Source 6)

  • Randomized trial, Certainty not rated.
  • Size: 107 women.
  • Who: Overweight or obese premenopausal women.
  • How long: 6 months.
  • Result: Withdrawals 11 (21%) intermittent vs 7 (13%) continuous; hunger 8 (15%) vs none.
  • Funding: mixed: charities (Breast Cancer Campaign, World Cancer Research Fund, Genesis Appeal), NIA intramural programme, Medical Research Council and Danish Research Council for Health and Disease, plus Tanita Europe BV, a body-composition scale maker.

Eighteen women withdrew from the study before 6 months (IER=11, CER=7) representing 21% IER and 13% CER subjects

A minority of women on 2 restricted days a week reported lack of energy, headaches, feeling cold and constipation, as well as poor concentration, bad temper and preoccupation with food. (Source 6)

  • Randomized trial, Certainty not rated.
  • Size: 107 women.
  • Who: Overweight or obese premenopausal women.
  • How long: 6 months.
  • Result: Physical symptoms 4 (8%) intermittent vs none continuous; psychological effects 8 (15%) vs 4 (7%)
  • Funding: mixed: charities (Breast Cancer Campaign, World Cancer Research Fund, Genesis Appeal), NIA intramural programme, Medical Research Council and Danish Research Council for Health and Disease, plus Tanita Europe BV, a body-composition scale maker.

A small number of the IER group (4, 8%), but none of the CER group experienced minor adverse physical symptoms including lack of energy, headaches, feeling cold and constipation

The same umbrella review found intermittent fasting was associated with reduced fat-free mass, and noted that adverse outcomes were missing from existing meta-analyses. (Source 4)

  • Review of reviews, Certainty not rated.
  • Size: 11 meta-analyses of 130 RCTs.
  • Who: Adults.
  • How long: median follow-up 3 months.
  • Result: Not given for 5:2 specifically in the abstract.
  • Funding: not stated.

IF was found to be associated with reduced fat-free mass.

What the evidence supports

In the same trial, fasting insulin and insulin resistance fell modestly in both groups, and slightly more with 2 restricted days a week. (Source 7)

  • Randomized trial, Certainty not rated.
  • Size: 107 women.
  • Who: Overweight or obese premenopausal women.
  • How long: 6 months.
  • Result: Between-group difference: fasting insulin -1.2 (-1.4 to -1.0) microU/ml; insulin resistance -1.2 (-1.5 to -1.0); both P = 0.04.
  • Funding: mixed: charities (Breast Cancer Campaign, World Cancer Research Fund, Genesis Appeal), NIA intramural programme, Medical Research Council and Danish Research Council for Health and Disease, plus Tanita Europe BV, a body-composition scale maker.

Limit of this finding: The difference was small and the P value borderline. It comes from a single trial.

Reductions in fasting insulin and insulin resistance were modest in both groups, but greater with IER than with CER; difference between groups for fasting insulin was -1.2 (-1.4 to -1.0) μU ml(-1) and for insulin resistance was -1.2 (-1.5 to -1.0) μU mmol(-1) l(-1) (both P = 0.04).

Over 3 months in women with a family history of breast cancer, a 2-day low-carbohydrate restriction gave larger falls in insulin resistance and body fat than daily energy restriction. (Source 8)

  • Randomized trial, Certainty not rated.
  • Size: 115 women.
  • Who: Overweight women aged 20 to 69 with a family history of breast cancer.
  • How long: 3 months of weight loss plus 1 month of maintenance.
  • Result: Body fat fell 3.7 kg in each intermittent group vs 2.0 kg with daily restriction (P = 0.007 and P = 0.019)
  • Funding: not stated.

Limit of this finding: The restricted days were also low in carbohydrate (under 40 g a day), so this is not a standard 5:2. It was short, and the authors call for longer-term safety studies.

In the short term, IECR is superior to DER with respect to improved insulin sensitivity and body fat reduction.

In adults with type 2 diabetes, 5:2 (500 to 600 kcal on 2 non-consecutive days) lowered HbA1c by an amount equivalent to daily energy restriction over 12 months. (Source 1)

  • Randomized trial, Certainty not rated.
  • Size: 137 randomised, 97 completed.
  • Who: Adults with type 2 diabetes, mean age 61, mean BMI 36, mean HbA1c 7.3%.
  • How long: 12 months.
  • Result: HbA1c -0.3% (intermittent) vs -0.5% (continuous), P = .65; between-group difference 0.2% (90% CI -0.2% to 0.5%), within the equivalence margin.
  • Funding: independent: University of South Australia postgraduate award (Carter) and National Health and Medical Research Council principal research fellowship (Clifton)

Limit of this finding: This was a non-inferiority trial. The authors note the population had well-controlled diabetes, which limits generalisability.

Intermittent energy restriction is an effective alternative diet strategy for the reduction of HbA1c and is comparable with continuous energy restriction in patients with type 2 diabetes.

An umbrella review of meta-analyses of RCTs found that 5:2 and modified alternate-day fasting were the only intermittent fasting types associated with statistically significant weight loss in adults with overweight or obesity, compared with regular diet. (Source 4)

  • Review of reviews, Certainty not rated.
  • Size: 11 meta-analyses of 130 RCTs (median 38 participants each)
  • Who: Adults, mostly with overweight or obesity.
  • How long: median follow-up 3 months.
  • Result: Body weight fell 1.67 kg after 3 months of 5:2 in participants with obesity.
  • Funding: not stated.

Limit of this finding: The comparator was regular diet, not calorie restriction. Of all 104 associations the review examined, 72% were graded very low quality, and the abstract gives no certainty grade for the 5:2 weight association.

MADF and the 5:2 diet were the only IF types associated with statistically significant weight loss in adults with overweight or obesity.

What the evidence does not support

Among overweight or obese premenopausal women, 2 restricted days a week gave the same weight loss over 6 months as daily 25% energy restriction. (Source 7)

  • Randomized trial, Certainty not rated.
  • Size: 107 women.
  • Who: Overweight or obese premenopausal women, mean BMI 30.6.
  • How long: 6 months.
  • Result: Weight change -6.4 kg (95% CI -7.9 to -4.8) with intermittent vs -5.6 kg (-6.9 to -4.4) with continuous restriction; P = 0.4 for the difference.
  • Funding: mixed: charities (Breast Cancer Campaign, World Cancer Research Fund, Genesis Appeal), NIA intramural programme, Medical Research Council and Danish Research Council for Health and Disease, plus Tanita Europe BV, a body-composition scale maker.

Limit of this finding: This compared equal weekly energy deficits, so it tests the pattern, not whether 5:2 makes people eat less.

Last observation carried forward analysis showed that IER and CER are equally effective for weight loss: mean (95% confidence interval ) weight change for IER was -6.4 (-7.9 to -4.8) kg vs -5.6 (-6.9 to -4.4) kg for CER (P-value for difference between groups = 0.4).

In the same diabetes trial, 5:2 was no better than daily restriction for fasting glucose, lipids or medication use, and weight loss did not meet the equivalence criteria. (Source 1)

  • Randomized trial, Certainty not rated.
  • Size: 137 randomised.
  • Who: Adults with type 2 diabetes.
  • How long: 12 months.
  • Result: Weight -6.8 kg (intermittent) vs -5.0 kg (continuous), P = .25; difference -1.8 kg (90% CI -3.7 to 0.07), not equivalent.
  • Funding: independent: University of South Australia postgraduate award (Carter) and National Health and Medical Research Council principal research fellowship (Clifton)

There were no significant differences between groups in final step count, fasting glucose levels, lipid levels, or total medication effect score at 12 months.

In a UK trial of 300 adults with obesity, simple 5:2 advice produced the same modest weight loss at one year as standard brief diet and activity advice. (Source 5)

  • Randomized trial, Certainty not rated.
  • Size: 300 adults.
  • Who: Adults with obesity.
  • How long: 1 year.
  • Result: At one year -1.9 kg (5:2 self-help) vs -1.8 kg (standard advice), p = 0.79; 18% vs 15% lost at least 5% of body weight.
  • Funding: independent (Medical Research Council)

Limit of this finding: About half of those who started completed the study.

Simple 5:2 advice and multicomponent weight management advice generated similar modest results.

In the HELENA trial, 5:2 at 25% energy on 2 non-consecutive days gave no more weight loss than daily restriction at 2 years, and more of the 5:2 group said they had stopped following it. (Source 9)

  • Randomized trial, Certainty not rated.
  • Size: 98 participants.
  • Who: Adults aged 35 to 65 with overweight or obesity, 49% female.
  • How long: 12-week intervention, follow-up to 102 weeks.
  • Result: Weight change at week 102 p = 0.63 between groups; 71.1% (5:2) vs 32.5% (daily restriction) reported not following, or only rarely following, their regimen between weeks 50 and 102.
  • Funding: not stated.

Limit of this finding: This is a follow-up analysis of the HELENA trial. The main 50-week paper could not be reached.

No significant difference between ICR and CCR regarding weight change at wk102 was observed (p = 0.63).

A Cochrane review of 22 RCTs covering several intermittent fasting types, periodic fasting among them, found little to no difference in weight loss compared with regular dietary advice. (Source 10)

  • Systematic review, Low certainty.
  • Size: 21 studies, 1,430 participants for this outcome.
  • Who: Adults with overweight or obesity.
  • How long: follow-up of at least 6 months, up to 12 months.
  • Result: MD -0.33 percentage points of weight lost (95% CI -0.92 to 0.26)
  • Funding: independent (no dedicated funding)

Limit of this finding: The review pools all intermittent fasting types. Its abstract gives no separate result for 5:2.

Compared to regular dietary advice, intermittent fasting may result in little to no difference in percentage from baseline weight loss (MD -0.33, 95% CI -0.92 to 0.26; 21 studies, 1430 participants; low-certainty evidence due to risk of bias).

Where the research disagrees

Whether 5:2 improves insulin sensitivity more than daily calorie restriction

  • Harvie and colleagues (Manchester, 2013), rct: Over 3 months, the 2-day low-carbohydrate restriction was superior to daily restriction for insulin sensitivity and body fat. (Source 8)
  • Carter and colleagues (South Australia, 2018), rct: In type 2 diabetes over 12 months, 5:2 showed no difference from daily restriction in fasting glucose, lipids or medication use. (Source 1)

A common belief, and what the research shows

The belief: 5:2 burns more fat or produces more weight loss than an ordinary diet.

What the research shows: In trials lasting 6 months to 2 years, 5:2 produced about the same weight loss as daily calorie restriction or standard advice. A Cochrane review found little to no difference compared with regular dietary advice (low-certainty evidence).

References

  1. JAMA Network Open. Effect of Intermittent Compared With Continuous Energy Restricted Diet on Glycemic Control in Patients With Type 2 Diabetes: A Randomized Noninferiority Trial. 2018. PMID 30646054, DOI 10.1001/jamanetworkopen.2018.0756. Read the source
  2. Johns Hopkins Medicine. Intermittent Fasting: What Is It, And How Does It Work? (expert Q&A with Mark Mattson, PhD). Read the source
  3. Academy of Nutrition and Dietetics (eatright.org). What is Intermittent Fasting? (Barbara Gordon, RDN, LD; reviewed by Academy staff RDNs, July 21, 2025). 2025. Read the source
  4. JAMA Network Open. Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials. 2021. PMID 34919135, DOI 10.1001/jamanetworkopen.2021.39558. Read the source
  5. PLOS ONE. A randomised controlled trial of the 5:2 diet. 2021. DOI 10.1371/journal.pone.0258853. Read the source
  6. International Journal of Obesity (PMC full text). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women (full text). 2011. PMID 20921964, DOI 10.1038/ijo.2010.171. Read the source
  7. International Journal of Obesity. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. 2011. PMID 20921964, DOI 10.1038/ijo.2010.171. Read the source
  8. British Journal of Nutrition. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. 2013. PMID 23591120, DOI 10.1017/S0007114513000792. Read the source
  9. Nutrients. Adherence and Dietary Composition during Intermittent vs. Continuous Calorie Restriction: Follow-Up Data from a Randomized Controlled Trial in Adults with Overweight or Obesity. 2021. DOI 10.3390/nu13041195. Read the source
  10. Cochrane Database of Systematic Reviews. Intermittent fasting for adults with overweight or obesity. 2026. PMID 41692034, DOI 10.1002/14651858.CD015610.pub2. Read the source
Share

0:00/0:00