36-hour fast

What it is

A 36-hour fast means going a full day and two nights with no calories, then eating. The largest trial of this pattern had participants alternate 36 hours of zero-calorie intake with 12 hours of unlimited eating for four weeks. Done once a week it is often called the monk fast; done every other day it is strict alternate-day fasting.

What the research says

The evidence is limited to small, short trials, mostly in healthy adults under 60. A four-week randomised trial of 36-hour fasts alternating with 12-hour eating windows reported about 4.5% body weight loss, less trunk fat and better cardiovascular markers with no adverse effects. A small trial comparing zero-calorie alternate-day fasting with ordinary daily calorie restriction found no significant difference in weight, body composition or lipids at 8 weeks. Hunger stayed elevated on fast days. No trial studied the once-a-week 36-hour 'monk fast' on its own, and none studied adults over 60 or people taking diabetes medicines.

Evidence grade: Limited evidence.

The schedule

• Pattern: weekly
• Fasting hours: 36
• Eating hours: 12
• Calorie limit on fast days: zero calories (water and non-caloric drinks) for 36 hours, as in Stekovic 2019

Why people choose it

People choose it because there is nothing to count on fast days: researchers in the main trial noted participants simply do not eat for one day. It has been studied mainly in healthy, non-obese adults over weeks to months, where weight and fat mass fell (in a four-week randomised trial against normal eating, and in a small single-arm study). In the one small trial that compared zero-calorie alternate-day fasting with ordinary daily calorie restriction in adults with obesity, weight loss was not significantly different. Hunger on fast days stayed high in an earlier study, and its authors doubted people could keep it up for long.

Cautions

• People taking insulin or sulfonylureas (glyburide, glimepiride, glipizide) or meglitinides: An expert review of fasting in diabetes names hypoglycaemia as the most immediate risk for people on these medicines. (Source 1)
• Pregnant or breastfeeding women, older adults of advanced age, and frail older adults: The same review says these groups have unique risks and should be dissuaded from intermittent fasting, especially if they have diabetes. (Source 1)
• People with an eating disorder or dementia: The review says fasting is likely to make these conditions worse and that these groups should not follow intermittent fasting regimens. (Source 1)
• People with a viral infection, and anyone starting a harsh fasting regime: The lead researcher of the main 36-hour fasting trial advised against fasting during a viral infection and advised consulting a doctor first. (Source 2)
• People with a transplant or taking immunosuppressants: The diabetes fasting review says people with immunodeficiencies, including solid organ transplant recipients on immunosuppression, should not fast. (Source 1)

What we did not find on harm

Trials of 36-hour fasting reported no serious adverse events, but they were small, short and in healthy adults under about 60. The harms found were persistent hunger and, from survey data on intermittent fasting in general, an association with disordered eating. No study measured muscle loss in older adults on this schedule. (Source 3)

What goes wrong

Hunger rose on the first fast day and stayed high for the whole study; the authors judged it unlikely people could continue this diet for long. (Source 3)

• Case series, Certainty not rated.
• Size: 16.
• Who: Non-obese healthy adults.
• How long: 22 days.
• Result: Hunger elevated throughout (P < 0.001); did not decrease on fasting days.
• Funding: not stated in the text retrieved.

Limit of this finding: Single-arm trial with no control group (before-and-after measurements in the same 16 people); very small and very short. Hunger measured with visual analogue scales weekly.

However, hunger on fasting days did not decrease, perhaps indicating the unlikelihood of continuing this diet for extended periods of time

In a national survey of Canadian adolescents and young adults, intermittent fasting in the past year was associated with eating disorder psychopathology in women, men and gender-diverse participants. (Source 4)

• Survey study, Certainty not rated.
• Size: 2,762.
• Who: Canadian adolescents and young adults.
• How long: self-report covering past 12 months and past 30 days.
• Result: 47.7% of women, 38.4% of men and 52.0% of TGNC participants reported intermittent fasting; significant associations with eating disorder psychopathology.
• Funding: not stated in the text retrieved.

Limit of this finding: Covers intermittent fasting in general, not 36-hour fasts specifically, and young people rather than adults over 50. A survey at one point in time cannot show which came first.

Intermittent fasting in the past 12 months and 30 days was significantly associated with eating disorder psychopathology among women, men, and TGNC participants

What the evidence supports

In a four-week randomised trial in healthy, non-obese middle-aged adults, strict alternate-day fasting (36 hours without calories, 12 hours of unlimited eating) reduced body weight by 4.5% and improved the fat-to-lean ratio. (Source 5)

• Randomized trial, Certainty not rated.
• Size: 60 participants randomised (about 30 per group); this number comes from the university press release (sciencedaily-2019-adf), not the paper's summary.
• Who: Healthy, normal-weight, middle-aged adults in Graz, Austria.
• How long: 4 weeks.
• Result: Body weight down 4.5% (paper); average calorie reduction 37% (paper summary; the press release says about 35%); ADF group lost an average of 3.5 kg (press release)
• Funding: not stated in the text retrieved.

Limit of this finding: Small, short trial in healthy non-obese adults; control group ate normally, not a calorie-restricted diet. The sample size, the 3.5 kg figure and the 35% figure come from the university press release (sciencedaily-2019-adf); the 4.5% and 37% figures come from the paper.

4-week ADF decreases the body weight by 4.5% and improves the fat-to-lean ratio

The same trial reported improved cardiovascular markers, less trunk fat and higher ketone levels, and the authors concluded the results support the safety of this pattern. (Source 5)

• Randomized trial, Certainty not rated.
• Size: 60 randomised (number from the university press release, sciencedaily-2019-adf; not given in the paper's summary)
• Who: Healthy, non-obese middle-aged adults.
• How long: 4 weeks.
• Result: Reduced fat mass, particularly trunk fat; increased beta-hydroxybutyrate even on non-fasting days.
• Funding: not stated in the text retrieved.

Limit of this finding: Surrogate markers only; no clinical outcomes.

ADF improved cardiovascular markers, reduced fat mass (particularly the trunk fat), improving the fat-to-lean ratio, and increased β-hydroxybutyrate, even on non-fasting days

In 16 non-obese adults who fasted every other day for 22 days (measurements taken after a 36-hour fast), the diet was followed by a 2.5% fall in body weight and a 4% fall in fat mass. (Source 3)

• Case series, Certainty not rated.
• Size: 16 (8 men, 8 women)
• Who: Non-obese healthy adults.
• How long: 22 days.
• Result: Weight -2.5 ± 0.5% (P < 0.001); fat mass -4 ± 1% (P < 0.001); fasting insulin -57 ± 4%.
• Funding: not stated in the text retrieved.

Limit of this finding: Single-arm trial with no control group (before-and-after measurements in the same 16 people); very small and very short.

Subjects lost 2.5 ± 0.5% of their initial body weight (P < 0.001) and 4 ± 1% of their initial fat mass (P < 0.001)

In the same pilot trial, no adverse effects were attributed to zero-calorie alternate-day fasting and 93% completed it; after 24 weeks, changes in body fat percentage and lean mass were more favourable than with calorie restriction. (Source 6)

• Randomized trial, Certainty not rated.
• Size: 26 randomised.
• Who: Adults with obesity, aged 18 to 55.
• How long: 8 weeks plus 24 weeks follow-up.
• Result: 93% completion; no significant difference in weight regain; % fat mass and lean mass changes favoured ADF at 24 weeks.
• Funding: not stated in the text retrieved.

Limit of this finding: Pilot trial; the follow-up comparison was secondary and small.

No adverse effects were attributed to ADF, and 93% completed the 8-week ADF protocol

What the evidence does not support

Fasting glucose and the hunger hormone ghrelin did not change significantly with three weeks of alternate-day fasting in non-obese adults. (Source 3)

• Case series, Certainty not rated.
• Size: 16.
• Who: Non-obese healthy adults.
• How long: 22 days.
• Result: No significant change in glucose or ghrelin from baseline.
• Funding: not stated in the text retrieved.

Limit of this finding: Single-arm trial with no control group (before-and-after measurements in the same 16 people); very small and very short.

Glucose and ghrelin did not change significantly from baseline with alternate-day fasting

In adults with obesity, zero-calorie alternate-day fasting produced weight loss that was not significantly different from ordinary daily calorie restriction (400 kcal a day less) after 8 weeks, despite a larger calorie deficit. (Source 6)

• Randomized trial, Certainty not rated.
• Size: 26 randomised (14 alternate-day fasting, 12 calorie restriction)
• Who: Adults with obesity (BMI 30 or more), aged 18 to 55.
• How long: 8 weeks, plus 24 weeks of unsupervised follow-up.
• Result: ADF -8.2 ± 0.9 kg vs CR -7.1 ± 1.0 kg; no significant between-group difference in weight, body composition, lipids or insulin sensitivity.
• Funding: not stated in the text retrieved.

Limit of this finding: Pilot study, very small; the abstract describes zero-calorie fast days but does not state the number of hours without food.

there were no significant between-group differences in change in weight (mean ± SE; ADF −8.2 ± 0.9 kg, CR −7.1 ± 1.0 kg), body composition, lipids, or Si

Where the evidence is mixed

In a separate group of people who had already practised strict alternate-day fasting for more than 6 months, no adverse effects were reported, and long-term fasting was associated with lower LDL cholesterol, a lower inflammatory marker and lower thyroid hormone T3. (Source 5)

• Survey study, Certainty not rated.
• Size: 30 long-term alternate-day fasters compared with healthy controls (group size from the university press release, sciencedaily-2019-adf; not given in the paper's summary)
• Who: Healthy adults already practising alternate-day fasting.
• How long: more than 6 months of prior practice.
• Result: No adverse effects reported; reduced sICAM-1, LDL and triiodothyronine (T3)
• Funding: not stated in the text retrieved.

Limit of this finding: Self-selected people who had already been fasting successfully; a comparison at one point in time, not a trial. Lower T3 is a sign of the body conserving energy; its long-term meaning is not established.

We found reduced levels sICAM-1 (an age-associated inflammatory marker), low-density lipoprotein, and the metabolic regulator triiodothyronine after long-term ADF

Where the research disagrees

Whether strict 36-hour alternate-day fasting is practical and safe to continue

• Stekovic and colleagues (Graz), rct: Safe for healthy non-obese adults for several months (Source 5)
• Heilbronn and colleagues (Pennington), case-series: Feasible short term, but hunger did not decrease, making it unlikely people could continue for long (Source 3)

A common belief, and what the research shows

The belief: Going 36 hours without food burns far more fat than an ordinary diet.

What the research shows: In the one randomised comparison found, zero-calorie alternate-day fasting and ordinary daily calorie restriction produced weight loss that was not significantly different after 8 weeks (8.2 kg vs 7.1 kg), in 26 adults with obesity.

References

1. Nutrients. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. 2019. PMID 31003482, DOI 10.3390/nu11040873. Read the source
2. ScienceDaily (source: Cell Press). Clinical trial shows alternate-day fasting a safe alternative to caloric restriction (press release on Stekovic et al. 2019). 2019. Read the source
3. American Journal of Clinical Nutrition. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. 2005. PMID 15640462, DOI 10.1093/ajcn/81.1.69. Read the source
4. Eating Behaviors. Intermittent fasting: Describing engagement and associations with eating disorder behaviors and psychopathology among Canadian adolescents and young adults. 2022. DOI 10.1016/j.eatbeh.2022.101681. Read the source
5. Cell Metabolism. Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans. 2019. PMID 31471173, DOI 10.1016/j.cmet.2019.07.016. Read the source
6. Obesity. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. 2016. PMID 27569118, DOI 10.1002/oby.21581. Read the source