Alternate-day fasting

What it is

Zero-calorie alternate-day fasting alternates a day with no calorie intake and a day of unrestricted eating. Because each fast runs from the last meal of one day to the first meal two days later, each fast lasts about 36 hours.

What the research says

The evidence for zero-calorie alternate-day fasting is thin: small, short trials. In an 8-week pilot RCT it matched daily calorie restriction for weight loss, and no adverse effects were attributed to it. In a 22-day study of non-obese adults, hunger stayed elevated on fast days. An umbrella review did not list it among the intermittent fasting types linked to significant weight loss. A meta-analysis of ADF trials, most of which used 25%-energy fast days rather than zero-calorie fasting, found weight loss but also loss of lean mass.

Evidence grade: Limited evidence.

The schedule

• Pattern: weekly
• Fasting hours: 36
• Fast days per week: 3.5
• Calorie limit on fast days: zero calories (water and calorie-free drinks only) on fast days, alternating with unrestricted eating days

Why people choose it

People may choose zero-calorie alternate-day fasting because it needs no calorie counting on fast days: nothing with calories is eaten. In a small 8-week pilot it gave the same weight loss as daily calorie restriction. An umbrella review did not find it associated with significant weight loss, unlike modified ADF and 5:2. Hunger on fast days did not fade in a 3-week study, and the authors doubted people could keep it up for long periods.

Cautions

• Older adults and people at risk of sarcopenia (muscle loss): A meta-analysis found lean mass fell with alternate-day fasting, and its authors warn it could make muscle loss worse in people at risk. (Source 1)
• 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 2)
• People taking insulin: A Johns Hopkins fasting researcher warns of unsafe hypoglycaemia during fasting periods for people with type 1 diabetes, who were not studied. (Source 3)
• Anyone who finds persistent hunger hard to tolerate: In a 22-day study, hunger on zero-calorie fast days stayed elevated rather than settling. (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 2)

What goes wrong

In non-obese adults who fasted every other day for 22 days, hunger rose on the first fast day and stayed elevated. The authors questioned whether people could keep the diet up for long periods. (Source 4)

• Case series, Certainty not rated.
• Size: 16 (8 men, 8 women)
• Who: Non-obese healthy adults.
• How long: 22 days.
• Result: Hunger elevated on fast days throughout (P < 0.001); weight fell 2.5% and fat mass 4%.
• Funding: not stated.

Limit of this finding: Single-arm trial with no control group.

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

The same meta-analysis found a significant reduction in lean mass with alternate-day fasting, and the authors flagged a risk for people prone to sarcopenia. (Source 1)

• Meta-analysis, Certainty not rated.
• Size: 7 RCTs, 269 participants.
• Who: Adults.
• How long: short-term.
• Result: Lean mass p = 0.002 vs control.
• Funding: independent: National Natural Science Foundation of China, Taishan Scholars Program, Beijing Municipal Administration of Hospitals.

Limit of this finding: Most included trials used 25%-energy fast days, not zero-calorie fasting.

We found a significant reduction in lean mass in the ADF group. This serves as a caution for patient populations at risk for sarcopenia because ADF could exacerbate muscle loss.

Trials in that meta-analysis reported mild headaches or light-headedness early on, and constipation in the first 2 weeks. (Source 1)

• Meta-analysis, Certainty not rated.
• Size: 7 RCTs.
• Who: Adults.
• How long: first weeks of treatment.
• Result: Adverse events described as infrequent and slight; no pooled rate given.
• Funding: independent: National Natural Science Foundation of China, Taishan Scholars Program, Beijing Municipal Administration of Hospitals.

Limit of this finding: Most included trials used 25%-energy fast days, not zero-calorie fasting.

A small number of participants experienced mild headaches or light-headedness in the early days of the trial.

What the evidence supports

In the same pilot, no adverse effects were attributed to zero-calorie ADF, and 93% completed the 8-week protocol. (Source 5)

• Randomized trial, Certainty not rated.
• Size: 14 in the ADF arm.
• Who: Adults aged 18 to 55 with obesity.
• How long: 8 weeks.
• Result: 93% completion; no adverse effects attributed to ADF.
• Funding: not stated.

Limit of this finding: With 14 people for 8 weeks, the trial could not detect uncommon harms. Older adults (over 55) were not included.

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

After 24 weeks of unsupervised follow-up, weight regain did not differ between the groups, and changes in fat mass and lean mass from baseline were more favourable after ADF. (Source 5)

• Randomized trial, Certainty not rated.
• Size: 26 adults.
• Who: Adults with obesity.
• How long: 24 weeks after an 8-week intervention.
• Result: No significant difference in weight regain; % fat mass and lean mass changes favoured ADF.
• Funding: not stated.

After 24 weeks of unsupervised follow-up, there were no significant differences in weight regain; however, changes from baseline in % fat mass and lean mass were more favorable in ADF.

The same study found alternate-day fasting feasible in non-obese adults, and fat oxidation was higher after the fasting period. (Source 4)

• Case series, Certainty not rated.
• Size: 16.
• Who: Non-obese healthy adults.
• How long: 22 days.
• Result: Fasting insulin fell 57%; average daily fat oxidation rose by at least 15 g.
• Funding: not stated.

Limit of this finding: Single-arm trial with no control group. The authors suggested that adding one small meal on fast days might make the approach more acceptable, which is the rationale for modified ADF.

Alternate-day fasting was feasible in nonobese subjects, and fat oxidation increased.

A meta-analysis of 7 RCTs of alternate-day fasting, mostly using 25%-energy fast days rather than zero-calorie fasting, found reductions in weight and BMI compared with control groups. (Source 1)

• Meta-analysis, Certainty not rated.
• Size: 7 RCTs, 269 participants.
• Who: Adults with obesity or normal weight.
• How long: short; trial lengths not given in the abstract.
• Result: Weight p < 0.00001 and BMI p < 0.00001 vs control; no difference in HDL, HOMA-IR or fasting glucose.
• Funding: independent: National Natural Science Foundation of China, Taishan Scholars Program, Beijing Municipal Administration of Hospitals.

Limit of this finding: The authors themselves describe the trials as having low methodological rigour and short interventions. Most included trials used 25%-energy fast days, so this is mainly evidence on modified ADF, not zero-calorie ADF.

In this meta-analysis, compared with the control group, the ADF group showed statistically significant reductions in weight (p < 0.00001) and body mass index (p < 0.00001).

What the evidence does not support

In an 8-week pilot RCT in adults with obesity, zero-calorie alternate-day fasting produced no more weight loss than daily calorie restriction, despite a larger energy deficit. (Source 5)

• Randomized trial, Certainty not rated.
• Size: 26 adults (14 ADF, 12 calorie restriction)
• Who: Adults aged 18 to 55 with obesity (BMI 30 or above)
• How long: 8 weeks, plus 24 weeks of unsupervised follow-up.
• Result: Weight change at 8 weeks -8.2 kg (ADF) vs -7.1 kg (calorie restriction), not significantly different; ADF had a 376 kcal/day larger energy deficit.
• Funding: not stated.

Limit of this finding: This was a small pilot trial.

At 8 weeks, ADF achieved a 376 kcal/day greater energy deficit; however, 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.

An umbrella review of meta-analyses of RCTs found that only modified ADF and 5:2, not zero-calorie ADF, were associated with statistically significant weight loss in adults with overweight or obesity. (Source 6)

• Review of reviews, Certainty not rated.
• Size: 11 meta-analyses of 130 RCTs.
• Who: Adults, mostly with overweight or obesity.
• How long: median follow-up 3 months.
• Result: No statistically significant weight-loss association reported for zero-calorie ADF.
• Funding: not stated.

Limit of this finding: No significant association may reflect too few trials rather than no effect.

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

A Cochrane review that included alternate-day fasting trials found little to no difference in weight loss compared with regular dietary advice. Its evidence on adverse events was very uncertain. (Source 7)

• Systematic review, Low certainty.
• Size: 22 studies, 1,995 participants overall.
• Who: Adults with overweight or obesity.
• How long: 6 to 12 months.
• Result: Weight MD -0.33 percentage points (95% CI -0.92 to 0.26); adverse events RR 1.45 (95% CI 0.64 to 3.28), very low certainty.
• Funding: independent (no dedicated funding)

Limit of this finding: The review pools all intermittent fasting types and reports no separate zero-calorie ADF result in its abstract.

Intermittent fasting may have little to no effect on adverse events but the evidence is very uncertain (RR 1.45, 95% CI 0.64 to 3.28; 7 studies, 619 participants; very low-certainty evidence due to risk of bias, inconsistency and imprecision).

What official bodies say

The Academy of Nutrition and Dietetics states that alternate-day fasting appears about as effective as a typical low-calorie diet. (Source 2)

• Official position, Certainty not rated.
• Size: not applicable.
• Who: General public.
• How long: not applicable.
• Result: not applicable.
• Funding: not stated.

Limit of this finding: This is a consumer page written by a dietitian and reviewed by Academy staff in July 2025. It is not a formal position paper.

Studies suggest that following an alternate-day fast is as effective as a typical low-calorie weight-loss plan.

Where the research disagrees

Whether zero-calorie alternate-day fasting is tolerable

• Catenacci and colleagues (2016), rct: It was safe and tolerable over 8 weeks in adults with obesity, with 93% completing. (Source 5)
• Heilbronn and colleagues (2005), case-series: Hunger on fast days did not decrease, suggesting people were unlikely to continue for long. (Source 4)

A common belief, and what the research shows

The belief: Going without any food every other day must lead to much more weight loss than normal dieting.

What the research shows: In a head-to-head pilot trial, zero-calorie ADF produced a larger energy deficit but no significantly greater weight loss than daily calorie restriction over 8 weeks. An umbrella review did not find it associated with significant weight loss.

References

1. Frontiers in Nutrition. Health Effects of Alternate-Day Fasting in Adults: A Systematic Review and Meta-Analysis. 2020. PMID 33330587, DOI 10.3389/fnut.2020.586036. Read the source
2. 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
3. Johns Hopkins Medicine. Intermittent Fasting: What Is It, And How Does It Work? (expert Q&A with Mark Mattson, PhD). Read the source
4. 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
5. 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
6. 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
7. 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