Fasting Evidence

Protocol

Alternate-day fasting: what the evidence shows

Alternating fed and fasted or very-low-calorie days

Updated 2 min read 12 citations

Non-stick saucepan of onion soup heating on a hob beside a salt pig
Springnuts · CC BY-SA 4.0 · Wikimedia Commons

What the evidence supports

Produces weight loss in trials; the largest comparisons find no advantage over daily restriction and notably higher dropout. [1]

Adherence is the recurring problem. A protocol that works in a trial and not in a life is not a better protocol.

Evidence strength across the fasting protocols profiled Of the 6 fasting protocols profiled, this shows how many sit at each level of evidence, with Alternate-day fasting graded established. Established3 protocolsLimited2 protocolsPromising1 protocols
Alternate-day fasting is graded established. Note what the grades do not say: a well-evidenced protocol is one we know works about as well as plain calorie restriction, not one that beats it. Grades assigned from trial design and replication.

The finding people miss

Across the randomised literature, fasting protocols produce weight loss comparable to continuous energy restriction of the same magnitude. Where a trial equates calories, the fasting arm rarely wins. This is consistently reported and consistently under-communicated, because "it works about as well as eating less" is a poor headline.

It is still a useful result. For some people a rule about when to eat is far easier to follow than a rule about how much, and adherence is the thing that actually determines outcomes over a year.

Side view of a mechanical personal weighing scale
PantheraLeo1359531 · CC BY 4.0 · Wikimedia Commons

Other protocols

Does alternate-day fasting work?
Produces weight loss in trials; the largest comparisons find no advantage over daily restriction and notably higher dropout.
Is it better than just eating less?
On the evidence, no — the outcomes converge when calories are equated. It is better if it is easier for you to sustain, which is a real advantage rather than a consolation.
Will I lose muscle?
Any energy deficit costs some lean mass. Adequate protein and resistance training are the mitigations with evidence behind them, and neither is specific to fasting.
Who should avoid it?
Pregnancy and breastfeeding, a history of disordered eating, type 1 diabetes, and anyone on medication requiring food or affecting glucose. Prolonged fasting adds refeeding risk on top.

References

Every citation below links to the original peer-reviewed record on PubMed or via DOI. Nothing here is a substitute for medical advice.

  1. Intermittent fasting for adults with overweight or obesity Garegnani LI, Oltra G, Ivaldi D, et al. · The Cochrane database of systematic reviews · 2026 · Meta-analysis DOIPubMed 41692034Full text
  2. Effects of time-restricted eating on markers of glucose metabolism and regulation in individuals with prediabetes or type 2 diabetes: a systematic review and meta-analysis of randomised controlled trials Viple F, Pedersen SS, Andersen TH, et al. · Diabetologia · 2026 · Meta-analysis DOIPubMed 42507108
  3. Digestible and Metabolizable Energy Intake in Humans: a Systematic Review Yoshimura E, Oi N, Abe K, et al. · Advances in nutrition (Bethesda, Md.) · 2026 · Systematic review DOIPubMed 41655947Full text
  4. The effects of low carbohydrate diets including intermittent fasting on body weight, glycaemia, and quality of life measures in adults with type 1 diabetes and overweight/obesity: a systematic review Zhen XM, Purcell AR, Glastras SJ, et al. · Nutrition, metabolism, and cardiovascular diseases : NMCD · 2026 · Systematic review DOIPubMed 42097900
  5. Comparison of Different Intermittent Fasting Patterns or Different Extents of Calorie Restriction for Weight Loss and Metabolic Improvement in Adults: A Systematic Review and Network Meta-Analysis of Randomized Controlled Trials Wu X, Ding Y, Cao Q, et al. · Nutrition reviews · 2026 · Meta-analysis DOIPubMed 40367516
  6. Dietary Interventions and Testosterone Levels in Obese Men: A Systematic Review Comparing the Mediterranean Diet, Ketogenic Diet, and Intermittent Fasting La Vignera S, Condorelli RA · Nutrients · 2026 · Systematic review DOIPubMed 42588040Full text
  7. Meta-analysis and meta-regression of intermittent fasting effects on glycaemic control in type 2 diabetes: Subgroup analyses and variability Anasanti MD, Hamzah N · Diabetes & metabolic syndrome · 2025 · Meta-analysis DOIPubMed 40849220
  8. The effects of intermittent fasting on sleep quality and cardiometabolic health outcomes in adults with overweight/obesity status: a systematic review and meta-analysis of randomized controlled trials Yong YN, McCubbin AJ, O'Driscoll DM, et al. · Sleep medicine reviews · 2025 · Meta-analysis DOIPubMed 41202521
  9. Food Intake According to Clock Gene Polymorphisms: A Systematic Review Campos LT, Fonseca PF, Rocha DMUP, et al. · FASEB journal : official publication of the Federation of American Societies for Experimental Biology · 2025 · Systematic review DOIPubMed 40787788Full text
  10. Intermittent fasting for weight management and metabolic health: An updated comprehensive umbrella review of health outcomes Hua Z, Yang S, Li J, et al. · Diabetes, obesity & metabolism · 2025 · Systematic review DOIPubMed 39618023
  11. Longer-term effects of intermittent fasting on body composition and cardiometabolic health in adults with overweight and obesity: A systematic review and meta-analysis Khalafi M, Maleki AH, Ehsanifar M, et al. · Obesity reviews : an official journal of the International Association for the Study of Obesity · 2025 · Meta-analysis DOIPubMed 39501676
  12. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials Semnani-Azad Z, Khan TA, Chiavaroli L, et al. · BMJ (Clinical research ed.) · 2025 · Meta-analysis DOIPubMed 40533200Full text