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Fasting Index

Every other day · intermediate

Alternate-day fasting

A fast day, then an ordinary day, repeating. Most trials allow about a quarter of normal intake on the fast day rather than nothing. It is the only fasting schedule a 2025 network meta-analysis found beat steady calorie restriction, and it is also the schedule people quit most often in the longest trial run on it.

Who should not do an alternate-day fast

  • Anyone taking insulin or a sulfonylurea, unless the schedule and the doses are managed with the prescribing clinician. A fast day is a hypoglycaemia risk on these drugs and the strict version is a full-day fast.
  • Anyone who is pregnant or breastfeeding.
  • Anyone with a current or past eating disorder. This schedule builds a restrict and refeed cycle into the calendar, which is the pattern most likely to entrench disordered eating.
  • Children and adolescents.
  • Anyone underweight, or with elevated nutritional requirements.
  • Anyone being treated for high LDL cholesterol. The longest trial of this protocol found LDL higher in the fasting arm at 12 months than in the daily restriction arm, so this is worth raising with the prescribing clinician rather than assuming the schedule is neutral.
  • Anyone on medication that must be taken with food, or requiring doses spread through the day.

This is not medical advice and no clinician has reviewed it. If any of the above applies to you, speak to a doctor before fasting rather than after.

A schematic timeline of a fast from 0 to 72 hours. A tone band along the top darkens as the body moves from fed to deeply fasted. Beneath it, liver glycogen is drawn as a stipple field that is largely spent within the first 24 hours. Below that, three traces on independent scales: glucose falls then levels, insulin falls steeply and stays low, ketones rise from about 12 hours and climb through the second and third day. The pattern is well described but individuals vary widely, and the horizontal scale breaks at 24 hours.
The hours of a fast. Above, the phases as a tone that darkens as the body moves from fed to deeply fasted. In the middle, liver glycogen as stipple, largely spent inside the first day. Below, three traces on independent scales: glucose, insulin, ketones. A schematic of a well-described pattern. Individuals vary widely, and the scale breaks at 24 hours.

What is actually happening

Alternate-day fasting puts a fast day between every pair of ordinary days. Two versions exist and they are not the same intervention. The strict version allows water and non-caloric drinks only. The modified version, which is what most trials actually test, allows about 25 percent of energy needs on the fast day, usually taken as one small meal. On a fast day insulin stays low and fat oxidation rises far enough to lift blood ketones, and in the strict version ketones remained elevated even on the eating days. The part that is easy to miss is the size of the deficit. Strict alternate-day fasting cut average intake by 37 percent in the trial that measured it, so this is a large calorie restriction rather than a change of timing. The eating day is supposed to absorb some of that, and in practice it does not. The longest trial prescribed 125 percent of energy needs on eating days and found participants ate less than that, while eating more than prescribed on the fast days.

What the evidence shows

  • In a 2025 network meta-analysis of 99 randomised trials and 6,582 adults, alternate-day fasting was the only intermittent fasting schedule to produce more weight loss than continuous calorie restriction, by 1.29 kg, at moderate certainty of evidence.
  • In that same network meta-analysis, no diet strategy outperformed any other among the 17 trials lasting 24 weeks or longer. The alternate-day advantage appears in the shorter trials and not in the longer ones.
  • In a one-year randomised trial of 100 adults with obesity, dropout was highest in the alternate-day fasting group at 13 of 34, or 38 percent, against 10 of 35 for daily calorie restriction and 8 of 31 for the no-intervention control.
  • In that trial, participants assigned to alternate-day fasting ate more than prescribed on fast days and less than prescribed on feast days, while the daily calorie restriction group generally met its targets. Its stated conclusion is that alternate-day fasting produced no superior adherence, weight loss, weight maintenance or cardioprotection.
  • LDL cholesterol was significantly higher at month 12 in the alternate-day fasting group than in the daily calorie restriction group in that trial, by 11.5 mg/dL with a 95 percent confidence interval of 1.9 to 21.1 mg/dL.
  • Two trials disagree about LDL cholesterol on this schedule. The one-year trial in adults with obesity found it higher in the fasting arm at 12 months, and the Cell Metabolism study of longer-term strict alternate-day fasting in healthy adults who were not obese reported it lower. They studied different people and different versions of the protocol, and the disagreement is unresolved.
  • Four weeks of strict alternate-day fasting in healthy middle-aged adults reduced average calorie intake by 37 percent and raised beta-hydroxybutyrate even on the eating days. The same paper reports lower LDL cholesterol and lower triiodothyronine after longer-term alternate-day fasting, which is a separate observation from the four-week randomised comparison. It carries a published correction in the same journal.
  • Alternate-day fasting produced larger changes in fat mass than time-restricted eating in a 2025 randomised trial in adults without obesity. The two schedules in that trial did not deliver matched calorie intakes, and neither was compared against ordinary calorie restriction.
  • In an eight-week randomised pilot of 26 adults with obesity, zero-calorie alternate-day fasting achieved a 376 kcal per day larger energy deficit than daily calorie restriction and still produced no significant difference in weight change, 8.2 kg against 7.1 kg.

What the evidence does not support

  • The 1.29 kg advantage over continuous calorie restriction is the single positive weight result on this page, and it comes from a network meta-analysis whose own subgroup of trials lasting 24 weeks or more found no strategy beat any other. It is not evidence of a durable advantage.
  • The one-year randomised trial built to answer this exact question concluded that alternate-day fasting did not produce superior adherence, weight loss, weight maintenance or cardioprotection against daily calorie restriction.
  • The evidence does not support alternate-day fasting as neutral for cholesterol. The longest trial found LDL significantly higher in the fasting arm at 12 months, and that result is rarely quoted next to the weight results from the same trial.
  • Claims that this schedule works through fasting physiology rather than through calorie reduction are not supported by the trials here. Where intake was measured the fasting arms were running large deficits, and in the pilot where the deficit was 376 kcal per day larger the weight loss was not.
  • The Cell Metabolism trial is cited widely for reporting no adverse effects. It ran four weeks in healthy adults who were not obese, it has no continuous calorie restriction arm, and it carries a published correction. This site has not read that correction and does not describe what it changes.
  • The trials of the zero-calorie version on this page ran four weeks and eight weeks. The one-year evidence is all modified alternate-day fasting at about 25 percent of energy needs on the fast day, and the two versions are not interchangeable.

How it is run

  • One fast day, then one ordinary day, repeating. Most trials use the modified version at about 25 percent of energy needs on the fast day, taken as a single small meal.
  • The eating day is an ordinary day. The trial that prescribed 125 percent of needs on eating days found people did not reach it, so eating extra to compensate is not an instruction the evidence supports.
  • The strict version allows water and non-caloric drinks only on the fast day. It has been studied over four weeks in healthy adults who were not obese, and it produced a 37 percent average reduction in calorie intake.
  • Adherence is the thing to plan for rather than the physiology. This schedule had the highest dropout of any arm in the only trial that ran it for a year.

What breaks an alternate-day fast

The rules differ by protocol. Check any food, drink or supplement against this protocol below.

See the full index

Sources

No clinician reviews this site. Sources on this page were verified against PubMed by the author, who is not a doctor or a dietitian. What that means.

Last updated 2026-08-16.

The phases of a fast covers the physiology behind every protocol on this site, hour by hour.