The 36-hour fasting protocol built to protect muscle

A 36-hour fast sounds extreme until you understand what it is actually trying to replicate. According to a physician who runs a structured fasting protocol with clients, a single well-executed 32 to 36 hour fast can approximate the metabolic benefits of a 5 to 7 day fast, without the biggest liability of prolonged fasting: muscle loss. The strategy is not about willpower or how long someone can go without eating. It is a biological setup built across three phases: glycogen depletion, metabolic transition, and refeed.

Why duration is the wrong metric

Fasting duration matters less than the biological conditions created around it. The core marker to track is ketones, not hours on a clock or urine strips. Rising ketones only happen once glycogen is depleted, insulin is normalized, and lipolysis is running at a high rate. A continuous ketone monitor, or a finger-stick device like Keto-Mojo using serum or interstitial fluid, gives a far more reliable signal than urine testing. The target during a fast is a serum ketone level of at least 1.5 by midday; starting the fast already in mild nutritional ketosis, around 1 to 1.2, is the goal for those following the full protocol.

The day before the fast is where the outcome gets decided

The three days surrounding a fast, prefast, fast, and postfast, drive most of the result, and the prefast day is where glycogen gets wiped out. That requires heavy resistance training, not cardio alone, since cardio is an efficiency play and resistance training forces the body to burn through glycogen without ever fully adapting to it. The recommended structure for the prefast day includes:

• Training: a lower body or whole body session at max intensity and load, followed by cardio-metabolic work, done earlier in the day.
• Nutrition: normal fast carbs around training (about 15 grams), then a light, zero-carb dinner such as salad and salmon, finished before 6 p.m.
• Movement: 60 minutes of low-intensity movement, like a walk, after that final meal.
• Heat exposure: a sauna session that evening to further upregulate energy demand, with 30 minutes as a reasonable target and hot water immersion as an alternative if no sauna is available.

During the fast: move more, not less

A common mistake is treating a fasting day as a day to rest. The opposite is closer to correct. The goal during the fasting window is roughly 20,000 steps plus structured movement sessions, because staying sedentary works against the intended energy demand. Caffeine and non-nutritive, zero-calorie drinks are fine and show no meaningful impact on ketone production in serial measurements. Electrolytes become important specifically when sauna use is heavy, since that is where most of the sodium loss in a 32 to 36 hour window actually comes from.

Clients using a GLP-1 medication such as tirzepatide (Mounjaro or Zepbound) as part of a broader protocol are advised to take it first thing on the prefast morning, since it takes roughly 24 hours to reach peak effect, timing it to align with the fasting window.

Refeeding is not optional and not casual

The refeed matters as much as the fast itself. The rule is bone broth first, for a minimum of two hours, before introducing any whole food. Bone broth is chosen deliberately because it is low effort on digestion and lacks leucine, an amino acid that can shut down ketosis. Breaking a fast directly with a heavy, high-protein meal, like a ribeye, is discouraged because gut motility and the gut's mucosal lining change during a fast, making it more vulnerable to anything abrasive. Once whole food starts, the sequence is protein first, then fat, then carbohydrates.

Training intensity should also drop on the fast day and the day immediately after it. High-intensity training without adequate fuel on those days is described as a fast path to injury; lifting at roughly half of normal intensity is a reasonable compromise if training happens at all.

Women are not disadvantaged by this approach

A common claim is that women produce more cortisol when fasting and should avoid it. The counterargument here is that female biology is actually more optimized for lipolysis and ketosis, largely due to differences in hormone-sensitive lipase and the rate of fatty acid oxidation triggered by adrenaline. The recommendation for women is not chronic, mild calorie restriction, but a short, intense fasting protocol followed by a full return to adequate food intake, essentially the same 36-hour structure already described rather than a separate, gentler version.

When not to fast

Fasting readiness has clear markers: fasting insulin under 10 and a visceral fat index under 10 are used as baseline criteria before recommending a demanding protocol. If ketones stay low midday during the fast despite genuine compliance, that is treated as a signal to stop, not push through. Illness, acute stress, and poor prior nutrition all blunt ketone production and increase the risk that a fast does more harm than good; in those situations, the protocol calls for pausing and resetting rather than forcing the fast to completion.