What Happens in Week 4 of a Prolonged Fast

By the fourth week of a prolonged fast, the human body has entered a state of metabolic adaptation that would not be fully explained by science for another century after it was first systematically documented.

The data for this account comes from a landmark 1915 study conducted at the Carnegie Nutrition Laboratory in Boston, directed by Francis Gano Benedict. The subject — Agostino Levanzin, a multilingual Maltese pharmacist with extensive prior fasting experience — completed a full 31-day fast on distilled water only, monitored daily by a multidisciplinary team of Harvard and Carnegie scientists. Week 4 of that experiment, roughly days 22 through 31, produced some of the study's most significant findings.

The Metabolic State at Week 4

By the fourth week, the body has moved through all three phases of fuel use. Phase one — burning stored carbohydrate (glycogen) — ended around day 13. Phase two — heavy fat catabolism with declining protein catabolism — dominated weeks two and three. By week four, the body has settled into what Benedict's team measured as maximum metabolic efficiency.

The respiratory quotient — the ratio of carbon dioxide produced to oxygen consumed, used to identify which fuel the body is burning — held consistently in the 0.71–0.76 range throughout week four. A quotient of 0.71 indicates nearly pure fat oxidation. The lowest recorded value in the entire study was 0.68, observed once during deep fat metabolism.

Protein catabolism had reached its minimum. Daily nitrogen excretion, which peaks around day four of a fast and then falls progressively, reached its lowest recorded level near the end of the fast: approximately 0.143 grams of nitrogen per kilogram of body weight per day. This protein-sparing effect mirrors what modern researchers have confirmed. The process of ketosis is specifically protective of lean tissue: ketones signal the body to preserve muscle mass and preferentially oxidise fat stores (Cahill, 2006; Longo & Mattson, 2014, Cell Metabolism).

Heat Production and Metabolic Rate

One of Benedict's most carefully measured variables was heat production — direct measurement of metabolic rate via precision calorimetry.

Heat production had been declining since the start of the fast, reaching its minimum on night 21: approximately 625 calories per 24 hours, down from roughly 836 calories on day 3. This represents a drop of approximately 25 percent in basal metabolic rate over the course of the fast.

Through week four, heat production rose slightly from this minimum — an observation Benedict noted but did not fully explain. Modern metabolic research (Leibel et al., 1995, New England Journal of Medicine) has since confirmed that the body's metabolic rate declines significantly during prolonged caloric restriction and fasting, and that this adaptation persists into recovery — one reason why the quality of refeeding matters as much as the fast itself.

Cardiovascular Changes in Week 4

Pulse rate, which had been declining across the entire fast, continued its gradual reduction into week four. The lowest recorded pulse was 73 beats per minute on day 23. Blood pressure in both systolic and diastolic measures was lower than at baseline.

These cardiovascular adaptations — reduced cardiac demand, lower blood pressure, reduced cardiac workload — are consistent with what modern therapeutic fasting research has documented. A 2019 study in Nutrients (Wilhelmi de Toledo et al.) confirmed that prolonged supervised fasting produces meaningful reductions in blood pressure and cardiovascular load in human subjects.

Heart sounds on clinical examination had become "less distinct" from day 11 onward, reflecting reduced cardiac workload rather than damage. No arrhythmia was recorded. The subject continued to walk, climb stairs, and participate in daily psychological testing throughout week four.

Mental and Psychological Condition

The psychological condition in week four showed the high day-to-day variability that had characterised the entire fast — but with specific notable highs.

On day 29, Levanzin wrote detailed, coherent, multi-page autobiographical notes. Nearly a month into a complete fast, his capacity for sustained writing remained fully intact.

Benedict's team observed: "The mental condition seemed to make a great difference in his whole make-up. On some days his faculties were very much keener than on others." Days of exceptional mental clarity alternated with days of drowsiness and reduced response speed. Word association quality remained high throughout — no senseless responses, coherent synonyms and compound words throughout.

This variability aligns with the neuroscience of fasting and BDNF (brain-derived neurotrophic factor) production during caloric restriction (Mattson et al., 2018, Nature Reviews Neuroscience). BDNF rises during fasting and supports cognitive function, but the effect fluctuates with hydration, sleep quality, and the body's acid-base balance — which accounts for the day-to-day highs and lows.

On day 31, Levanzin was photographed climbing stairs — clinical notes recorded: "No evidence of unsteadiness." Grip strength had declined from baseline, but physical function remained sufficient for independent movement.

Urine and Blood Data at Week 4

Daily urine analysis throughout the fast documented the ketosis pattern in detail — one of the earliest systematic recordings of nutritional ketosis in controlled human fasting. Beta-hydroxybutyrate (the primary ketone body) appeared in urine from early in the fast and continued throughout week four.

Total urine acidity had increased from the baseline, reflecting the mild metabolic acidosis characteristic of deep ketosis. No dangerous acid-base derangement was recorded. The acidosis was moderate and self-limiting.

After refeeding began on day 31, nitrogen excretion dropped to its absolute minimum: 0.058 g/kg — reflecting the body's post-fast priority of tissue rebuilding over nitrogen excretion.

The End of the Fast and What It Revealed

On day 31, the fast was broken with two lemons eaten whole, followed by oranges, honey, and grape juice. What followed confirmed one of the most important lessons of prolonged fasting: the refeeding phase is as critical as the fast itself.

The sudden reintroduction of food — particularly concentrated fructose and simple sugars — caused severe intestinal colic and required brief hospitalisation. This is what modern medicine now identifies as refeeding syndrome: the electrolyte shifts and intestinal stress that occur when a depleted body is suddenly given concentrated nutrition (Mehanna et al., 2008, BMJ). The syndrome was formally named and described only after World War II — but it is clearly documented here in 1912.

The lesson remains central to every modern fasting protocol: break a prolonged fast gradually, with small quantities of easily digestible foods, increasing portion and complexity over several days.

What Week 4 Tells Us About the Body

The data from Levanzin's fourth week of fasting reveals a body that has adapted far more comprehensively than most people expect. Metabolic rate has declined to conserve energy. Fat has become the near-exclusive fuel. Protein is being protected. The heart beats more slowly and efficiently. And despite 28–31 days without food, the mind can still compose coherent prose and the legs can still climb stairs.

This does not mean prolonged fasting is safe or appropriate for general use. The severe refeeding episode and the requirement for daily medical monitoring make clear that the risks are real. But the physiological data documents the human body's capacity for metabolic adaptation in extraordinary detail — measured in controlled conditions in 1912, and confirmed by modern research in the century since.

For the complete guide to intermittent fasting, get Intermittent Fasting in Practice on Amazon → [Amazon link]. Buy the book and claim 3 months free on our fasting app at https://www.fastinginpractice.com/redeem

Frequently Asked Questions

Is it safe to fast for 31 days? A 31-day complete fast is not something to attempt without experienced medical supervision. Levanzin had prior fasting experience, passed a full medical examination, and was monitored daily by physicians throughout. This article is historical context, not a protocol recommendation.

Does metabolic rate fully recover after a prolonged fast? Research suggests metabolic adaptation persists into recovery. The body does not immediately reset to its pre-fast metabolic rate — which is why food quality after a fast matters as much as the fast itself.

What is refeeding syndrome? Refeeding syndrome occurs when a depleted body receives concentrated nutrition suddenly, causing dangerous electrolyte shifts, particularly in phosphate, potassium, and magnesium. It is clearly documented in Benedict's 1912 data, though the clinical term came later.

Why does mental performance fluctuate during a long fast? Day-to-day mental variability during extended fasting appears to be driven by hydration, electrolyte balance, sleep quality, and the body's acid-base status during ketosis. Both the high-performing days and the slow ones are well-documented.

At week 4 of a fast, is muscle being burned? Protein catabolism is at its lowest by week 4, with nitrogen excretion reaching minimum levels. The body's protein-sparing mechanisms are most active during deep ketosis — fat is the overwhelmingly dominant fuel at this stage.

Related Articles

• What happens in week 3 of a prolonged fast
• The three phases of fuel use during a prolonged fast
• What happens to your body during a 30-day fast

This article draws on historical scientific research from 1915 and is for informational purposes only — not medical advice. Always consult a qualified healthcare provider before undertaking any prolonged fast.

Benedict, F.G. (1915). A Study of Prolonged Fasting. Carnegie Institution of Washington, Publication No. 203.