What Happens in Week 2 of a Prolonged Fast

The first week of a prolonged fast is the body's transition phase — burning through carbohydrate stores, establishing ketosis, adjusting to the absence of food. By the time week two begins, something more fundamental has happened: the body has almost fully completed its shift away from glucose as a primary fuel. What follows in days 8 through 14 is the settling of a new metabolic state, documented in remarkable detail by a landmark scientific study conducted in 1912 at the Carnegie Nutrition Laboratory in Boston.

Historical Context: The 1915 Benedict Study

In 1915, Francis Gano Benedict of the Carnegie Institution of Washington published A Study of Prolonged Fasting — arguably the most rigorous scientific investigation of extended fasting conducted up to that time. The subject, Agostino Levanzin (referred to as "L." in the study), was a Maltese pharmacist and self-described health experimenter who had previously undertaken a 37-day fast. He completed a full 31-day controlled fast at the laboratory, drinking only distilled water while a multidisciplinary team of Harvard and Carnegie scientists measured his every physiological variable daily.

The week-two period — roughly days 8 through 14 — represents one of the most significant metabolic transitions of the entire fast, and Benedict's measurements capture it with precision that modern researchers still find valuable.

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

The End of Glycogen: Days 8–13

By the end of the first week, the body has been steadily drawing down its glycogen stores — the carbohydrate reserves stored primarily in the liver and muscles. Benedict's measurements showed that on the very first day of the fast, Levanzin was burning approximately 68.8 grams of carbohydrate per day. By days 10 to 13, that figure had fallen to approximately 4 grams per day.

By day 13, carbohydrate combustion had effectively ceased. The non-protein respiratory quotient — the ratio of carbon dioxide exhaled to oxygen consumed — confirmed the shift. The number moved steadily toward 0.71 to 0.76 during week two, which is the range characteristic of almost pure fat combustion (pure fat combustion produces a respiratory quotient of approximately 0.71).

This is the defining metabolic event of week two: the body completing its transition from mixed fuel use to fat dominance. After day 13, virtually every calorie of energy Levanzin used came from his own stored fat and — to a smaller degree — from body protein.

Modern metabolic research confirms this general timeline, though most people who eat a standard Western diet deplete glycogen more quickly — within 24 to 72 hours of fasting — because they have fewer pre-fast glycogen stores than Levanzin, who had been eating one meal per day for the year before the experiment.

Protein Sparing: A Critical Adaptation

One of the most important findings of Benedict's week-two measurements was what happened to protein catabolism — the breakdown of body protein for energy. The body uses amino acids from protein as a backup fuel source, primarily through gluconeogenesis (producing glucose from non-carbohydrate sources). The question of how much muscle and organ tissue a prolonged fast consumes was central to the scientific debates of Benedict's era.

His measurements offered reassurance. Nitrogen excretion in the urine — a direct proxy for protein breakdown — peaked early in the fast around day 4, then fell progressively. By week two, daily nitrogen excretion per kilogram of body weight had dropped substantially from its peak. The body was actively conserving protein.

This protein-sparing mechanism is now understood through the lens of ketosis. As ketone production increases and the brain and other organs adapt to running on ketones rather than glucose, the body's demand for gluconeogenesis drops. Less glucose needs to be manufactured from amino acids, so less protein is broken down. Levanzin was losing primarily fat during week two — not muscle.

Modern research by Cahill (2006, Annual Review of Nutrition) confirms this process in detail, documenting how ketone adaptation progressively reduces the brain's glucose requirement and thereby reduces protein catabolism during prolonged fasting.

Energy Production Falls, Then Stabilises

Benedict's respiration calorimeter — which measured the direct heat output of the sleeping body — showed a progressive decline in energy production during the fast. By around day 21, heat production reached a minimum of approximately 625 calories per 24-hour period, down from around 836 calories early in the fast.

During week two, this decline was actively in progress. The body was reducing its basal metabolic rate — the energy it burns at rest — as a conservation measure. This is what modern researchers describe as metabolic adaptation: the body becoming more efficient during a caloric deficit to reduce the rate at which its reserves are depleted.

The approximate 25% reduction in BMR observed in Benedict's data parallels what subsequent studies — including the Minnesota Starvation Experiment by Ancel Keys and colleagues — have documented during prolonged caloric restriction and fasting. The body does not simply continue burning at its normal rate; it adapts downward.

Cardiovascular Adaptation in Week 2

Benedict documented a gradual but consistent decline in Levanzin's pulse rate and blood pressure throughout the fast, including during week two. The highest recorded pulse rate was around 100 beats per minute early in the fast; by day 23 it had fallen to approximately 73. Blood pressure — both systolic and diastolic — also declined.

The reduction in heart rate and blood pressure reflects reduced metabolic demand. With the body burning less energy, the heart does not need to pump as much blood per minute. This is not a sign of cardiac weakness — it mirrors the cardiovascular profile associated with endurance fitness. Modern therapeutic fasting research, including work by Wilhelmi de Toledo and colleagues (2019, Nutrients), documents similar beneficial cardiovascular adaptations during extended fasting under medical supervision.

By week two, Benedict noted that heart sounds had become slightly less distinct — but Levanzin remained mobile, participated in daily psychological testing, and showed no signs of cardiovascular distress.

Psychological Experience in Week 2

Benedict's team conducted daily psychological tests: memory for words, reaction time, word association, and grip strength. The findings during week two were nuanced.

There was no collapse in mental function. Levanzin continued to read, write, and engage in conversation. His word association responses remained coherent and intelligent — no senseless replies were recorded. However, day-to-day variability in performance was high. Some days brought what the subject himself described as remarkable mental clarity and focused energy. Other days brought drowsiness and sluggishness.

Benedict's team noted that the subject's psychological state on any given day was "the greatest single variable" in test performance. Days when Levanzin was cheerful and engaged consistently produced better cognitive results than days of low mood.

This fluctuating pattern is now widely recognised among modern extended fasters. Mental clarity during prolonged fasting is real — but it is not constant. It reflects the day-to-day variability in ketone availability, hydration, electrolyte balance, and stress levels. Mattson and colleagues (2018, Nature Reviews Neuroscience) document the neurological mechanisms behind fasting-induced cognitive improvement, including increased BDNF (brain-derived neurotrophic factor) production.

Physical Condition at the End of Week 2

At the conclusion of week two — around day 14 — Levanzin had lost approximately 7 to 8 kilograms from his starting weight, with the rate of daily weight loss having slowed considerably from the first few days. His abdomen was visibly less prominent in clinical photographs. He was ambulatory, continuing to climb stairs and participate in all research activities.

The transition from glycogen burning to fat burning was complete. His body had entered the sustained fat-catabolism phase that would continue through the remaining 17 days of the fast. The metabolic groundwork laid by week two — protein sparing, fat dominance, cardiovascular adaptation, metabolic rate reduction — created the physiological conditions that made the remainder of the fast possible.

Connecting 1915 to Today

Benedict's week-two findings anticipated several concepts that modern nutritional science would not formally describe for decades. The protein-sparing effect of ketosis. The metabolic rate adaptation during caloric restriction. The cardiovascular rest provided by reduced metabolic demand. The variability of cognitive performance during extended fasting.

What he measured in Boston in 1912 with state-of-the-art calorimetry equipment remains consistent with what modern researchers observe using far more sophisticated tools. The body's response to the second week of fasting has not changed — only our ability to explain the mechanisms has improved.

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Frequently Asked Questions

Is week 2 of a prolonged fast dangerous? The 1915 study showed no dangerous events during week two of a medically supervised complete fast in a healthy adult. Metabolic adaptations were occurring — reduced BMR, declining pulse, ketosis — but all within physiologically safe ranges. Prolonged fasting outside of medical supervision carries risks that were not present in Benedict's controlled setting.

Does hunger disappear by week 2? Benedict's subject reported that the acute hunger of the first few days had resolved well before week two. By days 4–7, hunger was absent. Levanzin described occasional mild appetite during week two but nothing like the acute hunger of the early fast.

Why does weight loss slow down in week 2? The large early weight loss reflects glycogen depletion (with its associated water) and the initial adjustment in metabolic rate. By week two, the body has adapted and is consuming fat at a more conservative rate. True fat loss continues but produces smaller day-to-day scale changes than the water weight losses of the first few days.

What happens to muscle in week 2? Nitrogen excretion data showed protein catabolism declining from its peak around day 4. By week two, protein-sparing mechanisms were operating — the body was conserving muscle protein and running predominantly on fat. Some protein catabolism continued throughout, but at a rate far below the peak.

Can I reproduce this at home? Benedict's study was conducted under full medical supervision with daily clinical monitoring, blood work, and immediate access to medical care. His results should not be used to justify attempting a 31-day fast at home. The study is a scientific reference — not a protocol to replicate.

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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.

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