What Happens on Days 6-7 of a Fast

By the time a fast reaches days 6 and 7, something remarkable has happened in the body. The metabolic emergency of the first few days — the blood sugar fluctuations, the hunger signals, the fatigue of glycogen depletion — has largely resolved. In its place, a more settled metabolic state has emerged. Fat is now the primary fuel. The body's protein conservation mechanisms are in operation. And the mind, depending on the day, can be either unusually sharp or profoundly still.

This is what a landmark 1915 scientific study documented in meticulous detail.

The Historical Study

In 1915, Francis Gano Benedict published A Study of Prolonged Fasting through the Carnegie Institution of Washington. It remains one of the most scientifically thorough investigations of extended human fasting ever conducted. Benedict's team — comprising Harvard physicians, chemists, psychologists, and physiologists — observed a single subject (Agostino Levanzin, referred to as "L." in the study) through a full 31-day fast on distilled water only.

Every day of the fast was measured: body weight, pulse rate, blood pressure, rectal temperature, respiratory gas exchange, urine composition, and daily psychological tests. The study provides an unparalleled day-by-day window into what the body actually does during prolonged fasting.

Days 6 and 7 fall in the middle of the first major metabolic transition — the body's shift from mixed glycogen-and-fat burning to near-total fat dependence.

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

The Metabolic Landscape at Day 6-7

Glycogen Is Almost Gone

On the first day of Levanzin's fast, Benedict's team measured carbohydrate combustion at 68.8 grams per day — the body burning through liver and muscle glycogen at its maximum rate. By days 6–7, that number had fallen dramatically.

The respiratory quotient (a measure of which fuel the body is using) had moved decisively toward fat. Where early fasting shows a mixed picture — carbohydrate and fat both contributing — by the end of the first week, carbohydrate combustion had dropped to a small fraction of its starting value. By day 13, it had effectively ceased.

This matches modern understanding of glycogen depletion timelines. For most people eating a typical diet, glycogen stores (held in the liver and muscles) are depleted within the first 12–48 hours of fasting. Levanzin had been eating only one meal a day before the experiment, which may have meant his glycogen stores were already lower than average. Regardless, by day 6–7 of a full fast, glycogen is not a meaningful fuel source.

Modern research by Cahill (2006, Annual Review of Nutrition) confirmed the sequence Benedict documented: glucose first, then fat (as fatty acids and ketone bodies), with protein catabolism minimised throughout through the protein-sparing mechanisms of ketosis.

Fat Has Taken Over

At days 6–7, the body is running almost entirely on fat. Two forms:

• Free fatty acids — released directly from fat stores and burned by muscle and organ tissue
• Ketone bodies — produced in the liver from fatty acids and used by the brain, heart, and other organs that cannot directly burn fatty acids

Levanzin's urine showed clear evidence of ketone body production from the early days of the fast, and by day 6–7 the ketosis was well established. This is one of the earliest systematic scientific recordings of nutritional ketosis in a fasting human — a significant contribution of Benedict's work.

Longo & Mattson (2014, Cell Metabolism) described this metabolic shift in modern terms: ketone bodies produced during fasting not only fuel the brain but also act as signalling molecules that reduce oxidative stress and inflammation. Benedict observed the end-state; modern research explains the mechanism.

Protein Catabolism Is Declining

One of the most reassuring findings from Benedict's study for anyone concerned about muscle loss during fasting: protein catabolism peaks early in the fast and then declines.

Nitrogen excretion (the best proxy measure for protein breakdown) peaked on day 4 of Levanzin's fast and then fell progressively as the fast continued. By the end of the first week, nitrogen excretion per kilogram of body weight was already substantially below its day-4 peak.

This reflects the protein-sparing effect of ketosis: when ketone bodies are available as brain fuel, the body reduces its need to manufacture glucose from amino acids (gluconeogenesis). The breakdown of muscle protein decreases as the body fully commits to fat as its fuel.

Modern research (Cahill, 2006; Keys et al., 1950, Minnesota Starvation Experiment) confirms this pattern: prolonged fasting with adequate fat reserves produces far less protein wasting than initially feared, particularly once ketosis is established.

Body Temperature and Heart Rate at Day 6-7

Benedict's team measured Levanzin's rectal temperature and pulse rate throughout the fast.

By days 6–7:

• Body temperature had declined modestly. The decrease was small — the body maintains core temperature within a narrow range even during prolonged fasting — but a slight downward trend was visible.
• Pulse rate was declining gradually. From its initial values, pulse had begun the steady fall that would continue across the fast, reflecting the reduced metabolic demand on the cardiovascular system.

These findings align with what modern researchers describe as beneficial cardiovascular adaptations to fasting. Reduced resting heart rate and lower blood pressure during extended fasting are now understood as the heart working less hard against a lower metabolic load — consistent with what Wilhelmi de Toledo et al. (2019, Nutrients) documented in clinical fasting studies.

Physical Function: What Could Levanzin Actually Do?

This is the question people most want answered about extended fasting, and Benedict's documentation provides a concrete answer.

At days 6–7, Levanzin was ambulatory, performing daily psychological tests, and carrying out normal daily activities around the laboratory. He was not bedridden. He experienced variable energy — some days of lassitude, some of surprisingly good physical capacity.

He had no genuine hunger. After the first 2–3 days of fasting, Levanzin reported that the sharp hunger signals of the early fast had disappeared entirely. By day 6, he was neither hungry in the acute sense nor fighting constant cravings. This disappearance of hunger in the middle phase of an extended fast is one of the most consistently reported phenomena across fasting literature, from Sinclair's 1911 case collection to modern clinical reports.

Grip strength was being measured regularly by dynamometer. There was some decline over the course of the fast, but not dramatic early-stage weakness.

Physical tasks remained possible. Benedict photographed Levanzin climbing stairs on day 31 — and if that was achievable at day 31, days 6–7 represent a point where physical function was largely intact.

Mental State: The Variable Days

Day 6–7 falls into what modern fasters describe as the "settling in" phase — past the difficult first few days, before the deeper exhaustion of the final week.

Benedict's psychological testing showed high day-to-day variability. Some days Levanzin's reaction times were sharp and his word associations were excellent. Other days he reported drowsiness and slower responses. The study noted: "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."

By day 6–7, Levanzin was in the phase he himself described as including "remarkable periods of mental clarity." He was writing, reading, and participating actively in the research. He had not entered the more difficult territory of the third and fourth weeks.

This is consistent with Mattson et al. (2018, Nature Reviews Neuroscience), who described the mechanisms by which fasting supports brain function: ketone bodies provide a stable, efficient fuel for neurons; BDNF (brain-derived neurotrophic factor) rises during fasting, supporting cognitive function and mood; and the reduction in blood glucose variability removes the "brain fog" associated with glucose fluctuations.

The variability Benedict observed is also well-recognised in modern fasting communities: some days are remarkably clear, others feel heavy. This is partly down to hydration, electrolyte status, sleep quality, and activity level — factors Benedict's team noted but could not fully control in 1912.

Weight Loss at Day 6-7

By the end of the first week, Levanzin had lost approximately 4–5 kilograms from his starting weight of 60.6 kg. The weight loss rate had already begun slowing from the high of day 1 (which was primarily water and glycogen — each gram of glycogen is stored with approximately 3 grams of water).

By days 6–7, the body was losing weight primarily through true fat oxidation, with a smaller component of protein catabolism. This is the slower, steadier weight loss phase that extended fasters describe after the rapid first-few-days drop.

Modern research (Leibel et al., 1995, New England Journal of Medicine) confirmed that metabolic adaptation begins within the first week of prolonged fasting, reducing energy expenditure as the body conserves resources. Benedict observed this in real-time: total heat production began declining across the fast, reaching its minimum around day 21.

What This Means for Everyday Fasting

Most people doing intermittent fasting never reach days 6–7 of a continuous fast. But understanding what happens at this stage has practical value:

The fear of extended fasting is often that the body enters a catastrophic breakdown — muscle rapidly consumed, metabolism destroyed. Benedict's careful measurements show the opposite. By days 6–7, the body is running efficiently on fat, protein catabolism is already declining, and physical and mental function are largely preserved.

This doesn't mean extended fasting is appropriate for everyone. It means the biology is more resilient than most people assume.

For the complete guide to fasting protocols, including how to approach extended fasts safely, get Intermittent Fasting in Practice on Amazon → [Amazon link]. Buy the book and claim 3 months free on the fasting app at https://www.fastinginpractice.com/redeem.

Frequently Asked Questions

Do you feel hungry on day 6 of a fast? Most people do not feel acute hunger by day 6. After the first 2–3 days, hunger signals change character — the sharp, urgent glucose-driven hunger of early fasting gives way to something quieter. Benedict documented this clearly: Levanzin reported no significant hunger during the mid-fast period.

Is fat burning still happening on days 6-7? Yes — fat burning is the dominant activity. By this point in a complete fast, carbohydrate stores are depleted and fat (as fatty acids and ketone bodies) is supplying most of the body's energy needs. Ketones are being produced continuously.

What about muscle loss on days 6-7? Protein catabolism peaked around day 4 in Benedict's study and was already declining by days 6–7. The protein-sparing mechanism of ketosis reduces the body's need to break down muscle for glucose. Muscle loss during extended fasting is real but much less dramatic than most people fear once ketosis is established.

Are days 6-7 the hardest part of a prolonged fast? Not typically. Days 1–3 are usually the hardest (hunger, fatigue, adaptation). Days 6–7 are often described as a plateau — neither easy nor especially difficult. Benedict's data supports this: no dramatic physical or mental crises during the mid-first-week period.

Did Benedict's subject feel well at day 6-7? Generally yes, with day-to-day variability. Levanzin walked, read, wrote, and performed tests. Some days felt sharp, others tired. No physical crisis was recorded during this period.

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.

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