Dry Fasting for 5 Days: What the Biometrics Actually Showed
BiohackingBlood glucose never crashed across five days with zero food and zero water — and that’s the data point that captures why dry fasting produces results that challenge typical fasting models.
What Was Measured
Nick Urban at Outliyr conducted a five-day dry fast in Austin summer heat and tracked a dense set of biomarkers daily: a continuous glucose monitor, blood ketone meter, recovery wearable, InBody bioimpedance scanner, grip dynamometer, and cognitive tests. The rigor distinguishes this from anecdotal dry fasting accounts — the protocol was designed to surface what actually changes, not what adherents believe changes.
The primary findings span several systems:
Glucose regulation: Blood glucose held between 78 and 110 mg/dL throughout, achieving 100% time-in-range despite the absence of any caloric intake. The body produced glucose endogenously from fat metabolism, a process known as gluconeogenesis. The expected crash did not materialize.
Ketone production: Blood ketones peaked at approximately 1.5 mmol/L by day three, settling around 1.2 mmol/L — mild nutritional ketosis, not the deep ketosis often reported in extended water fasts. The dry fast produced a shallower ketone curve than equivalent water fasting periods typically show.
Body composition: Total weight dropped approximately 23 pounds over five days, with roughly 6 pounds regained during refeeding. Body fat measured at 5.6 pounds of actual fat loss, with visceral fat showing a 70% reduction from baseline (16.8 to 5.0 cm²) when measured after full rehydration. Skeletal muscle was preserved and measured as slightly increased post-rehydration.
Cardiovascular and recovery metrics: Resting heart rate fell from 56 to the low 40s bpm. The recovery score peaked at 92% on the final fasting day. HRV rebounded strongly after rehydration.
Performance: Breath-hold (BOLT score) improved 33%, from 32 to 42 seconds. Grip strength dipped temporarily and recovered. Reaction time slowed modestly toward day five.
The Dehydration Artifact Problem
A critical methodological finding from the report is what Urban calls the “dehydration artifact”: bioimpedance scales, including clinical InBody units, misread severely dehydrated bodies. His exit-day scan showed an apparent nine-pound muscle loss that vanished entirely upon rehydration. Any body composition reading taken during or immediately after a dry fast is, in his framing, “noise, not signal.”
This matters for interpreting the circulating dry fasting literature. Studies and case reports that measure body composition at the end of the fast — before rehydration — are capturing fluid redistribution, not tissue change. The pre-to-post comparison should always use a post-rehydration baseline.
Day two represented the objective low point across most metrics in this protocol. Day three marked the physiological turning point, with recovery scores and subjective state improving despite continued fasting.
The So What
This dataset doesn’t establish dry fasting as safe or effective for general audiences — it documents what one rigorously self-tracked individual observed under controlled conditions. The glucose data challenges assumptions that extended fasting without water leads to metabolic dysregulation; the body’s gluconeogenic response appears robust. The dehydration artifact finding has practical implications for anyone interpreting self-tracking data: measurement timing relative to rehydration status meaningfully changes what the numbers mean. For biohackers using wearables and CGM, the more useful takeaway may be methodological — how to measure before, during, and after any extended protocol without conflating dehydration state with tissue-level change. Related: how continuous biosensing is changing what biohackers can actually measure.
Source: Outliyr
Content created with AI assistance and reviewed for accuracy.
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