Summary of "All the Health Effects of a 5 Day Water Fast. [Study 55 - Detailed Analysis]"

Study design / protocol

3-day gradual refeed in the lab:
1. Day 6: ~30% of calculated maintenance calories
2. Day 7: ~60% of maintenance
3. Day 8: 100% of maintenance

Non-esterified fatty acids
Serum and urine β‑hydroxybutyrate (ketones)
Body weight, waist circumference, BMI
Blood pressure (systolic & diastolic)
Triglycerides, total cholesterol, LDL, HDL
Insulin, blood glucose, IGF‑1
Broad metabolome (carbohydrate / fatty-acid / amino-acid metabolites heatmap)
(Longer/insiders version included immune, thyroid, and additional metabolic details)

Non-esterified fatty acids and serum β‑hydroxybutyrate rose substantially by day 3 and day 5, indicating a switch from carbohydrate to fat/ketone fuel.
Ketones were detectable in urine as well.

Systolic blood pressure fell by ~4 mmHg by day 5; reductions persisted at day 8 and at 1 month but not at 3 months.
Diastolic blood pressure dropped substantially during the fast (e.g., ~77 → 66 mmHg by day 5) but did not show the same longer-term persistence as systolic.

Triglycerides increased during the fast and remained elevated in many participants at day 8 and to some extent at 3 months; there was large inter‑individual variability.
Total cholesterol and LDL increased during the fast (sometimes outside the normal band) then fell back toward normal by 1 month.
HDL decreased during the fast and increased by 1 month.

Insulin and blood glucose dropped markedly during fasting (insulin reduced by ~60%); effects were short‑lived and rebounded after refeeding.
IGF‑1 decreased during the fast (largest effect at day 5), stayed reduced at day 8, then rebounded with high variability by 1 month.

Metabolomics showed a clear shift away from carbohydrate-related metabolites toward fatty‑acid and amino‑acid metabolic signatures during the fast.
Bile-acid markers decreased during fasting.
Many metabolomic changes largely reversed with refeeding by day 8 / day 38.

If attempting a multi-day water fast, consider medical supervision and a controlled refeed (gradual calorie reintroduction: ~30% → 60% → 100% over 3 days) to reduce metabolic shock.
Expect rapid metabolic switching: increased circulating fatty acids and ketones within 2–3 days.
Expect substantial short-term weight loss, but much of it may be regained by 3 months unless accompanying lifestyle changes are made.
Monitor key markers while fasting if possible: blood pressure, blood glucose/insulin, ketones, and a lipid panel — responses vary substantially between individuals (notably triglycerides).
Be aware of transient adverse or ambiguous changes: fasting can raise triglycerides and total/LDL cholesterol while fasting; these often normalize after refeeding but vary by person.
Short-term reductions in insulin and IGF‑1 (lower growth signaling) are possible during the fast but typically rebound quickly after eating.
Measuring ketones (serum or urine) can confirm the metabolic shift toward fat metabolism.
Individual variability means personalized assessment and supervision are recommended, especially for people with preexisting conditions.

Study population was normal-weight adults; results may differ for overweight or obese individuals.
Some potentially important outcomes (immunity / thyroid / additional metabolic details) were not shown in the shorter analysis; full data are available in the extended content.
Markers such as triglycerides showed large inter‑individual variability; some increases persisted longer than expected.
Many beneficial changes were transient (returned after refeeding), so fasting should be viewed as one tool rather than a guaranteed long-term solution.

Video presenter: Nicholas Verhoeven (Physionic — doctoral candidate in molecular medicine; master’s in exercise physiology)

Primary study referenced: “Five-day water-only fasting decreases metabolic syndrome risk factors and increases anti‑aging biomarkers without toxicity in clinical trials [normal weight individuals]” (as cited in the video)
Video / series: Physionic — “All the Health Effects of a 5 Day Water Fast. [Study 55 - Detailed Analysis]” (short and long/insiders versions)