A 72-Hour Water Fast Reshapes Hormones, Cholesterol, and Even Your Glycome

Going without food for three days is not something most people do by accident. But a new pilot study published in the Croatian Medical Journal set out to map exactly what happens inside the body during a 72-hour water-only fast, and the results go well beyond the expected drop in blood sugar. Researchers tracked 22 biochemical parameters, plus detailed profiles of plasma glycans and immunoglobulin sugar attachments, in five healthy adults. The findings are preliminary, not definitive, but they sketch a surprisingly intricate picture of systemic remodeling.

Cholesterol, CRP, and Thyroid Hormones All Shifted During the Fast

The study, led by Lauc G et al. and published in June 2026, enrolled five healthy volunteers aged 40 to 65 (four male, one female) who fasted on water alone for 72 hours. Blood was drawn at three points: before fasting (T0), immediately after fasting (T1), and 11 days later at refeeding follow-up (T2). Total cholesterol increased significantly at T1, with similar non-significant trends in LDL-cholesterol and triglycerides. C-reactive protein (CRP), a standard marker of inflammation, also rose significantly at T1, then fell back toward baseline at T2. Thyroid hormones told a consistent story: TSH and free T3 both dropped at T1 in all five participants, a pattern the authors describe as consistent with a well-documented "low-T3 fasting state," before recovering at T2. Glucose and insulin fell at T1 and partially rebounded at T2, consistent with reduced signaling through the mTORC1 pathway, a key regulator of cell growth and metabolism.

The Glycome Finding Adds an Unexpected Layer

What sets this study apart from most fasting research is its focus on glycans: the complex sugar structures attached to proteins circulating in the blood. Plasma N-glycan profiles showed significant changes at T1 in five specific traits, including shifts in low-branching, high-branching, trisialylated, trigalactosylated, and high-mannose structures. IgG Fc glycosylation, which is linked to both inflammatory status and biological aging, showed subtle alterations. IgA glycosylation at three sites also shifted, in a pattern distinct enough that the researchers suggest it may follow separate regulatory mechanisms from IgG. Nearly all glycomic changes partially reversed at T2, pointing to the fast as their direct cause. This matters because, as prior research on widespread biological changes during extended fasting has shown, the body's response to food deprivation is far more systemic than simple calorie arithmetic.

Why These Shifts Could Relate to Autophagy

The authors frame several of their findings in terms of autophagy, the cellular housekeeping process by which cells break down and recycle damaged components. After roughly 12 to 24 hours without food, glycogen stores deplete and the body shifts toward using fat for fuel. This metabolic switch, combined with falling insulin and mTORC1 activity and rising AMPK signaling, creates conditions that are theoretically favorable for autophagy induction. The mild CRP increase at T1 is described as consistent with concurrent autophagy activation, since early-phase autophagy can produce transient inflammatory signals. The cholesterol rise also fits: cholesterol has a documented role in autophagosome membrane formation via the protein GRAMD1A. Understanding these evolutionary adaptive responses to fasting has become increasingly relevant as obesity rates rise and fasting-based therapies attract clinical interest. That said, autophagy was not measured directly in this study. No autophagy-specific markers, such as the LC3-II/LC3-I ratio or autophagic flux, were tested. The autophagy interpretation is hypothesis-generating, not confirmed.

What a Five-Person Sample Can and Cannot Tell Us

The study's authors are candid about its limitations. Five participants, only one of whom was female, provide very limited statistical power. There was no control group fasting on normal calories for comparison. The limitations common to prolonged fasting trials are well-documented: small samples and missing control arms make it difficult to determine which changes are caused by fasting itself versus other variables. A separate study in Molecular Metabolism found that prolonged fasting inflammation and platelet activation risks may be a concern for people with pre-existing cardiovascular conditions, a population not represented here. The single-arm design also means there is no way to know whether the participants' cholesterol or glycan shifts fall within normal day-to-day biological variation. The authors acknowledge that denser sampling during the fast and refeeding period would give a more complete mechanistic picture. On the positive side, the protocol was feasible and well-tolerated: no participant dropped out, and the transient increases in liver enzymes AST and ALT were small.

Where Fasting Research Needs to Go Next

This pilot study is best read as a feasibility and hypothesis-generating exercise. The researchers identify several specific targets for follow-up: direct measurement of LC3-II/LC3-I ratio, autophagic flux, leptin, and IGF-1 to test the autophagy hypothesis; a larger and more sex-balanced cohort; and a non-fasting comparison arm. A parallel finding from research on plasma lipidome remodeling during a 36-hour fast published in a Lancet journal showed that even shorter water-only fasts produce significant metabolite shifts, suggesting that 72-hour studies are likely to reveal even more dramatic changes with adequate sample sizes. Reviews have noted that scarce research on longer fasting durations beyond 48 hours leaves major gaps in understanding sustained physiological effects, and this study begins to fill that gap, carefully. The glycomic findings are particularly ripe for follow-up: if fasting reliably alters IgG and IgA glycosylation in ways that track inflammatory or biological age markers, that could have implications for understanding how fasting interventions affect long-term immune function. For now, the honest summary is that three days without food reshapes the body in more ways than most people expect, and science is only beginning to map the territory.

Evidence rating

INVESTIGATIONAL — 61/100

Caveats:

• Pilot study with only five participants (four male, one female) — results are preliminary and cannot be generalized.
• Single-arm design with no control group makes it impossible to isolate the effect of fasting from other variables.
• Autophagy was not directly measured; claims about autophagy induction are speculative and hypothesis-generating only.
• Funding source was not disclosed by the authors.

Single-arm interventional pilot study enrolling five healthy adults. The extremely small sample provides insufficient statistical power, and the absence of a control group means the observed changes cannot be causally attributed to fasting alone. Autophagy was not directly assessed; mechanistic interpretations remain speculative. Findings are preliminary and have not been independently replicated.

References

1. Gordan Lauc, Petar Brlek, Luka Bulić, et al. (2026). Systemic metabolic, hormonal, and glycomic remodeling during a 72-hour fast in healthy adults: a pilot study. Croatian Medical Journal. https://doi.org/10.3325/cmj.2026.67.226
2. Scientists reveal how seven days of fasting transforms the human body | ScienceDaily. ScienceDaily. https://www.sciencedaily.com/releases/2024/02/240212134551.htm
3. Critical Assessment of Fasting to Promote Metabolic Health and Longevity | Endocrine Reviews | Oxford Academic. academic.oup.com. https://academic.oup.com/edrv/article/43/2/170/6641566
4. Mark Ezpeleta, Sofia Cienfuegos, Shuhao Lin, Vasiliki Pavlou, Kelsey Gabel, Krista A Varady (2023). Efficacy and safety of prolonged water fasting: a narrative review of human trials. Nutrition Reviews. https://doi.org/10.1093/nutrit/nuad081
5. Brian V. Hong, Christopher H. Rhodes, Joanne K. Agus, et al. (2023). A single 36-h water-only fast vastly remodels the plasma lipidome. Frontiers in Cardiovascular Medicine. https://doi.org/10.3389/fcvm.2023.1251122
6. Karolina Kapłon, Marta Napiórkowska-Mastalerz, Aelita Bredelytė, Barbara Ciastek (2026). Brain-Derived Neurotrophic Factor as a Molecular Bridge Between Fasting, Neuroplasticity, and Metabolic Regulation: Clinical and Translational Insights. Nutrition Reviews. https://doi.org/10.1093/nutrit/nuag089