A 3-Day Fast Raised Resting Metabolism by 14%, Not Lowered It: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Researchers in Vienna set out to test one of the most persistent fears about fasting: that going without food causes the body to slow its metabolism to "conserve energy," making weight loss harder and leaving people sluggish. To test this directly, they measured resting energy expenditure (REE) — the number of calories the body burns at complete rest — before and during a 72-hour water-only fast, using indirect calorimetry, the gold-standard method for measuring metabolic rate from oxygen consumption and carbon dioxide production. For background on what a multi-day fast actually involves, see how to do a 72-hour fast safely and what happens to your body hour by hour when you fast.

Who Was Studied

Participant profile: Healthy, non-obese adult volunteers with no metabolic disease, studied in a controlled clinical research setting.

How the fasting protocol worked in this study: Participants consumed only water for 72 hours. Resting energy expenditure was measured by indirect calorimetry at baseline and again at set intervals during the fast, alongside blood sampling for hormones including norepinephrine, insulin, and glucagon.

What the Researchers Found

Resting Energy Expenditure

• Resting energy expenditure rose rather than fell across the 72-hour fast, directly contradicting the "starvation mode" assumption that short fasting slows metabolism.
• The increase in REE was closely tied to a sharp rise in plasma norepinephrine — roughly a two-and-a-half-fold increase from baseline — pointing to the sympathetic nervous system as the driver of the metabolic uptick.
• Insulin levels fell substantially over the fasting period, consistent with the shift away from glucose availability and toward fat mobilization.

Hormonal Changes

• Norepinephrine rose sharply, consistent with the body's stress-hormone response mobilizing stored fat for energy.
• Glucagon increased modestly, supporting glucose release from liver glycogen stores early in the fast.
• Insulin dropped, removing the main brake on fat breakdown and allowing free fatty acids to become the dominant fuel source.

What Did Not Change

• There was no evidence of an early metabolic shutdown or energy-conserving slowdown within the 72-hour window studied.
• Thyroid hormone (T3) suppression — the mechanism understood to eventually lower metabolic rate in much longer fasts or severe calorie restriction — was not yet a dominant factor at this short duration.

What the Researchers Concluded

The authors concluded that short-term fasting does not trigger an immediate energy-conserving "starvation response." Instead, resting metabolic rate rises in the first 72 hours, driven primarily by increased norepinephrine release as the body mobilizes fat stores for fuel.

What This Means If You Fast

• The "starvation mode" myth doesn't hold up for short fasts. A 24, 48, or 72-hour fast is unlikely to tank your metabolism — if anything, the early data points the other way. See does intermittent fasting slow your metabolism? for more on this myth.
• The energy boost has a hormonal cause, not a magical one. The rise in norepinephrine during fasting is the same stress-hormone response responsible for the mental alertness and steady energy many people report during extended fasts.
• This doesn't mean longer is always better. This study only covers 72 hours. Much longer fasts or prolonged severe calorie restriction are understood to eventually suppress T3 and lower metabolic rate as the body adapts to sustained energy scarcity — a different phase than the one studied here.
• Daily intermittent fasting windows (like 16:8) are a different scenario entirely. This study used a continuous 3-day fast, not a daily eating-window pattern — see what is extended fasting and is it safe? for how the two compare.
• Electrolyte and hydration support still matters during any multi-day fast, regardless of what's happening to resting metabolism — see electrolytes and intermittent fasting.

Study Limitations

• Very small sample size (n=9), which limits how confidently the findings generalize to a broader population.
• Short study duration (72 hours) — the findings say nothing about metabolic changes during much longer fasts (7+ days) or repeated fasting cycles.
• No long-term follow-up to see how REE trends beyond day 3.
• All participants were healthy adults; findings may not apply to people with metabolic disease, low body weight, or older adults.
• As a pre-post design without a non-fasting control group measured over the same period, some of the change could theoretically reflect factors other than fasting itself, though the hormonal correlation with norepinephrine strengthens the causal interpretation.

Source

Zauner, C., Schneeweiss, B., Kranz, A., Madl, C., Ratheiser, K., Kramer, L., Roth, E., Schneider, B., & Lenz, K. (2000). Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. American Journal of Clinical Nutrition, 71(6), 1511–1515. PMID: 10648253

Frequently Asked Questions

Does fasting slow down your metabolism?

According to this study, not in the short term. Resting energy expenditure actually increased by about 14% after 72 hours of fasting, driven by a rise in norepinephrine, rather than decreasing as the "starvation mode" theory predicts.

How long can you fast before metabolism actually slows down?

This study only measured up to 72 hours, during which metabolism rose. Metabolic slowdown is understood to become a factor with much longer fasts or prolonged severe calorie restriction, when thyroid hormone (T3) production drops and lean muscle mass may be affected.

Why does resting energy expenditure go up during a fast?

The increase appears to be driven mainly by a sharp rise in norepinephrine, a stress hormone that helps mobilize stored fat for energy. This hormonal shift appears to temporarily raise the body's calorie-burning rate rather than lower it.

Is the "starvation mode" myth completely false?

Not entirely — it's more accurately described as a myth for short-term fasting. Very prolonged fasting or chronic severe calorie restriction can eventually suppress metabolic rate through different mechanisms, such as reduced thyroid hormone output and loss of lean tissue.

Does this study apply to intermittent fasting like 16:8 or OMAD?

Not directly — this study looked at a continuous 72-hour fast, not a daily eating-window pattern. Daily intermittent fasting protocols involve much shorter fasting periods and haven't shown the dramatic metabolic changes seen here, though they may share some of the same underlying hormonal mechanisms in smaller magnitude.

Related Research and Articles

• Does intermittent fasting slow your metabolism?
• Intermittent fasting and metabolism: what science says
• What happens to your body hour by hour when you fast
• Is it safe to fast for 24 hours or longer?
• How to do a 72-hour fast safely
• Does eating frequently boost your metabolism?
• What is extended fasting and is it safe?

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