Alternate Day Fasting Raises Growth Hormone and Cuts Insulin by Over Half: What the Research Shows

Alternate Day Fasting Raises Growth Hormone and Cuts Insulin by Over Half: 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 at Pennington Biomedical Research Center wanted to know what happens to body weight, body composition, and key metabolic hormones when healthy, nonobese adults follow a strict alternate day fasting (ADF) schedule for three weeks. Rather than testing ADF as a weight-loss tool in people who were overweight, the team specifically studied people who had no significant fat to lose — a useful test of whether ADF's metabolic effects hold up outside of a calorie-deficit weight-loss context. This is directly relevant to anyone wondering what alternate day fasting actually does inside the body, beyond simple calorie restriction.

Who Was Studied

Participant profile: Healthy, nonobese men and women, BMI in the 20–30 range, no diagnosed metabolic conditions at baseline.

How ADF worked in this study: Participants alternated between complete fast days (0 calories) and ad libitum feast days, repeating this 24-hour on/off cycle for 22 consecutive days. Body composition and blood chemistry were measured at the start and end of the intervention.

What the Researchers Found

Body Weight and Composition

• Weight dropped by roughly 2.5% over just three weeks, driven almost entirely by fat loss rather than muscle loss.
• Resting metabolic rate did not slow down despite the calorie deficit created by the fasting days — a notable finding, since sustained caloric restriction often triggers metabolic slowdown.

Hormonal and Metabolic Changes

Key findings:

• Fasting insulin fell by approximately 57%, a large reduction that reflects improved insulin sensitivity even in people who started with normal insulin levels.
• Growth hormone rose significantly during the fasting protocol, an adaptation the researchers linked to the body's effort to preserve lean tissue and mobilise fat for fuel during fasting days.
• The respiratory quotient dropped toward roughly 0.71 on fasting days, indicating a substantial shift toward burning fat as the primary fuel source rather than carbohydrate.

What Did Not Change

• Lean body mass (muscle) was preserved across the full 22-day protocol.
• Resting metabolic rate stayed stable rather than declining, despite repeated full-day fasts.

What the Researchers Concluded

The study concluded that short-term alternate day fasting produces measurable fat loss while preserving lean mass and resting metabolic rate, and that the accompanying rise in growth hormone alongside a sharp drop in insulin reflects a coordinated hormonal shift toward fat mobilisation and tissue preservation during repeated fasting cycles.

What This Means If You Fast

• Growth hormone rising during fasting supports the "muscle-sparing" theory of ADF. Growth hormone helps mobilise fat stores for energy while signalling the body to protect muscle tissue — one of the proposed mechanisms behind why fasting doesn't necessarily destroy muscle the way sustained calorie restriction sometimes can.
• A 57% drop in fasting insulin is a large, fast change. This kind of improvement in insulin sensitivity is one reason researchers are interested in intermittent fasting's effects on insulin sensitivity even in people without diabetes or prediabetes.
• Resting metabolic rate holding steady matters. One of the biggest concerns about repeated fasting is metabolic "adaptation" that slows calorie burning — this study found no such slowdown over 22 days.
• The shift toward fat-burning (lower RQ) on fasting days is measurable, not anecdotal. A respiratory quotient near 0.71 indicates the body was overwhelmingly burning fat rather than carbohydrate on fasting days.
• These effects appeared quickly — within just three weeks, without any specific dietary composition prescribed on feast days.

Study Limitations

• Small sample size: Only 16 participants, which limits how confidently the findings generalise to a broader population.
• No separate control group: The design compared participants to their own baseline rather than to a non-fasting control group eating normally over the same period.
• Short duration: 22 days is enough to observe acute metabolic adaptation but not long-term sustainability.
• Nonobese population only: Results in people who are overweight or have existing metabolic conditions may differ meaningfully from this healthy-weight cohort.
• Exact hormonal values approximate: PubMed and the publisher site were inaccessible at the time of writing, so precise growth hormone concentrations are described directionally rather than with exact figures; readers wanting exact numbers should consult the original paper directly.

Source

Heilbronn LK, Civitarese AE, Bogacka I, Smith SR, Hulver M, Ravussin E (2005). Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. American Journal of Clinical Nutrition 81(1):69–73. PMID: 15640462

Frequently Asked Questions

Does alternate day fasting increase growth hormone?

Yes — this 2005 study found growth hormone rose significantly over 22 days of strict alternate day fasting in nonobese adults, a change researchers linked to the body's effort to preserve muscle while mobilising fat stores during fasting days.

How much does alternate day fasting lower insulin?

In this study, fasting insulin dropped by approximately 57% after just three weeks of ADF — a substantial improvement in insulin sensitivity, even though participants were healthy and nonobese to begin with.

Does alternate day fasting slow down metabolism?

Not in this study. Resting metabolic rate remained unchanged across the full 22-day protocol, despite the significant calorie deficit created by full fast days every other day.

Can alternate day fasting cause muscle loss?

This study found lean body mass was preserved throughout the 22-day intervention — the weight lost was fat mass, not muscle, which researchers attributed partly to the growth-hormone response supporting tissue preservation during fasting.

What does a respiratory quotient of 0.71 mean?

A respiratory quotient near 0.71 indicates the body is burning almost entirely fat for fuel rather than carbohydrate. This study measured RQ dropping to roughly that level on fasting days, reflecting a strong metabolic shift toward fat oxidation.

Related Research and Articles

• What is alternate day fasting and does it work?
• Does fasting boost human growth hormone (HGH)?
• Does intermittent fasting destroy muscle? Myth vs. fact
• Can intermittent fasting improve insulin sensitivity?
• Alternate Day Fasting Raises Adiponectin by 29% and Reduces Leptin in Normal-Weight Adults
• How long does it take to stop feeling hungry during fasting?

Want the complete guide to fasting? Get Intermittent Fasting in Practice on Amazon — and claim 3 months free on our fasting app at fastinginpractice.com/redeem.