How Fasting Affects Your Hormones

Quick answer: Fasting triggers a coordinated hormonal cascade: insulin drops, growth hormone surges, norepinephrine rises, and your body shifts from glucose-burning to fat-burning mode. These changes are well-documented and generally beneficial, though hormonal responses can differ between men and women.

Hormones govern almost everything your body does — energy production, fat storage, muscle building, mood, sleep, reproduction, and stress response. When you fast, you are not just reducing calorie intake. You are fundamentally changing your hormonal environment.

Understanding these changes explains why fasting does what it does — and why it does not work the same way for everyone.

Insulin: The Master Switch

Insulin is the hormone most directly and immediately affected by fasting. It deserves its own dedicated article — see our full guide on fasting and insulin resistance — but here is the summary.

When you eat, insulin rises. Its primary job is to shuttle glucose from your blood into cells for energy or storage. Insulin also promotes fat storage and inhibits fat breakdown.

When you fast, insulin drops. Within 12-16 hours of your last meal, insulin levels fall to baseline. This low-insulin state:

Unlocks fat stores for energy (lipolysis)
Allows cells to resensitize to insulin's signal
Permits growth hormone release (insulin suppresses GH)
Enables autophagy to activate

Insulin is the hormonal fulcrum of fasting. Nearly every other hormonal change during a fast is either a direct consequence of falling insulin or occurs because low insulin permits it.

Growth Hormone: The Preservation Signal

Human growth hormone (HGH) rises dramatically during fasting. Hartman et al. (Journal of Clinical Endocrinology and Metabolism, 1992) measured a five-fold increase in growth hormone pulsatility during a two-day fast.

This is not subtle. It is one of the most dramatic hormonal changes fasting produces.

Growth hormone during fasting serves several purposes:

Preserving lean mass. Growth hormone is strongly anti-catabolic. It protects muscle tissue from breakdown during periods of food scarcity. This is a key reason why fasting does not cause the muscle loss many people fear.

Promoting fat oxidation. Growth hormone stimulates lipolysis — the release of fatty acids from fat tissue — making them available as fuel.

Supporting cellular repair. Growth hormone contributes to tissue maintenance and repair, working alongside autophagy during fasted states.

The growth hormone response is significant enough that we have written a dedicated article on growth hormone and fasting.

One important note: eating — particularly carbohydrates and protein — rapidly suppresses growth hormone. This is mediated partly through insulin (which directly inhibits GH release) and partly through elevated blood glucose. This is why the growth hormone benefits of fasting require an actual fasted state, not just calorie reduction.

Norepinephrine: The Alertness Signal

Norepinephrine (noradrenaline) increases during fasting. Zauner et al. (American Journal of Clinical Nutrition, 2000) documented rising norepinephrine levels during a multi-day fast, corresponding with increased metabolic rate.

Norepinephrine during fasting:

Increases metabolic rate by promoting thermogenesis
Enhances alertness and focus — the "clear-headed" feeling many fasters report
Promotes fat breakdown by activating hormone-sensitive lipase in fat tissue
Mobilizes glycogen from the liver during early fasting

This is an evolutionary adaptation. When food was scarce, becoming sluggish and foggy would have been a death sentence. Instead, the body ramps up alertness and energy availability to support the hunt for food.

Cortisol: The Misunderstood Hormone

Cortisol rises modestly during fasting, and this worries people. Cortisol has a reputation as the "stress hormone" associated with belly fat, muscle wasting, and metabolic damage.

Context matters enormously here.

Chronic cortisol elevation — from ongoing psychological stress, sleep deprivation, or overtraining — is genuinely harmful. It promotes visceral fat storage, impairs immune function, and breaks down muscle tissue.

Acute, transient cortisol elevation — as occurs during fasting, exercise, or waking up in the morning — is normal and functional. During fasting, cortisol helps maintain blood glucose through gluconeogenesis and supports the mobilization of energy stores. It works in concert with norepinephrine and growth hormone.

Nair et al. (American Journal of Physiology, 1995) showed that the cortisol increase during short-term fasting was modest and physiological, not pathological. The cortisol response during a 16-24 hour fast is comparable to the cortisol spike from moderate exercise.

If chronic cortisol elevation is a concern for you, the more productive interventions are managing psychological stress, optimizing sleep, and moderating exercise intensity — not avoiding intermittent fasting.

Leptin and Ghrelin: The Hunger Hormones

Ghrelin is your hunger signal. Produced primarily in the stomach, it rises before habitual meal times and drops after eating. During fasting, ghrelin increases — this is what makes you feel hungry.

Importantly, ghrelin operates on a schedule. It spikes at times you normally eat, then subsides even if you do not eat. This is why experienced fasters report that hunger comes in waves rather than building continuously. Your body adapts its ghrelin release pattern to your new eating schedule, typically within one to two weeks (Frecka and Mattes, Physiology and Behavior, 2008).

Leptin is the satiety hormone, produced by fat cells. It signals to your brain that energy stores are adequate. Leptin decreases during fasting — which makes sense, as the body is signaling reduced energy availability.

With consistent intermittent fasting, the leptin-ghrelin dynamic normalizes. Hunger becomes more predictable and manageable. Many long-term fasters report that they become less hungry overall, not more — their appetite regulation improves.

Thyroid Hormones

The thyroid hormones T3 and T4 regulate metabolic rate, body temperature, and energy production. Chronic calorie restriction can significantly lower T3, slowing metabolism.

Short-term fasting has a more nuanced effect. Webber and Macdonald (Metabolism, 1994) found that T3 decreases become significant only with extended fasting lasting several days. Intermittent fasting protocols of 16-24 hours have minimal impact on thyroid function in most people.

However, this is an area where individual variation matters. Some people — particularly women and those with existing thyroid conditions — may be more sensitive to fasting-related thyroid changes. Monitoring how you feel and getting thyroid levels checked if symptoms arise (persistent fatigue, cold intolerance, hair loss) is reasonable.

Sex Hormones: Where Gender Matters

This is where the conversation about fasting and hormones becomes critical, because men and women often respond differently.

Testosterone

In men, short-term fasting does not appear to reduce testosterone. Moro et al. (Journal of Translational Medicine, 2016) found that men following 16:8 time-restricted eating maintained testosterone levels while reducing body fat — a net positive for hormonal health, since excess body fat converts testosterone to estrogen via aromatase.

Estrogen and Reproductive Hormones in Women

Women's reproductive hormones are more sensitive to energy availability signals. The hypothalamic-pituitary-gonadal (HPG) axis — which regulates estrogen, progesterone, LH, and FSH — can downregulate when the body perceives insufficient energy availability.

Kumar and Kaur (Journal of Mid-Life Health, 2013) described how caloric restriction and negative energy balance can disrupt menstrual cycles. While the research specifically on intermittent fasting and menstrual health is still developing, some women report cycle irregularities when fasting aggressively.

This does not mean women should avoid fasting. It means that women may benefit from:

Starting with shorter fasting windows (12-14 hours) and gradually extending
Being more attentive to total calorie adequacy during eating windows
Considering circadian-aligned eating patterns that work with hormonal cycles
Modifying fasting during the luteal phase (post-ovulation) when progesterone is high and energy needs increase

For a comprehensive discussion, see our guide on intermittent fasting for women.

The Hormonal Timeline of a Fast

Here is roughly what happens to your hormones as a fast progresses:

0-4 hours: Insulin is still elevated from your last meal. Hormones are in "fed state" mode. Nutrients are being absorbed and stored.

4-8 hours: Insulin begins dropping. Growth hormone starts its gradual rise. The body begins transitioning from fed to fasted state.

8-12 hours: Insulin reaches baseline. Glycogen stores are being tapped. Growth hormone continues rising. Norepinephrine begins increasing.

12-16 hours: Low insulin environment is firmly established. Growth hormone is significantly elevated. Fat oxidation increases. Ghrelin may spike (hunger wave) then subside.

16-24 hours: Peak hormonal fasting state. Growth hormone is high, insulin is low, norepinephrine is elevated. Autophagy is ramping up. Ketone production begins.

24-48 hours: Growth hormone continues rising. Cortisol increases modestly. Ketone levels climb further. The body is fully in fat-burning, repair-focused mode.

For a complete breakdown of every stage, see our hour-by-hour fasting guide.

How Fasted Helps

Understanding your hormonal response to fasting is one thing — consistently practicing it is another. Fasted helps you track your fasting duration so you can see where you are in the hormonal timeline. The customizable schedules let you find the protocol that works for your body, whether that is a standard 16:8 or a gentler 14:10 that respects your hormonal sensitivity. The insights and stats features help you correlate your fasting patterns with how you feel over time.

Frequently Asked Questions

Does fasting increase cortisol dangerously?

No, not in typical intermittent fasting protocols. The cortisol increase during a 16-24 hour fast is modest and transient — comparable to the cortisol response from exercise. Chronic cortisol elevation from ongoing stress, sleep deprivation, or extreme dieting is a separate concern. If you are already under significant stress, consider shorter fasting windows.

Will fasting affect my menstrual cycle?

It can, particularly with aggressive fasting protocols or insufficient calorie intake during eating windows. Women's reproductive hormones are sensitive to energy availability. Starting with shorter fasts, eating enough during your window, and monitoring your cycle are sensible precautions. If you notice irregularities, reduce fasting duration and consult your healthcare provider.

Does fasting lower testosterone?

Research does not support this concern for standard intermittent fasting in men. The combination of low insulin, high growth hormone, and reduced body fat from fasting may actually support healthy testosterone levels. Severe, prolonged caloric restriction can lower testosterone, but that is a different scenario.

How does fasting affect sleep hormones?

Some people report improved sleep with intermittent fasting, particularly when they stop eating several hours before bed. Not eating close to bedtime can improve melatonin signaling and sleep quality. However, going to bed very hungry can disrupt sleep for some individuals. Finding the right balance — typically finishing your last meal 3-4 hours before sleep — works for most people.

Do hormonal changes from fasting persist long-term?

The acute hormonal shifts (insulin drop, GH rise, norepinephrine increase) occur during each fast and resolve when you eat. However, the downstream effects — improved insulin sensitivity, better leptin signaling, normalized ghrelin patterns — become more established with consistent practice over weeks and months.

What to Read Next

This article is for informational purposes only and does not constitute medical advice. People with hormonal conditions, thyroid disorders, or reproductive health concerns should consult their healthcare provider before starting any fasting protocol.