Fasting and testosterone: does fasting raise or lower it?

The reproductive axis is sensitive to energy availability

Testosterone production is regulated by a signalling chain. The hypothalamus releases GnRH in pulses; the pituitary releases luteinising hormone; the testes respond by producing testosterone. Energy deprivation can reduce the drive at the top of this chain.

That response is biologically coherent. During a sustained shortage, the body prioritises immediate fuel needs over reproduction. It should not be interpreted as permanent damage from every missed meal.

What the 83-hour experiment found

Aloi and colleagues studied six healthy young men during fed conditions and two fasting sessions lasting about 83 hours. Fasting reduced LH secretion and total and free testosterone. When researchers delivered GnRH in physiological pulses, the fall in LH was prevented, while the pituitary could still respond to a bolus challenge.

The experiment supports a fasting-related reduction in hypothalamic GnRH drive. It is mechanistically strong but very small, male-only and far longer than common 16:8 routines. It cannot tell us the long-term clinical effect of intermittent fasting in a broader population.

Shorter windows: a different question

Moro and colleagues randomised 34 resistance-trained men to eight weeks of a 16:8 time-restricted pattern or a normal meal distribution while training continued. The time-restricted group lost more fat and showed lower testosterone and IGF-1, yet fat-free mass and strength were maintained.

This is a valuable warning against reading a blood marker without the outcome. It does not prove that lower testosterone never matters. It shows that over eight weeks, in this group and training context, the measured hormonal change did not translate into lost strength or lean mass.

Why results can differ between studies

Eating-window duration is only one variable. Total calories and protein, the size and timing of the energy deficit, body-fat change, training load, sleep, sampling time, sex, age and baseline metabolic health can all change interpretation. Testosterone has a daily rhythm, so morning and afternoon samples are not interchangeable.

Longer fasting and chronic low energy availability are also different from a time-restricted pattern that still supplies adequate energy. Studies should be compared by actual intervention, not by the shared label “fasting.”

What twin research adds

Kuijper and colleagues studied adult male twins and siblings and found substantial genetic contributions to testosterone and related hormones. A separate study of male twins linked nutritional factors—including fat intake and body weight—to sex-hormone variation, illustrating that diet and acquired body composition still matter within genetically informative designs.

These studies support two simultaneous statements: baseline hormone levels are partly heritable, and environmental or nutritional differences can still influence them. They do not tell us whether two monozygotic twins will show the same testosterone drop during a 24-hour fast.

A useful contradiction

During fasting, growth hormone can rise while testosterone and IGF-1 fall or fail to rise. That pattern makes sense once the response is viewed as fuel adaptation instead of “anabolic hormone optimisation.” The body can mobilise fat and preserve glucose while temporarily reducing reproductive signalling and growth-related output.

Common claims, checked

“Fasting boosts testosterone because it boosts GH.” These hormones are regulated differently. One can rise while the other falls.

“A lower testosterone value means muscle loss.” Not automatically. Duration, symptoms, repeated measurements, training and body composition matter.

“16:8 always lowers testosterone.” One small trained-male trial found a reduction, but this is not a universal effect across people or protocols.

“Twin heritability means lifestyle cannot change hormones.” False. Heritability describes variation in a population under particular conditions; it does not make a trait fixed.

From research to self-tracking

Track eating-window consistency, total intake, training performance, sleep, perceived recovery and symptoms. If symptoms such as persistent low libido, fatigue or menstrual disturbance occur, consumer tracking is not a substitute for medical assessment. Do not infer testosterone from mood, a wearable or body weight alone.

Explore in TwinPare

TwinPare Health & Fitness can organise the behaviour and recovery context around an eating pattern. It does not diagnose hypogonadism, measure testosterone or determine the genetic contribution to a result.

Safety note

Persistent low energy availability can affect reproductive and bone health in all sexes. People with menstrual disturbance, fertility concerns, diabetes, medication use, eating-disorder history, pregnancy, breastfeeding, adolescence, frailty or chronic illness should seek appropriate clinical guidance before restrictive fasting.

Sources and methodology

fasting-induced hypogonadotropic hypogonadism in men. J Clin Endocrinol Metab. 1997;82:1543–1548. doi:10.1210/jcem.82.5.3947.

(16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14:290. doi:10.1186/s12967-016-1044-0.

male twins. Hum Reprod. 2007;22:2153–2159. doi:10.1093/humrep/dem145.

levels in male twins. Genet Epidemiol. 1988;5:43–59. doi:10.1002/gepi.1370050105.

Editorial method: long fasting, time-restricted eating and chronic low energy availability were not treated as equivalent. This article is educational and is not medical advice.

• Aloi JA, et al. Pulsatile intravenous GnRH administration averts
• Moro T, et al. Effects of eight weeks of time-restricted feeding
• Kuijper EAM, et al. Heritability of reproductive hormones in adult
• Bishop DT, et al. The effect of nutritional factors on sex hormone