Intermittent Fasting Increases Sexual Behavior in Male Mice

A new study reports that intermittent fasting in a 24-hour on-off cycle increases sexual activity in male mice by reducing serotonin levels in the brain. This effect appears to be linked to a decrease in the availability of tryptophan, an essential amino acid required for serotonin synthesis. The research, published in Cell Metabolism, was conducted by the German Center for Neurodegenerative Diseases (DZNE) in collaboration with Qingdao University and the University of Health and Rehabilitation Sciences in China.

While fasting is widely studied for its metabolic and longevity-related effects, this study highlights a behavioral change not previously associated with dietary restriction: increased sexual activity. The researchers suggest the findings could inform future investigations into the effects of fasting on libido in humans.

A behavioral shift, not a physiological one

The initial goal of the study was to investigate how fasting might influence the fertility of male mice. The researchers were surprised to find that older male mice subjected to long-term intermittent fasting produced more offspring than expected. Despite age-related declines in sperm quality and testosterone levels, these males fathered significantly more litters than controls.

Subsequent analysis revealed that this increase in reproductive success was due to changes in behavior. The fasted males engaged in more frequent mating than their non-fasted counterparts. This behavioral increase in sexual activity compensated for age-related declines in reproductive physiology.

“It was a bit of detective work to uncover the real cause. Eventually, we realized: it is a matter of behavior. The fasting males had significantly more sexual contacts than mice that could eat freely. In other words, these animals had an unusually high frequency of mating and, as a result, an unusually high number of offspring for their age. Their mating behavior more than compensated for the age-related physiological limitations.”

Dr. Dan Ehninger.

Intermittent fasting and delayed onset of effects

The fasting protocol began when the mice were two months old. The animals alternated between 24 hours of unrestricted feeding and 24 hours of water-only access for a period of 22 months. During this time, they were housed without contact with females. Only later were they introduced to females raised under normal feeding conditions.

Increased sexual activity was also observed in younger mice that followed the same fasting protocol for six months. However, the effect was not present in mice that underwent the same diet for only a few weeks, suggesting that the behavioral change requires a longer duration of dietary restriction to manifest.

“For intermittent fasting to increase sex drive, it takes some time. Based on our experiments, the minimum duration appears to be somewhere between six weeks and six months.”

Dr. Yu Zhou.

Serotonin as a key factor

Analysis of brain chemistry in the sexually active mice revealed reduced concentrations of serotonin, a neurotransmitter that typically acts to suppress sexual behavior. Serotonin production in the brain relies on tryptophan, which must be obtained from the diet or derived from internal protein reserves.

The mice subjected to intermittent fasting consumed approximately 15% fewer calories than control animals, including less tryptophan. This reduction appears to underlie the observed decline in serotonin levels and may account for the increase in sexual behavior.

Relevance to humans remains unclear

Whether this mechanism applies to other types of dietary restriction or is unique to the specific fasting protocol used in the study remains to be determined. It also remains unclear whether similar effects would occur in humans. However, serotonin’s role in regulating sexual desire is well-established. For example, selective serotonin reuptake inhibitors, commonly used to treat depression, are known to reduce libido in some patients.

The study's authors suggest that fasting may offer a non-pharmacological option to help address certain forms of low sexual desire. In particular, they highlight potential implications for hypoactive sexual desire disorder, a condition more commonly reported among older adults.

Reference: Xie K, Wang C, Scifo E, et al. Intermittent fasting boosts sexual behavior by limiting the central availability of tryptophan and serotonin. Cell Metabolism. doi: 10.1016/j.cmet.2025.03.001

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