Japanese researchers have discovered that NMN supplementation can mitigate age-related auditory decline in mice, offering a promising avenue for the prevention of hearing loss.


1. NMN Inhibits Age-Related Hearing Loss

Mice, like humans, naturally experience hearing loss as they age. To confirm this, we compared the hearing of 25-week-old mice (approximately equivalent to humans aged 30) with that of 12-week-old mice (approximately equivalent to humans aged 21).

The study found that the auditory brainstem response (ABR) thresholds (the sound intensity threshold that stimulates brainstem electrical activity) of the 25-week-old mice were higher than those of the 12-week-old mice, indicating that hearing loss had occurred in the mice at 25 weeks of age.

To determine the effects of NMN on age-related hearing loss, 12-week-old mice were fed 500 mg/kg of NMN daily for 13 weeks. The results showed that compared to the control mice that were given water, the NMN-treated mice had a significantly lower ABR threshold, indicating that NMN inhibited the progression of age-related hearing loss.

Studies have shown that NAD+ levels in multiple organs decline with age, leading to chronic diseases. However, when measuring the NAD+ levels in the cochlea (the structure in the ear responsible for hearing) of young and old mice, no age-related decrease in NAD+ levels was detected.

This can be explained by the relatively young age of the “older” mice in this study. Previous research has shown that the cochlear NAD+ levels of 12-month-old mice (approximately equivalent to 43-year-old humans) are lower.

To further investigate how NMN prevents hearing loss, the researchers measured changes in cochlear gene activation. They found that NMN led to changes in ion homeostasis. Based on these results, the authors speculate that NMN may counteract age-related imbalances of iron and zinc in the inner ear.


2. NMN Improves Hearing Mechanisms

Elevating NAD+ Levels

As people age or due to certain diseases, NAD+ levels in the body gradually decline. NAD+ is crucial for various physiological functions, such as cellular energy metabolism and DNA repair.

In the auditory system, adequate NAD+ levels help maintain the normal function of inner ear cells, such as hair cells and spiral ganglion cells, ensuring the proper transmission and processing of sound signals. This, in turn, plays a protective and restorative role in hearing.


Enhancing Cellular Energy Metabolism

Cells such as hair cells in the inner ear require a large amount of energy to perform functions like sound signal transduction. NAD+, derived from NMN, participates in the process of oxidative phosphorylation within cells, providing the energy currency ATP.

When NMN supplementation increases NAD+ levels, it enhances the energy metabolism of inner ear cells, allowing hair cells and others to more effectively perceive sound vibrations and convert them into neural impulses, thereby improving the function of the auditory system and, to some extent, restoring hearing.

For hearing loss caused by impaired energy metabolism in inner ear cells due to aging, noise exposure, or other factors, NMN helps restore normal cell function and reduce hearing loss by enhancing energy metabolism.


Exerting Antioxidant Effects

Oxidative stress is one of the key factors contributing to hearing loss, as excessive reactive oxygen species (ROS) can damage inner ear cells. NMN increases NAD+ levels, which activates several antioxidant enzymes, such as SIRT1.

SIRT1 regulates the cellular antioxidant defense system, promoting the expression and activity of antioxidant enzymes like superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px). These enzymes help clear ROS from the inner ear, reducing oxidative damage and protecting inner ear cells from free radical attack, thereby improving hearing.

For example, in models of hearing loss induced by noise or drugs, NMN supplementation can lower oxidative stress levels in inner ear tissues, reduce damage to hair cells and nerve fibers, and improve hearing function.


Regulating Inflammatory Responses

Inflammatory responses in the inner ear are also one of the common causes of hearing loss.

NMN, by regulating NAD+-dependent signaling pathways such as the SIRT1 pathway, can suppress the expression and release of inflammation-related factors such as tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β). This helps reduce inflammation in the inner ear, alleviating the damage caused by inflammation to auditory cells and nerves, thereby improving hearing.