Food hardness influences the progression of age-related hearing loss in mice

C57BL/6J and DBA/2J mice are often used for hearing research because of their early onset and progression of age-related hearing loss (AHL). Here, we report that the hardness of the diet affects the progression of AHL in these mice. When C57BL/6J mice and DBA/2J mice were fed a pellet-type or powder-type standard AIN93M diet, the pellet diet significantly promoted AHL. AHL promotion was eliminated by crushing the pellet diet to a powder. Subsequently, when C57BL/6J mice were fed the pellet-type AIN93M diet obtained from three different manufacturers, two of them significantly promoted AHL. The hardness of the diets was measured, and it was found that the two diets that promoted AHL were significantly harder than the other diet. Next, we attempted to reduce diet hardness by replacing some nutritional ingredients with dried eggs or phosphatidylcholine (PC), and we succeeded in obtaining brittle diets with lower hardness values. Then, C57BL/6J mice were bred with brittle diets for 6 months and the promotion of AHL was suppressed to the equivalent level as the powder diet. Furthermore, when senescence-accelerated mice, SAMP8, were fed a brittle diet for one year, the progression of AHL was also suppressed; however, it did not affect other aging indexes, such as mental and physical performance. We also confirmed that a high-fat pellet diet, which is soft even in pellet form, did not promote AHL. Time-restricted feeding (tRF), which is a chrono-nutritional method to delay aging, ameliorated the promotion of AHL by the hard AIN93M pellets in C57BL/6J mice. These results indicate that the physical form and hardness of diets affect the progression of AHL in mouse models.