Clinical and experimental studies have shown the masticatory complex (the skeletal and soft tissues associated with feeding) to be phenotypically plastic with regard to dietary material properties. Harder, tougher diets generally produce larger skeletal elements with greater bone density. However, less is known about the plasticity of the chewing musculature. Muscles are composed of two primary fiber types: slow-twitch (type I) and fast-twitch (type II). Fast-twitch fibers produce rapid, powerful contractions but fatigue more quickly than slow-twitch fibers. Here we use a rodent model to examine the plasticity of the masseter muscle fibers in response to inter-individual variation in dietary material properties. We hypothesize that rats raised on hard, tough diets will have more fast-twitch muscle fibers with greater cross-sectional area, representing an increase in force production by the masseter in response to the dietary material properties.

Male Sprague-Dawley rats were raised from weaning (3 weeks) to adulthood (16 weeks) in two dietary treatment groups: a “hard/tough” pellet diet and a “soft” meal diet. Post-sacrifice, fixed muscles were embedded in paraffin and divided into anterior, middle, and posterior regions. To visualize fast-twitch fibers, 6 sections per region per individual were stained with a primary antibody for the fast isoforms of myosin heavy chain (MHC) and a fluorescent secondary antibody. Images were collected at 4x using a fluorescent confocal microscope. ImageJ was used to collect total muscle cell area, fast-twitch cell area, and slow-twitch and fast-twitch cell counts. Mann-Whitney U tests were used to compare cell count and area between the treatment groups (n=5/group, α = 0.05).

Preliminary results indicate that hard/tough diets tend to be associated with a decrease in slow-twitch fiber count and an increase in fast twitch fiber cross-sectional area in the rat masseter. This allows for a greater proportion of fast-twitch muscle fibers in a given muscle volume. We propose that this change reflects an increase in muscle force production during feeding on mechanically challenging food items. The results of this study allow for greater understanding of skeletal muscle growth postnatally through adulthood, and provides insight into the consequences of material properties of diet for the function and dysfunction of the human masticatory complex. Future studies are needed to address how muscle fiber plasticity changes with age and with intra-individual variation in the material properties of diet.