Abstract
There has been a recent trend in human dietary habits toward a semi-fluid diet. Thoughtherehave been many studies on the effect of a semi-fluid diet on various biological functionsand the development of the oral and maxillofacial unit, no studies havebeen conducted onregressive changes in the jaw and masticatory muscles of the elderly who are forced to switchto a semi-fluid diet due to reductions in oral functions caused by aging. Therefore, the presentstudy was conducted to investigate the effect of a semi-fluid diet on the masticatory musclesof aged rats. Male Wistar rats were used in the present study. They were raised on a hard dietfor four months until they were fully grown, and were then allowed to freely eat powdered foodhaving the same composition as the hard diet until reaching the age of 21 or 30 months (experimental groups). For comparison, rats were raised on a hard diet until reaching the ageof 21 or 30 months (control groups). The temporal muscle and the superficial and deep layersof the masseter muscle were removed from each rat and stained using actomyosin ATP ase todetermine the mean ratio of the components of muscle fibers, the mean ratio of the area ofmuscle fibers, and the mean of minor-calibers muscle fibers. Type-1 fibers were not detectedin the temporal muscle or the superficial and deep layers of the masseter muscle in the presentstudy. The glycogenase activity of fibers of types 2A and 2B was high, and anaerobic ATPsynthesis was active. In particular, the oxidase activity of type-2Afibers was high, indicatingthat these fibers were durable and highly resistant to fatigue.The findings of the study wereas follows:
(1) The composition ratio of type-2A fibers decreased in the superficial layer of the masseter muscle in both the experimental and control groups, but increased in the deep layer of themasseter muscle in the control groups. The mean ratio of the area of muscle fibers decreasedin the superficial layer of the masseter muscle in the experimental group.
(2) The mean caliber of fibers of types 2A and 2B decreased in the control group over time, and these decreases were marked in the deep layer of the masseter muscle.
(3) Although there were significant differences between the experimental and control groups 21 months after birth, virtually no differences were seen between the two groups30 months after birth.
These findings suggest that the mastication of a hard diet has some effect on the maintenance of occlusal force, and that regressive changes in masticatory muscles are mainly attributable to the aging process after a certain age.
