Long-term effects of altered functional demands induced by diet consistency throughout growth on adult rat masticatory muscle fibres.

Objective The functional profile and fibre characteristics of masticatory muscles may change depending on their functional demands. This in vivo experimental animal study aimed to evaluate the influence of diet consistency on fibre-type distribution and cross-sectional area in adult masticatory muscles using a rat model.Design Twenty-four male Wistar rats were divided into two groups of 12 and fed exclusively either a hard or soft diet following weaning. At 38 weeks, corresponding to adulthood, the masseter and digastric muscles were dissected. Immunofluorescence staining was performed to identify muscle fibre types (I, IIA, IIB, IIX), and morphometric analysis performed to measured fibre cross-sectional area and minimum diameter.Results In the masseter muscle, rats fed a soft diet exhibited a lower percentage of type IIA fibres (p = 0.006) and a higher percentage of type IIX fibres (p = 0.007) than rats fed a hard diet. No significant differences were found between groups in average fibre cross-sectional area or minimum diameter. In the digastric muscle, differences were detected only in fibre-size and not in fibre-type distribution. Rats fed a soft diet had larger average cross-sectional area and minimum diameter of type I fibres (p = 0.004; p = 0.003) and type IIB fibres (p = 0.007; p = 0.007) than those fed a hard diet.Conclusions Following soft-diet feeding, the masseter muscle was composed of less fast, fatigue-resistant fibre types (less IIA and more IIX). No such fibre-type distribution differences were observed in the digastric muscle, although some fibre types were larger in soft-diet-fed animals. These results indicate an adaptive response to reduced masticatory loading.