This systematic review had the purpose to collect existing data concerning the influence of food hardness on mastication in adults.
The review was listed with PROSPERO (CRD42017069760) and was directed following with PRISMA and CRD (Centre for Reviews and Dissemination, University of York) statement. A database search of articles issued from 1998 up to December 2018 was carried-out using four databases: Pubmed, Cochrane Library, Web of Science and Scopus. The lists of references of the articles selected for the review were read to identify any other relevant studies. The included publications were analysed for level of evidence, study design, sample characteristics, test of mastication, primary outcomes and key results. A quality assessment of the articles included in the review was performed.
1686 articles were found through database searching. The studies that complied with all the inclusion criteria and were considered for the conclusive analysis were 38 and their methodological quality was scored as moderate/low. The findings of the analysed articles were consistent, despite the presence of different methodologies and the lack of a complete control of the bias. They revealed that the majority of the chewing parameters, which were gathered in four groups: 1) number of cycles, 2) sequence duration, 3) muscle activity and 4) coordination and amplitudes and shape of mandibular displacements, increased in the transition from soft to hard food.
Hard-diet in adults has an impact on the masticatory function increasing almost all the physiological masticatory parameters, muscle coordination and changes of masticatory side.

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