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Background Type I Diabetes mellitus (DT1) is a multifactorial disease with various genetic and environmental factors involved in its pathophysiology. Several nutritional factors such as the timing of exposure to solid foods may increase the risk of DT1. The aim of this systematic review was to investigate the association between the time of introducing solid food into infant's nutrition during the first year of life and the risk of developing DT1. Methods PubMed and Science Direct were searched from inception to October 2019 for observational studies that investigated the above association. No restrictions on language or publication date were applied. Specific inclusion and exclusion criteria were set for the selection of articles. Title and abstract screening were performed by two independent researchers and data were extracted from shortlisted articles. Results In total, 76 studies were screened, and 7 articles met the inclusion criteria. The analysis shows that early exposure (before or during 3 or 4 months) and subsequent exposure (during or after 6 or 7 months) was associated with an increased risk of developing DT1 and/or pancreatic islet autoimmunity, compared to an intermediate exposure period (4-6 months, 4-5 months or 3-6 months). Conclusions The evidence suggests that an interim exposure period of genetically predisposed infants to DT1 from 4 to 5 months of age may reduce the risk of developing DT1 in later life as compared to early or later exposure. However, due to the limitations of the studies, further investigation is required to inform DT1 prevention practices. Key messages There is some evidence indicating that early exposure and later exposure to solid food may increase the risk of developing DT1 in children with a genetic predisposition. Due to the methodological heterogeneity across the studies included, more reliable data from large prospective studies and randomized clinical trials are needed.