Background Gestational diabetes mellitus (GDM), defined as any degree of glucose intolerance with onset during pregnancy, is increasing worldwide, mostly because obesity among women of reproductive age is continuously escalating. GDM is associated with adverse maternal and fetal outcomes. The aim of this article was to systematically review literature on the effectiveness of nutritional factors before or during pregnancy to prevent GDM. Methods We assessed the primary prevention of GDM through nutritional factors, as diet and supplements. We searched on PubMed, Cochrane Databases and ClinicalTrials.gov from inception to June 2016. Clinical trials and adjusted prospective cohort studies were included. Results Eight clinical trials and twenty observational studies assessing the association between dietary factors and primary prevention of GDM were included. Furthermore, six clinical trials and two observational studies related to supplements were also added. Only two nutritional interventions were found to significantly reduce the incidence of GDM, besides the supplements. However, the observational studies showed that a higher adherence to a healthier dietary pattern can prevent the incidence of GDM, especially in high risk population before getting pregnant. Conclusions The results indicate that there may be some benefits of some nutritional factors to prevent GDM. However, better-designed studies are required to generate higher quality evidence. At the moment, no strong conclusions can be drawn with regard to the best intervention for the prevention of GDM.