Background: Studies have consistently shown that patients with epilepsy could benefit from ketogenic diets (KDs). Recent evidence suggests that KD could be used in the treatment of central nervous system (CNS) diseases. The aim of this systematic review was to investigate the use and efficacy of KD, modified Atkins diet (MAD) and medium-chain triglyceride (MCT) diet in infants, children, adolescents, and adults with CNS diseases. Methods: This systematic review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Main databases, i.e. EMBASE, PubMed and PsycINFO, were searched on 4 December 2019. Only randomized clinical trials (RCTs) were included and only if they reported KD, MCT or MAD interventions on patients with CNS diseases. Results: Twenty-four publications were eligible for inclusion (n = 1221). Twenty-one publications concerned epilepsy, two concerned Alzheimer’s disease (AD), and one concerned Parkinson’s disease (PD). All studies regarding epilepsy reported of seizure reduction compared to baseline. MCT did not significantly change regional cerebral blood flow (rCBF) in patients with AD, but MAD significantly improved memory at 6 weeks (p =.03). KD significantly improved motor and nonmotor functions in patients with PD at 8 weeks (p <.001). There was a trend towards fewer adverse effects in MAD compared to KD. Conclusion: In conclusion, various forms of KDs seem tolerable and effective as part of the treatment for epilepsy, AD and PD, although more investigation concerning the mechanism, efficacy and adverse events is necessary.