Disturbances in glucose metabolism and glutamate neurotransmission feature in the pathophysiology of psychotic disorders. Ketogenic diet (KD) is a high-fat, low-carbohydrate diet that restricts glucose and forces metabolism of ketones, which serve as alternative energy substrates for the brain. KD is an established treatment for intractable epilepsy. However, we lack the randomized controlled trials (RCT) evidence regarding potential effects of KD on psychotic symptoms in humans. This randomised, controlled pilot study aims to investigate: 1. feasibility of a Modified Ketogenic Diet (MKD) intervention protocol in psychotic inpatients, 2. potential impact of MKD intervention on psychotic symptoms, depressive and anxiety symptoms, and functioning in patients with psychotic symptoms / psychotic disorder. A 6-week randomised KD pilot study will be carried out in psychotic inpatients (aimed n=40) at Niuvanniemi Hospital and Kuopio University Hospital, Finland. In the KD group, carbohydrate consumption is limited to 15-20 g/day to activate ketosis. The control group will have their ordinary hospital meals. A number of different assessment will be carried out at time points 0, 1 week, 3 weeks and 6 weeks.