Bipolar affective disorder (BPAD) is a chronic mental disorder, characterised by mood swings, alternating between depression and manic or hypomanic episodes. Unfortunately, in some patients pharmacological treatment does not bring satisfactory results, and a certain group of patients shows resistance to treatment. Therefore, other treatment methods are sought after, including a change in diet. The most promising nutrition model is the ketogenic diet. In the presented case study of a male patient, thanks to the introduction of the ketogenic diet, full remission of the disease was achieved, doses of lamotrigine were reduced and quetiapine was completely discontinued. Previously, neither lamotrigine monotherapy nor combined treatment with quetiapine achieved euthymia. The effects of the diet may be related to, among others, the influence on ionic channels and increase in blood acidity (similarly to mood stabilisers), increase in gamma-aminobutyric acid (GABA) concentration, modulation of GABAA receptors and blocking of AMPA receptors by medium-chain fatty acids. The ketogenic diet influences glutamate metabolism and nerve cell metabolism, which uses ketone bodies as energy sources. Ketosis can also stimulate the biogenesis of mitochondria, improve brain metabolism, act as a neuroprotective factor, as well as increase glutathione synthesis and reduce oxidative stress. However, there is a need for carefully planned studies, with an appropriate representative group, to verify the potential benefits and risks of introducing the ketogenic diet in patients with BPAD.