Inclusion Criteria:  Probable (clinically or laboratory) or definite ALS according to the revised version of the El Escorial World Federation of Neurology criteria loss of Amyotrophic Lateral Sclerosis Functional Rating Scale Revised (ALSFRS-R) of ≥ 0.33 points per month since onset (first paresis), based on the formula: (48 - score at screening visit) / (months between onset and screening visit) age ≥ 18 years continuously treated with 100 mg riluzole per day for at least 4 weeks capable of thoroughly understanding all information given and giving full informed consent according to good clinical practice (GCP)  Exclusion Criteria:  hyperinsulinism pyruvate decarboxylase deficit disturbance of fatty acid oxidation disturbance of gluconeogenesis acute porphyria metabolism disorders which prevent utilization or degradation of ketone bodies severe gastro-esophageal reflux severe renal insufficiency previous participation in another interventional study within the preceding 4 weeks tracheostomy pregnancy or breast-feeding females evidence of a major psychiatric disorder or clinically evident dementia