Patients with liver diseases, especially decompensated cirrhosis, commonly have weight loss and muscle wasting. It is known that such patients have poorer clinical outcomes than patients with similar diagnoses but without such weight loss or muscle wasting. If the problem is just deprivation of nutrients, it would be expected that the provision of some type of nutrition should result in better outcomes. Nutrients in addition to food, or in place of food when food is not taken in sufficient amounts, can be provided in a manner whereby the patient voluntarily consumes them by drinking various nutrient formulations. Nutrients can also be provided in an involuntary manner; tubes can be placed in the vein (parenteral nutrition) or intestinal tract (enteral nutrition) and nutrient solutions infused through them. All of these nutritional interventions have associated economic costs and also can produce a variety of complications (including vomiting, diarrhoea, and altered metabolic functions (for example, high blood sugar)). Thus, it is important to determine if such nutritional interventions (that is, the provision of nutrients in some manner other than just as food) do result in improvements in clinical outcomes. Since the best way to make such a determination is to undertake randomised trials, in which patients are assigned by chance to receive, or not receive, one or another of these treatments, this systematic review was undertaken to identify and summarise this information. Randomised trials comparing patients with liver diseases who were assigned to receive parenteral nutrition, enteral nutrition, or oral nutritional supplements to similar patients assigned not to receive any nutritional intervention were collected. The three nutritional interventions were considered separately. In addition, within each category of nutritional intervention, patients with medical conditions were compared separately from patients with surgical conditions. Thus there were six primary analyses, medical patients receiving or not receiving parenteral nutrition, surgical patients receiving or not receiving parenteral nutrition, medical patients receiving or not receiving enteral nutrition, surgical patients receiving or not receiving enteral nutrition, medical patients receiving or not receiving supplements by mouth, and surgical patients receiving or not receiving supplements by mouth. The outcomes of interest were mortality, hepatic morbidity (ascites, gastrointestinal bleeding, encephalopathy), quality of life, adverse events, infections, cost, duration of hospitalisation, jaundice, postoperative complications (only for the surgical trials), and nutritional outcomes (for example, body weight). A total of 37 randomised trials were identified. All but one had a high risk of systematic error (bias, that is overestimation of benefits and underestimation of harms). When the data were combined, most of the analyses failed to demonstrate a difference. There were some significant differences observed. These were that 1) parenteral nutrition reduced serum bilirubin more rapidly and improved one type of nutritional outcome (nitrogen balance) in medical patients with jaundice, and may have reduced some postoperative complications; 2) enteral nutrition may have improved nitrogen balance in medical patients, and reduced postoperative complications in surgical patients; and 3) supplements reduced the occurrence of ascites and also may have decreased the number of infections. Furthermore, the receipt of supplements (especially ones containing branched‐chain amino acids) may have been helpful in the treatment of patients with hepatic encephalopathy. No significant effects were seen from the use of supplements in surgical patients. None of these observed benefits can be said to be definitively present because of the presence of methodologic flaws in the trials, which may have produced an overestimation of the observed effect. Moreover, due to too few patients included in the trials with two few outcome measures, both spurious significant findings and spurious insignificant findings cannot be excluded. The data are not strong enough to justify a recommendation to use these nutritional interventions routinely. We need well‐designed and well‐conducted randomised trials to prove that such therapy is indeed efficacious.