Alzheimer s disease is the most frequent cause of dementia in elderly people and it is one of the leading causes of death among older individuals. Weight loss is a frequent clinical finding in Alzheimer s disease patients, and it is actually listed as a symptom consistent with the diagnosis of Alzheimer s disease, but its significance has not been clearly understood until now. This review examines the role of the components of energy balance in determining weight loss in AD patients, on the basis of data collected from the literature. We also considered the possible causes of anorexia in AD patients. In the last years many researchers investigated the possible role of abnormally high energy expenditure, or low energy intakes, or both, to explain weight loss in these patients. Studies on energy intake, energy expenditure and body composition in AD patients have been reviewed. The results of published studies do not seem to support the hypothesis of an hypermetabolic state or inadequate energy intake in AD patients, but further studies, with greater samples are necessary in the future to investigate weight loss in AD patients. A better understanding of this finding could be important to obtain the maximal lifespan of demented patients.