The intermediate and advanced stages of Alzheimer's disease (AD) are frequently associated with weight loss (WL), but WL may even precede the onset of cognitive symptoms. This review focuses on the possible aetiologic and temporal relationships between AD and WL. When WL occurs some years before any signs of cognitive impairment, it may be a risk factor for dementia due to deficiency of several micronutrients, such as vitamins and essential fatty acids, and consequent oxidative tissue damage. The leptin reduction associated with WL may also facilitate cognitive decline. The mechanisms potentially inducing WL in AD include lower energy intake, higher resting energy expenditure, exaggerated physical activity, or combinations of these factors. A hypermetabolic state has been observed in animals with AD, but has not been confirmed in human subjects. This latter mechanism could involve amyloid assemblies that apparently increase the circulating cytokine levels and proton leakage in mitochondria. WL may be caused by patients' increased physical activity as they develop abnormal motor behaviour (restlessness and agitation) and waste energy while trying to perform daily activities. During the course of AD, patients usually find it increasingly difficult to eat, so they ingest less food. AD-related neurodegeneration also affects brain regions involved in regulating appetite. The caregiver has an important role in ensuring an adequate food intake and controlling behavioural disturbances. In conclusion, WL is closely linked to AD, making periodic nutritional assessments and appropriate dietary measures important aspects of an AD patient's treatment.