AimsThe aim of this study was to investigate cognitive performance and BMI of patients with dementia living in their own homes with family members, nursing homes and alone.DesignA prospective observational cohort study with a quantitative design.MethodMini–mental state examination (MMSE) scores and BMI were examined with a sample of Slovak patients (N = 428). Patients were followed up 12 months later after the first examination.ResultsCognitive decline was significantly faster for patients living in nursing homes and for solitary patients. BMI consistently decreased in the follow‐up examination and this drop was stronger in patients living alone and in nursing homes. Patients with VaD manifested a stronger BMI decline as compared with AD patients. This study suggests that impoverished conditions such as nursing homes or social isolation of solitary people contribute to stronger progress in dementia. Healthcare professionals need to implement meaningful activities for institutionalized people and for people who are living alone to eliminate the negative impact of an impowerished environment on patient's cognitive functioning.