The family of the man complained to Scottish Public Services Ombudsman Jim Martin over the care he received at the Maple Villa Care Home in Livingston before his death in 2009.
Mr Martin ordered NHS Lothian to apologise to his family after upholding complaints that it had failed to provide proper nutrition, general personal care and stimulus for the man.
A report yesterday revealed his family had complained that the meals he received were very small and no-one made sure he had regular drinks. His granddaughter said this meant when he was admitted to hospital in July 2009, shortly before his death, he was severely dehydrated.
Upholding the complaint, the ombudsman said there was no evidence to show the patient had been eating and drinking agreed amounts, so NHS Lothian could not be satisfied that he was properly cared for in this regard.
Mr Martin said he was particularly concerned by what he had found, given that the home was a specialist unit caring for patients with challenging aspects of dementia. "As the number of people with Alzheimer's increases, hospitals and care homes must have policies and procedures in place to assess and respond to such patients' needs," he said.
Melanie Hornett, nurse director at NHS Lothian, said she was writing to the family to offer a formal apology. "We accept all the recommendations made by the ombudsman and have already implemented changes," she said.