Our loved ones can have difficulty eating due to a host of different problems -- medicines that make them nauseated, abdominal discomfort from constipation, changes to their senses of taste and smell making foods unappetizing, arthritis making it painful to raise a spoon or fork, vision problems making it hard for them to find the food in order to eat it, difficulty swallowing (called "dysphagia"), gastrointestinal disorders such as gastroparesis (which often responds well to a pureed diet) making them feel uncomfortably full after just a bite or two ... the list is endless. Just the other day, I saw a post from a caregiver who discovered the problem was thrush.
There's a pretty comprehensive article on how to make sure our loved ones get enough nutrition at:
As for the bouts of agitation ... An astoundingly high percentage of elderly dementia patients have pain that is untreated or under-treated, since our loved ones often can't explain that they are in pain, or where the pain is or what it's like, especially in the later stages. For example, studies have found that most (74%) demented nursing home residents have some pain and the majority (70%) are untreated or under-treated.
Untreated or under-treated pain can produce agitation and behavioral problems due to the suffering they are enduring. And, of course, it can also result in refusal to eat.
There's a good article on pain management in dementia patients at:
Also, there is a good workshop on how to evaluate pain in dementia patients, at: