It's probably not your imagination that a loved one with severe dementia appears increasingly fragile over time. Weight loss is common at this stage, even among individuals who for a time were gaining weight because they'd forget they'd just finished a meal and eat more. Now the mind is not only unable to read the body's hunger signals, but the appetite itself is downshifting. Eventually, the body will want less as it begins to shut down.
For now, here's an appropriate response:
Try calorie-dense nutrition beverages to supplement an indifferent appetite.
Offer softer, more easily swallowed foods (scrambled eggs, thickened soup, yogurt).
Check to see if any mouth issues (ulcers, ill-fitting dentures, cavities, or infections) are a possible root cause. If you sense mouth pain, see if you can get a dentist to do a check -- preferably one with experience treating patients with dementia.
Do mention weight loss and a loss of interest in food to the doctor.
But don't be overly alarmed. These changes are part of the disease process.