The most common reversible cause of anorexia and weight loss in older people is depression. Decline in appetite — a key feature of many forms of depression — results from imbalances in various brain neurotransmitters, including serotonin that signals the hypothalamus, causing people to eat less.

Older depressed adults may also develop constipation, which leads to a sense of fullness and less appetite.

In more extreme cases of depression, patients can become psychotic and lose touch with reality. They may suffer from hallucinations or delusions that further limit their food consumption.

Severely depressed patients may refuse to eat entirely. Older depressed people often experience feelings of loneliness, which is another factor involved in lower levels of food consumption.

Defined as impairment of memory or other cognitive abilities to the extent that patients need help from others, dementia afflicts 10 percent of people 65 and older, and nearly 50 percent of those 85 and older.

Patients with dementia have twice the risk of developing anorexia than those without dementia.

Sometimes these patients have a loss of appetite; other times they are indifferent to eating or simply forget to eat.

Dementia patients in particular experience a decline in sense of smell, which further impairs appetite.

Many dementia sufferers become chronically agitated, which can disrupt their eating habits. As they continue to decline, they also often develop difficulty swallowing food and liquids.