Some causes related to cardiovascular disease:  Loss of appetite can be related to significant heart failure.

Some causes related to other disorders:  Many significant medical conditions can cause loss of appetite.  One of the most common is depression.

Significance: Appetite loss is not only itself a morbid condition that deserves medical attention, but it also may be a sign of a significant underlying medical problem.  Following heart attacks or heart surgery, depression is very common, and appetite loss is an important sign of depression.  Depression after a heart attack or surgery significantly increases mortality, and needs to be specifically treated.