When the stomach is dilated, either after a meal or in some abnormal condition, this makes the dog unwilling to eat. When the stomach is empty, a hormone called ghrelin is secreted, which plays a role in stimulating the appetite. Watch out if your dog is stealing or sneaking food. This may cause him to have a distended stomach and lose interest in food at meal times.
Diseases and Pain
Diseases that are associated with vomiting or nausea may cause dogs to develop food aversion. Some diseases result in a reduced sense of smell, and therefore a lack of appetite. Other disorders that can cause low appetite include low phosphorus intake and vitamin B1 deficiency (Read and Harrington, 1981).
Psychogenic (Mental Health)
Neurotransmitters in the body may inhibit appetite such as serotonin.
Stress and anxiety can also lead to anorexia. Emotional stress (e.g. loss of companion), is known to cause a temporary loss of appetite in dogs.
Some medications can cause anorexia as a side effect. These drugs include but are not limited to analgesics, anti-inflammatories, diuretics, antibiotics, and chemotherapy (Delaney et al., 2006). Let your vet know when your dog refuses to eat after medication changes.