A decrease in appetite can be partial or complete. Dogs are typically not inclined to eat when they feel unwell; however, a decrease in appetite is not specific for a given disease. A sudden decrease in appetite is likely to be abnormal and often is an important and early sign of disease despite its lack of specificity. A change to a more palatable type of food may temporarily correct the problem, but may also mask the presence of underlying illness.
An increase in appetite may be a normal response to increased need for nutrients such as exercise, cold ambient temperatures, and pregnancy. Many diseases also increase nutritional needs, but most of these are associated with decreased appetite because the dog does not feel well. Other diseases do not cause a feeling of illness, especially early on in the illness, but may interfere with the way food is digested, absorbed, or converted to usable energy. Such diseases include:
It should also be noted that certain drugs can produce an increase in appetite.
Weight gain can be the result of increase in fat, muscle, or fluid. Obesity is a disease that is characterized by too much fat. Like people, dogs tend to gain weight as they age, especially if their activity levels decrease. If the food is especially tasty and if it is associated with social interactions, dogs will tend to eat more food than they need to maintain body weight.
Fluid gain can be caused by the accumulation of fluid in the abdominal and chest cavities or by excessive fluid in the tissues under the skin. Fluid in the abdominal cavity is not usually noticed until it causes a marked increase in abdominal size. Fluid in the chest cavity causes a change in the respiratory rate or effort. Fluid under the tissues of the skin, called edema, can be caused by:
Weight loss may represent a loss of fat, muscle, or body water. Healthy dogs lose weight from exercising more and eating less involves loss of fat. Fat loss continues until they body stores of fat are consumed and subsequent weight loss will be due to muscle loss. This is typically caused by underfeeding the dog.
Some diseases affect metabolic processes and muscle tissue can be broken down before fat stores have been consumed. Affected dogs often fell ill and do not eat normally.
Loss of body water is called dehydration and occurs when the intake of water is not sufficient to cover water losses.