One of the first symptoms noticed in a newly diabetic pet is a sudden loss of weight, usually despite increased hunger and food intake. This happens because with a shortage of insulin, the animal can no longer receive nourishment at the cellular level and will use both muscle and fat as energy sources. 
Untreated diabetics, or poorly-treated ones, may continue to lose weight until serious complications ensue. Complications can include dehydration and/or ketoacidosis, either of which can be deadly within 48 hours.
If the underweight is caused by diabetes, the only treatment that's known to work nearly always is insulin given with injections. This treatment should begin immediately, to reduce complications from hyperglycemia, as well as those mentioned above.
Our bodies normally are "fueled" by metabolizing glucose; they are able to do this provided they have enough insulin (normally or by injection). When there's not enough insulin to allow the body to turn its glucose into energy, it begins metabolizing fat to fuel its cells.
Once on regular doses of insulin, the animal's weight will generally return quickly, possibly even tending toward overweight.  Because the glucose is once more able to enter body cells, it no longer passes from the body in the urine  and the system no longer needs to use fat as its energy source. If a previously-regulated pet begins losing weight with no weight loss plan, it can mean that he or she needs an adjustment of insulin dosage; the present dose may not be correct. See regulation. It can also signal that the insulin currently in use may no longer be effective for that particular pet. A vet visit is suggested to be certain insufficient insulin dose or a need to switch insulins is the only problem.
- ↑ Caninsulin Guide-Page 3. Intervet. Archived from the original on 2006-08-20.
- ↑ Insulin and Weight Gain. Mayo Clinic.
- ↑ Managing Your Diabetes. Joslin Diabetes Center.
- Dog Weight Chart-Visuals to Determine Weight-from Purina
- General Characteristics of Protein Degradation in Diabetes & Starvation National Academy of Sciences-US 1978
- Diabetes Mellitus-CVC Proceedings-Type I Diabetes Schall, William D., 2009, DVM 360