Why Is My Dog Losing Weight While Eating Well?

by Katherine Khoo October 09, 2025 7 min read

A dog that keeps eating, sometimes even more than usual, yet continues to lose weight is giving you an important message: something isn’t right. Unexplained weight loss often points to an underlying medical problem that needs a veterinarian’s attention. There are numerous diseases that cause weight loss in our canine companions and below are ten common disease causes. We’ve included information on how each one leads to weight loss and the key signs and symptoms to look out for.

Canine Kidney Disease

Kidney failure in dogs is one of the most common reasons for a senior dog losing weight while still eating. Normally, the kidneys filter toxins from the blood and remove them through urine. They also keep electrolytes, fluids, and proteins in delicate balance. When the kidneys become diseased, they can’t perform these critical tasks, and the body starts to break down.

Dogs with kidney disease experience unexplained weight loss as they advance through the stages of kidney disease. Other warning signs include vomiting, diarrhea, bad breath with a chemical odour, increased thirst and urination, and overall lethargy. Older dogs may also develop urinary tract infections, which can lead to accidents or blood in the urine.

Vets usually confirm kidney disease by finding elevated kidney enzymes on bloodwork and a low urine concentration on urinalysis. They may order urine cultures, X-rays, or an ultrasound for a clearer picture. Treatment often involves fluid therapy (including subcutaneous fluids at home), a prescription kidney diet, and medications to manage nausea or slow the disease.

Canine Liver Disease

The liver acts like a giant chemical plant, storing energy, processing nutrients, and clearing toxins. When it’s damaged by infection, inflammation, or toxins, these vital jobs stall. Your dog may keep eating but still lose weight because nutrients aren’t processed properly and energy stores run low.

Common symptoms include yellowing of the eyes or gums (jaundice), a swollen belly from fluid buildup, pale stools, vomiting, diarrhea, and overall tiredness.

Diagnosis usually starts with blood tests that show elevated liver enzymes. Your vet may recommend an abdominal ultrasound or specific bile-acid tests. Treatment depends on the cause but may include a special liver-support diet, supplements like milk thistle and dandelion which can be found in Augustine Approved Faith's Cleanse & Detox Supplement Powder for Dogs (Liver, Kidney, Skin, Coat & Gut), antioxidant SAMe, antibiotics for infections, or medications to reduce inflammation and improve liver function.

Canine Dental Disease

Dental disease isn’t just about bad breath. It can seriously affect your dog’s weight. Infected gums or loose, painful teeth make chewing difficult. Many dogs start swallowing food whole or avoiding kibble, so they take in fewer calories even though they still appear hungry.

Look for drooling, bleeding gums, pawing at the mouth, a sudden preference for soft foods, or foul-smelling breath.

A vet will perform a thorough oral exam and may take dental X-rays to check roots below the gum line. Treatment often includes a professional cleaning under anesthesia, tooth extractions if needed, and antibiotics for infection. Going forward, daily brushing with dog toothbrush and toothpaste, and regular check-ups keep the mouth healthy and your dog’s weight stable.

Cancer

Cancer can rob a dog of weight in several ways. Tumours may steal nutrients, cause chronic inflammation, or release substances that speed up metabolism, leading to muscle wasting even when your dog eats well.

Warning signs vary with the type of cancer but can include lumps that grow or change, persistent coughing, unexplained bleeding, chronic vomiting or diarrhea, and a noticeable drop in energy.

Reaching a cancer diagnosis can look a bit different depending on the type of cancer. Your vet may recommend blood work, X-rays, ultrasound, or a biopsy to identify the cancer type. Treatment depends on the diagnosis and may involve surgery, chemotherapy, or targeted medications to slow tumor growth and help maintain weight.

Adrenal Gland Disease (Addison’s or Cushing’s)

The adrenal glands sit near the kidneys and produce hormones that control stress responses, blood pressure, and metabolism. When these glands misfire, weight loss follows.

Addison’s disease means the glands don’t make enough hormones. Dogs may show weakness, vomiting, diarrhoea, shaking, or even collapse.
Cushing’s disease is the opposite—too much cortisol. Dogs may develop a pot-bellied shape, thin skin, hair loss, heavy panting, and muscle breakdown despite a good appetite.

Diagnosis involves specialized blood tests and sometimes ultrasound. Lifelong medication is usually needed and these are usually hormone replacement for Addison’s or drugs that reduce cortisol for Cushing’s.

Diabetes

Normally, the dog’s pancreas releases insulin, which drives glucose from the blood into the cells for energy production. Diabetes happens when the dog’s body can’t use sugar for energy because it lacks insulin or can’t use it properly. The cells “starve,” so the body burns fat and muscle to use as alternative energy sources. Despite a strong appetite, diabetic dogs will lose weight rapidly.

Key symptoms include excessive thirst and urination, unexplained weight loss, tiredness, and sometimes cloudy eyes (cataracts) or sweet-smelling breath.

Diagnosis is straightforward with blood and urine tests. Most diabetic dogs need daily insulin injections and a consistent, high-fiber diet. With proper care, many live long, active lives.

Heart Disease

A weak or failing heart can cause a dog to burn more calories just to breathe and move. Poor circulation also means nutrients and oxygen don’t reach tissues efficiently, leading to gradual weight loss and muscle wasting.

Typical signs include coughing (especially at night), fast or laboured breathing, exercise intolerance, fainting spells, and a swollen belly from fluid buildup.

Your vet will likely perform chest X-rays, an echocardiogram (heart ultrasound), and blood pressure checks. Treatment focuses on medications to reduce fluid, strengthen the heartbeat, and control blood pressure, along with dietary changes and tailored exercise plans. A CoQ10 supplement is sometimes recommended for dogs with heart problems and this Bark & Whiskers Ubiquinol Liquid Pump Supplement For Dogs is a highly absorbable CoQ10 formula. It’s simple to use, no messy, oily pills to cut open, and it stays fresh with an airless pump.

Inflammatory Bowel Disease (IBD)

IBD is chronic inflammation of the stomach or intestines. Even when a dog eats plenty, the inflamed gut can’t absorb nutrients properly, so nutrients and calories pass right through.

Watch for repeated vomiting, diarrhoea that may contain mucus or blood, loud stomach gurgling, gas, and a dull, dry coat. Please take note that IBD can also cause unexplained weight loss in dogs with few or no symptoms.

Diagnosis can involve stool tests, blood work, ultrasound, and sometimes endoscopy with intestinal biopsies. Treatment often includes a prescription hypoallergenic or novel-protein diet, antibiotics, and anti-inflammatory medications to calm the intestinal lining. Selected probiotic strains and herbal blend like this Four Leaf Rover (GUT GUARD) For Leaky & Imbalanced Guts can help to control inflammation in the digestive tract, improve digestion, and restore healthy bacteria colonies.

Megaesophagus

The esophagus is a tube that carries food from the mouth to the stomach. When it loses its normal muscle tone, food pools or comes back up before it reaches the stomach, so the dog never digests or absorbs the nutrients properly. This condition is known as megaesophagus.

Classic signs include effortless regurgitation (not forceful vomiting), coughing, nasal discharge, and frequent pneumonia from inhaling food.

X-rays or a special contrast “swallow study” confirm the diagnosis. Management focuses on feeding your dog in an upright position, often with a special “Bailey chair,” offering small, frequent meals, and sometimes medications to improve motility or reduce reflux.

Parasites

Many dogs with parasites are asymptomatic. These intestinal parasites like roundworms, hookworms, whipworms, or tapeworms, steal nutrients right from the digestive tract. A dog can eat normally yet stay thin or even become pot-bellied.

Look for diarrhoea, a dull coat, visible worms or eggs in the stool, scooting, or chronic belly discomfort. Puppies are especially vulnerable.

Your vet will check a stool sample for eggs or larvae and prescribe the right deworming medication. Regular fecal checks and monthly parasite preventives help keep future infestations away.

When to See the Vet

Hopefully, you have a better idea of the various reasons your dog might be losing weight despite having a good appetite. Any unexplained weight loss, especially over a few weeks, deserves prompt veterinary attention. Bring notes about your dog’s eating habits, activity level, and any other symptoms such as vomiting, coughing, or changes in thirst or urination.

Supporting Your Dog at Home

Once your vet has figured out why your dog is losing weight, he or she will speak with you about how to treat that particular condition. Sometimes, weight gain can be achieved by simply increasing the amount of food. For dogs with muscle wasting, sometimes adding amino acids supplement to their diet can help to mitigate the condition. Do consult with your vet before making any dietary changes and continue to monitor your dog’s progress.

Track weight: Weigh your dog every couple of weeks to spot changes early.
Feed a balanced diet with good quality protein: Choose high-quality food suited to age and health needs.
Stay on schedule: Keep up with dental care, parasite prevention, and routine check-ups.
Watch for changes: Note increased drinking, coughing, vomiting, or behaviour shifts and share them with your vet.

A dog that eats well but loses weight isn’t just “getting older.” From kidney or liver disease to parasites or diabetes, many treatable conditions can be behind the problem. Knowing the warning signs and getting a prompt veterinary exam gives your dog the best chance to recover and maintain a healthy weight.