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While weight loss in dogs is considered more worrisome than weight gain, it's still important investigating what causes sudden, rapid weight gain in dogs.

 Weight gain is considered when a dog's caloric intake exceeds his metabolic demands. In other words, there is an imbalance between the dog's energy intake and energy burning. 

Generally, weight gain in dogs deserves evaluation when the weight gain consists of more than 10 percent of body weight. 

Obesity instead is generally considered when the weight gain exceeds 15 percent of the ideal body weight set by standardized weight charts. 

While being fed excess food and not exercising enough is a main cause of obesity in dogs, there are also several medical conditions that may cause rapid weight gain in dogs.

To narrow down potential causes of rapid weight gain in dogs, it helps categorizing the weight gain in different categories.

dog increased weight

Weight Gain and Excess Eating

When there is weight gain and excess eating, one may gain reassurance in knowing that that there is an explanation for the weight gain as the intake of excess food is to blame for the fat accumulation; however, there are medical conditions that can be associated with excess eating in dogs (polyphagia) that warrant diagnostic evaluation.

1) A Matter of Boredom

Weight gain due to excess eating may be due to boredom. Bored dogs may eat more given the opportunity.

The food may be free-fed fed ad libitum by dog owners or the dog may scavenge for food by raiding the trash can.

2) Owners Overfeeding

In other words, dogs overeating because it's easy for dog owners to get carried away with doling out table scraps.

Many dog owners can't resist the pleading eyes and drooling, and end up repeatedly feeding their dogs at the table. While the food given may seem little, it can quickly add up if you're feeding scraps on top of your dog's daily ration of food. 

"If you feed less, your dog may be hungry because the stomach never feels full. If this is a problem, you can add canned green beans (low sodium) or canned pumpkin (not the pie filling kind), to bulk up the fiber and add only a few calories. This will make the dog feel fuller." Dr. Wally

3) Reduced Physical Activity

As dogs move around less, they'll be more likely to pack pounds as all those calories must go somewhere if they aren't burned.

Spayed and neutered dogs often are know to gain weight. This is because paying and neutering alter the dog’s metabolism and appetite. The metabolism is slowed down and the appetite is increased. When these two changes are combined, weight gain is likely to occur, explains veterinarian Dr. Ivana Crnec in the article "does spaying make a dog fat?"

In the case of neutered male dogs, another factor that contributes to weight gain is the fact that neutering often leads to substantial changes in a dog's propensity to roam, urine mark and mount. 

When these hormonally-induced behaviors reduce, up to the point of sometimes disappearing from the dog's behavioral repertoire, it can lead to increase weight due to decreased physical activity.

Did you know? A study showed that female beagles were eating more food and gaining more weight after being spayed. 

Dog owners can prevent their recently spayed/neutered dogs from gaining weight rapidly by reducing caloric intake by up to 30 percent after spaying. Consult with your vet for specific dietary recommendations after spaying or neutering your dog.

"Spayed (dogs) have a lower metabolic rate and therefore need less food intake, if you continue to feed them the same amount of food after the spay there is a high risk that they will put on weight. Having understood this point all you have to do is to modify their diet, monitor their weight on a monthly basis and make changes to their diet accordingly so that they stay trim." ~Dr. Scott Nimmo

4) The Role of Medications

Certain drugs, may cause increased appetite in dogs. For example, phenobarbital, a popular anti-seizure drug given at high dosages may cause excess eating and so can steroids.

5) Underlying Medical Conditions

Health conditions that may increase appetite may include endocrine disorders such as Cushing's disease, diabetes and hypothyroidism.

Simply put, hypothyroidism is a medical condition characterized by decreased production of thyroid hormones. 

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Decreased thyroid function is directly linked with weight gain. This is because the lack of thyroid hormones slows down the metabolism. Regardless of what they eat and how much they exercise, dogs with hypothyroidism are quite likely to gain weight.

Did you know? Research has shown that, when dog owners measure their dog's  food by volume by using cups, this may often lead to either under or overfeeding. This is because it can be difficult to accurately measure a cup exactly level to the top, especially when it comes to kibble that comes in large sizes. 

In the case of dogs needing to lose weight, veterinary nutritionist Dr. Deborah E. Linder at Tufts University recommends that dog owners measure out their dog's food by weighing it with a kitchen scale.

Weight Gain and Reduced/Normal Appetite

If your dog is gaining pounds by the day, but yet he is eating normally or perhaps he's even not eating enough, things may not add up. It could be that you are overfeeding or that your dog has a slow metabolism.

6) Genetic Predisposition

 Some dogs may be genetically predisposed to obesity. For example, some dog breeds are particularly predisposed to gaining weight such as basset hounds, dachshunds, beagles, cocker spaniels, and Labrador retrievers.

Did you know? A study on Labrador retrievers has revealed why Labradors are always so hungry. 

7) A Matter of Aging 

Age can also be a factor considering that, as dogs age, they undergo a reduced metabolic rate. 

Also, senior dogs suffering from orthopedic problems may move around less, yet dog owners fail to reduce caloric intake. 

If your dog is moving less around due to illness, mobility problems or old age, consult with your vet for a quick calculation of your dog's caloric needs. 

Reduced metabolic rates can also be caused by medical conditions such as hypothyroidism.

Did you know? Statistics by The Association for Pet Obesity Prevention (APOP) show that an estimated 58 percent of dogs in the Unites States are overweight or obese.

Weight Gain due to Enlargements

Sometimes the weight gain may not be due to increased body fat, but rather some enlargement or swelling of dog body parts. In many cases, this may be very noticeable, but sometimes not. 

8) Presence of Ascites

For instance, a dog may gain pounds from ascites; the accumulation of fluid in the dog's abdomen. 

Dogs with ascites typically present an enlarged belly. Ascites in dogs may be due to serious conditions such as malfunctioning organs.

9) Presence of Edema

Another cause of weight gain may be edema, the buildup of fluids, which can be seen under the form of swollen legs in dogs with congestive heart failure.

Since the heart no longer pumps effectively, fluid starts building up in the body. Affected dogs may have swollen legs and fluid in the dog's abdomen may accumulate.  When fluid accumulates in the lungs, the condition is known as pulmonary edema.

10) Enlargement of Organs

  Organomegaly is also a cause of weight gain. In this case, a dog's organs such as the liver or spleen may enlarge causing increased weight gain.

11) Increase in Body Mass

Less concerning, but worthy of mentioning, dogs may gain weight from an increase in lean body mass (the total weight of your dog's body minus all fat mass) as seen from increased exercise in athletic dogs (greyhounds, sled dogs etc).


  • DVM360, Obesity in dogs, Part 1: Exploring the causes and consequences of canine obesity
  • Jeusette I, Detilleux J, Cuvelier C, et al. Ad libitum feeding following ovariectomy in female Beagle dogs: effect on maintenance energy requirement and on blood metabolites. J Anim Physiol Anim Nutr (Berl) 2004;88:117-121.

Photo Credits:

  • Flickr, Creative Commons, Dale My buddy Andy CCBY2.0

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