Why fat Labradors can blame their genes
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A genetic mutation makes some Labradors and flat-coated retrievers constantly hungry while burning fewer calories, say scientists.
This "double whammy" effect means dog owners must pay special attention to keeping their pets slim.
The mutation was found in one-in-four Labrador retrievers and two-thirds of flat-coated retrievers.
"It shows us the power of our genes to change how we feel about food," the lead scientist says.
Dr Eleanor Raffan of the University of Cambridge said dogs, like humans, have genes that influence both interest in food and metabolic rate.
"If we get dealt a genetic hand of cards that makes us feel hungry or always want to eat, it takes greater effort to stay slim," she told BBC News.
The study builds on previous research into a mutation in a gene known as POMC.
The POMC gene and the brain pathway it affects are similar in dogs and humans.
Not only do dogs with the mutation get hungrier in between meals but they also use around 25% less energy at rest, meaning they don't need to consume as many calories.
"Dogs with this genetic mutation face a double whammy: they not only want to eat more, but also need fewer calories because they're not burning them off as fast," says Dr Raffan.
More than 80 pet Labradors dogs took part in the study.
They were given various tests, including the 'sausage in a box' test, where they were tempted with a hidden treat they could see and smell.
Dogs with the POMC mutation tried much harder to get at the sausage than dogs without it, indicating greater hunger.
Flat-coated retrievers were allowed to sleep in a special chamber that measured the gases they breathed out revealing those with the POMC mutation burn about 25% fewer calories than dogs without it.
Kathryn Taylor has a "big chunky black lab" called Leo, who took part in the study.
She said he will eat anything, including a bowl of salad from the table, runner beans from vegetable beds and has been known to dig up carrots.
"You can feed him and he'll still be wanting more food," she said, adding that they work hard to keep him slim including using a slow feeder bowl.
As many as two in three dogs (34-59%) in rich countries are now overweight.
The Labrador has the highest levels of obesity and has been shown to be more obsessed with food than other breeds.
They make successful working and pet dogs because they are also relatively easy to train.
We may have unconsciously selected for Labradors with the mutation because they adore food and will do anything for a biscuit.
The research, published in the journal Science Advances, was funded by the Wellcome Trust and the Dogs Trust.
Follow Helen on Twitter @hbriggs.