A police force has defended its decision to arrest the parents of a 15 stone 11-year-old on suspicion of neglect for allowing their son to become obese.

The un-named 49-year-old man and 44-year-old woman, both from the King's Lynn area, are currently on bail pending further inquiries after concerns were raised about the boy's welfare.

Norfolk police have said that intervention at this level was "very rare" and only took place where other attempts to protect the child had been unsuccessful.

A spokesperson for the force said that officers from the child abuse investigation unit worked closely with health and social services to deal with "sensitive issues such as obesity and neglect of a child".

The Sun reported that the 5ft 1in boy has a body mass index of 41.9 – meaning the boy is classed as very overweight – and that his mother had told the newspaper that she and her partner had been trying to keep him slim by encouraging the child to play active games on his Wii console.

"The idea they could take away my son scares me so much," said the woman, who added that she and her partner had spent 50 minutes in a cell following their arrest. "His weight isn't that much of a big deal. I'm chubby and the whole of my husband's side of the family is big. It's genetics – you can be genetically fat."

The boy's father was reported to have said: "He's always been big. He was born with shovels for hands and spades for feet. Everyone on my side of the family is big, there's nothing we can do about it."

Doctors at the Queen Elizabeth hospital in King's Lynn are believed to have contacted police after the boy, who was reportedly already known to social services, was twice brought in for treatment in March.

The parents voluntarily attended a police station and were interviewed under caution on suspicion of neglect and child cruelty. They were arrested in March on suspicion of child neglect and cruelty under section 1 of the Children and Young Persons Act 1933.

The police spokesman added: "Any action taken by any agency will be subject to a joint strategy between all partner agencies and will always be taken with the welfare of the child and their protection from harm as paramount.

"While it is inappropriate to comment on this case specifically, it is important to stress that intervention at this level is very rare and will only occur where other attempts to protect the child have been unsuccessful."

A five-year-old girl was taken into care in Wales in 2012 after her weight reached more than 10 stone. The girl weighed 10st 5lbs – more than three times the weight of a healthy child her age – in August 2012, and she was put into foster care by Newport council.