NINTENDO'S Wii Fit game has attracted the ire of a childhood health expert for telling healthy children they are fat.

The game assesses players' fitness levels based on their body mass index, labelling them underweight, ideal weight, overweight or obese.

The body mass index is a measure of a person's weight relative to height, but experts say this is not an appropriate measure for children because it does not account for their age or stage of development.

The $149.95 Wii Fit went on sale in Australia in May and includes a weight- and motion-sensitive balance board, which players can use for yoga, muscle workouts, aerobic exercises and balance games.

Shirley Alexander, a staff specialist in childhood and adolescent weight management services at the Children's Hospital at Westmead, said body mass index was not ideal for measuring the fitness of adults, let alone that of children.

Nintendo said the body mass index calculations were based on adult measurements and might not be accurate for children.

"Anyone that's even overweight can actually be fit, [although] the more overweight you are, the less likely you are to be fit," Dr Alexander said.

"But with children they have more fatty tissue at different periods of their life and they have a larger body surface area relative to adults … the age of the child has got to be taken into account."

Asher Moses