Healthy teenage girls who diet regularly show worrying signs of malnutrition, Sydney researchers have found.

The largest study of its kind shows pressure to be thin could be causing teenage girls serious harm, potentially preventing them from developing properly.

The study of 480 girls, between 14 and 17, attending school in Sydney's northern suburbs and on the central coast, found those who dieted oftenwere deficient in a number of nutrients and biochemical, including calcium and protein, as well as haemoglobin, which is vital for transporting oxygen in the blood.

Many students in the study were dieting even though, on average, they were not overweight.

The low levels of calcium were particularly worrying because Calcium is used as a signalling molecule for every cell in the body.

Most researchers believe the amount of calcium consumed in a person's teenage years sets the basic level available for the rest of their life.

Media messages presenting excessively thin women as having an ideal body shape, or public health campaigns making girls overly aware of not consuming too many calories, could be to blame for dieting, said Dr Grant, who is the head of the Australasian Research Institute at the Sydney Adventist Hospital.