Children whose parents are thin are likely to be very slim themselves due to "skinny genes", new research suggests.
Youngsters whose parents are at the lower end of the healthy weight range are three times more likely to be regarded as thin (weighing less than a healthy weight range) than those whose parents are overweight.
The study recorded the height and weight of parents and up to two children from 7,000 families over a five-year period.
Analysis of body mass index (BMI) found that when both parents were at the lower half of the ideal BMI range, the chance of the child being thin was 16.2%, compared with 7.8% when both parents were in the upper half.
These children had a BMI of 18.5 or under (regarded as "thin"), compared with a healthy weight range of 18.5 to 24.9. A BMI of 25 to 29.9 is overweight and over 30 is obese.
The study found that the chance of a child being thin (BMI under 18.5) was just 5.3% when both parents were overweight and only 2.5% for children whose parents were obese.
Today's study, published in the journal Archives of Pediatrics and Adolescent Medicine, suggests thinness may be inherited, with children of thinner parents being likely to be genetically predisposed to a lower body weight.
Lead author Dr Katriina Whitaker, from University College London's department of epidemiology and public health, said: "We know from other studies that children's weights are correlated with those of their parents, but previous research has tended to focus on obesity rather than the other end of the spectrum."
Professor Jane Wardle, from the same unit, added: "Parents are often concerned if their child is thin, but it may just be their 'skinny genes'.
"All genes have two versions, called alleles.
"We might think of weight-related genes as having a 'skinny' and 'curvy' allele.
"Thinner parents are likely to have more of the skinny alleles, increasing the chance of passing them on to their children.
"A child who inherits more of the skinny alleles from their parents will be naturally thinner."
The researchers concluded: "These results are consistent with the idea that many cases of thinness are likely to represent the low end of the healthy distribution of weight and, as such, are likely to have a primarily genetic origin."