Childhood obesity is not related to whether a baby was breastfed in infancy, a US study suggests.
Some studies have linked breast feeding with lower obesity rates
The American Journal of Clinical Nutrition study contradicts previous research which suggested breastfeeding could protect against later obesity.
It measured the body fatness of 313 American children aged five and found no difference between those who were breastfed and those who were not.
But this should not diminish the many benefits of breastfeeding, it added.
Lead researcher Dr Hillary Burdette at the Children's Hospital of Philadelphia said there was much interest in whether breastfeeding or the delayed introduction of complementary foods or both can reduce the risk of obesity later.
But she said many studies had conflicting results, so the team tried to devise a new technique to measure their subjects' body fatness, or adiposity, using a specially created X-ray machine.
Earlier studies had used a body mass index, which divides a person's weight in kilograms by the square of their height in metres.
The children were enrolled in the study at the age of three, and their mothers reported information about feeding practices about the first year of life.
The mothers were also asked when their children started eating infant cereal, baby food, mashed and regular table food and sweetened beverages such as fizzy drinks and fruit juice.
When the children reached an average of five years old they had their body fat measured using the X-ray machine.
The results were then adjusted for factors which may have influenced the child's body fat including maternal obesity, socio-economic status and maternal smoking.
The study found: "Regardless of the duration of breastfeeding, breastfed children did not have significantly less fat mass than did children who were never breast fed.
"When fat mass was compared between groups of children who had different combinations of breastfeeding and formula feeding and children who were never breastfed, the differences were not significant."
The team said it was possible that some of the protective effects of breastfeeding against later obesity were "overestimated" when BMI was used as a measure.
It concluded: "Clinicians should be cautious about concluding that breastfeeding protects against later obesity solely on the basis of studies using BMI.
"This in no way, however, diminishes the importance of recommending breastfeeding for its multiple and other benefits to mother and child."
Ursula Arens, spokeswoman for the British Dietetic Association, said the method the researchers used for measuring body fat was more accurate than methods just based on weighing.
"Previous research has suggested that breast-fed babies may be better able to regulate their energy intakes over formula-fed babies, but the outcome of this study does not support this view.
"It does not challenge the many positive health effects for mother and child, of breast-feeing, but 'obesity prevention', at least up to age five, does not seem to be a benefit that can now be promoted.
She added: "There have been many developments in the formulation of infant milk, and the products on the market today better match breast-milk features than previous products."