Breastfeeding may not actually protect children against obesity later in life, according to doctors.
Breastfeeding protects babies from a range of diseases
Previous studies have suggested breast milk may prevent obesity but two studies in this week's British Medical Journal appear to refute that theory.
The studies, which together looked at more than 5,000 people, found no evidence to support the claim.
However, researchers said women should continue to breastfeed infants because of its many other health benefits.
In the first study, Professor Cesar Victoria and colleagues at the Universidade Federal de Pelotas in Brazil looked at 2,250 men who had been born in 1982 and for whom detailed breast feeding information was collected in early childhood.
They examined these men when they signed up to serve in Brazil's army at the age of 18. Around 17% were overweight and 8% were obese.
However, the researchers found no link between the length of time they were breastfed as infants and their current weight.
In the second study, Leah Li and colleagues at the Institute of Child Health in London looked at the records of 2,631 British children.
They examined data on how long they were breast fed and their respective body mass indexes - an accepted measurement of obesity.
Again, the researchers found no link between the amount of time these children were breastfed and their current weight.
"We found no evidence that breastfeeding influenced body mass index or obesity," they wrote.
The Brazilian researchers said the findings should not detract from efforts to encourage women to breastfeed babies.
The World Health Organization recommends mothers breastfeed their newborn infants for six months because of health and psychological benefits.
Previous studies have shown breastfeeding can reduce a child's chances of contracting a variety of viruses and diseases, ranging from colds to asthma.
The researchers said: "Regardless of the role breastfeeding may have in preventing obesity, it has been consistently associated with many advantages for the mother and child, ranging from decreased childhood mortality to a likely protection against breast cancer.
"The continued protection, promotion and support of breast feeding remains a major public health priority."
Belinda Phipps of the National Childbirth Trust said it was difficult to say whether breastfeeding protected against obesity.
"The jury is still out," she told BBC News Online.
"A number of papers have suggested that it may have a protective effect.
"However, obesity is complex and many factors are involved."
But she added: "There are many benefits to breastfeeding and whether or not it protects against obesity certainly is not the most important one."