Breast is still best for young babies but scientists are now questioning for how long.
Mother's milk is full of special nutrients, hormones and antibodies that are passed on to infants to help them to resist infections, respiratory illness and diarrhoea.
But new research reports that doctors at the Institute of Child Health has raised questions about the optimal duration for breast-feeding.
Professor Alan Lucas and his team have found that young adults who had been breast-fed for more than four months have stiffer arteries, an early marker of heart disease, than people who had been bottle-fed or breast-fed for a shorter time.
"The longer the duration of breast-feeding the stiffer we found arteries to be in 20-28 year-old men and women," Lucas told Reuters.
But the flexibility of arteries was the same in people who had never been breast-fed and those who were nursed for less than four months.
The doctors stressed that their findings, reported in The British Medical Journal, are purely observational and they have not established a causal link.
They recommended that women continue to breast-feed their babies because of the advantages it has for both the child and mother.
"There are many pluses for breast-feeding," said Lucas.
"We need to do more work on the optimal duration of breast-feeding for people in the West and we need to do more work on whether there are other things we can do, like changing diet in adulthood, that could remove this risk altogether."
All of the 331 people who took part in the study were young so the results are less likely to be related to other risk factors for heart disease such as smoking, social class and size.
"We took a range of factors that we measured during this study and found that none of those things explained the relationship between breast-feeding duration and later artery stiffness," Lucas added.
Although the researchers have not established the mechanism by which breast milk could lead to arterial stiffness, one theory they have put forward is that breast-milk was not meant to get babies started in life and then take up a high-fat Western diet.
Lucas said that results of animal studies support the theory.
Ian Booth, of the University of Birmingham in England, said the study should not alter the current recommendations for breast-feeding which suggest that women nurse their babies for the first four to six months of life.
Breast-feeding is particularly important in developing countries where it is vital for infant survival and health.
"Independent corroboration in different populations is required before the potential impact of these observations can be assessed," Booth said in a commentary on the research. - Reuters