Heart disease risk linked to long periods of breastfeeding.
Prolonged breast-feeding is linked to stiffening of the artery walls in early adulthood, a study in the British Medical Journal has found.
Researchers said that there should be no change to the advice that 'breast is best', but the study raised questions over how long mothers ought to breast-feed for.
The study looked at 331 people who had been born at Cambridge Maternity Hospital between 1969 and 1975.
Scientists from the Medical Research Council measured the participants for arterial distensibility - stiffness in the walls of the arteries - which is an early sign of cardiovascular disease.
They found that people who had been breast-fed for four months or more had greater stiffness in their arteries than those who had been bottle fed or breast-fed for only the first few months of their lives.
People with greater stiffness of the arteries also had higher cholesterol levels and blood pressure.
Every extra two months of breast-feeding led to a rise in cholesterol and blood pressure.
The researchers suggested that breast-feeding could affect the way fats are produced in the body and result in the early formation of fatty streaks in the arteries.