By Jeremy Laurance
The slogan "breast is best" has been used to promote mother's milk for decades, but breast milk may not be quite as good as it has been billed.
Although breast feeding still offers babies the best start in life, enthusiasm for its benefits among doctors and medical researchers has led to its benefits being overestimated.
A recent study suggests that breast feeding offers little protection against the development of high blood pressure compared with bottle feeding, contrary to earlier claims.
High blood pressure is a main cause of heart attacks and strokes, which together take 180 000 lives a year in Britain. Even a small increase in blood pressure above normal levels increases the risk.
Researchers have suggested that the higher levels of salt in bottled or formula milk until 1980, when salt levels were reduced, contributed to higher blood pressure in infancy and that these effects persisted into adulthood.
They have also suggested that long-chain fatty acids, which are only present in breast-milk, play an important part in the development of healthy blood vessels, which protect against high blood pressure.
But a team of researchers at St George's Hospital Medical School in London, who reviewed 24 studies examining breast-feeding and blood pressure, found no proof of a connection.
Those that reported noticeable differences were mostly small studies whereas larger-scale research showed little difference. That suggested any effect of breast-feeding was likely to be slight, at best.
Their report, published in the British Medical Journal, follows conflicting debate about the effectiveness of breast-feeding in reducing obesity in later life. Researchers are split on whether mother's milk may help to protect children against becoming overweight, compared with those babies who are bottle-fed.
Two studies that were published in the BMJ last month failed to find any significant link between breast-feeding and being overweight or obese in later life. One of those studies followed 2 600 British children and the other investigated the health of 2 250 Brazilian children who had been breast-fed for varying lengths of time as babies.
Neither found a correlation between breast milk and being overweight at age 18, although they did find evidence of other health benefits.
Researchers have also pointed out that the incidence of breast-feeding in the United States and Britain has increased since 1990, yet so has childhood obesity.
In the latest study, the epidemiologist Christopher Owen and colleagues from St George's say the likeliest explanation for the impression that breast-feeding protects against high blood pressure is "publication bias". This is the tendency for medical journals to publish studies with positive results while rejecting those with negative findings.
They conclude that the effect of breast-feeding on blood pressure is likely to be of "limited clinical or public health importance".
However, they add that it should be encouraged on the basis of its other advantages in promoting development of the nervous system, protecting against allergies, reducing cholesterol in later life and strengthening the mother/child bond. "The case for breast feeding rests on a combination of short and long-term benefits," they say.