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Is breast really best?

Some studies show that breast-feeding protects babies against asthma, while others suggest it may actually increase the risk. So which is right? A vitamin may hold the key, reports Oliver Gillie

06 November 2002

Breast-fed babies are more likely than bottle-fed babies to develop asthma, a recent study concludes. But a dozen other studies have come to various contradictory conclusions, leaving doctors, midwives and mothers in a quandary. Human milk has evolved for human babies – so can it be possible that breast is not always best?

The answer may lie in the content of human milk, which depends on the mother's diet and lifestyle. Our increasingly unnatural lifestyle – spending more time indoors or in cars, away from the sun – means that breast milk is often deficient in vitamin D, especially in winter. A small amount of vitamin D is taken in breakfast cereal, eggs, oily fish and margarine, but most of it is obtained by the action of the sun on the skin in summer. In winter the sun is not strong enough.

Extreme deficiency of vitamin D causes rickets, a disease of the bones that was common in Britain before it became mandatory for vitamin D to be added to food. But vitamin D also plays a key role in the maturation of the immune system and so may be important in the prevention of asthma and allergies.

The research findings linking asthma with baby feeding have puzzled experts, as each new piece of research seems to contradict earlier observations. The latest study of 1,037 children born in Dunedin, New Zealand, found that children who were breast-fed for an average of 21 weeks were more than twice as likely to suffer from asthma at the age of nine than children who were bottle-fed.

Dr Malcolm Sears, the lead investigator of the Dunedin study and now professor at McMaster University, in Hamilton, Ontario, told The Lancet: "We are absolutely not telling people not to breast-feed. There are many good reasons for breast-feeding – we have just taken one of these away."

Professor Sears believes breast-feeding may increase the risk of asthma because it reduces the risk of infections. This explanation is based on the so-called "hygiene theory". Children who have older siblings, and children who attend nurseries from an early age, suffer less from asthma. According to the hygiene theory, they are more exposed to infections that stimulate their immune system and make them resistant to asthma.

Two years before the Dunedin study was published in The Lancet, a similar study of 2,602 children born in Perth, Western Australia, was published in the British Medical Journal, but the researchers came to the opposite conclusion. The BMJ summarised the findings in a "key message". It said: "Exclusive breast-feeding for at least four months is associated with a significant reduction in the risk of asthma ... at age six years."

So what are we to believe? Is breast best? There are at least a dozen other studies from all over the world that have tried to answer the question. Five have found that breast-feeding is associated with an increased risk of asthma or wheezing, and six have found that breast-feeding protects against asthma or wheezing. During the first two years of life, breast-feeding protects against infections that may trigger wheezing or asthma, and so breast-feeding protects against asthma in the first two years of life. Even so, we are left with a head-to-head contradiction between different studies that assessed asthma at a later age.

Mothers who have asthma in the family, and so have an increased risk that their children will develop the disease, are faced with a difficult choice. Perhaps their babies will do better if they are bottle-fed.

Places where breast-fed babies have been found to suffer least from asthma have a lot of sunshine. Perth in Western Australia, at 32 degrees latitude, is renowned for its sunny climate; Kenya and Brazil, where breast was also found to be best, are in the tropics. But babies born and breast-fed in England or Dunedin, in the extreme south of the south island of New Zealand, are more vulnerable to asthma. The mothers in England and New Zealand may have received very little sunlight during the final months of pregnancy and so have been deficient in vitamin D.

Britain is not only far north geographically, but it also has a cloudy maritime climate that provides among the fewest hours of summer sun of any industrial country in the world. For much of the British summer, cloud obscures the sun, absorbing the ultraviolet light necessary for the formation of vitamin D in the skin. And during six months of the year – October to March – the sun is not strong enough to make any vitamin D at all.

In Sweden, asthma has increased more in the north of the country, where the summer season is shortest, than in the south, according to a study carried out at the University of Gothenburg. Swedish children who were born during the autumn and winter, when vitamin D reserves in the body are at their lowest, were found to be most vulnerable to asthma. On the other hand, in the highlands of New Guinea and in several parts of rural Africa, asthma has been found to be rare. In urban South Africa and in Harare, the capital of Zimbabwe, a significant percentage of children (3 to 6 per cent) have signs of asthma, whereas in nearby rural areas it is extremely uncommon.

In Europe, children who live on farms, where they regularly play outdoors, suffer less from asthma than do city children. In the United States, the prevalence of asthma varies greatly from area to area but has been found to be greatest in poor inner-city areas. Air pollution has been blamed for the concentration of asthma in cities because it is known to trigger asthma attacks, but air pollution also filters out ultraviolet light, often preventing city dwellers from producing as much vitamin D as they need. People in cities also have less exposure to sunlight because opportunities for outdoor living are limited.

In the US, asthma has increased much faster among black people than it has among whites. A person with a black skin has to spend up to six times as long in the sun to produce the same amount of vitamin D as a white-skinned person. Asthma is more frequent among the poor, but this does not account for the dramatic increase in asthma among black people, according to a study undertaken at Rutgers University in New Jersey. Among higher income families, black children are almost twice as likely to suffer from asthma as are white children, and among the lowest income groups, 12.2 per cent of black children suffer from asthma compared with 9.6 per cent of white children.

It is only recently that scientists have begun to understand that vitamin D plays an important part in controlling the immune system. Vitamin D alters the production of substances called cytokines, which are made by certain white blood cells. Cytokines are responsible for the inflammation of tissue that occurs in allergies and asthma. However, this knowledge of the importance of vitamin D for immunity does not yet seem to have left the laboratory and impinged on medical practice.

Vitamin C – or fruits that are rich in vitamin C – has a beneficial effect on lung function. Vitamin E (obtained from seeds and vegetable oils) and beta-carotene (obtained from red and orange vegetables such as carrots and maize) also seem to benefit lung function. The importance of vitamin D has not yet been proved in direct trials. However, it is known that if mothers are deficient in vitamin D, as many are, then babies too will be deficient when they are breast-fed. Bottle feeds contain vitamin D as a normal part of the formula, and once infants are weaned they will obtain some vitamin D from margarine, butter and supplemented cereals.

The UK Committee on Medical Aspects of Food Policy (Coma) recommends that lactating women should take 10 micrograms per day of additional vitamin D – the equivalent to two teaspoons of cod-liver oil a day. This is in order to provide for good growth of the baby's bones – but it may also be useful in preventing asthma. In summer, mothers can get plenty of vitamin D if they are able to sunbathe for 20 minutes a day in full sun (10 minutes each side), but they should take care not to burn.

In the 1940s, free or subsidised vitamins, including vitamin D, were provided along with milk and concentrated orange juice to all pregnant and lactating women under the Welfare Foods Scheme. However, as families have become more affluent this scheme has become progressively restricted, so by 1971 only women whose families were on supplementary benefit or family income supplement benefited from free Welfare Foods. Could this be one of the reasons why asthma in children and young people in the UK has increased by around 50 per cent over the past 30 years?

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