Now research on premature babies at the Institute of Child Health has demonstrated for the first time that good nutrition in the early years can significantly influence mental ability in later life. The researchers looked not at the long-running breast-versus-bottle controversy, but at premature babies who had been fed two different types of formula food. They found that those given enriched food were likely to develop more quickly than those who had been fed a regular formula food.
The hunt is now on to find what exactly is in breast milk and enriched formula food that can give a baby a better start in life, with benefits that can last well into adulthood. One of the latest studies, based on a New Zealand examination of 1,000 children, carried out over 18 years, showed that longer breast-feeding resulted in children with a higher intelligence quotient at eight years old. They were also more advanced in reading and maths at 13, and performed better in final exams at school.
But the problem with studies like these is that, however well designed, they are at risk of being confounded by the fact that it is the mother who chooses how she will feed her baby. In an ideal study, mothers would be chosen at random and either breast- or bottle-feed their babies, to iron out any socio-economic bias, but that approach has never proved to be a realistic proposition.
Over the past two or three decades, breast-feeding has been the choice practised by mothers from a higher socio-economic group than average, who tend to be better educated non-smokers, with higher incomes and living standards,and whose children have been born in a stable relationship.
"There isn't one randomised trial comparing breast-feeding and formula feeding, and in the absence of that there is always going to be some doubt about the causes of the differences in performance," says Professor Alan Lucas, who led the latest research team at the University of London's Institute of Child Health.
In a previous study, he and his colleagues got the nearest yet to a randomised study of breast milk benefits by taking babies whose mothers had chosen not to breast-feed, and allocating half of them to formula food and the remainder to human milk from a milk bank. Because it was not biased by the mothers' choice, that study has provided the most compelling evidence yet that human milk may promote neural development in pre-term babies.
In his latest research, published in the British Medical Journal, he and his team looked at 360 premature babies, and found that those who were fed standard formula milk, rather than a nutrient-enriched formula, had reduced verbal IQ scores when they reached the age of seven or eight years.
Verbal IQs below 85 were found to be twice as common among children who had been fed the standard formula; the difference was especially marked among boys.
"What we are saying is that cognitive function may be impaired by sub- optimal neonatal nutrition. Our study was with premature babies, but there is evidence that nutrition may be important for long-term cognitive function in all groups of babies," says Prof Lucas.
The problem is that no one is really sure which of the many ingredients of both formula food and breast milk is responsible for the improved mental ability. It may be one component or several, or the result of an interaction between different constituents.
The enriched food given to the better-performing babies in the study by Prof Lucas contained extra amounts of almost all of the 36 listed ingredients that both preparations had, including protein, carbohydrate, zinc and folic acid, as well as vitamins, sodium, copper and calcium.
"We don't know what is responsible, but I would suggest that protein is important in pre-term babies, because they are growing so rapidly.
"Whether it applies to full-term babies, we don't yet know," says Prof Lucas.
A powerful lobby believes that long-chain polyunsaturated fatty acids (LCPUFAs) should take the credit, and in particular docosahexaenoic acid, or DHA, which is known to be involved in the development of the brain. Research by a team at the University of Dundee found that infants fed on food enriched with LCPUFAs achieved better scores in problem-solving at the age of 10 months than babies reared on traditional formula food.
LCPUFAs are also found in breast milk, but work at the San Paolo Hospital in Italy, reported in The Lancet, found that the concentration of DHA dropped significantly in the breast milk of mothers who smoked.
Prof Lucas says that although there is increasing awareness that good early nutrition is beneficial for neural development, the jury is still out on whether or not LCPUFAs should be used to supplement formula food.
"In Canada, the authorities are beginning to promote breast-feeding on the grounds that it may well promote cognitive development, but both they and the American authorities support not adding long-chain lipids as a food supplement. The Canadian position is that the evidence for doing so is not there," he says.
"In Europe, it is a hotly debated issue, and some people feel very strongly that it should be added. It is therefore very difficult to give mothers advice at the moment, when the debate is still going on."
An added twist in the debate is that in his research, LCPUFAs were not added to either of the two formula foods used, yet a big difference in development was still observed.
Meanwhile, mothers are making up their own minds. For Anabel Hands, who lives in Ascot, there was never any doubt that she would breast-feed her children: "I was aware of the research about the benefits," she says, "and so all three of my children have been breast-fed. My seven-year-old is way ahead in her class, with a reading age of about 11. The speech of my second, at two-and-a-half, is stunning, while my third is a very alert baby.
"It's not only intelligence that is affected; they are healthier too. I consciously decided to stop early with my first child, and I think that was social pressure. With my second, I had more confidence and was very late introducing her to solids. The difference in health between my first and second has been noticeable."
In the trial milk which resulted in higher IQ, the enriched ingredients were protein, fat, maltodextrin, sodium, energy, potassium, chloride, calcium, phosphorus, copper, zinc, vitamins D, E, K, B1, B2, B6, B12, and C, niacin, iodine, folic acid, pantothenic acid, biotin, choline, and taurine
This has naturally occurring compounds which affected the immune system and health of the baby including:
Fatty Acids which destroy viruses like chicken pox;
Hormones that help the baby's digestive tract to mature much faster;
Lysozyme that kills off bacteria;
T-lymphocytes that kill infected cells and boost the immune system;
Interferon that kills viruses;
Macrophages which kill microbes in the gut;
Lactoferrin that ties up iron production so it cannot be used by bacteria to grow;
Feeding babies with breast milk is also associated with lower rates of bacterial meningitis, insulin dependent diabetes, colitis, botulism, ear infections, necrotising enterocolitis, diarrhoea and respiratory tract infections