Many new mothers come to dread the arcane growth charts produced by baby clinics to assess whether their new arrival is being under or overfed.



Now research is beginning to confirm what many mothers have long suspected - that the most commonly used growth charts, based on babies fed high-protein formula milk, wrongly classify lean but healthy babies as underweight. What's more, by encouraging mothers to overfeed their babies, the charts may be setting perfectly healthy children on the path to obesity.



The most popular growth chart, produced by the US National Center for Health Statistics (NCHS), has been used for nearly 30 years to provide a reference against which to judge the growth of a new baby. Introduced in 1977, when rampant obesity had yet to manifest itself, its main aim was to make sure babies didn't suffer from malnutrition. What is now being increasingly recognised is that these charts were based on babies that were atypically heavy: almost all of them had been bottle-fed and came from white, middle-class families in Ohio.



The charts were revised by the US Centers for Disease Control and Prevention (CDC) in 2000 to include more breastfed infants, but this has little bearing on the preceding 23 years. "They have skewed infant nutrition towards overfeeding for decades," says Bert Koletzko of the University of Munich in Germany, who heads a major European programme known as Earnest to investigate the effects of infant nutrition on obesity in adult life. "You could say we've had avoidable obesity as a result."



Last week, Koletzko presented preliminary results from Earnest showing that babies fed high-protein formula milk put on weight far faster and more extensively than those fed breast or low-protein formula milk. Previous studies reached the same conclusion, but these relied on simply watching how a group of babies turned out, without being able to isolate the effect from other factors, such as wealth or the smoking habits of parents.



Koletzko's study is the first to demonstrate a link after being able to dictate what the babies ate and control for these other factors. In all, 1000 infants in five European countries randomly received breast milk, low-protein formula milk or high-protein formula milk and were monitored until the age of 2.



At any given age, babies in the high-protein group weighed around twice as much above the norm as the highest-scoring babies in other groups. "Children in the high-protein group were significantly heavier," says Koletzko, who presented the results at a conference on the impact of infant and maternal nutrition on childhood obesity in Budapest, Hungary, on 20 April. He is now following the babies to school age to see whether those who were heaviest during the first two years of life are at greater risk of obesity later on, as earlier studies have suggested.



As to why high-protein formula milk should make babies heavier, Koletzko believes that the extra amino acids could drive production of the hormones insulin and insulin-like growth factor 1 (IGF-1). Insulin encourages fat storage in infants, while IGF-1 promotes body growth generally. Koletzko found that levels of IGF-1 were higher in the high-protein group, as were levels of C-peptide, a metabolite of insulin.



A adviser to infant food manufacturers says that protein levels in baby milk have more than halved over the past 25 years, and that companies would be happy to supply brands with lower protein content if this was proven to cut obesity.



Though Koletzko's results have yet to be published in full, there is growing acceptance that the NCHS charts are out of date. Last month, representatives from 31 European countries met in Brindisi, Italy, to discuss whether to adopt new charts. These are based on a study coordinated by the WHO of 8500 children in six diverse countries, all of whom were breastfed and reared in optimal circumstances, free from poverty, illness and malnutrition (see Graphs).



The concern that infants receiving formula may be overfed was a critical factor behind the WHO study, says Laurence Grummer-Strawn of the CDC, who is coordinating efforts to assess the new charts in the US. He declined to comment on Koletzko's results, but says that "rapid growth in infancy has been shown to be associated with increased obesity".



Other researchers in Budapest were keen to see the new charts brought in. "Everyone is pleased they're being changed," says Lucilla Poston of King's College London, who has been studying how a mouse's diet during pregnancy affects the prospects of its offspring becoming obese in adulthood (see "Eating for two", below).



"The reaction I get from breastfeeding mothers when I explain the new chart is one of relief, because they describe feeling heavy pressure from health visitors using the old charts to feed up babies they themselves think are perfectly healthy," says Peter Aggett of the University of Central Lancashire, a member of a UK panel which evaluated the charts. "It means we have a much better standard to operate to in the first, crucial two years of life."



"Breastfeeding mothers describe feeling heavy pressure to feed up babies they themselves think are perfectly healthy"As for Koletzko, he believes his results constitute new and powerful grounds for accelerating replacement of the old NCHS charts. He points out that infancy is probably the only time in life when an individual's entire diet can be carefully controlled. "Infants only get one form of nutrition," he says. "You can't do that in 10-year-olds."

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