“Bottle-fed babies are primed for a life of obesity,” reported the Daily Express. Babies who put on weight too fast in their first months are more likely to become fat, the newspaper added.
The story comes from two studies which looked at the effect of giving nutrient-enriched formula to babies who were born too small for their age. The studies found that, at 5–8 years old, children who were given the enriched formula had more body fat than those who were given normal formula. This suggests that faster weight gain as a baby causes children to gain a higher proportion of fat tissue (fat mass) when they’re older.
The results of these two studies seem to support previous research that suggests “overfeeding” in infancy – in this case by using nutrient-enriched formula – increases the risk of obesity later in life. These findings were independent of factors such as gender, height in childhood or socioeconomic status. However, the studies had some limitations. Both studies had a high drop-out rate, which could undermine the reliability of the results. Also, the studies did not look at children who had normal birth weight. Finally, it is not clear if the early feeding influenced the appetite and diet of the children as they grew or if it independently influenced fat mass.
The study did not measure obesity, as defined by Body Mass Index (BMI). Instead, it looked at the children’s fat mass. As the children were not followed up into adolescence and adulthood, it is incorrect to say that these children were “primed for a life of obesity”.
Where did the story come from?
The study was carried out by researchers from University College London, University Hospital Nottingham, Leicester General Hospital, Royal Hospital for Sick Children in Glasgow, the Wishaw General Hospital, the Southern General Hospital in Glasgow and the Danone Research centre for Specialized Nutrition in the Netherlands. It was funded by the Medical Research Council (UK) and other organisations, with contributions from Farley’s Health Products and Nutricia Ltd.
The study was published in the peer-reviewed American Journal of Clinical Nutrition.
The Daily Express’s claim that bottle milk makes children obese and that “breast is still best if you want your child to be slim” is incorrect. The study compared children who were fed enriched or normal formula, and the former group was found to have more fat tissue later. Likewise, the Daily Mail’s headline that “Baby formula milk could make your child obese” and The Guardian’s “Bottle-feeding babies can lead to adult obesity, says study” were also misleading.
What kind of research was this?
This research comprised two randomised controlled trials. They looked at the body composition of children who were given extra nutrition to encourage growth because they were born small for their gestational age. The authors point out that previous observational studies have suggested that “overnutrition” and rapid growth in infancy may increase the risk of obesity later, but that the results of these studies could have been affected by both genetic and lifestyle factors. Randomised controlled trials are the best type of study to look at the effects of certain interventions. By selecting subjects at random and having a control group, they eliminate bias.
What did the research involve?
The researchers recruited newborn babies as soon as possible after birth from 10 UK hospitals to take part in the two studies. Study 1 recruited babies between 1993 and 1995, and study 2 between 2003 and 2005. All the babies were born at full term (after 37 weeks) but were small for gestational age (SGA). The babies in study 1 were below the 10th percentile for their gestational age and those in study 2 were below the 20th percentile, according to UK growth charts.
Babies of the mothers who had already decided to bottle feed were randomly assigned to receive either standard formula (the control group) or a nutrient-enriched formula (the intervention), which had a higher protein and energy content designed to promote rapid growth. The formulas were given until the babies were nine months old in study 1 and until they were six months old in study 2. A total of 545 babies were originally enrolled in the two studies, and in study 1 a reference group of 175 breastfed babies was also recruited.
Researchers followed up babies between 1999 and 2002 in study 1 and between 2008 and 2009 in study 2. In study 1, the children’s body composition was measured by a nurse at home, using “bioelectric impedance analysis”, a standard technique to measure the proportion of fat and lean body mass. In study 2, a method called “deuterium dilution”, which measures total body water, was used to calculate fat-free mass. In both studies, researchers estimated fat mass using callipers to measure skin fold thickness.
They used standard statistical techniques to analyse the effects of early feeding on body fat later.
What were the basic results?
The researchers followed up 243 of the original 545 infants enrolled in the study. In both studies, fat mass in those who had been given normal formula was lower than in those given the enriched formula (after adjustment for sex) at 5–8 years of age.
- In study 1, children who had been on normal formula had 38% less fat mass than those in the enriched formula group (95% confidence interval [CI] -67% to -10%).
- In study 2, children who had been on normal formula had 18% less fat mass than those in the enriched formula group (95% [CI] -18% to -0.3%).
In a separate non-randomised analysis, the researchers also found that babies who had grown faster were more likely to have a higher proportion of fat mass in childhood. This suggests that the rate of growth is the important factor in determining later fat mass.
A further analysis suggested that in the group of breastfed babies, faster weight gain in infancy was also associated with greater fat mass later.
How did the researchers interpret the results?
The researchers say their results suggest that there is a causal link between overfeeding and faster infant growth and a higher risk of obesity later. This link is independent of genetic or lifestyle factors. These results have implications, they suggest, for the prevention of obesity, which should begin in infancy.
These two well-conducted studies show that small for gestational age (SGA) babies who were fed enriched formula to promote rapid growth had higher proportions of body fat in later childhood. However, as the authors note, a causal link has not been established. It is possible that genetic factors influenced the babies’ appetites and, therefore, “overfeeding” and later obesity. It is interesting to note that among breastfed babies, those who grew more rapidly also had higher fat mass later.
As the authors note, the study had several limitations:
- Most importantly, the study had a poor follow-up rate. In study 1, 51.2% of the children were followed up, and in study 2 only 36.6% were followed up. Although these studies had large sample sizes initially, in any randomised controlled trial a completion rate of above 80% would be expected in order to increase the reliability of the results.
- The study involved SGA babies. It is not clear if the findings would apply to babies of normal birth weight.
- It is possible that there were inaccuracies in the techniques used to measure body fat, which is not a measure of obesity.
- It is also possible that the children’s diets after bottle feeding influenced the later fat measurement.