Tuesday November 8 2011
Obesity at all ages is a major public health problem
“Bigger babies are more likely to become obese,” exclaims the Daily Mail, also reporting that parents should not assume their overweight children “are going to grow out of it”.
This study measured the weight and length of over 44,000 infants at six-monthly intervals between ages one and 24 months. Children who moved up more than two weight categories in early life were more likely to be obese at age five and 10. These children were more than twice as likely to be obese by age five compared to those who experienced less change in weight categories. They were also 75% more likely to be obese at 10 years old.
The study also found that babies who started off in the higher weight categories were more likely to be obese in later childhood than those who started off smaller. However, the biggest babies – bigger than 90% of children their age – were excluded from the study, and so the effect on this group is not known.
This study highlights the possibility that excess increases in an infant’s weight could be associated with obesity in later childhood. Whether this could be associated with overweight and obesity into adulthood, or related health problems, cannot be assumed from this study.
The majority of infants are unlikely to experience the level of weight change observed in this study. Parents should continue to follow the advice of the health practitioner who monitors their baby’s growth.
Where did the story come from?
The study was carried out by researchers from Harvard School of Public Health in the US and was funded by the Centers for Disease Control and Prevention. The study was published in the medical journal Archives of Pediatrics & Adolescent Medicine.
The Daily Mail states that parents should not assume their overweight children “are going to grow out of it”, and reports that Dr Taveras, the study leader, “hopes the findings will put an end to the idea that large gains in adiposity are normal for babies”. Both these statements are broadly justified by this study.
What kind of research was this?
This was a cohort cohort study that followed children aged one month through to 10 years measuring their length and weight at six-monthly intervals up to the age of 24 months, and then looking at how this related to levels of obesity at age five and 10 years.
Obesity is a major public health problem due to the large number of diseases linked to the condition. Some previous studies have found that weight gain in early life can predict later obesity, but researchers say these studies did not use accurate ways of assessing weight gain in infancy. This study used an established way of comparing a baby’s individual weight and length measurements with the average range of values from other babies of the same age and gender. This method uses “growth chart percentiles”, shown as curved lines on a graph. Plotting a baby’s weight and length on this chart shows what proportion (or percentile) of the range they fall into. A girl in the 95th percentile, for example, weighs more than 95% of girls her age, but less than 5%.
The aim of this study was to examine the link between moving upwards across weight-for-length percentiles in the first 24 months of life, and the prevalence of obesity at five and 10 years.
What did the research involve?
The study analysed length and weight measurements from 44,622 US children, taken at six-monthly intervals between the ages of one and 24 months. The child’s length and weight were plotted on the growth chart and the researchers could see which of the standard percentile groups (5th, 10th, 25th, 50th, 75th, 90th and 95th) the child was in (i.e. how they compared to other children of the same age and gender). Plotting on the chart at each check-up point shows whether the child is staying in the same percentile group or whether they are crossing into other percentile groups. The researchers investigated whether crossing two or more of these percentile boundaries was linked to the prevalence of obesity at five and 10 years.
Only infants with at least two measurements between one and 24 months were included. A total of 122,214 measurements were used in the main analysis of this study. These measurements were plotted against standard charts to assess the child’s weight-for-length percentile and whether this increased or decreased in the first 24 months of life.
At five and 10 years the children were re-measured. Children were classed as obese if they had a body mass index (a combined measure of height and weight) greater or equal to the 95th percentile for their age group and gender, i.e. they were heavier than 95% of other children their age and gender.
The analysis compared those who increased by two or more percentiles compared with those who crossed less than two percentile groups. The researchers excluded infants higher than the 90th percentile group as these children could not change their percentile upwards by two groups. Some of the analysis took into account the potential effect of ethnicity, which is known to be associated with differences in obesity prevalence.
What were the basic results?
Obesity prevalence was 11.6% when the children were five years old, and 16.1% at age 10 years. In the first six months of life, 43% of infants crossed upwards two or more percentile groups; there was less change between six and 24 months.
Infants with a higher weight-for-length percentile at any time between one and 24 months were more likely to be obese at five or 10 years than those starting at a lower percentile. In other words, those that were larger compared to their peers when infants, were more likely to be obese when older.
Crossing upwards two or more percentiles in the first six months of life was associated with a higher risk of obesity at ages five and 10 years compared to those that crossed less than two groups. For example, in those who started in the 75th to 90th percentile group, but had increased by two or more percentile boundaries, obesity prevalence was 32.9%. This compared to 19.7% in those who had seen no percentile change, an absolute difference of 13.2%.
When looking at the first 24 months, the researchers found that youngsters who had increased by two or more percentile groups had more than twice the odds of being obese at five years of age (OR 2.08, 95% CI 1.84 to 2.34) compared to those who had crossed less than two groups. They were also 75% more likely to be obese at 10 years (OR 1.75, 95%CI 1.53 to 2.00). From the results reported it is not possible to calculate the absolute difference in the prevalence of obesity in the two groups.
How did the researchers interpret the results?
The study authors concluded, “crossing two or more weight-for-length percentiles in the first 24 months of life is associated with later obesity”. Crossing two percentiles during the first six months is associated with the highest risk of obesity at five and 10 years. They report that “efforts to curb excess weight gain in infancy may be useful in preventing later obesity”.
They suggest the crossing of percentiles should “trigger a discussion between parents and their paediatric providers of what's contributing to the rapid gains”.
This study, using a large amount of data collected over many decades, highlights an important association between weight gain in the first 24 months and the subsequent risk of obesity at five and 10 years. It also confirms that children who are larger than their peers in infancy are more likely to continue to be overweight or obese into later childhood.
A strength of this study is that it used standard growth charts and gender-specific percentiles to measure change in body weight relative to height. These growth charts are already used in standard medical practice to compare a child’s measurements with others of their same age and gender to identify potential growth and weight problems.
A limitation of the study is that it did not adjust for other factors that can influence weight, such as family socioeconomic status. This may have introduced error into the results. Further studies that take into account influential factors such as this would be valuable to confirm these findings.
While this study shows us there is a potential association between excess weight change and risk of obesity in later childhood, the way it was designed means that it cannot tell us what caused the weight change. However, the causes of obesity are well established and are typically a combination of dietary, exercise and genetic factors. Having said that, whether obesity in early life is associated with overweight and obesity in adulthood – or its related health problems – is not something that can be assumed from this study.
This study raises the possibility that the standard system used to monitor an infant’s change in weight and length could be used to identify those who may be at risk of continued weight problems as they grow older. It may also provide an opportunity for discussion about why the child’s weight has changed so much. Parents should continue to follow the advice of the health practitioner who monitors their baby’s growth.