Monday September 8 2008
Activity levels and diet are known factors in obesity
“Pollution is making children fat, say experts”, reported The Independent on Sunday. It said that a new Spanish study found that exposure to a range of common chemicals while in the womb sets up a child to ‘grow up stout’. The newspaper suggests that this may be helping to drive the worldwide obesity epidemic. The study measured levels of hexachlorobenzene (HCB), a pesticide, in the umbilical cords of children and found that those with the highest levels were more than twice as likely to be obese six years later.
The study has some limitations and the researchers mention some of these, including that the children’s diet and activity levels were not considered; two important known determinants of BMI. To have a clearer idea of the effect of environmental pollutants, more research that considers these factors is needed. Obesity is a complex disorder with a number of contributing factors. Pollutants are unlikely to be the sole cause of the obesity epidemic – in fact, this study found no statistically significant link between HCB and obesity (only the link with ‘overweight’ was significant).
Where did the story come from?
Dr Agnes Smink and colleagues from the Centre for Research in Environmental Epidemiology at the Institut Municipal Investigacio Medica in Barcelona and other academic institutions across Spain, carried out the research. The study was funded by the Spanish Ministry of Health, Instituto de Salud Carlos III, Fundacio La Caixa and the European Commission. The study was published in the (peer-reviewed) medical journal: Acta Paediatrica.
What kind of scientific study was this?
In this longitudinal study, the researchers aimed to investigate the effect of organochlorine compounds (OCs), chemicals used in agriculture and industry, on children in the womb. These chemicals are resistant to degrading naturally and can build up in the food chain. They were particularly interested in hexachlorobenzene (HCB), used as a pesticide to protect seeds against fungus. Previous research has found an association between OCs and body size, and they wanted to look at exposure to HCB in the womb and its effects on the child’s weight and BMI in later life.
To do this, the researchers looked at the concentration of HCB in the umbilical cord blood of 405 children born in Menorca from mid 1997 to mid 1998. Originally, 482 children had been recruited, but umbilical cord blood was only available from 405 of them. The researchers kept in contact with the remaining 77 for comparison. Preterm births were excluded. When mothers were recruited, questionnaires were used to collect data from mothers on their age, education, socio-economic status, number of children, whether or not they smoked during the first trimester, how much alcohol they drank, their pre-pregnancy weight and their diet during pregnancy. Infant height and weight was measured at birth and feeding practices were reported by mothers in interviews at six months and one year after birth. The children’s height and weight were measured when they were 6.5 years old.
The researchers then assessed the link between concentrations of HCB in cord blood and weight and BMI at age 6.5 years. There were three different statistical models that took into account different factors: one that took into account child age and sex, a second that took into account sex, maternal age, height, pre-pregnancy overweight or obesity, education and number of children and a third that also took into account the weight of the child at birth.
What were the results of the study?
The researchers report that they detected OCs in all 405 cord blood samples. They found no differences in BMI at 6.5 years between the 405 children who had had their umbilical cord blood measured for OCs (BMI of 16.6) and the 77 who had not (BMI of 16.8).
Children with higher HCB levels in cord blood were heavier and had a higher BMI than children with low HCB levels. When they adjusted for various factors, they found a statistically significant increase in BMI ‘related to prenatal exposure to hexachlorobenzene’. Children in the highest exposure group of HCB were 2.5 times more likely to be overweight. The researchers also report that they were three times more likely to be obese, but this was not statistically significant.
What interpretations did the researchers draw from these results?
The researchers report that prenatal exposure to HCB increased the risk of being overweight at age 6.5 years. This link is independent of socio-economic status, maternal education, and number of children, maternal obesity and weight at birth. This effect is stronger in children whose mothers smoked. Overall, they conclude that additional studies are needed to “assess directly whether HCB at current concentrations of exposure increases health risks on children such as obesity’”.
What does the NHS Knowledge Service make of this study?
This study suggests that prenatal exposure to HCB is linked with height and weight at age 6.5 years. There are several points to highlight however:
- The researchers note that they interviewed mothers at six months and one year after birth when they asked about ‘feeding practices’. Importantly though, there was no assessment of diet during the children’s toddler years, and diet is probably one of the most important environmental factors affecting weight. The researchers add that ‘some now-known relevant variables were not included, like… improved diet variables’. The study also did not account for activity levels in childhood. Given the known contribution of diet and activity levels to being overweight and obese, studies that assess ‘causal’ links should consider these factors.
- The researchers also note that this was a small study cohort.
Due to these limitations, confidence in the results is limited. It would be best to see these results replicated in other studies that take into account diet and activity levels during childhood before coming to any conclusions. Such studies are helpful in adding pieces to the puzzle, but they must take into account other obvious factors to reliable quantify the link with a new exposure.
Sir Muir Gray adds...
Maybe, but much less important than the simple equation: when energy input exceeds energy output, the waistline grows.