The make-up and volume of gut bacteria in toddlers, when they are 3.5 years old, is predictive of body mass index (BMI) at age 5, irrespective of whether they are born prematurely or not, according to new research.

The findings, being presented at this year's European Congress on Obesity (ECO) in Dublin, Ireland, also identified differences in the bacteria that colonise the gut seen in adults living with obesity, suggesting that changes in the gut microbiota that predispose to adult obesity begin in early childhood.

"The gut microbiota is emerging as an important early-life factor able to influence weight gain in childhood and later life," said Gael Toubon from Inserm, France.

"Our findings reveal how an imbalance in distinct bacterial groups may play an important role in the development of obesity."

The makeup of the gut microbiota grows and changes in the first few months and years of life and disruption to its development is associated with conditions in later life including inflammatory bowel disease, Type 1 diabetes, and childhood obesity.

However, the associations between gut microbiota and both changes in BMI during childhood and paediatric overweight remain unclear, and information on infants born preterm is scarce.

To find out more, the study led by researchers from Inserm, Universite Paris Cite and Sorbonne Paris Nord in France investigated how the gut microbiota of children at 3.5 years was associated with their BMI at 5 years old and changes in their BMI between 2 and 5 years old.

Overall, 143 preterm infants (born less than 32 weeks of gestational age) were included and 369 full-term infants (born more than 33 weeks of gestational age). Stool samples were collected at 3.5 years.

Genetic microbiota profiling revealed a positive association between BMI z-score (a measure of body weight based on height for each age group by sex) at 5 years and the ratio of gut bacteria Firmicutes to Bacteroidetes that are directly involved with obesity.

The more Bacteroidetes (compared to Firmicutes), the leaner individuals tend to be.

"The reason these gut bacteria affect weight is that they regulate how much fat we absorb," Toubon said, adding "Children with a higher ratio of Firmicutes to Bacteroidetes will absorb more calories and be more likely to gain weight."

The analysis also found that greater abundances of three categories of bacteria -- Eubacterium hallii group, Fusicatenibacter, and Eubacterium ventriosum group -- were identified as a risk factor for a higher BMI z-score.

And greater number of three types of bacteria -- Eggerthella, Colidextribacter, and Ruminococcaceae CAG-352 -- were associated with a lower BMI z-score.