Understanding the association between mother's education level and effectiveness of a child obesity prevention intervention: a secondary analysis of an RCT

Objectives: to explain differences in effectiveness of paediatrician-led motivational interviewing (MI) in decreasing body mass index (BMI) between children of mothers with low or high education level.

Design: secondary analysis of a randomised control trial.

Setting and participants: individually randomized controlled trial previously conducted from 2011 to 2013 in the province of Reggio Emilia (Emilia-Romagna Region, Northern Italy). Eligible participants included in the trial were 372 (187 in the MI group and 185 in the control group) overweight children (BMI percentile >= 85th and < 95th) aged between 4 and 7 years, residing in the province of Reggio Emilia and under the care of paediatrician for >= 12 months. The intervention included 5 MI sessions based on the transtheoretical model of addiction and behavioural change delivered at 1, 4, 7, and 12 months after the baseline visit, when families had to define specific goals in changing physical activity (PA) and diet behaviours.

Main outcome measures: primary: BMI score variation (ΔBMI) from baseline to 12 months; secondary: percentage of changes in parent-reported PA and dietary behaviours.

Results: a significant effect of MI on ΔBMI in children whose mothers had high education level (ΔBMI = -0.62; 95%CI -0.92;-0.32) were observed. Children of women with high education level in MI group had more improvements in set unstructured PA, decreasing screen time and sweet snacks consumption, while children with less educated mothers had improvements in consuming more vegetable soup and less desserts, sweet snacks, and sugary beverages. Highly educated mothers chose for their children to drink fewer sugary beverages and to increase PA. Less educated mothers most frequently chose as goals having breakfast, eating more fruit and vegetables, eating fewer snacks, and having less screen time. Overall achievement was similar in the two strata for diet goals, but higher for PA goals in the high education level stratum.

Conclusions: MI intervention was not effective in reducing BMI in children of mothers with low education level. This does appear to be weakly or not associated with goal choices and achievement within MI, it is rather an effect of unmeasured behaviours which possibly mediate association between MI and BMI reduction.