Early weight loss consistently predicts longer-term outcomes. However, longer-term outcomes are often assessed from baseline. Since this approach inherently includes initial weight loss within the longer-term change variable, it is unclear if early improvement predicts later improvement. This study illustrated this distinction by examining both if early standardized BMI (zBMI) change predicted later zBMI change and if early zBMI change predicted overall change from baseline.
Secondary analysis was conducted using a database of Mexican American middle school students with a BMI Percentile ≥85 who had been part of the treatment group of a six-month school-based obesity intervention (n=222). Height and weight were measured at baseline, three-, six-, and 12 months, and were used to calculate zBMI at each time point. Controlling for baseline BMI percentile, four separate linear regression models were developed to compare how early zBMI change (first three months) predicted later zBMI change (from months three to six, and months three to 12) and how early zBMI change (first 3 months) predicted change from baseline at six and 12 months.
At baseline, participants (47% female) were 12.06 ± 0.61 years old and had an average BMI Percentile of 94.97 ± 4.46. Early change in zBMI (from baseline to three months) did not predict change in zBMI from three to six months or from three months to a year (β: -0.03, p=0.70 and β: -0.02, p=0.77). However, early zBMI change significantly predicted change in zBMI from baseline to six months and from baseline to one year (β: 0.73, p<0.001 and β: 0.48, p<0.001).
The findings of this study indicate that response in the first three months of intervention is not indicative of later response. Further investigation to understand how the timing of weight loss during an intervention contributes to longer term outcomes is warranted.