295 - "Rapid Infant Weight Gain" Does Not Accurately Reflect Infant Accelerated Weight Velocity

Publication Number: 295.3626

Jessica G. Woo, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States; Tim J. Cole, University College London, London, England, United Kingdom; Charles Wood, Duke University School of Medicine, Durham, NC, United States

Background: "Rapid infant weight gain" is a widely used risk factor for childhood obesity, defined as an increase over time in attained weight-for-age (WFA) z-score exceeding 0.67 units. WFA z-score assesses an infant’s current weight (g) relative to the cross-sectional distribution of infant weight at a specific age. By contrast, weight velocity (WtVel) z-score depicts an infant’s observed weight gain (g/month) relative to expected weight gain during a specific time interval. The relationship between “rapid infant weight gain” and WHO WtVel z-scores across different ages and time intervals has not been explored.

Objective: To evaluate how specific examples of rapid infant weight gain relate to WHO weight velocity z-scores and the likelihood of having “accelerated weight velocity”, defined as a WtVel z-score>2.0.

Design/Methods: We used both the WHO WFA and the WHO WtVel standards to express a 0.67 increase in WFA z-score as a WtVel z-score conditional on four factors: a) sex; b) starting age (1, 3, 6, 12 or 18 months); c) starting WFA z-score (-1, 0 or +1), and d) time interval between measurements (1, 2, 3 or 6 months, excluding 1-month intervals beyond 12 months of age). For example, a girl with a 0.67 increase in WFA z-score from -1.0 at 1 month increasing to -0.33 at 2 months reflects a starting age of 1 month, a starting WFA z-score -1.0, and a 1-month interval. For each scenario the corresponding weight increment in grams was converted to a WtVel z-score using the WHO weight velocity calculator.

Results: Across all 118 examples, a WFA z-score increment of 0.67 translated to a WtVel z-score ranging from +0.3 to +3.9. Using an arbitrary cutoff of WtVel z-score >2.0, only 37% of rapid infant weight gain scenarios resulted in “accelerated weight velocity” (Figure 1). At older starting ages, higher starting z-scores and shorter intervals, the 0.67 z-score increment corresponded to significantly higher WtVel z-score than at younger ages, lower starting z-scores and longer intervals (Figure 2).

Conclusion(s): A 0.67 increment in WFA z-score, dubbed “rapid infant weight gain” corresponds to a wide range of different weight velocity z-scores, depending on the sex, starting age, starting z-score and time interval between measurements. Increases in WFA z-score of 0.67 over a shorter time interval, especially at older ages, imply greater accelerated weight velocity than for shorter intervals or younger ages. Results for “rapid infant weight gain” are uninterpretable unless the corresponding starting age and time interval are pre-specified.

Distribution of WtVel z-scores corresponding to +0.67 Change in WFA z-scores


WHO Weight Velocity z-scores by Sex, Interval, Starting WFA z-score and Starting Age, Corresponding to +0.67 Changes in WFA z-scores
Least square means +/- SE from adjusted models with all parameters included.

Distribution of WtVel z-scores corresponding to +0.67 Change in WFA z-scores


WHO Weight Velocity z-scores by Sex, Interval, Starting WFA z-score and Starting Age, Corresponding to +0.67 Changes in WFA z-scores
Least square means +/- SE from adjusted models with all parameters included.