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Fat Free Mass Index and Anthropometric Indexes for Newborns


Researchers wanted to explore the link between three anthropometric indexes taken at birth and their relationship with fat mass and predicting obesity. The evidence suggests that at the time of birth the three indicators are linked to the fat free mass index, however, by five months of age, weight-for-length, and body mass index are strongly linked to fat mass.


At the time of birth, newborns undergo various measurements and examinations to ensure that they are in good health. Part of the examination includes measuring the length and weight of newborns, as these measurements are used to derive indexes such as weight-for-age, weight-for-length, as well as body mass index. These three indicators, also known as anthropometric indexes, are thought to give insight into body composition, specifically fat mass, which even early on in life is thought to predict the likelihood of obesity and related diseases, such as cardiovascular disease. Fat mass describes the amount of adipose tissue (fat) relative to the total mass of the body.  Fat free mass refers to the amount of water, bone, and muscle relative to the total mass of the body.

Despite the prominent use of these anthropometric indexes, however, it is still unclear whether these measurements are even related to fat mass. Due to the lack of research, it is possible that these indicators are actually related to the fat free mass index, thereby impacting their usefulness as tools for predicting obesity. Therefore, the purpose of this study was to determine whether weight-for-age (WFAZ), weight-for-length (WFLZ), and body mass index (BMIZ) are linked to fat mass as assumed and whether they are useful tools in determining future body composition.

This study, which was published in the American Journal for Clinical Nutrition, observed 1027 infants born in Colorado between the years 2009 and 2014. Information on length and weight was collected at two times during this study, the first of which took place the day after birth and the second of which took place at five months after birth. These values were used to determine the anthropometric indexes of WFAZ, WFLZ, and BMIZ. In addition, during these two visits, measurements were taken to estimate fat mass and fat-free mass in kilograms.

The findings indicate that at the time of birth, all three anthropometric indexes are strongly associated with fat free mass index rather than fat mass. In contrast, at the five-month mark, the data indicates that both WFLZ and BMIZ are strongly correlated with fat mass, rather than fat-free mass. Consequently, researchers determined that both WFLZ and BMIZ were more useful in determining body composition than WFAZ. Additionally, evidence from the study indicates that body mass index is the most useful tool to determine fat gain in infants during the first five months of life, as changes in body mass index were linked to changes in the percentage of fat mass.

Therefore, these findings suggest that contrary to popular belief measuring for the anthropometric indexes of weight-for-age, weight-for-length, and body mass index at the time of birth is not useful, as they are more closely related to the fat free mass index rather than fat mass. However, as the child ages, WFLZ and BMIZ become useful in determining adiposity, as these become strongly associated with fat mass as demonstrated by the results obtained at the five-month visit.


Written by Sonia Parmar, BSc


Perng W, Ringham BM, Glueck DH, Sauder KA, Starling AP, Belfort MB, Dabelea D. 2017. An observational cohort study of weight-and length-derived anthropometric indicators with body composition at birth and 5 months: the Healthy Start study. Am J Clin Nutr. doi: 10.3945/acjn.116.149617.

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