A model for fat free mass in humans from very premature neonates to young adults


Objectives: To describe the maturation of fat free mass (FFM) from prematurity to adulthood as a function of postmenstrual age (PMA) using nonlinear mixed effect modeling.

Background:  A model for FFM for adults has been well described, with different formulae for males and females (1).  An extension to this model using data from age 3 years to 30 years has been presented by Al-Sallami (2).  However there is no model linking neonates, infants to adult FFM. For neonates, infants and children there is a paucity of published individual observations of FFM, but many papers publishing mean data, using a variety of measurement methods.

Method: 1,952 FFM and post menstrual age (PMA) observations obtained from 5,298 subjects were used to develop a model for fraction of adult FFM predicted from (1) using sex, weight and height.  Post-menstrual age (PMA) ranged from 24 weeks to 30 years.  49% of observations were from males.

Data was sourced from a pooled set comprising: 1) Published individual data from 2 papers, with 54 individual observations from PMA 24 weeks – 5 years; 2) Published mean data from 13 papers, with 119 means reported  from PMA 24 weeks – 10 years; 3) 1,779 individual observations from PMA 3 years  to 30 years from the Al-Sallami et al.

All observations were weighted by the 1/sqrt(N) where N is the number of measurements contributing to each observation.

Results:  Girls even from very premature ages have a FFM similar to that predicted from adults based on height, weight and sex. Boys on the other hand have lower than predicted FFM until around the onset of puberty and approach adult predictions in early adulthood.

Conclusions: The model we have developed may be used to predict FFM from very premature to young adult ages. This will allow a better understanding of how to use FFM to account for body composition differences and better predict parameters such as clearance and volume of distribution in humans.


1.              Janmahasatian S, Duffull SB, Ash S, et al. Quantification of lean bodyweight. Clin Pharmacokinet 2005; 44: 1051-1065.

 2.           Al-Sallami H, Goulding A, Taylor RW, Grant AM, Williams SM, Duffull SB. A semi-mechanistic model for estimating fat free mass in children [www.page-meeting.org/?abstract=2063]. 20. 2011.