High ratio of resting energy expenditure to body mass in childhood and adolescence: a mechanistic model.

Background: Children have lower resting energy expenditure (REE) but higher ratio of resting energy expenditure to body mass (REE/BM) than do adults. This well-known observation has never been quantitatively explained.

Objectives: The aim of the present study is to understand the high REE/BM in childhood and adolescence.

Design: A mechanistic REE/BM model is proposed. Literature data on REE, BM and the masses of four high metabolic rate organs (i.e., liver, brain, heart and kidneys) of 1-18 y boys and girls were used to evaluate the proposed REE/BM model. Specifically, we tested the hypothesis that the magnitude and variation of the REE/BM can be predicted by a combination of four variables, including relative cellularity, growth energy expenditure for constructing new cells, fraction of body mass as individual organs/tissues, and their specific resting energy expenditure.

Results: The REE/BM provided by the literature is 54 kcal/kg per day at 1 year, decreasing to 26.0 kcal/kg per day at 18 years. Model-predicted REE/BM can account for 97.0% and 100.7% of the literature-reported REE/BM in males and females, respectively. The average differences between literature-reported and model-predicted REE/BM are 1.1 kcal/kg per day in boys and -0.3 kcal/kg per day in girls.

Conclusion: The high REE/BM ratio in childhood results from two model variables: the high fraction of body mass as liver, brain, heart, and kidneys, and their high specific resting metabolic rates. The proposed REE/BM model promotes understanding of the REE, and allows a rational establishment of energy requirements for children and adolescents.