Contribution Of Organ Mass On Resting Energy Expenditure In Male Athletes

Fat-free mass (FFM) is a major determinant of resting energy expenditure (REE) in athletes. However, FFM is metabolically heterogeneous, as each component tissue has a specific metabolic rate. Organs such as the brain, heart, liver, and kidneys, may contribute more to REE because of their relatively high specific metabolic rates. PURPOSE: To investigate the contribution of high-metabolic rate organs including the brain, heart, liver, and kidneys to REE in male athletes. METHODS: A total of 45 Japanese male athletes (age, 20 ± 1 years) were included. REE was determined by indirect calorimetry using the Douglas bag technique. Dual-energy X-ray absorptiometry and magnetic resonance imaging were used to evaluate body composition and measure the cross-sectional areas of the brain, heart, liver, and kidneys, respectively. The volume of each organ was calculated from the sum of all areas multiplied by the slice thickness. Volumes were converted into masses using the organ densities. The energy expenditure of each organ was calculated using organ-specific metabolic rates and expressed as a percentage of REE. Multiple regression analysis was used to investigate the contribution of organ tissue mass on the variance of REE. p < 0.05 was considered statistically significant. RESULTS: The mean body weight and FFM of the athletes were 73.5 ± 10.1 kg and 64.1 ± 6.9 kg, respectively. Although each organ was a small proportion of the body weight, the mean energy expenditure of the brain, heart, liver, and kidneys accounted for 17.7 ± 1.7%, 7.1 ± 1.0%, 16.5 ± 1.7%, and 8.5 ± 0.9% of the REE, respectively. The total energy expenditure of the four high-metabolic rate organs constituted approximately 50% of the REE. Furthermore, FFM singularly accounted for 57.5% of the variability in REE, with notable contributions from the brain and liver, collectively amounting to 56.1%. CONCLUSION: The mass of the liver and brain could influence REE. The findings can lead to a detailed understanding of individual differences in the REE of athletes.