Resting energy expenditure and fat mass

Inter-individual variance in resting energy expenditure (REE) is important in interpreting (i.e. normalizing) or even predicting metabolic rate. Fat-free mass (FFM) explains 70-80 % of variance in REE. FFM is regarded as the metabolically active, oxygen-consuming body cell mass. By contrast, fat mass (FM) is the metabolically inert lipid compartment of the body. FM (in kg) is however a dependent contributor to the variance in REE, due to the energy requirement of adipose tissue. The specific metabolic rate of lean tissue ranges from 54 kJ/kg for skeletal muscle to 1841 kJ/kg for heart and kidney, whereas the specific metabolic rate of adipose tissue is low 1l·3-14·3kJlkg lipid.
In contrast, regression equations from population analyses give different relationships between the effect of either FFM or FM on REE, i.e. the ratio between the regression coefficients of FFM and FM on REE ranged between 1·5: 1 and 7: 1, suggesting that each kg of lean tissue exerts a 1·5 – 7 times greater effect on REE than did each kg of fat tissue. These discrepancies may result from by differences in age between study populations or the amount of fat tissues which might affect the secretion of metabolically active adipose tissue-derived hormones such as leptin, resitine or adiponectin.
This study by Bosy-Westphal et al provides a systematic analysis of the relationship between REE, FFM,
They used a database of 1306 women and a linear regression model, and analysed the contribution of FM to the total variance in REE at different grades of adiposity (ranges of body %FM). After adjusting for age, the relative contribution of FM on REE variance increased from low (>10 %FM) to normal (> 10 to 30 %FM) and moderately elevated (> 3cto 40 %FM) grades of adiposity but decreased sharply at high (>40 to 5O %FM) and very high (> 5O %FM) grades of adiposity according to the ratio between regression coefficients. These data suggest that the specific metabolic rate of fat tissue is reduced at high adiposity.
Resting energy expenditure: Fat mass: Fat-free mass: Women

Bosy-Westphal et al 2009 grade of adiposity affects the impact of fat mass on resting energy expenditure in women Brit J Nutrition vol 101 pp 474-477

Martin Eastwood
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