Fat-free mass as well as lean soft tissue mass is a surrogate for skeletal muscle mass and is often used for the normalization of several physiological variables or for the diagnosing of low muscle mass in older adults. However, both fat-free mass and lean tissue mass include nonskeletal muscle components such as the fat-free component of adipose tissue fat cells. A technique known as water-fat MRI provides a noninvasive and radiation-free assessment of the fat-free component of adipose tissue in humans. However, if this method is impractical or unavailable, some authors suggest that a constant value for the fat-free component of adipose tissue can be used as an indirect estimate. The purpose of this review is to examine the fat fraction percentage of white (subcutaneous) adipose tissue in adolescents and young/middle-aged/older adults measured by water-fat MRI and provide discussion on how the fat-free adipose tissue values from the water-fat separation method compare with the constant value used in previous studies. Calculated mean values for the percentage of fat fraction in subcutaneous adipose tissue were 86.9% in the overall sample, 86.4% in adolescents (3 studies), and 87.1% in young, middle-aged and older adults (7 studies). This is similar to the 85% value proposed in the classical studies but in the majority of studies the 85% estimate was outside of the 95% confidence interval (CI) of the water-fat MRI estimate. There may be several factors to consider that may affect the fat fraction percentage (e.g. reliability of the MRI estimate, age, sex, obesity, etc.), however, at this time there is insufficient evidence to determine the effect of each of these variables. If the measurement is reliable, then this might suggest that the 85% constant may need to be altered to better reflect the water-fat MRI estimate.
Keywords: Adipose tissue fat content; dual X-ray absorptiometry; water-fat MRI.
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