8. Sex-, Age-, and Ethnicity-Dependent Variation in Body Composition: Can There Be a Single Cutoff?
KeywordsBody composition assessmentMuscle massSarcopeniaNormative valesCirrhosis
Body composition measurements are an essential component for the evaluation of nutritional and health status in clinical assessment . The association between body fat and health risks is well known, and body mass index (BMI) is commonly used as a marker of increased body fat. The progress of the techniques to analyze body composition has enabled the accurate quantification of its compartments and increased the importance of body composition as a key determinant not only of health but also as a prognostic factor in several clinical conditions. It is also highlighted that muscle mass is the major determinant of negative outcomes in clinical settings . Although BMI has a good correlation with increased body fat at the population level, there are several conditions, like aging or chronic diseases, where low muscle can occur in obese individuals (sarcopenic obesity), and BMI alone cannot identify it . For this reason, weight and BMI are not enough to identify body composition abnormalities at the individual level, particularly in clinical situations.
Several body composition methods can be used to assess muscle mass compartments. Depending on the employed technology, different terminologies can be used to identify muscularity. The most commonly used terms are skeletal muscle mass [from computerized tomography (CT) and magnetic resonance imaging (MRI)], appendicular lean mass or appendicular lean soft tissue, a marker of appendicular skeletal muscle mass [from dual X-ray absorptiometry (DXA)], and fat-free mass [from DXA, air displacement plethysmography (ADP), and bioelectrical impedance analysis (BIA)] . In all these assessments, muscle mass is usually adjusted for body size, using height squared to create an index (cm2/m2 or kg/m2).
Determinants of Body Composition Variability
Sex as a Determinant of Body Composition Variability
The differences in body composition by sex can be noted since birth. Boys in general have an increased lean mass (mainly due to muscle mass) compared to girls resulting in a heavier weight from birth to the first decade of life . The dimorphism in body composition becomes more notable under the hormonal influence during adolescence, when girls increase their weights due to increments in fat mass, while boys increase their lean mass, leading to a higher lean to fat ratio in boys than in girls. This distinctive pattern of body mass accretion is also found in adulthood, with women having a lower percentage of lean mass, regardless of ethnicity . Another sex-related body composition difference is the muscle mass distribution. In all ages during adulthood, men have a greater muscularity of the upper body compared to women. Although men also have a greater muscularity in the lower body until the fifth decade, the difference between the sexes decreases because of the steeper decline in this regional muscle mass in men. From the fourth to ninth decade, quadriceps thickness assessed by ultrasound reduces by 50% in men and 30% in women, resulting in similar amounts of this muscle in men and women after the eighth decade .
The sex-related differences in body composition are usually recognized, and most of the cutoffs to identify low muscularity are specific for sex. The only exception is the recommendation from the new European Working Group on Sarcopenia in Older People (EWGSOP) consensus for calf circumference measurement, a marker of muscle mass used in epidemiological studies . Despite all the very well-known sex-related differences in muscle mass, they still recommend a single cutoff, derived from a study performed only in women, notwithstanding other studies have showed that this measurement is also different in older men and women [7, 8].
Age as a Determinant of Body Composition Variability
The age-related changes in muscle mass have a negative impact on health and quality of life of the elderly. These alterations start very early, in the third decade, but muscle mass loss and its consequences are usually evident only after the fifth decade . The main factors accounting for age-related variations in body composition seem to be hormonal changes and decline in physical activity.
The loss of muscle mass is positively associated with the loss of muscle strength in a nonlinearly relation, with muscle strength declining at a faster rate than muscle mass . While the depletion of muscle mass after the fifth decade is at an annual rate of 1%, the annual decline rate for muscle strength is 1.5% between the fifth and sixth decade and 3% from the seventh decade onwards . The greater impact of age on muscle strength than mass can be explained by remodeling in muscle composition with aging, such as an increased conversion of type II muscle fibers (fast) into type I (slow) and the increase in fat deposition within the muscle fibers (i.e., myosteatosis). These age-related alterations promote a decrease in muscle quality, which may precede the loss of muscle mass.
There is also a difference in the rates of age-associated muscle mass depletion between sexes. Although men had more muscle mass, mainly in the upper limbs, they also have a steeper decrease after the fourth decade. This decrease is even more pronounced in lower limb muscles [5, 12, 13]. The modifying effect of sex on age-related muscle loss has been demonstrated by muscle mass assessment from different body composition analysis methods, including ultrasound, DXA, and calf circumference measurement. Ethnicity may also modify the effect of sex in muscle mass depletion with aging. The age-related decline in muscle mass in African American, Whites, and Asians men is two times faster than in women, but in Hispanic men this rate could be three to four times higher than in Hispanic women [14, 15]. An exception to this age-related decline in muscle mass was found in Asian women from China, who didn’t show any relationship between muscle mass and age . All these sex, age, and ethnicity interactions should be considered during the identification of low muscularity.
The term sarcopenia, defined by a progressive loss of muscle mass and muscle strength, was initially implemented to name this age-related muscle loss, but sarcopenia is now considered a muscle disease, where both muscle quantity and quality are diminished, and loss of muscle strength is recognized as its key component for negative outcomes .
Ethnicity as a Determinant of Body Composition Variability
Another important determinant of body composition variability is ethnicity. Ethnic differences are associated with environmental conditions and genetic polymorphism . African Americans have the highest amount of muscle mass, followed by Whites, Hispanic, and Asians, as demonstrated by several studies using different techniques [10, 14, 15]. One hypothesis explaining these differences would be the longer extremities of African Americans compared to that of other ethnicities, as muscularity is usually assessed by appendicular lean mass from DXA. However, this hypothesis was rejected when the analysis was controlled by appendicular bone lengths . Besides the modification effect of age on muscle loss between sexes as discussed above, ethnicity also impact the rates of decline in muscle mass among women and men. African American women and Hispanic men have the highest age-associated decline rate of muscle mass across ethnicities and sex, respectively .
Variations in muscle mass among different countries from East to South and West Asia have also been reported. Therefore, reference values to define low muscularity (myopenia) can differ among these regions . The use of a cutoff value not specific for ethnicity may result in an over- or underestimation of myopenia prevalence.