Body Composition Applications During Aging

During the adult life span, body weight generally increases slowly and progressively until about the seventh decade of life, and thereafter, declines into old age. An increased incidence of physical disabilities and comorbidities is likely linked to aging-associated body composition changes. Characterization of the aging processes has identified losses in muscle mass, force, and strength, which collectively are defined as 'sarcopenia.' Little is known about the overall rate at which sarcopenia develops in otherwise healthy elderly subjects, if this rate of progression differs between women and men, and the underlying mechanisms responsible for age-related sarcopenia. Peak SM mass is attained in the young adulthood years and slowly declines thereafter. During the latter adult years, SM decreases more rapidly as body fat becomes more centralized. Anthropometric equations have been developed for predicting appendicular skeletal muscle (ASM = SM of the limbs) in the elderly where sarcopenia was defined as ASM (kg)/height2 (m2) less than two standard deviations below the mean of the young reference group. In the elderly men, the mean ASM/ height2 was approximately 87% of the young group. The corresponding value in women was approximately 80%. Table 2 shows the estimated prevalence's of sarcopenia in the same survey sample for each ethnic group, by age and sex. The same authors have reported that obese and sarcopenic persons have worse outcomes than those who are nonobese and sarcopenic.

Even in healthy, weight-stable elderly persons, changes in body composition over a 2-year period can include decreases in SM mass and bone mineral content with corresponding increases in IMAT and VAT, after adjusting for their baseline values, despite no detectable changes in physical function or food intake.

In adults, excess abdominal or VAT is recognized as an important risk factor in the development of coronary heart disease and non-insulin dependent diabetes mellitus. Waist circumference and the waist:hip ratio are commonly used to predict visceral fat accumulation in epidemiological studies. However, waist circumference is unable to differentiate VAT from SAT. As a result, persons with similar waist circumferences could have markedly different quantities of VAT and abdominal SAT. Skinfold thickness has been used as a continuous variable grading adiposity or adipose tissue distribution within study populations.

The most accurate measurement of VAT requires imaging techniques (MRI and computed tomography (CT)), which are expensive and not readily available in many clinical settings. Figure 3B shows an MRI-derived cross-sectional image at the L4-L5 level with adipose tissue depots identified. The AT located between muscle bundles (IMAT; Figure 3) and visible by MRI and CT may be negatively associated with insulin sensitivity. In the elderly, greater IMAT (as suggested by lower skeletal muscle attenuation by CT) is associated with lower specific force production. Currently, there is no simple or clinic-based method to measure adipose tissue located between the muscle groups, defined in our laboratory as intermuscular adipose tissue (IMAT). IMAT has been reported to be significantly negatively correlated with insulin sensitivity and higher in type 2 diabetic women compared to nondiabetic women.

Sex and race differences in body composition are well established in adults. Men acquire higher peak SM mass than women and some evidence exists suggesting that men may lose SM faster than women with age. Moreover, it is well established that women have a larger amount of total body fat or total adipose tissue than men. Among races, African-American adult men and women have larger amounts of SM than Asian and Caucasians even after adjusting for differences in body weight, height, age, and skeletal limb lengths.

Efforts are ongoing to better understand variations in IMAT as a function of age, race, and level of fatness. IMAT deposits appear comparable in size in adult African-Americans, Asians, and Caucasians at low levels of adiposity but accumulate as a greater proportion of TAT in African-Americans compared to Caucasians and Asians subjects (58 g IMAT/kg TAT in African-Americans; 46 g IMAT/kg TAT in

Table 2 Prevalance (%) of sarcopeniaa

in the New Mexico Elder Health Survey, by age, sex, and ethnicity, 1993-1995

Age group (years) Men

Women

Hispanic

Non-Hispanic whites

Hispanics

Non-Hispanic whites

(n = 221)

(n = 205)

(n = 209)

(n = 173)

<70 16.9

13.5

24.1

23.1

70-74 18.3

19.8

35.1

33.3

75-80 36.4

26.7

35.3

35.9

>80 57.5

52.6

60.0

43.2

Appendicular skeletal muscle mass/height2 (kg/m2) less than two standard deviations below the mean value for the young adults from Gallagher D, Visser M, De Meersman RE et al. (1997) Appendicular skeletal muscle mass: effects of age, gender, and ethnicity. Journal of Applied Physiology 83: 229-239.

Adapted from Baumgartner RN, Koehler KM, Gallagher D et al. (1998) Epidemiology of sarcopenia among the elderly in New Mexico. American Journal of Epidemiology 147: 755-763.

Appendicular skeletal muscle mass/height2 (kg/m2) less than two standard deviations below the mean value for the young adults from Gallagher D, Visser M, De Meersman RE et al. (1997) Appendicular skeletal muscle mass: effects of age, gender, and ethnicity. Journal of Applied Physiology 83: 229-239.

Adapted from Baumgartner RN, Koehler KM, Gallagher D et al. (1998) Epidemiology of sarcopenia among the elderly in New Mexico. American Journal of Epidemiology 147: 755-763.

Caucasians; 44 g IMAT/kg TAT in Asians). Across race groups, VAT deposits also appear comparable in size at low levels of adiposity but with increasing adiposity VAT accumulates more in Asians and Caucasians compared to IMAT, although accumulation rates for IMAT and VAT do not differ in African-Americans. While the association between greater amounts of abdominal or VAT and increased insulin resistance and the metabolic syndrome is well established compared to the peripherally located SAT, the role of the IMAT compartment in the metabolic alterations leading to the development of insulin resistance warrants further investigation, especially as it may influence race/ethnicity differences in dys-glycemia. Collectively, sex and race differences exist in body composition in children and adults.

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