Tissues and Organs

The aforementioned models do not allow for subregion and/or specific organ and tissue measurements. For example, skeletal muscle mass (SM) is contained within the FFM compartment. SM represents the single largest tissue in the adult body and is equivalent to ^40% of body weight in young adults, decreasing to ^30% of young values at elderly ages. SM is one of the more difficult components to quantify. Estimates of SM are commonly derived from anthropometry, total body potassium, and DXA using modeling approaches previously described. The use of magnetic resonance imaging (MRI) in body composition research has allowed for a good estimation of SM, adipose tissue (AT), and select organs in vivo, in all age groups with no risk to the participant (Figure 2). Moreover, AT distribution, including subcutaneous adipose tissue (SAT), visceral adipose tissue (VAT), and intermuscular adipose tissue (IMAT) is also measurable using a whole-body multislice MRI protocol (Figure 3). In studies relating body composition to energy expenditure, high metabolic rate organs including liver, kidneys, heart, spleen, and brain are also measurable using MRI.

Bone mineral content and bone mineral density of specific body sites (e.g., radius, hip, lumbar spine) are most commonly measured using DXA. Bone mass and microarchitecture are important determinants of bone strength, with microarchitectural deterioration being one of the specific changes associated with

Figure 2 3D reconstructed image of whole-body scan (from MRI). Skeletal muscle (red); adipose tissue (green); bone, organs, and residual (yellow); intermuscular adipose tissue (pink).

Figure 2 3D reconstructed image of whole-body scan (from MRI). Skeletal muscle (red); adipose tissue (green); bone, organs, and residual (yellow); intermuscular adipose tissue (pink).

Figure 3 Cross-sectional images from (A) upper arm, (B) trunk (L4-L5 level), (C) mid-thigh, and (D) mid-calf in an elderly female volunteer. IMAT, intermuscular adipose tissue (pink); SM, skeletal muscle (red); SAT, subcutaneous adipose tissue (green); VAT, visceral adipose tissue (blue).

osteoporosis. Using high-resolution microcomputed tomography (micro-CT) and computer software, detailed analysis of three-dimensional (3D) architecture is feasible and allows microstructural 3D bone information to be collected.

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