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CHAPTER24 I Nutrition and Bone Health 619

Chronic low l!Iletary calcium and loss

otbone mass

Initial bone mass

Increased IPTH~

OsteoclastFIGURE 24-4 Effects of persistently elevated serum

BLOONSES!

High concentration of parathyroid1IItI resorption hormone (PTH) on bone mass.

(Copyright John J.B. Anderson and

Sanford C. Carner.)

Final bone mass

Incomplete replacement

of bone mass

Boneresorption

Bone remains intact

FIGURE 24-5 Interaction between osteoblasts and osteoclasts in bone remodeling. The role of parathyroid hormone (PTH) in stimulating osteoclasts to resorb bone (upper) is contrasted with the inhibitory action of estrogens on osteoblasts that negate the action of PTH (lower)

(Copyright John J.B. Anderson and

Sanford C. Carner.)

Osteoblast Osteoclast

N-telopeptides (NTXs), and plasma tartrate-resistant acid phosphatase (TRAP). Osteocalcin, considered a bone for- mation marker, is also released following the resorption of bone matrix; therefore interpretation of its blood values is complicated by both formation and resorption, which typically occur simultaneously at several different skeletal sites.

BONE MASS

Bone mass is a generic term that refers to bone mineral content but not to bone mineral density. Bone mineral content (BMC) is more appropriately used in assessing the amount of bone accumulated before the cessation of growth (height gain), whereas bone mineral density (BMD) is better used to describe bone after the developmental period is completed. These measurements are often used interchangeably, but BMD is more useful for monitoring bone changes in adults. However, neither BMC nor BMD provides information on the micro architectural (three-

dimensional) structural quality of bone tissue (i.e., index of risk of fracture).

Measurement of Bone MineralContent and Bone Mineral Density

Bone densitometry measures bone mass on the basis of tis- sue absorption of photons produced by one or two mono- energetic x-ray tubes. Dual-energy x-ray absorptiometry (DEXA) is available in most hospitals and many clinics for the measurement of the total body and regional skeletal sites such as the lumbar vertebrae and the proximal femur (hip). Results of BMC measurements are expressed as grams of mineral per centimeter and BMD in grams per centimeter squared is calculated from the BMC divided by the width of the bone at the measurement site.

Computerized tomography (CT) may also be used to measure BMD (a true volumetric density) of the spine, but this technique has not yet been developed to measure the limbs. In the near future, more penetrating instruments are expected that will be able to assess both bone density and the 3-D quality of the bone tissue at different skeletal sites.