Nutrition Science and Everyday Application - beta v 0.1

INTRODUCTION TO BONE HEALTH 497
Figure 9.1. This cross-section of a flat bone from the skull shows the spongy bone lined on either
side by a layer of compact bone.
BONE GROWTH, MODELING, AND REMODELING
Bones change in shape, size, and position throughout the life cycle. During infancy,
childhood, and adolescence, bones are continuously growing and changing shape through
two processes: growth (or ossification) and modeling. In the process of modeling, bone tissue
is dismantled at one site and built up at a different site, which influences the shape of the
bone. During childhood and adolescence, more bone is deposited than dismantled, so bones
grow in both size and density, reaching 90 percent of peak bone mass by age 18 in girls and
age 20 in boys. 2 Peak bone mass is reached by age 30, at which point bones have reached
their maximum strength and density. Factors affecting peak bone mass include sex, race,
hormones (e.g., estrogen and testosterone), nutrition (e.g., calcium and vitamin D intake),
physical activity, and behavioral factors like smoking. These factors will be discussed in more
detail when we discuss osteoporosis.
In adulthood, our bones stop growing and modeling, but they continue to go through
a process of bone remodeling, in which bone tissue is degraded and built up at the same
location. About 10 percent of bone tissue is remodeled each year in adults. Bones adapt their
structure to the forces acting upon them, even in adulthood. This is why physical activity
increases bone strength, especially when it involves weight-bearing activities. For example,
tennis players can have measurably higher bone mass in the arm they use for play compared
with the other arm. 3 Ultimately, bones adapt their shape and size to accommodate function.
The dynamic nature of bone means that new tissue is constantly formed, and old, injured,
or unnecessary bone is dissolved for repair or for calcium release. The cell type responsible
for bone resorption, or breakdown, is the osteoclast. Osteoclasts are continually breaking