Download Children and Bone

Children and Bone
Jennifer Brittain
October 24, 2002
Osteoporosis

$13.8 billion/year in healthcare costs (U.S.)
– “pediatric disease with a geriatric outcome”
– Key is PREVENTION
– “Peak Bone Mass” (PBM)
– Optimize bone mineral gains throughout
childhood and adolescence
Bone Physiology
“Osteoblasts”—(OB’s) cells responsible for
formation of bone tissue
 “Osteoclasts”—(OC’s) cells responsible for
the breakdown of bone tissue
 Bone growth= activity of OB’s greater than
that of OC’s
 Bone resorption= activity of OC’s greater
than that of OB’s

Bone Physiology
– “Proper skeletal formation, growth and repair
are critically dependent on the accurate
orchestration of all the processes participating
in the formation of endochondral bone at the
growth plate.”
– Bone Biology, Chapter 3
Basis of Bone Growth

Bones grow in:
– Width/diameter—involves formation externally
and resorption internally
– Length—enlargement of chondrocytes
(cartilage cells) at the growth plate, followed by
replacement with bone cells for mineralization
Growth Plate

The epiphyseal plate is the cartilaginous
region separating bone into two distinct
parts.
 The plate survives if the cartilage growth
keeps pace with osteoblast invasion.
 When osteoblast activity increases beyond
cartilage growth, the plate disappears and
the “epiphyseal line” is seen.
Bone Growth in Childhood

Process begins approximately 6 weeks after
fertilization and continues through
adolescence.
 Some parts of the skeleton are developing
through approximately 25 years of life.
Growth Rate

The most rapid skeletal growth is seen in
infancy.
 A slow deceleration in the rate is seen
around the age of 3 and on through puberty.
 Slight growth spurt appears around 8 years
of age and distinctly decelerates
immediately before puberty.
Gains in Mass

Most rapid gains in bone mass seen during
adolescence
 ~25% of PBM acquired within the 2-year
period at peak height velocity(rate).
 Maximal rates of bone mineral gains follow
behind the peak height velocity (~612months)
Bone Mineral

Accrual of bone mineral continues after
longitudinal growth has occurred.
 At peak growth velocity, 90% adult stature
attained and only 57% of total bone mineral
content (BMC)
 90% of peak bone mass is gained by 18 yrs
 5-12% bone mineral density gained during
third decade
Terminology

Bone mineral content (BMC) is the
measurement of amount of mineral in the
bone (grams).
 Bone mineral density (BMD) is the
measurement of mineral per area (grams per
cm2)
Bone mineral determinants

60-80% PBM is genetic
– Likely that the effects are greater in mineral
acquisition than in loss

Racial differences (reduced risk of
osteoporosis in blacks)
– Some seen before puberty
– Others emerge in late adolescence
Determinants cont.

Hormonal status (systemic):
– Growth hormone (GH)
 Major influence before puberty
 Longitudinal bone growth impaired with deficiency
 Deficiency leads to reduced bone mineral/failure to
acquire at expected rate
 Acts at growth plate by enhancing production of
cartilage cells and then inducing IGF1 synthesis (a
local regulator responsible for expansion of
chondrocytes at growth plate)
More bone mineral
determinants

Hormones cont.
– Thyroid
 Deprived levels lead to deleterious effects on bone
growth
 Direct effect on bone
 Indirect effect on GH and IGF-1
 High levels increase growth rate and advance bone
age
Hormone Regulation cont.

Estrogen
– Direct effects on growth plate
– Responsible for epiphyseal growth plate fusion
(end of puberty) in females and males

Glucocorticoids
– Pharmacological doses cause stunted growth
– Role in chondrocyte differentiation and
hypertrophy
Exercise and Bone

Weight-bearing activity below a
physiological threshold will lead to
excessive resorption. Ex. Bone loss in
spaceflight.
 Within physiological range, bone is
maintained.
 Bone is gained as load/strain increases
Exercise Cont.

Greatest influence before and during
puberty
 Bone mass increases due to early intense
activity are carried into adulthood
– Ex. BMD of former gymnasts, runners and
dancers as high as 8-12% greater than agematched controls ( years later)
Diet and Bone

Protein:
– Important role in bone development

Influences peak bone mass
– Protein malnutrition in developmental years can
increase risk for osteoporosis and fracture later in life
– Low intake impairs synthesis and action of IGF-1
(essential factor for longitudinal growth)
– Significant association between ingested protein and
bone mass gain in prepubertal children
• Bonjour, JP et al. 2001
More Dietary Elements

Calcium
– Skeleton stores 95% of body’s calcium
– In 2001, 90% of adolescent girls and 50% of
boys consumed less than optimal Calcium
amounts. Bachrach, 2001
– Contributes to bone mineral accrual during
early adulthood and helps prevent loss at
maturity
Diet Cont.

Vitamin D
– Deficiency is a major cause of rickets in children and
osteomalacia in adults

Expansion of zone of growth plate coupled with impaired
matrix calcification
– Promotes mineralization
– Suggestion that vitamin D receptors are found on OB
and OC precursors Johnson, 1996
– Increases absorption of calcium (and phosphorous)
Overview

Peak bone mass is key to prevention of
osteoporosis.
 Bones grow in length and mass due to a
number of processes working together.
 Exercise can benefit bone health.
 Protein, calcium, and vitamin D are three
important dietary components of bone
health.