Milk-and-Dairy-Products-in-Human-Nutrition-FAO

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Milk and dairy products in human nutrition
4.4.4 Bone-remodelling transient
The bone remodelling transient is a temporary alteration in the balance between
bone formation and bone resorption brought about by any factor (e.g. drugs,
hormones or nutrients that alter either secretion of parathyroid hormone 25 or its
action on bone) that affects bone remodelling (Heaney, 2001). According to Heaney
(2001), because the remodelling activity is spread out over several months (several
weeks in growing children, approximately three months in young adults and 6–18
months in older adults), nutritional interventions that alter remodelling produce a
temporary phase lag between the normally coupled destructive and constructive
components of the bone-remodelling process. This phase lag, when observed as an
external balance or a dual-energy X-ray absorptiometry time course (commonly
used to measure BMD), is the remodelling transient. Steady-state effects of any
given nutrient intake can only be ascertained by measurements made after the
transient has passed. The significance of the transient for nutritional interventions is
both that early effects will always be misleading and that one can draw no inference
whatsoever about the new steady state from what one observes during the transient
phase (Heaney, 2001).
4.4.5 Limitations of studies using bone mineral density as an end point
BMD is used to define peak bone mass in young adults and is generally accepted to
be a strong predictor of fractures in the elderly (see Bischoff-Ferrari et al., 2007).
Bone mineral content in adults at any time is dependent on peak bone density
achieved during development and subsequent bone loss; therefore, low BMD can
result from poor bone accretion, accelerated bone loss or both (see Small, 2005 and
references therein). BMD is often used as a surrogate measure of efficacy in clinical
trials of osteoporosis therapies because even though studies with fracture incidence
as a primary end point provide the most meaningful assessment, these trials typically
require large numbers of patients and often take at least three years to generate
sufficient data (Small, 2005). Although the majority of clinical trials with calcium
or dairy product supplementation in children and adolescents that have been completed
to date show a positive effect of intervention on bone mass, they are generally
too short (one to three years) to address the question of whether it is the temporary
adaptation of bone tissue to the alteration in calcium intake that leads to peak bone
mass (Matkovic et al., 2005). The increase in bone mass observed in those shortterm
studies could be explained to a large extent by the bone-remodelling transient
(see “Bone-remodelling transient”, above). However, to conduct controlled feeding
trials for sufficiently long periods for bone properties to change may not be practical
except for animal studies (Weaver, 2008).
4.4.6 Osteoporosis
Osteoporosis is a condition of low bone mass with increased risk of fracture. Bones
can get to that state through acquiring low peak bone mass during growth and/
or through accelerated bone loss later in life as depicted in Figure 4.1. Worldwide
25 Parathyroid hormone (PTH) regulates the quantity of remodeling activity.