Vitamin D and Health

calcium intake, although this is more common in developing countries, and in children with
inadequate phosphorus intake.
6.18 Osteomalacia, like rickets, develops as a result of vitamin D deficiency. It commonly presents in adults
as severe aching in bone and muscles and proximal muscle weakness making standing up and walking
difficult and painful and results in a marked waddling gait. Osteomalacia arises from a disorder in the
physiological process of bone turnover where the mineralisation phase of bone remodelling is
impaired. It can occur in children with rickets and there are also reports of adolescents presenting
with symptoms of osteomalacia (Ladhani et al., 2004; Ward et al., 2005; Das et al., 2006). When
vitamin D deficiency is implicated in the aetiology of osteomalacia there is usually evidence of
secondary hyperparathyroidism. Osteomalacia can also be caused by kidney or liver damage, which
can interfere with vitamin D metabolism.
6.19 Osteoporosis is a progressive skeletal disorder generally associated with ageing. It is characterised by
reduced bone strength due to loss of bone mass and deterioration in the micro-architecture of
trabecular bone which increases bone fragility and, as a consequence, risk of fracture (WHO, 1994).
Fractures are most common at sites where trabecular bone predominates; i.e., at the spine, wrist and
hips. It can affect both sexes, but women are at greater risk mainly due to the decrease in oestrogen
production after the menopause, which accelerates bone loss to a variable degree. Factors which
affect bone mass will influence the risk of developing osteoporosis (see paragraphs 6.15-6.16).
Assessment of bone health
6.20 In studies which have examined factors influencing bone health, the most clearly defined and clinically
relevant endpoint is bone fracture. In most studies, however, intermediate outcome measures are
used to assess bone strength. Measurement of areal bone mineral density (BMD), the quantity of
mineral present per given area of bone (g/cm 2 ), is the most common proxy measure of bone strength
and fracture risk. The most widely used technique to measure BMD is dual-energy x-ray
absorptiometry (DXA) which has high reproducibility and low radiation dose (DH, 1998). Other
techniques include quantitative computed tomography (QCT) which allows three-dimensional
assessment of the structural and geometric properties of the skeleton but the equipment is expensive
and the radiation dose is relatively high. Peripheral QCT (pQCT) has a much lower radiation dose and
allows three-dimensional assessment of the lower arms and legs and volumetric measures of BMD
(g/cm 3 ). Ultrasound methods are also used; however, the clinical relevance of ultrasound bone
measures is less well understood.
6.21 Although there is a relationship between BMD and fracture risk, the extent of the relationship is not
clear. BMD measurements do not provide a complete assessment of bone strength; other factors that
contribute include bone size, shape, architecture, and turnover (Ammann & Rizzoli, 2003).
Additionally, BMD obtained by single or dual-energy techniques, is an areal density measurement
(g/cm 2 ) derived by dividing bone mineral content (BMC) by the scanned area of bone. It does not
measure volumetric density of the bone or the mineralised tissue within the bone (Prentice et al.,
1994). Since both BMC and BMD are influenced by the size, shape and orientation of the bone, this
limits its use in cross sectional studies of factors influencing bone health unless adjustment is made for
the confounding influence of size. Various methods have been used to adjust areal BMD to more
closely represent volumetric BMD especially at the spine, including calculation of bone mineral
apparent density (BMAD) (Faulkner et al., 1995). Bone mineral measurements are more useful in
prospective studies where changes are assessed over time.
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