Chemistry Reference
In-Depth Information
hyperparathyroidism and a marked increase in serum PTH, potentially
resulting in increased bone calcium mobilization and accompanying bone
loss (Malabanan et al., 1998; Lips, 2001). In addition, calcium absorption is
compromised at serum 25(OH)D levels
50 nmol/l (Heaney et al., 2003).
Even when serum 25(OH)D is as high as 80-100 nmol/l, slight elevations in
PTH can be observed, although levels remain within the normal range
(McKenna and Freaney, 1998; Lamberg-Allardt et al., 2001). In elderly
subjects, it is only when serum 25(OH)D exceeds 100 nmol/l that serum
PTH concentration becomes minimal (Dawson-Hughes et al., 1997;
Kinyamu et al., 1998). Furthermore, at serum 25(OH)D concentrations
>
100 nmol/l, no disturbances in vitamin D-dependent physiological func-
tions are observed (Peacock, 1995). Therefore, serum 25(OH)D
>
100 nmol/l,
but
<
250 nmol/l, above which toxicity may occur (see above), is considered
'adequate' (Zittermann, 2003).
12.3.8.2.
Dietary Vitamin D Requirements
Establishing dietary requirements for vitamin D is difficult because
sunlight makes a highly significant contribution to the functional marker of
vitamin D [serum 25(OH)D]. In the United States, the adequate intake (AI) is
5 mg/day for children aged 4-17 years, between 0 and 10 mg/day for adults
aged 18-50 years, 10 mg/day for adults aged 51-70 years and 15 mg/day for
adults over 70 years (Food and Nutrition Board, 1997). The difference in
requirements between children and older adults relates to the reduced ability
for dermal synthesis in older subjects. In the United Kingdom, there is no
reference nutrient intake (RNI) for vitamin D for people aged 4-64 years
based on a lack of evidence that individuals in this age group rely on dietary
intake for adequate vitamin D status (Department of Health, 1998). How-
ever, the RNI for people aged 65 and over is 10 mg/day, which was considered
to help safeguard against vitamin D deficiency and its adverse effects on bone
health (Department of Health, 1998).
12.3.8.3.
Vitamin D Toxicity
It has been suggested that the concentration of pre-vitamin D in the skin
of Caucasians reaches an equilibrium within 20 min of UVB exposure (Holick,
1995), with any excess 7-dehydrocholesterol being converted to inactive meta-
bolites such as tachysterol and lumisterol. Therefore, UVB exposure beyond
the minimal erythemal dose does not increase vitamin D production further
(Vieth, 1999). This explains why it is impossible to get toxicity from UVB
radiation, even after prolonged exposure. Such inherent feedback regulatory
control mechanisms do not exist for oral exposure to vitamin D. There are,
