Chemistry Reference
In-Depth Information
the seasonal decline in serum 25(OH)D by
>
50% (McKenna et al., 1995).
However, in countries where the fortification of milk is not mandatory, such
as the United Kingdom and Ireland, milk contributes
<
10% of the mean
daily intake of vitamin D (Hill et al., 2004). Stability studies show that on
exposure to light, there is slight loss of vitamin D
3
from fortified milk
(Renken and Warthesen, 1993), although this is unlikely to be caused during
normal handling and storage of milk. Air exposure does not affect stability in
milk (Renken and Warthesen, 1993).
12.3.8. Vitamin D Status and Requirements
12.3.8.1. Measurement of Vitamin D Status
Serum or plasma 25(OH)D [including 25(OH)D
2
and 25(OH)D
3
]is
considered to be the best index of vitamin D nutritional status because it
closely reflects the amount produced in the skin and ingested in the diet
(Holick, 1990a; Parfitt, 1998). In addition, measurement of 25(OH)D is
used routinely for the detection of vitamin D deficiency (Preece et al., 1975;
Holick, 1996). Several analytical techniques have been used to measure
25(OH)D in serum or plasma, such as HPLC (Shepard et al., 1979; Bouillon
et al., 1984; Hollis and Frank, 1985), radioimmunoassay (RIA) (Hummer
et al., 1984; Hollis and Napoli, 1985; Hollis et al., 1993), competitive protein
binding (CPB) assay (Belsey et al., 1971; Bouillon et al., 1976) and recently
developed enzyme immunoassays (EIA) (Lind et al., 1997). However, many
of these techniques can give quite different results when compared with each
other (Lips et al., 1999). There is a need for standardization of laboratory
methodology for serum 25(OH)D so that results from various populations
can be compared. The establishment of a vitamin D external quality assess-
ment scheme (DEQAS) operated by Charing Cross Hospital, London, UK, is
designed to ensure analytical reliability of vitamin D assays within labora-
tories worldwide.
Although serum 25(OH)D is dependent on a variety of factors including
season, latitude and race, a typical normal range for serum 25(OH)D in adults
living in northern Europe is between 25 and 200 nmol/l (Keane et al., 1998).
Defining adequacy of vitamin D status has proved difficult and there is no
consensus among researchers as to what cut-off level of 25(OH)D should be
applied (McKenna and Freaney, 1998). According to Scharla (1998), bone
disease such as rickets in children and osteomalacia in adults are reported
with serum 25(OH)D levels below 12.5 nmol/l, although some studies demon-
strate that these conditions also develop at higher serum 25(OH)D concen-
trations (
25 nmol/l) (Basha et al., 2000). Serum 25(OH)D levels
<
50 nmol/l,
which are characteristic of vitamin D insufficiency, result in secondary
