Fat-Soluble Vitamins and Vitamin C in Milk and Milk Products - Advanced Dairy Chemistry - page 534

the antirachitic activity of either vitamin D 2 or D 3 (Ovesen et al., 2003a). The
vitamin D content of foods is usually described in mg or international units
(IU), where 1 mg ¼ 40 IU.

12.3.2. Vitamin D Synthesis in the Skin

In vertebrates, the cholesterol-like precursor, 7-dehydrocholesterol,
present in the skin epidermis, undergoes photolysis when exposed to UVB
light of wavelength 290-315 nm to yield a variety of photoirradiation pro-
ducts, including tachysterol, lumisterol and pre-vitamin D 3 . Pre-vitamin D 3
then undergoes spontaneous thermal rearrangement to vitamin D 3 . The fate
of ergo-calciferol in plants and lower organisms such as fungi follows a
similar pathway to chole-calciferol. Because of the skin's ability to synthesize
the vitamin upon exposure to appropriate sunlight, vitamin D is an essential
nutrient only when sunlight is limited. For this reason, some scientists con-
sider vitamin D to be as much a hormone as a nutrient (Singh, 1985; Ball,
1992; Park, 1997).
As a consequence of dermal synthesis upon exposure to UVB radia-
tion, serum levels of vitamin D in Northern populations exhibit a clearly
evident seasonal variation, with highestlevelsattheendofsummer
(August-September) and lowest levels at the end of winter (February-
March) (Stamp and Round, 1974; van der Wielen et al., 1995; and for
reviews, see Lips, 2001; Ovesen et al., 2003b; Zittermann, 2003). It has
been suggested that in northern regions at a latitude of 408 Nsuchas
Boston, the skin is incapable of synthesizing vitamin D from November to
March (Webb et al., 1988). For regions > 508 N, such as Edmonton, this
period extends from October to March. At higher latitudes during winter,
sunlight must pass a much longer distance through the atmosphere and
most UVB light is absorbed, thus preventing any effective UV radiation
from reaching the skin (Holick, 2004).

12.3.3. Absorption, Metabolism and Excretion

Dietary vitamin D is absorbed from the jejunum and ileum and, like
other fat-soluble vitamins, is transported in chylomicrons and lipoproteins
via the lymphatic system (Endres and Rude, 1994). Absorption is by non-
saturable passive diffusion and the efficiency of absorption is estimated to be
60-90% (Van den Berg, 1997). Vitamin D absorption efficiency is unaffected
by vitamin D status (Fraser, 1983). Vitamin D absorption has been suggested
to be facilitated by long-chain fatty acids (Holmberg et al., 1990), while others
suggest that the elimination of vitamin D from the gastrointestinal lumen is
increased by a high fibre diet (Batchelor and Compston, 1983).