Chemistry Reference
In-Depth Information
of the retinol isomers. Photochemical isomerization of vitamin A com-
pounds occurs both directly and indirectly via a photosensitizer (Pesek
and Warthesen, 1990). The ratios and quantities of cis isomers produced
differ depending on the photoisomerization. The type of packaging material
has a significant effect on net retention of vitamin A activity in food exposed
to light during storage. Vassila et al. (2002) reported vitamin A losses
ranging from 15.1 to 73.6% in whole milk stored in various flexible mono-
layer and multilayer co-extruded pouches held under fluorescent light at
48C for up to 7 days. Zygoura et al. (2004) reported losses of 8.8-50.9%,
depending on the packaging material, when pasteurized milk was stored
under fluorescent light at 48C for 7 days.
12.2.5.
Vitamin A Status and Requirements
Worldwide, vitamin A deficiency is a major public health problem and
the most important preventable cause of blindness. The earliest signs of
deficiency are loss of sensitivity to green light, followed by inability to
adapt to dim light and finally inability to see in dim light (night blindness).
More prolonged or severe deficiency leads to xerophthalmia, which involves
keratinization of the cornea, followed by ulceration, which leads to irrever-
sible damage to the eye that causes blindness. In addition to clinical defi-
ciency, more than 200 million children suffer from vitamin A inadequacy in
the absence of clinical signs of acute deficiency (WHO, 1996). These children
generally show a higher mortality rate and a greater incidence of severe
infection than do vitamin A-sufficient children. Symptoms of vitamin A
deficiency are associated with a low intake of fat and lipid malabsorption
syndrome (Olson et al., 2000). Vitamin A deficiency symptoms also occur in
protein-energy deficiency, regardless of whether or not the intake of vitamin
A is adequate. This is due to impairment of the synthesis of plasma RBP. Zinc
is also required for the synthesis and secretion of RBP and TTR; therefore,
zinc deficiency influences the mobilization of vitamin A from the liver and its
transport into the circulation (Food and Nutrition Board, 2001).
When adequate retinol is provided in the diet, there are no known
clinical effects of consuming diets low in carotenes over the short term
(Food and Nutrition Board, 2000). However, carotene-deficient diets were
reported to increase various measures of oxidative susceptibility (Dixon et al.,
1998; Lin et al., 1998), but this is of uncertain relevance with regard to clinical
outcomes.
Current estimates of vitamin A requirements are based on the intake
required to maintain a reserve concentration of at least 20 mg retinol/g of liver
tissue (Olson, 1987; Food and Nutrition Board, 2001). This concentration is
