Chemistry Reference
In-Depth Information
a single unit (NRC, 1989). Nutritionally, 1 mgRE
¼
1 mg of all-trans-retinol
¼
2 mg of supplemental (in oil) all-trans-
-carotene
¼
6 mg of dietary all-trans-
-
carotene
¼
12 mg of other dietary pro-vitamin A carotenoids. When defining
RE, it was assumed that the efficiency of absorption of pro-vitamin A
carotenoids was relatively good. Recent studies document, however, that
absorption of carotenoids is much lower and appears to be quite variable.
In addition, a number of factors such as protein-energy malnutrition, zinc
deficiency, dietary fat, alcohol, infections and degree of food processing/food
matrix affect the bioavailability and bioconversion of retinol and carotenoids
(Parker et al.,1999; van het Hoff et al.,1999). Based on these and other studies,
it is estimated that 1 retinol activity equivalent (mg RAE) is equal to 1 mgof
all-trans-retinol, 2 mg of supplemental all-trans-
-carotene, 12 mg of dietary
all-trans-
-carotene or 24 mg of other dietary pro-vitamin A carotenoids (e.g.
-carotene,
-cryptoxanthin) (Parker et al., 1999; Food and Nutrition Board,
2001; Trumbo et al., 2001).
Most retinoid and carotenoid analyses have been performed by parti-
tion RP-HPLC on octadecylsilane (C18) columns, but C30 columns have
proven useful for certain demanding separations of carotenoids (Sander et al.,
1994). The most commonly used detectors are UV and visible light absor-
bance detectors, although fluorescence detectors (retinol and retinyl esters are
fluorescent but other retinoids and most carotenoids are not), electrochemical
detectors and mass spectrometers are used also (Furr, 2004). Detailed reviews
of HPLC analysis of retinoids and carotenoids, including sample preparation
techniques, have been published (Barua et al., 2000; Song et al., 2000). A
number of analytical methods have been described for the determination of
vitamin A in milk and milk-based infant formulae either alone (Strobel et al.,
2000; Miyagi et al., 2001) or with other fat-soluble vitamins (see vitamin E
section for details).
12.2.2.
Absorption, Metabolism and Excretion
Higher animals must obtain vitamin A from the diet, either as the
preformed vitamin or as a pro-vitamin carotenoid such as
-carotene. The
major dietary forms of preformed vitamin A are long-chain fatty esters of
retinol (Plack, 1965). In Western countries, the intake of preformed retinol
or retinyl esters typically accounts for 25-75% of the total vitamin A
intake; the rest is provided by pro-vitamin A carotenoids (Harrison,
2005). The retinyl esters must be hydrolysed prior to absorption. The
presence of dietary fat in the intestine can promote retinyl ester digestion
by stimulating the secretion of pancreatic enzymes and bile salts, which
serve to form mixed micelles of lipids, and by providing products of lipid
digestion (i.e. lysophospholipids, monoglycerides and free fatty acids),
