Agriculture Reference
In-Depth Information
colon of horses, were found to synthesize sufficient vitamin B
12
to meet their needs.
The structure of vitamin B
12
was established in 1955.
v
i t a m i n
a
a n D
P
r of v i t a m i n
a C
a r o t e n o i D s
The term
vitamin A
refers to a group of compounds possessing the biological activ-
ity of all-
trans
-retinol. They are important for vision, cell growth, communication
between cells, and differentiation of cells into specific functional types. They are vari-
ably soluble in lipids and organic solvents but insoluble in water. Following digestion
and release from the dietary matrix, their absorption is enhanced by bile salts and
pancreatic lipase, which promote formation of small lipid droplets (micelles) in the
intestinal lumen, within which retinyl esters, such as retinyl palmitate (a combination
of retinol and palmitic acid), are dissolved and hydrolyzed. Retinol is taken up by the
mucosal cells lining the intestine (enterocytes) and incorporated into chylomicrons
(lipoprotein particles) that are transported into the lymph and then into the general
blood circulation. The chylomicrons deliver retinyl esters, some unesterified retinol,
and carotenoids to the liver and other (extrahepatic) tissues. Retinyl esters that have
been stored in the liver are hydrolyzed before transport to the peripheral tissues in
association with a complex of retinol-binding protein and transthyretin.
Over 600 carotenoids have been identified, but fewer than 60 have provitamin A
activity. Those that do undergo conversion in the body to retinol. One of the most
abundant and most active is β-carotene, which can be split into two retinol molecules;
other common dietary carotenoids, α-carotene and β-cryptoxanthin, potentially
yield one. This bioconversion may occur during digestion, followed by absorption of
retinol as described. In some species, including humans, provitamin A carotenoids
also may be absorbed intact and are subject to postabsorption bioconversion. If not
converted to retinol, some carotenoids may function as antioxidants.
Historically, vitamin A activity has been expressed in international units (IU),
with 1 IU equivalent to 0.3 µg of retinol. IUs are still used in the literature, but the
Food and Agriculture Organization of the United Nations/World Health Organization
(FAO/WHO) has adopted retinol equivalents (REs), and the U.S. Institute of Medicine
(IOM) has adopted retinol activity equivalents (RAEs), with 1 RE or 1 RAE equiv-
alent to 1 µg of retinol. Although these two systems are equivalent in expressing
vitamin A activity of preformed vitamin A compounds, they differ in their assign-
ment of activity to carotenoids. Furthermore, the vitamin A activity of provitamin
A carotenoids varies with the species fed, its vitamin A status, and the nature of the
diet. Thus, it is often preferable to use mass as the basis for estimating contributions
of provitamin A carotenoids to the requirement for vitamin A. The IOM has set 1
RAE equal to 12 µg of β-carotene or 24 µg of α-carotene or β-cryptoxanthin in food
matrices consumed by humans.
WHO/FAO and IOM estimates of requirements and recommended intakes of vita-
min A for humans are not identical but tend to be similar for comparable age classes.
Recommended daily dietary allowances range from 300 RAE for 1- to 3-year-old
children to 1300 RAE for lactating females. Whole milk, cheese, butter, eggs, organ
meats (particularly liver), and fish are important sources of vitamin A in U.S. diets.
The main natural form of vitamin A in these foods is esters (e.g., retinyl palmitate).
