Introduction.
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Dark green vegetables (like spinach), orange vegetables (carrots, sweet
potatoes) and some fruits (mangoes) contain carotenoids, some of which can
be transformed by the body into vitamin A. However, the absorption and
transport are quite complex and misunderstood [Erdman et al.,
absorption and transport of carotenoids, Ann NY Acad Sci 1993; 691:76-85].
In at least one experiment, increased consumption of dark green vegetables
didn't improve vitamin A status [de Pee et al., Lancet 1995; 346:75-81].
Here are a few of the possible reasons:
Digestion of the food matrix.
-----------------------------
Beta-carotene absorption can be as low as 1-2% from raw vegetables such as
the carrot. Particle size on uncooked foods is particularly important;
pureed or finely chopped vegetables yields considerably higher
beta-carotene absorption compare to whole or sliced raw vegetables.
Mild heating, such as steaming, appears to improve the extractability of
beta-carotene from vegetables and also beta-carotene bioavailability.
However, additional heating can transform the naturally occurring trans
double bonds into cis configurations, which reduces the biological value
of beta-carotenes. For example, canning of sweet potatoes and carrots
results in the conversion of 75% of all-trans B-carotene to 13- or 9-cis
isomers.
Formation of lipid micelles.
----------------------------
Dietary fat stimulates bile secretion which facilitates the emulsification
of fat-soluble vitamins into lipid micelles within the small intestine.
Without micelle formation, carotenoids are poorly absorbed. Very low
levels of dietary fat substantially reduces human carotene absorption.
Intestinal parasites severely limit carotenoid absorption.
Uptake of carotenoids by mucosal cells.
---------------------------------------
Because many of the hundreds of carotenoids in foods differ only slightly
in structure, it is possible that there is competition between carotenoids
for uptake into mucosal cells and for absorption.
Conclusion.
-----------
Animal sources of vitamin A (eggs, whole fish, liver) seem more
bioavailable.
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A few related abstracts:
------------------------
Zhou JR. et al., The crystalline form of carotenes and the food matrix in
carrot root decrease the relative bioavailability of beta- and
alpha-carotene in the ferret model, Journal of the American College of
Nutrition. 15(1):84-91, 1996 Feb.
OBJECTIVE: The objective of this study was to investigate the relative
bioavailability (BV) of beta-carotene (BC) and alpha-carotene (AC)
from different extracts of carrots utilizing the ferret model.
METHODS: Five groups of animals (n = 6) were provided free access to
a carotenoid-free, vitamin A-adequate diet and tap water for 2 weeks,
then for 3 consecutive days 10 mg of BC was provided in 30--40 mL of one
of the following fluids as partial replacement for tap water along with
diet: 1) commercial BC beadlets dispersed in distilled water (control), 2)
non-heated carrot juice, 3) heated carrot juice, 4) non-heated isolated
carrot chromoplasts, or 5) heated carrot chromoplasts. The animals were
killed and samples of blood and tissues (liver, adrenal, lung, kidney,
spleen) were collected and analyzed for AC and BC concentrations.
RESULTS: The tissue analysis of BC concentrations indicated that
BC-beadlet-supplemented animals had significantly higher BC concentrations
than other groups (p<0.01). Carrot chromoplast-supplemented animals had
significantly higher tissue BC and AC concentrations than carrot
juice-supplemented animals. Heat treatment tended to reduce the relative
BV of carotenoids but the differences between heated and non-heated juices
or isolated carrot chromoplasts were not significantly different.
CONCLUSIONS: These results suggest that the food matrix, probably
pectin-like fibers, and the crystalline form of carotenoids in carrot
chromoplast are the primary factors that reduce the relative BV of
carotenoids from carrot juice.
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Jonsson L., Thermal degradation of carotenes and influence on their
physiological functions, Advances in Experimental Medicine & Biology.
289:75-82, 1991.
Raw carrot juice contains a considerable amount of alpha- and
beta-carotene, which makes carrot an excellent source of vitamin
A. Heat treatment of the juice at temperatures comparable to those
at pasteurization and boiling does not change the carotenes, while
heating at temperatures used during sterilization results in
rearrangement of the carotene molecules and a decrease in total
carotenes. The all-trans alpha- and beta-carotenes appear partly as
cis-isomers, especially the 13-cis-isomer. Isomerization of the
carotenes leads to a decrease in their vitamin A activity. Carotenes
also seem to be anticarcinogens but the extent to which this property
is influenced by isomerization is still unknown.
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Hussein L. el-Tohamy M, Vitamin A potency of carrot and spinach carotenes
in human metabolic studies, International Journal for Vitamin & Nutrition
Research 60(3):229-35, 1990.
Changes in plasma retinol and carotenoids was measured in 17 young
males after daily ingestion of grated carrots, carrot juice or spinach
leaves for 2 weeks. Regression equations showed that the supply of
3350 and 4750 micrograms carotenes from 78 ml carrot juice
(prepared from 185 g carrots) or 91 g grated carrots, respectively
were adequate in maintaining plasma retinol at a constant level in
subjects with initial plasma retinol of 1.2 mumol/l. Under similar
experimental conditions, 280 g boiled spinach leaves providing
12,700 micrograms carotenes were required to maintain plasma
retinol at a constant level. Apparent carotene digestibilities of 47
and 81% were obtained with carrot and spinach, respectively.
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Best wishes,
Jean-Louis
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