Genox

Carotenoids

Introduction

Carotenoids are red, yellow and orange pigments that are widely distributed in nature.24  Although specific carotenoids have been identified in photosynthetic centers in plants, bird feathers, crustaceans and marigold petals, they are especially abundant in yellow-orange fruits and vegetables and dark green, leafy vegetables.25  Of the more than 700 naturally occurring carotenoids identified thus far, as many as 50 may be absorbed and metabolized by the human body.26  To date, only 14 carotenoids have been identified in human serum.26

Carotenoids absorb light in the 400-500 nm region of the visible spectrum. This physical property imparts the characteristic red/yellow color of the pigments.  Carotenoids contain a conjugated backbone composed of isoprene units, which are usually inverted at the center of the molecule, imparting symmetry.  Changes in geometrical configuration about the double bonds result in the existence of many cis and transisomers.  Hydroxylated, oxidized, hydrogenated or ring-containing derivatives exist.  Hydrocarbon carotenoids are classified as carotenes while those containing oxygen are known as xanthophylls.27

Absorption, Metabolism and Distribution in Humans

Carotenoids are absorbed from the intestine with the aid of dietary fat and incorporated into chylomicrons and very low and low density lipoproteins for transport in the serum.28 Approximately 1 percent circulate in serum on high and low density lipoproteins.29 The different structural features possessed by carotenoids account for selective distribution in organ tissue, biological activity and provitamin A potency, or in vivo conversion to vitamin A.  Due to the hydrophobic character, carotenoids are associated with lipid portions of human tissues, cells and membranes.  In general, 80-85% of carotenoids are distributed in adipose tissue, with smaller amounts found in the liver, muscle, adrenal glands and reproductive organs.29 The major serum carotenoids are beta-carotene, alpha-carotene, lutein, zeaxanthin, lycopene and cryptoxanthin. Smaller amounts of polyenes such as phytoene and phytofluene are also present.29

As adipose tissue is the largest body pool for carotenoids, serum concentrations are fairly constant and slow to change during periods of low intake.  The estimated half-life is estimated to be 11-14 days for lycopene, alpha-carotene, beta-carotene, lutein and zeaxanthin.30, 31 Evidence for the existence of more than one body pool has been published.31

Serum levels reflect lifestyle choices and dietary habits within and between cultures.26  Variations can be attributed to different intakes, unequal abilities to absorb certain carotenoids, and different rates of metabolism and tissue uptake.29  Decreased serum levels occur with alcohol consumption,32 the use of oral contraceptives,33 smoking 33, 34 and prolonged exposure to UV light.46

Vitamin A Potency
alpha-Carotene, beta-carotene and beta-cryptoxanthin can be converted to retinol (vitamin A) in the intestine and liver by the enzyme 15-15’-beta-carotenoid dioxygenase.29 Such in vivo formation of retinol appears to be homeostatically controlled, such that conversion to retinol is limited in persons having adequate vitamin A status.35  In a recent study, increases in serum beta-carotene levels did not result in concomitant increases in serum retinol.31  Therefore, the toxic effects of large doses of vitamin A are not produced by ingestion of these carotenes, even at high levels of intake.

Prevalence in Food, Effects of Cooking and Differences in Absorption
Approximately 80-90% of the carotenoids present in green, leafy vegetables such as broccoli, kale, spinach and brussel sprouts are xanthophylls, whereas 10-20% are carotenes.  Conversely, yellow and orange vegetables including carrots, sweet potatoes and squash contain predominantly carotenes.  Up to 60% of the xanthophylls and 15% of the carotenes in these foods are destroyed during microwave cooking.  Of the xanthophylls, lutein appears to be the most stable.36 Effects of freezing, conventional cooking and prolonged storage are unknown.

Several factors influence the differential absorption of carotenoids from food:

  • Cooking:  Although cooking reduces the carotene content of food, it also disrupts cellular membranes and liberates nutrients.  Therefore, carotenes are absorbed more efficiently from cooked versus uncooked foods.31
  • Supplements versus purified sources:  Serum levels of beta-carotene were almost 20% higher in persons who consumed purified beta-carotene in a capsule compared to those ingesting an equal amount from cooked carrots.31
  • Competitive absorption and the influence of other dietary carotenoids:  The absorption of alpha-carotene appears to be greater than beta-carotene from the same food source.  Moreover, large doses of purified beta-carotene may decrease intestinal absorption of other carotenoids.  Absorption of lutein from broccoli was decreased with concurrent ingestion of purified beta-carotene.31

Antioxidant Defense:  Cancer and Disease Prevention

The established efficacy of beta-carotene in quenching singlet oxygen and intercepting deleterious free radicals and reactive oxygen species makes it part of the diverse antioxidant defense system in humans.37, 38  Reactive oxygen species have been implicated in the development of many diseases, including ischemic heart disease, various cancers, cataracts and macular degeneration.39  Because the conjugated polyene portion of beta-carotene confers its antioxidant capability and all carotenoids possess this structural feature, research efforts have been directed at evaluating the efficacy of other carotenoids in the prevention of free radical-mediated diseases.  Indeed, in vitro experiments have demonstrated that lycopene, alpha-carotene, zeaxanthin, lutein and cryptoxanthin quench singlet oxygen and inhibit lipid peroxidation.40  Oxidized metabolites of lutein, zeaxanthin and lycopene have been isolated and identified, and may provide direct evidence for the antioxidant action of these carotenoids.26

In addition to antioxidant capability, other biological actions of carotenoids include the ability to enhance immunocompetence41 and in vitro gap junction communication,42 reduce or inhibit mutagenesis and inhibit cell transformations in vitro.43,44

Many epidemiological studies have established an inverse correlation between dietary intake of yellow-orange fruit and dark green, leafy vegetables and the incidence of various cancers, especially those of the mouth, pharynx, larynx, esophagus, lung, stomach, cervix and bladder.1,2 While a number of protective compounds may be responsible for this observation, the co-incidence of carotenoids in these foods has been noted. Because nutritionists and medical professionals currently recognize the occurrence of a large number of distinct carotenoids in food, interest in their functions and biological impact on health is burgeoning. Genox currently measures retinol and the following carotenoids in serum:

·  Lutein

·  Total Lycopene

·  Zeaxanthin

·  alpha-Carotene

·  beta-Cryptoxanthin

·  beta-Carotene

·  Lycopene

·  Total beta-Carotene

Lutein
This xanthophyll exists in the retina. It functions to protect photoreceptor cells from light-generated oxygen radicals, and thus plays a key role in preventing advanced macular degeneration.18 Lutein possesses chemopreventive activity, induces gap junction communication between cells and inhibits lipid peroxidation in vitro more effectively than beta-carotene, alpha-carotene and lycopene.42 High levels of lutein in serum have been inversely correlated with lung cancer.26  Lutein occurs in mango, papaya, oranges, kiwi, peaches, squash, peas, lima beans, green beans, broccoli, brussel sprouts, cabbage, kale, lettuce, prunes, pumpkin, sweet potatoes and honeydew melon.26 Commercial sources are obtained from the extraction of marigold petals.26  Lutein is not converted to vitamin A in vivo.29

Zeaxanthin
In addition to lutein, zeaxanthin exists in the retina and confers protection against macular degeneration.18 Zeaxanthin is also prevalent in ovaries and adipocyte tissue.45 Dietary sources include peaches, squash, apricots, oranges, papaya, prunes, pumpkin, mango, kale, kiwi, lettuce, honeydew melon and yellow corn.26  This xanthophyll is not converted to vitamin A in vivo.29

Alcohol consumption has been shown to influence lipid peroxidation.  Anhydrolutein, an oxidative by-product of lutein and zeaxanthin, was higher in plasma after alcohol ingestion, while concentrations of these xanthophylls were reduced. Lutein and zeaxanthin may therefore have protective effects against LDL oxidation.32

Lycopene
The all-trans isomer of this carotenoid is typically quantified in serum, although signals for 9-, 13- and 15-cis isomers are detectable and account for as much as 50% of the total lycopene.47 In experiments performed in vitro, lycopene quenched singlet oxygen more efficiently than alpha-carotene, beta-carotene, zeaxanthin, lutein and cryptoxanthin.40  Lycopene induces gap junction communication, inhibits lipid peroxidation and has displays chemopreventive activity.42 Serum levels of lycopene are inversely correlated to the risk of developing cancer in the pancreas and cervix.49 This carotenoid has been identified in tissues of the thyroid, kidneys, adrenals, spleen, liver, heart, testes and pancreas.45  The red color of fruits and vegetables such as tomatoes, pink grapefruit, watermelon and the skin of red grapes and red guava is due to lycopene. Other dietary sources include papaya and apricots. Lycopene is not converted to retinol in vivo.29

beta-Cryptoxanthin
beta-Cryptoxanthin is capable of quenching singlet oxygen.40 It occurs in oranges, mango, papaya, cantaloupe, peaches, prunes, squash, and is used to color butter.26 beta-Cryptoxanthin is converted to retinol in vivo.29

beta-Carotene
The all-trans isomer of this carotenoid is the major source of dietary retinoids, due to its high provitamin A activity.29 One molecule of trans-beta-carotene can theoretically provide two molecules of trans retinaldehyde in vivo. Signals for 13- and 15-cis isomers of beta-carotene are also observed in the carotenoid profile and account for 10% or less of the total beta-carotene in serum. beta-Carotene quenches singlet oxygen, induces gap junction communication and inhibits lipid peroxidation.42 High serum levels of beta-carotene are correlated with low incidences of cancer in the mouth,3 lung,5 breast,8 cervix,7 skin50 and stomach.5 beta-Carotene has been identified in tissues of the thyroid, kidney, spleen, liver, heart, pancreas, fat, ovaries and adrenal glands.45,47 Dietary sources include mango, cantaloupe, carrots, pumpkin, papaya, peaches, prunes, squash, sweet potato, apricots, cabbage, lima beans, green beans, broccoli, brussel sprouts, kale, kiwi, lettuce, peas, spinach, tomatoes, pink grapefruit, honeydew melon and oranges.26

alpha-Carotene
This carotenoid is similar to beta-carotene in its biological activity, but quenches singlet oxygen more effectively.40 alpha-Carotene improves gap junction communication, prevents lipid peroxidation and inhibits the formation and uptake of carcinogens in the body.42  High serum levels have been associated with lower risks of lung cancer. With one half the provitamin A potency of beta-carotene,29 alpha-carotene also restores normal cell growth and differentiation. Serum levels are usually between 10 and 20% of the values for total beta-carotene. Dietary sources include sweet potatoes, apricots, pumpkin, cantaloupe, green beans, lima beans, cabbage, kale, kiwi, lettuce, peas, prunes, peaches, mango, papaya, squash and carrots.

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