Carrot

Beta-carotene

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β-Carotene is an organic compound and classified as a terpenoid. It is a strongly-coloured red-orange pigment abundant in plants and fruits. As a carotene with beta-rings at both ends, it is the most common form of carotene. It is a precursor (inactive form) of vitamin A.

The structure was deduced by Karrer et al. In nature, β-carotene is a precursor to vitamin A via the action of beta-carotene 15,15'-monooxygenase. β-Carotene is also the substance in carrots that colours them orange. β-Carotene is biosynthesized from geranylgeranyl pyrophosphate. Isolation of beta-carotene from fruits abundant in carotenoids is commonly done using column chromatography. The separation of beta-carotene from the mixture of carotenoids is based on the polarity of a compound. Beta-carotene is a non-polar compound, so it is separated with a non-polar solvent such as hexane. Being highly conjugated, it is deeply colored, and as a hydrocarbon lacking functional groups, it is very lipophilic.

Pro-vitamin A activity

Plant carotenoids are the primary dietary source of pro-vitamin A worldwide, with β-carotene as the most well-known pro-vitamin A carotenoid. Others inlcude α-carotene and β-cryptoxanthin. Carotenoids are absorbed into the small intestine by passive diffusion. One molecule of β-carotene can be cleaved by a specific intestinal enzyme into two molecules of vitamin A.

Absorption efficiency is estimated to be between 9-22%. The absorption and conversion of carotenoids may depend on the form that the β-carotene is in (cooked vs. raw vegetables, in a supplement), intake of fats and oils at the same time, and the current levels of vitamin A and β-carotene.

Researchers list the following factors that determine the pro-vitamin A activity of carotenoids:

• Species of carotenoid
• Molecular linkage
• Amount in the meal
• Matrix properties
• Effectors
• Nutrient status
• Genetics
• Host specificity
• Interactions between factors

Symmetric/Asymmetric Cleavage

In the β-carotene molecule, chain between the two cyclohexyls rings cleave either symmetrically or asymmetrically. Symmetric cleavage is done by an enzyme called beta-carotene-15,15'-dioxygenase in the human body. This symmetric cleavage gives two equivalent retinal molecules and each retinal molecule further reacts to give retinol (vitamin A) and retinoic acid. β-Carotene is also asymmetrically cleaved into two asymmetric products. The product of asymmetric cleavage is β-apocarotenal (8',10',12'). Asymmetric cleavage reduces the level of retinoic acid significantly.

Conversion factors

Until recently, vitamin A activity in foods was expressed as international units (IU). This is still the measurement generally used on food and supplement labels. However, it is difficult to calculate the total vitamin A activity in the diet in terms of IU, because both the absorption and conversion of carotenoids, as compared with retinol, are variable. The unit retinol equivalent (RE) was developed by the Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO) in 1967. More recently in 2001, the US Institute of Medicine proposed retinol activity equivalents (RAE) for their Dietary Reference Intakes.

International Units

1 RE = 3.33 IU vitamin A activity from retinol

1 RE = 10 IU vitamin A activity from β-carotene

(In Canada, Health Canada sets 1 RE = 6.667 IU from β-carotene.)

Retinol Equivalents (REs)

1 RE = 1 µg retinol

1 RE = 6 µg β-carotene (In Canada, Heath Canada sets 1 RE = 2 µg β-carotene.)

1 RE = 12 µg other provitamin A carotenoids

Retinol Activity Equivalents (RAEs)

1 RAE = 1 µg retinol

1 RAE = 2 µg all-trans-β-carotene as a supplement

1 RAE = 12 µg of all-trans-β-carotene in a food matrix

1 RAE = 24 µg other provitamin A carotenes in a food matrix

Sources in the diet

β-Carotene contributes to the orange color of many different fruits and vegetables. Vietnamese gac (Momordica Cochinchinensis Spreng.) and crude palm oil are particularly rich sources, as are yellow and orange fruits, such as mangoes and papayas, orange root vegetables such as carrots and yams and in green leafy vegetables such as spinach, kale, sweet potato leaves, and sweet gourd leaves. Vietnam gac and crude palm oil have by far the highest content of β-carotene of any known fruit or vegetable, 10 times higher than carrots for example. However, Gac is quite rare and unknown outside its native region of SE Asia, and crude palm oil is typically processed to remove the cartenoids before sale to improve the color and clarity.

The average daily intake of β-carotene is in the range 2–7 mg, as estimated from a pooled analysis of 500,000 women living in the USA, Canada and some European countries.

The U.S. Department of Agriculture lists the following 10 foods to have the highest β-carotene content per serving.

Side effects

The most common side effect of excessive β-carotene consumption is carotenodermia, a harmless condition that presents as a conspicuous orange skin tint arising from deposition of the carotenoid in the outermost layer of the epidermis. Chronic, high doses of β-carotene supplements have been associated with increased rate of lung cancer among those who smoke. Additionally, supplemental beta-carotene may increase the risk of prostate cancer, intracerebral hemorrhage, and cardiovascular and total mortality in people who smoke cigarettes or have a history of high-level exposure to asbestos.

Beta-carotene and lung cancer in smokers

Beta carotene increases the probability of lung cancer in cigarette smokers. When retinoic acid is liganded to RAR-beta (Retinoic Acid Receptor beta), the complex binds AP1 (Activator Protein 1). AP1 is a transcription factor that binds to DNA and in downstream events promote cell proliferation. Therefore, in the presence of retinoic acid, the retinoic acid:RAR-beta complex binds to AP1 and inhibit AP-1 from binding to DNA. In that case, AP1 is no longer expressed, and cell proliferation does not occur. Cigarette smoke increases the asymmetric cleavage of beta-carotene, decreasing the level of retinoic acid significantly. This can lead to a higher level of cell proliferation in smokers, and consequently, a higher probability of lung cancer.

However, no lung damage has been detected in those who are exposed to cigarette smoke and take a physiologic dose beta-carotene (6 mg) in contrast to pharmacologic dose (30 mg). Therefore, the oncology from beta-carotene is based on both cigarette smoke and high daily doses of beta-carotene.

Compendial status

• British Pharmacopoeia

See also

• Vitamin A
• Lycopene
• Lutein
• Zeaxanthin

References

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2. ^ "22040 β-Carotene BioChemika, purum, ≥97.0% (UV)". http://www.sigmaaldrich.com/catalog/ProductDetail.do?lang=en&N4=22040. Retrieved Oct. 21, 2009. 
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4. ^ P. Karrer, A. Helfenstein, H. Wehrli, A. Wettstein (1930). "Pflanzenfarbstoffe XXV. Über die Konstitution des Lycopins und Carotins". Helvetica Chimica Acta 13: 1084–1099. doi:10.1002/hlca.19300130532. 
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8. ^ Food and Agriculture Organization/World Health Organization (1967). Requirement of Vitamin A, Thiamine, Riboflavin and Niacin.. FAO Food and Nutrition Series B. Rome. 
9. ^ Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium and Zinc. Washington, DC: National Academy Press. 2001. 
10. ^ ^{a b} Beta-carotene, Natural Health Product Monograph, Health Canada
11. ^ Koushik, A.; Hunter DJ, Spiegelman D, Anderson KE, Buring JE, Freudenheim JL, Goldbohm RA, Hankinson SE, Larsson SC, Leitzmann M, Marshall JR, McCullough ML, Miller AB, Rodriguez C, Rohan TE, Ross JA, Schatzkin A, Schouten LJ, Willett WC, Wolk A, Zhang SM, Smith-Warner SA. (2006). "Intake of the major carotenoids and the risk of epithelial ovarian cancer in a pooled analysis of 10 cohort studies.". Int J Cancer 119 (9): 2148–54. PMID 16823847. 
12. ^ "USDA National Nutrient Database for Standard Reference, Release 21". http://www.ars.usda.gov/Services/docs.htm?docid=17477. Retrieved 2009-07-24. 
13. ^ Stahl W, Heinrich U, Jungmann H, et al. (1998). "Increased Dermal Carotenoid Levels Assessed by Noninvasive Reflection Spectrophotometry Correlate with Serum Levels in Women Ingesting Betatene". Journal of Nutrition 128 (5): 903–7. PMID 9567001. 
14. ^ Tanvetyanon T, Bepler G (July 2008). "Beta-carotene in multivitamins and the possible risk of lung cancer among smokers versus former smokers: a meta-analysis and evaluation of national brands". Cancer 113 (1): 150–7. doi:10.1002/cncr.23527. PMID 18429004. 
15. ^ Beta-carotene, MedlinePlus
16. ^ Russel, R.M. (2002). "Beta-carotene and lung cancer.". Pure Appl. Chem. 74 (8): 1461–1467. 
17. ^ British Pharmacopoeia Commission Secretariat (2009). "Index, BP 2009". http://www.pharmacopoeia.co.uk/pdf/2009_index.pdf. Retrieved 4 February 2010. 

External links

• A11CA02
• USDA Webpage on β-carotene Content of Gac - Fatty Acids and Carotenoids in Gac (Momordica Cochinchinensis Spreng) Fruit.