Comprehensive Guide to Beta-Carotene
The splitting of Beta-Carotene (and other carotenes) into retinol within intestinal cells is well regulated to help guard against Vitamin A toxicity. The retinol that is formed from Beta-Carotene enters the chylomicron and is metabolized from that point forward as preformed Vitamin A. Chylomicrons primarily deliver Beta-Carotene to the liver, where they are repackaged within another lipoprotein carrier system known as the very-low-density lipoprotein.
Beta-Carotene (and other carotenoids) enters the bloodstream from the liver and is transported to peripheral tissue by very-low density lipoproteins and low-density lipoproteins (VLDL and LDL, which is the remnant particle of VLDL after triglycerides are removed by fat cells, muscle fibers and other tissues). In contrast, Vitamin A is transported form the liver attached to retinal-binding protein (RBP). Beta-Carotene is stored in fat tissues, and the adrenal glands, testes, ovaries, rather than the liver and is responsible for the yellowish tinge to the skin when large amounts are stored (carotenodermia). However, carotenodermia is considered to be a nonpathological, reversible condition; not associated with any health risks. Some conversion of Beta-Carotene may take place in the liver and lungs. About 40-60% of Beta-Carotene is absorbed from food. Of interest is the fact that Beta-Carotene supplements are better absorbed than carotenes from food. Beta-Carotene comprises 20-25% of the total serum carotene level.
Antioxidant: Beta-Carotene is an antioxidant and does not need to be converted into Vitamin A to perform antioxidant functions.
Immune System: Beta-Carotene appears to enhance thymus gland function and increases interferon's stimulatory action on the immune system.1
Other Functions: as described below, Beta-Carotene exhibits a number of immune-enhancing and anti-cancer propertied, and has therefore, been tested in patients with immune-compromised states, precancerous, and cancerous conditions, as well as in patients at high risk in developing certain cancers.
Compromised Immune Function
A number of studies reveal that older subjects can enhance various aspects of immune function through the supplementation of at least 15 mg of Beta-Carotene (25,000 I.U.) per day. The immune system tends to weaken as humans age, thus researchers have examined various nutrients that may prevent or reverse age-related decline in immune function. High doses of Beta-Carotene have been used in the treatment of immune compromised states and studies on normal human volunteers indicate that supplementation with 180 mg (300,000 I.U.) of Beta-Carotene per day, significantly increased in the number of T-helper cells by approximately 30% after seven days of supplementation, with a 30% increase in a total T-cell count after 14 days. This may be of great significance in HIV/AIDS patients, who have low T-helper cell counts and other parameters of immune function compromise.2-5
At this time it is inadvisable to give high dose Beta-Carotene supplementation (50,000 I.U. or greater) to patients who smoke one pack of cigarettes per day or more. The Alpha-Tocopherol, Beta-Carotene study and the CARET study suggested that Beta-Carotene, in these cases, may slightly increase the risk of lung cancer, although this needs confirmation.6,7 However, Beta-Carotene does demonstrate a number of anti-cancer properties and has been shown to reverse leukoplakia – a pre-cancerous condition of the oral cavity, as well as early-stage cervical dysplasia, a pre-cancerous condition of the uterine cervix.8-13 In the Linxian China study, the combination of modest dosages of Beta-Carotene, Vitamin E, and selenium significantly reduced stomach and esophageal cancers, as well as total cancer incidence in high-rish individuals, compared to other vitamin and mineral combinations.26 Beta-Carotene is an antioxidant, an immune system modulator and enahances cellular differentiation of epithelial cells. All of these effects are associates with the prevention of cancer and the reversal of some early stage cancers and states of dysplasia (pre-cancerous states).4-13
Beta-Carotene supplementation has been shown to decrease oxidation of LDL-cholesterol, but to a lesser degree than Vitamin E. In this regard, it may help to reduce the risk of cardiovascular disease, as oxidized LDL-cholesterol appears to be more inclined to narrow arteries as part of the atherosclerotic process that leads to heart disease and ischemic stroke. However, evidence is stronger for Vitamin E. Both Vitamin E and Beta-Carotene are transported through the bloodstream within VLDL and LDL lipoproteins, where they are able to act as antioxidants in regards to reducing the oxidation of fatty acids and cholesterol within these lipoproteins (VLDL and LDL).14,15,16 The Physicians Helath Study failed, to show a benefit in cardiovascular disease reduction with Beta-Carotene supplementation of 50 mg (83,333 I.U.), taken every other day for 12 years. However, a subgroup analysis of these 22,000 medical doctors showed that of the 333 physicians prior history of heart disease, Beta-Carotene supplementation produced a small reduction in risk of fatal and non-fatal heart attack.30 A number of prospective studies have suggested that higher intakes of Beta-Carotene is associated with a significant reduction in heart attack and stroke, as highlighted in the Western Electric Study in Chicago and a study of Italian women by A Tavani, et al.28,29
1. Compromised Immune Function: 50,000 I.U., but a dosage of up to 3,000,000I.U. has been used in short term studies2-5
2. HIV/AIDS: 50,000 I.U., twice daily has been used with some success2,27
3. Oral Leukoplakia: 50,000-1000,000 I.U. per day9,10
4. Cervical Dysplasia: 50,000-100,000 I.U. per day12,13
5. Cancer Treatment Support: 75,000-100,000 I.U. per day (lung cancer would be an exception)11
6. Heart Diseases and Cardiovascular Health: 10,000-75,000 I.U.14,30
7. General Wellness: 10,000-25,000 I.U. is commonly consumed
Bile Acid Sequestrants, such as cholestyramine and colestipol may decrease absorption of Beta-Carotene (as they do other fat-soluble vitamins).19,20
Proton Pump Inhibitors such as omeprazole are known to decrease Beta-Carotene absorption.21
Other drugs that impair Beta-Carotene absorption include:
- mineral oil23
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Santos MS, Meydani SN, Leka L, Wu D, Fotouhi N, Meydani M, et al. Natural killer cell activity in elderly men is enhanced by B-Carotene supplementation. Am J Clin Nutr 1996;64:772-7.
The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. N Engl J Med 1994;330:1029-35.
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Liu T, Soong SJ, Wilson NP, Craig CB, Cole P, Macaluso M, et al. A case control study of nutritional factors and cervical dysplasia. Cancer Epidemiol Biomarkers Prev 2. 1993;2(6):525-30.
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deVet HC, Knipschild PG, Willebrand D, Schouten HJ, Sturmans F. The effect of Beta-Carotene on the regression and progression of cervical dysplasia: a clinical experiment. J Clin Epidemiol 1991:44:273-93.
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Diarrhea and Constipation. In: Berkow R, Fletcher AJ, Beers MH, et al, editors. The Merck Manual of Diagnosis and Therapy. 16th ed. Rahway, NJ: Merck Research Laboratories; 1992. p. 810.
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Blot WJ, et al. Nutrition intervention trials in Linxian China: supplementation with specific vitamin/mineral combinations, cancer incidence, and disease-specific mortality in the general population. J Natl Cancer Inst 1993;85:1483-92.
Fryburg DA, Mark RJ, Griffith BP, et al. The effect of supplemental beta-carotene on immunologic indices in patients with AIDS: a pilot study. Yale J Biol Med 1995;68(1-2):19-23.
Daviglus ML, Orencia AJ, Dyer AR, et al. Dietary vitamin C, beta-carotene and 30-year risk of stroke: results from the Western Electric Study. Neuroepidiology 1997;16(2):69-77.
Tavani A, Negii E, D'Avanzo, et al. Beta-carotene intake and risk of nonfatal acute myocardial infarction in women. Eur J Epidemiol 1997;13(6):631-7.
Hennekens CH, et al. Lack of effect of long term supplementation with beta-carotene on the incidence of malignant neoplasms and cardiovascular disease. N Engl J Med 1996;334:1145-9 and 1189-90.