Tuesday, 29 May 2012
Beta carotene is one of the most important naturally occurring antioxidants. It is a fat-soluble pigment found in palm oil and plants (notably carrots and many colorful vegetables, and fruits) and in the sea alga Dunaleilla salina and D. bardawil. Natural beta carotene supplements are derived primarily from D. salina. Beta carotene is one of the major dietary carotenoids and one of the most biologically active of approximately 800 carotenes and more than 1,000 carotenoids present in food. It is responsible for the orange or yellow colors of many fruits and vegetables. In the human body, beta carotene is found in lipids and in fat tissues. Sometimes beta carotene is called provitamin A because it is more easily converted to vitamin A (retinol) in the liver than other carotenoids. Beta carotene is considered to be a conditionally essential nutrient because it becomes essential when vitamin A intake is low.
Beta carotene consists of a chain of 40 carbon atoms, with conjugated double bonds and a ring structure at each end of the chain. Depending on the positions of the molecular groups attached to the carbon chain, naturally occurring beta carotene may be:
Synthetic beta carotene is primarily all-trans.
In plants and alga, beta carotene and other carotenoids attract light for photosynthesis and provide protection from toxic forms of oxygen. Beta carotene is a powerful antioxidant because it destroys toxic free radicals, including singlet oxygen—an oxygen atom that is missing an electron and is very damaging to human tissue if not taken up quickly and "deactivated."
Vitamin A Precursor
Vitamin A is obtained in the diet from animal products or is made in the liver from beta carotene and other carotenoids. Vitamin A is essential for:
In sub-Saharan Africa about three million children under the age of five suffer from an eye disorder, caused by vitamin-A deficiency, that can lead to blindness and death. Although red palm oil, a traditional African food, contains high provitamin A, its substitution by imported cooking oils has reduced this dietary source in many homes. Many vegetables and fruits also contain provitamin A, but are not always consumed in adequate amounts. Vitamin A deficiency is the leading cause of blindness worldwide.
In the 1920s vitamin-A deficiency was linked to stomach cancer and to precancerous conditions in the epithelial (lining) cells of the throat and lungs. In 1977, vitamin A supplementation was shown to inhibit certain cancers and to reduce the growth of certain tumors in at-risk animals.
Dietary Beta Carotene
Carotenoids, including beta carotene, that are obtained from food may have:
Epidemiological studies that looked at cancer rates and diet found that at least five daily servings of green, orange, red, and yellow vegetables and fruits appeared to significantly reduce the risk of stomach, lung, prostate, breast, head, and neck cancer, and possibly slow the progression of others. In 1971, a large human study linked cancer death rates to low levels of beta carotene in the blood. Subsequent studies linked high blood levels of dietary beta carotene to lower cancer risk. However, more recent evidence links these results to a combination of antioxidants found in fruits and vegetables, rather then to beta carotene alone. High beta carotene levels in the blood may be associated with a reduced risk of asthma.
Supplemental Beta Carotene
Supplemental beta carotene has been claimed to:
However, there is very little evidence that supplemental beta carotene is an effective cancer-preventing substance, except perhaps in those with poor nutrition or low baseline levels of beta carotene in the blood. Additional studies have shown that beta-carotene supplements do not reduce the risk of cancer, heart disease, or cataracts.
Yet supplemental beta carotene does appear to increase the amounts of some types of immune-system cells. Studies have shown that women with low dietary intake or low blood levels of beta carotene are at increased risk for cervical dysplasia (abnormal cell growth) and cervical cancer. Another study found that beta-carotene supplementation provided some protection against the recurrence of colorectal adenoma (benign tumors) in patients who neither smoked nor drank alcohol. However in patients who used tobacco and/or alcohol the risk of recurrence increased with beta-carotene supplementation. Studies of smokers and/or those who had been exposed to asbestos initially found an increase in lung cancers among those receiving supplemental beta carotene (at least 20 mg per day), particularly among those who also consumed large quantities of alcohol. However, follow-up studies found no such increases in lung cancer in those taking beta carotene.
The Age-Related Eye Disease Study found that a combined supplement of beta carotene, vitamin C, vitamin E, zinc, and copper reduced the risk of disease progression and vision loss in people with advanced macular degeneration. The supplement did not slow disease progression in those with early-stage macular degeneration.
One study found that supplementation with a mixture of antioxidants—beta carotene, alpha-tocopherol, and plant sterols—lowered cholesterol levels in the blood.
Beta carotene at 25,000 international units (IU) daily may be useful for treating psoriasis, a skin condition. Beta carotene supplements also are used to treat acne. Two 25,000-IU supplements daily, in combination with other supplements, are sometimes used to treat stomach ulcers.
Measuring Beta Carotene
The recommended dietary allowance (RDA) for beta carotene has not been established and most foods are not labeled with vitamin A content. There are two incompatible systems for quantifying beta carotene. IUs are used most often for nutritional labeling:
The second system uses retinol equivalents (RE):
Dietary Beta Carotene
Daily values (DVs) are determined from the RDA. They are based on a 2,000-calorie diet and usually are expressed as a percentage of an RDA.
Carrots and sweet potatoes that are more orange contain more beta carotene. New carrot cultivars that contain more beta carotene have been developed and high-beta-carotene sweet potatoes are being introduced into sub-Saharan Africa to treat vitamin-A deficiency.
Other foods that contain beta carotene include:
According to the Institute of Medicine, a daily intake of 3–6 mg of beta carotene will keep the blood level within the range associated with a lower risk for chronic diseases. The recommended daily diet of five or more servings of fruits and vegetables provides 3–6 mg of beta carotene (if carrots, sweet potatoes, papaya, apricots or other very high carotenoid food is used, the RDA can be met in a single serving). In contrast, the average American diet contains 1.3–2.9 mg daily. Vegetarians may have twice as much beta carotene in their blood as compared to non-vegetarians, because they generally consume a lot more greens and fruits.
Beta carotene in food is found within oil or a matrix of sugars and proteins; and, therefore, the absorption of beta carotene by the body varies greatly. The elderly, and those with bad digestion and liver trouble may be at risk for poor absorption from a diet with adequate beta carotene.
Animal sources of vitamin A are more easily absorbed than plant sources of beta carotene, particularly if the vegetables and fruits are eaten raw or whole. Although beta carotene can be converted to vitamin A in the body, it has its own unique physiological functions and beta carotene and vitamin A are not totally identical in the health benefits they deliver. So it is good to eat sources of both. While supplementation is helpful to those who have trouble absorbing adequate beta carotene, getting all or some beta carotene through food sources rather than supplements alone is by far the best. This is substantiated by research showing there are many beneficial carotenoids in foods, and that they may also work together synergistically to optimize health.
Supplemental Beta Carotene
Beta carotene supplements are inexpensive and readily available over-the-counter. They are available as pills, powders, and oils and they vary greatly in potency. Some supplements contain a mixture of carotenoids. There is a major problem with shelf life stability for beta carotene, as it "oxidizes" quickly when in pure form. When buying a supplement of it, shelf life stability or the presence of such stabalizers as vitamin E can guarantee biological activity of the capsule.
Supplemental intake of beta carotene probably should not exceed 3–15 mg per day. Common preparation of supplemental beta carotene include:
A typical dosage of beta carotene for treating cancer is 75,000–150,000 IU daily. Absorption of beta carotene in nutritional supplements can be 70% or more. There is no established maximum daily intake for beta carotene.
Some common beta carotene nutritional supplements include:
Manufacturers often supplement food with beta carotene. One study showed that bakery products enriched with beta carotene increased beta carotene levels in the blood.
Antioxidants such as beta carotene often work together with other antioxidants and an excess or deficiency of one can inhibit the other. The Food and Nutrition Board of the Institute of Medicine does not recommend beta carotene supplementation except in cases of vitamin A deficiency.
Pregnant and nursing mothers should limit their intake of supplemental beta carotene to 6 mg per day or less.
Even long-term high-dosage use of supplemental beta carotene appears to be non-toxic. Daily doses of 30 mg or more over a long period may cause carotenosis (carotenodermia), a yellowing of the skin, which is harmless and reversible. In contrast, very high daily doses of vitamin A are very dangerous and damage the liver and other organs, as well as provoke hair loss).
Drugs and other substances that may interfere with beta-carotene absorption include:
The absorption of luteine, another carotenoid antioxidant, may be reduced if taken in conjunction with beta carotene.