Beta-Carotene
Description
Beta-carotene is one of more than 400 carotenoids found in nature. Carotenoids give fruits and vegetables their red and yellow colors. Carotenoids are also abundant in green produce, where they are found inside the chloroplasts and chlorophyll.
As a general rule, the deeper the green, red or yellow color, the higher the concentration of beta-carotene.
Some of the best sources of beta-carotene are:
| carrot | squash |
| bell pepper | red pepper |
| spinach | sweet potato |
| apricot |
The carotenoids in green leafy vegetables are absorbed two to three times better than those in either red or yellow vegetables. Dunaliella bardawil, a sea-water algae, has been found to be unusually rich in beta-carotene content.
Beta-carotene is converted to vitamin A (retinol) in the gut, hence the name "provitamin A." This conversion requires adequate levels of protein, thyroid hormones, bile, zinc and vitamin C. The conversion is also affected by the age and overall health of the individual.
Inadequate beta-carotene intake may be a factor in the development of cancers of the skin, uterine, cervix, gastrointestinal tract, and lung. Data from a number of prospective and retrospective studies show an association between low dietary intake and serum levels of vitamin A and/or beta-carotene levels, and increased rates of epithelial cancers. However, some studies contradict these findings.
Supplementation with beta-carotene may prevent certain types of cancers. A number of reports were published between 1979 and 1986 which demonstrated an association between lung cancer and previous dietary intake of beta-carotene. For example, individuals who ate two or more servings per week of dark green leafy vegetables (rich in beta-carotene) were found to have 50% less lung cancer than those who did not. In another study, tobacco smokers who consumed higher levels of beta-carotene had a lower incidence of lung cancer.
However, it is also possible people who consume foods rich in beta-carotene may "tend to avoid some real cause of cancer in meat or other foodstuff of which they consume less."
The U.S. National Cancer Institute (NCI) funds several large-scale prospective studies of cancer risk and beta-carotene intake. NCI studies are designed to find if daily beta-carotene supplements can:
1. reverse precursor lesions
2. prevent the progression of precursor lesions to overt malignancies
3. reduce the incidence of malignancy
4. reduce cancer mortality
One NCI trial involves 22,000 male US physicians randomly divided between a beta-carotene supplement group (30 milligrams every other day), a group taking aspirin with or without beta-carotene, and a placebo group. The results from this trial and others are pending.
Method of Action
Beta Carotene has a dual role, as a precursor to vitamin A (which it also teams up with) and an antioxidant in its own right. Hence, it is mainly used against cancer, eye and skin diseases and various forms of diseases involving free radicals e.g. immunodeficiency, infection and inflammation.
Several mechanisms have been proposed to explain the anti-carcinogenic effects of beta-carotene. Peto et.al., have suggested since beta-carotene is an efficient free radical scavenger of singlet oxygen, it may act to reduce the toxic effects of this naturally occurring oxidant. A second explanation is based on dioxygenase, the enzyme which oxidizes beta-carotene to retinol. Dioxygenase may be present in tissues other than the gut and liver. The conversion of provitamin A to vitamin A at these other sites may produce potentially protective retinol deposits in sites where carcinogenesis would be taking place.
Beta-carotene can act as a cellular sunscreen against the photo-oxidative and free radical-induced damage caused by singlet oxygen resulting from ultraviolet radiation. Beta-carotene supplementation may decrease the skin redness experienced by light-skinned humans exposed to sunlight, allowing for longer initial exposure periods in the sun when tanning.
Therapeutic Approaches
Considerable experimental evidence shows animals are able to develop and remain healthy on a diet devoid of any carotene. However, this is not true of vitamin A, which is essential to both development and maintenance of health. Carotenes are not converted to vitamin A very efficiently. For example, in one study, infants given cooked spinach absorbed only 5% of the carotene.
Several studies show, children suffering from a deficiency of vitamin A do not benefit from carotenes, therefore questioning the utility of beta-carotene and other provitamin A carotenoids as vitamin A substitutes in infancy. Carotenes do not cross the placental barrier. A newborn's blood may show one-fifth to one-tenth the level of carotenes in maternal blood.
Blood carotenoid levels reflect beta-carotene intake. In one trial of well nourished adults, a daily beta-carotene supplement of 30 milligrams (5,000 retinol equivalents) nearly tripled baseline plasma carotenoid levels.
Eighty to ninety percent of ingested vitamin A is absorbed, with only a slight decrease in absorption with increasing doses. Beta carotene's absorption is much lower, in the range of 40 to 60%, but declines rapidly as the dosage increases.
On a fat-free diet, absorption of beta-carotene is nearly halved. This information is important to the user. For example, drinking concentrated wheat grass juice on an empty stomach without any dietary fat may reduce the potential absorption of beta-carotene considerably. Illness may also interfere with absorption of beta carotene, particularly diarrhea, celiac disease (gluten enteropathy) and steatorrhea, since in these conditions fat absorption is impaired.
Acne
There has been some excitement over the effects of retinoic acid on the skin, with respect to wrinkles etc. The hopes for acne appear to have been unrealized but it does seem to support the condition of the skin, generally.
Asthma
Beta-carotene maintains the health of the epithelium in the respiratory tract. For this reason, it is being increasingly studied as a benefit in asthma therapy.
Gastric Ulcers
In animal experimental studies, it has been shown beta-carotene and vitamin A can reduce the severity and frequency of gastric ulcers. This may be due to its action as a free radical scavenger of singlet oxygen.
Immunity
Beta-carotene has been shown in animal experiments to potentiate interferon's stimulatory action in the immune system, also inhibiting its suppressor effects on host effector cells. Further, retinol and retinoic acid exert an inhibitory effect on macrophage function.
In a study using normal health human volunteers, significant increases of helper/inducer T cells and total T-cells were observed 7 days after supplementing with 180 milligrams a day (300,000 I.U.) of beta-carotene.
[A combination of supplements is also popular e.g. 200,000 iu of Beta Carotene taken together with 25,000 iu of vitamin A.]
Lung Cancer
The best evidence beta-carotene traps free radicals and reduces lung tumor development comes from animal experiments. This effect has been confirmed in retrospective and prospective human epidemiological studies. One prospective study followed 99 persons over a ten year period. The results showed that individuals with low serum levels of beta-carotene in 1974 had an increased risk of subsequent squamous-cell carcinoma of the lung. The risk of lung cancer appeared to increase in a linear fashion with decreasing serum levels of beta-carotene. This linear relationship is in agreement with other similar studies. These findings suggest persons smoking tobacco give serious attention to their carotene intake.
Photosensitivity Disorders and Erythropoietic Protoporphyria (EPP)
Beta-carotene is a recognized treatment for photosensitivity disorders, especially erythropoietic protoporphyria, a condition characterized by elevated levels of porphyrin (in blood, skin and feces) and sensitivity to visible light.
When porphyrin levels are elevated the patient experiences a burning sensation, followed by edema and redness of the skin, upon exposure to sunlight. In EPP, it is necessary to maintain serum carotene levels at between 600-800 mcg/dl, although benefits may experienced for one to two months after the start of therapy.
Among the photosensitive disorders benefiting from beta-carotene supplementation are:
solar urticaria
discoid lupus erythematosis
pyoderma
glomerulenephritis
polymorphous light eruption
Radiation Side-effects
In a study using CBA mice given total-body-gamma-radiation (450-750 rads), vitamin A or beta-carotene supplements decreased mortality and prolonged survival time. Supplementation reduced all signs of radiation toxicity including weight loss, thymus involution, lymphopenia, and adrenal gland hyperaemia.
Toxicity Factors
Beta-carotene may be ingested for long periods of time with virtually no risk of toxicity (hypercarotenemia). However, an individual may develop an innocuous yellow skin pigmentation called carotenodermia (also referred to as xanthosis cutis).
Reports of neutropenia or menstrual irregularity caused by excessive carotene intake are questionable. Although carotene blood levels in these patients were high (221 to 1,007 mcg/dl), other subjects consuming considerably higher levels of beta-carotene and other carotenoids from carrots, for example, have not reported these symptoms. Symptom-free subjects ingested 300,000 to 600,000 I.U. per day for extended periods of time (equivalent to consuming 4 to 8 pounds of raw carrots a day).
Hypercarotenemia, or caroteniod toxicity, results from an excessive intake of beta-carotene. It does not lead to hypervitaminosis A. Change in skin pigment color (carotenodermia) usually associated with hypercarotenemia appears first in the palms of the hand, along the naso-labial folds, and later in those areas where sweating is most marked and where the skin is thick, such as the soles of the feet and groin area.
The absence of carotenodermia does not dismiss the possibility of excess carotenoid intake. Patients with lipoid nephrosis or nephritis can have carotinemia from excessive beta-carotene intake although their skin does not discolor. Further, high concentrations of carotene in blood leading to carotenodermia may be pathological in origin, such as: tuberculosis, disturbed lipid metabolism, failure to oxidize carotene, or myxoedema.
In animals, attempts at producing acute toxicity or death (LD50) have failed, even following abnormally high amounts of carotene. Doses up to 1,000 milligrams per kilogram of weight in rats and rabbits supplemented for extended periods of time with beta-carotene have produced no signs of embryotoxicity, interference in reproductive functioning, mutagenicity, or tumorogenicity.
Beta-carotene's storage occurs throughout the body, including the liver, adipose tissues, the adrenals, testes or ovaries, and the skin. Because it is stored in the latter tissue, excess total body stores become evident by the change in skin color (carotenodermia).
Occasionally, cases of carotenodermia are reported where no excess dietary intake or supplementation of beta-carotene or other carotenoids is found. In such cases, it may be indicative of a deficiency in those compounds or nutrients required in the conversion of beta-carotene to retinol.
Food colorings from carotenoids such as beta-carotene are used in the food industry to give processed foods particular colors. Such carotenoid food colorings are listed as GRAS by the FDA.
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