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http://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/#risk
What are the health risks of too much vitamin A?
Hypervitaminosis A refers to high storage levels of vitamin A in the body that can lead to toxic symptoms. There are four major adverse effects of hypervitaminosis A: birth defects, liver abnormalities, reduced bone mineral density that may result in osteoporosis (see the previous section), and central nervous system disorders [1,48-49].
Toxic symptoms can also arise after consuming very large amounts of preformed vitamin A over a short period of time. Signs of acute toxicity include nausea and vomiting, headache, dizziness, blurred vision, and muscular uncoordination [1,48-49]. Although hypervitaminosis A can occur when large amounts of liver are regularly consumed, most cases result from taking excess amounts of the nutrient in supplements.
The IOM has established Tolerable Upper Intake Levels (ULs) for vitamin A that apply to healthy populations [1]. The UL was established to help prevent the risk of vitamin A toxicity. The risk of adverse health effects increases at intakes greater than the UL. The UL does not apply to malnourished individuals receiving vitamin A either periodically or through fortification programs as a means of preventing vitamin A deficiency. It also does not apply to individuals being treated with vitamin A by medical doctors for diseases such as retinitis pigmentosa.
Retinoids are compounds that are chemically similar to vitamin A. Over the past 15 years, synthetic retinoids have been prescribed for acne, psoriasis, and other skin disorders [50]. Isotretinoin (Roaccutane® or Accutane®) is considered an effective anti-acne therapy. At very high doses, however, it can be toxic, which is why this medication is usually saved for the most severe forms of acne [51-53]. The most serious consequence of this medication is birth defects. It is extremely important for sexually active females who may become pregnant and who take these medications to use an effective method of birth control. Women of childbearing age who take these medications are advised to undergo monthly pregnancy tests to make sure they are not pregnant.
What are the health risks of too many carotenoids?
Provitamin A carotenoids such as beta-carotene are generally considered safe because they are not associated with specific adverse health effects. Their conversion to vitamin A decreases when body stores are full. A high intake of provitamin A carotenoids can turn the skin yellow, but this is not considered dangerous to health.
Clinical trials that associated beta-carotene supplements with a greater incidence of lung cancer and death in current smokers raise concerns about the effects of beta-carotene supplements on long-term health; however, conflicting studies make it difficult to interpret the health risk. For example, the Physicians Health Study compared the effects of taking 50 mg beta-carotene every other day to a placebo in over 22,000 male physicians and found no adverse health effects [54]. Also, a trial that tested the ability of four different nutrient combinations to help prevent the development of esophageal and gastric cancers in 30,000 men and women in China suggested that after five years those participants who took a combination of beta-carotene, selenium, and vitamin E had a 13% reduction in cancer deaths [55]. In one lung cancer trial, men who consumed more than 11 grams/day of alcohol (approximately one drink per day) were more likely to show an adverse response to beta-carotene supplements [1], which may suggest a potential relationship between alcohol and beta-carotene.
The IOM did not set ULs for carotene or other carotenoids. Instead, it concluded that beta-carotene supplements are not advisable for the general population. As stated earlier, however, they may be appropriate as a provitamin A source for the prevention of vitamin A deficiency in specific populations [1].
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วิตตามินเอ ในรูปแบบเม็ด 25000 IU ดีไหมคะ
