Читать книгу The Vitamin Cure - Monte Lai - Страница 10

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VITAMIN A (RETINOL)

Vitamin A is also known as the anti–dry eye vitamin. In 2000 BCE, physicians in ancient Egypt and Greece already knew that beef liver could cure night blindness. Of course, they had no way of knowing that this was because beef liver contains high amounts of vitamin A. In 1913, Lafayette Mendel and Thomas Osborne were able to isolate a “lipid-soluble” substance from chicken eggs and fish liver and called it vitamin A. In 1930, the chemical structure of vitamin A was identified as retinol.

Vitamin A enhances vision, allowing us to differentiate people from objects in our surroundings in dim light. In the eye, vitamin A binds to opsin, a photosensitive protein, to form rhodopsin. The photosensitivity of rhodopsin essentially establishes our eyesight and vision.

Vitamin A is also called the “anti-infective vitamin.” The body’s first-line defense against infections involves the skin, respiratory tract, digestive tract, and urinary tract. Vitamin A activates immune cells in these tissues.

Since our bodies cannot make vitamin A, it has to be obtained from foods. Preformed vitamin A and provitamin A are two kinds of vitamin A found in foods. Vitamin A from animal-based foods, such as beef liver, is preformed vitamin A, which the body uses promptly. Plants, such as carrots, contain carotenes. Carotenes are made up of provitamin A, which must be converted to vitamin A in the digestive tract before our bodies can utilize them.

What Are the Symptoms of Vitamin A Deficiency?

 • Xerophthalmia. Xerophthalmia, or abnormal dryness of the conjunctive cornea of the eye, is the most common symptom in vitamin A–deficient children. Symptoms related to vitamin A deficiency in adults include night blindness and diminished adaptability of eyesight under dim light or at nighttime. In severe cases, patients might be afraid of driving or being outdoors alone at night because of their weakened vision.

 • Eye disease and blindness. Severe vitamin A deficiency can cause blindness. Pregnant women deficient in vitamin A have an elevated risk of giving birth to babies with dry eye syndrome and even blindness. Globally, about half a million children suffer from blindness related to vitamin A deficiency; it is the most preventable cause of blindness in children. Hippocrates, the celebrated physician of Greek antiquity, told his patients who had eye diseases to eat cooked beef liver, which is rich in vitamin A.

 • Infectious diseases. Infectious diseases can consume and deplete vitamin A stored in the liver, causing vitamin A deficiency. Measles infections depleting vitamin A can also lead to dry eye syndrome and xerophthalmia or even blindness in children.

 • Thyroid malfunction. The thyroid gland produces thyroid hormones T3 and T4. T3 is the active form of the thyroid hormone, and vitamin A enhances the conversion of T4 to T3. Vitamin A deficiency can hinder this conversion and cause thyroid malfunction.

What Are the Causes of Vitamin A Deficiency?

 • Lipid malabsorption. Lipids, or fats, facilitate the absorption of lipid-soluble vitamin A in the intestines, so lipid malabsorption can reduce the absorption of vitamin A, resulting in vitamin A deficiency. Lipid malabsorption is often caused by a blockage of pancreatic or gallbladder juice secretion, Crohn’s disease, celiac disease, or inflammatory bowel disease.

Prevention and Treatment of Diseases

 • Prevention. Meta-analysis confirms that vitamin A can prevent lung cancer (82), bladder cancer (44), stomach cancer (107), glioma (70), cervical cancer (50), asthma (38), and cataracts (49).

 • Treatment. High doses of vitamin A can be used as an adjuvant therapy for treating blood cancers (45).

Which Food Items Are Vitamin A Rich?

Beef liver and fish liver oil contain high amounts of vitamin A. Milk and eggs are also excellent sources of vitamin A. Vegetables and fruits contain high levels of carotenoids.

This list of food items that are rich in vitamin A and beta-carotene is adapted from information provided by the USDA.

Daily reference value of vitamin A is 5,000 IU according to the 2013 FDA food-labeling guidelines.

How to Calculate the Dosage of Vitamin A

International Units (IU) are commonly used to quantify vitamin A in food items and supplements.

 • 1 IU of vitamin A is equal to 0.3 mcg of retinol

 • 1 IU of beta-carotene is equal to 0.15 mcg of retinol

 • 1 IU of beta-carotene in food is equal to 0.025 mcg of retinol

What Are the Recommended Dietary Allowances for Vitamin A?

The upper daily intake limit of vitamin A is 10,000 IU.

Vitamin A Supplements

 • Dosage. The common dosage for vitamin A supplements ranges from 5,000 to 25,000 IU. Vitamin A supplements can be taken either daily or weekly. The recommended dose is 3,000 IU daily or 20,000 IU weekly.

 • Types. Vitamin A supplements are mainly composed of retinol, retinyl acetate, and retinyl palmitate. Their weight-to-unit conversion factors are as follows:◦ 0.3 mcg of retinol is equal to 1 IU of vitamin A◦ 0.344 mcg of retinyl acetate is equal to 1 IU of vitamin A◦ 0.55 mcg of retinyl palmitate is equal to 1 IU of vitamin A

 • Fish liver oil. Fish liver oil contains vitamin A, vitamin D, and omega-3 fatty acids, including EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). There are two types of fish liver oil supplements: regular and fermented. Fermented fish liver oil supplements are more expensive compared to regular fish liver oil supplements. The fermentation process enhances the contents and purity of vitamin A, vitamin D, and the omega-3 fatty acids. The vitamin A in fish liver oil is a natural retinyl palmitate, which is the same as synthetic retinyl palmitate.

 • Melanoma. Vitamin A can prevent melanoma. Clinical studies have shown that taking a dose of 4,000 IU of vitamin A daily for six months reduced the risk of melanoma by 40%. The protective effect was more profound in women than in men.

 • Wrinkles. Vitamin A reduces wrinkles, making older skin look younger. Topical vitamin A at a dose of 0.4% applied three times weekly was effective in reducing wrinkles within weeks.

Safety Issues

 • Overdose. Long-term consumption of high doses of vitamin A may cause hypervitaminosis A. Symptoms include blurred vision, lack of appetite, dry skin, and muscle weakness. High doses of vitamin A can cause liver damage as well as osteoporosis.

 • Pregnant women. During pregnancy, women should take no more than 25,000 IU of vitamin A supplement daily to avoid hypervitaminosis A, which can lead to birth defects in infants.

 • Recommended daily dose. Vitamin A is lipid-soluble and, once absorbed, will be retained in the body for a long time. It is recommended that you take no more than 10,000 IU of vitamin A per day.

What Types of Drugs May Interact with Vitamin A?

 • Alcohol can exacerbate liver damage caused by high doses of vitamin A. Drinking alcoholic beverages with beef liver slices as a side dish is prevalent in some Asian cultures, which can lead to vitamin A toxicity. Furthermore, cholesterol-lowering drugs that hinder lipid absorption may also diminish vitamin A absorption.

 • Long-term consumption of high doses of vitamin A can decrease vitamin K absorption, leading to vitamin K deficiency. Vitamin A enhances the drug action of warfarin. People who take warfarin to treat and prevent blood clots together with high-dose vitamin A should consult with health-care professionals.