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.
| FOODS | PORTION | VITAMIN A CONTENT, IU | % DAILY REFERENCE VALUE |
|---|---|---|---|
| Beef liver (cooked) | 1 slice (68 g) | 21,566 | 431 |
| Sweet potato (cooked) | ½ cup | 19,218 | 384 |
| Pumpkin (cooked) | ½ cup | 19,065 | 381 |
| Carrots (raw) | ½ cup | 10,692 | 214 |
| Spinach (cooked) | ½ cup | 9,433 | 214 |
| Cantaloupe | ½ | 9,334 | 187 |
| Mango | 1 | 3,636 | 73 |
| Broccoli (cooked) | ½ cup | 1,207 | 24 |
| Milk | 1 cup | 395 | 8 |
| Eggs | 1 | 270 | 5 |
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?
| 1–3 years | 1,000 IU |
| 4–8 years | 1,333 IU |
| 9–13 years | 2,000 IU |
| 14 years and older | 3,000 IU (men); 2,333 IU (women) |
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
