Vitamin A is a crucial nutrient for maintaining healthy vision, immune function, and cell growth. It plays a particularly important role in promoting eye health. Additionally, Vitamin A possesses antioxidant properties that help protect cells from damage caused by free radicals. Research has also shown that Vitamin A can reverse the process of cell tumor formation, contributing to cancer prevention[1].
However, excessive supplementation of Vitamin A can lead to conditions like osteoporosis and fractures[2]. Therefore, understanding how to supplement Vitamin A for maximum health benefits is vital. Let’s delve into this article to gain a better understanding of the functions of this vitamin in the human body.
What is Vitamin A?
Vitamin A is a nutrient that contains biologically active structures like retinol, retinal, retinoic acid, and retinyl ester. It is a fat-soluble vitamin[2]. Vitamin A can be categorized into preformed vitamin A and provitamin A, both of which are obtained from the diet[3].
The main forms of vitamin A are retinol and retinyl ester, which are long-chain fatty acids.
Provitamin A refers to carotenoids such as alpha-carotene, beta-carotene, and beta-cryptoxanthin.
Vitamin A is primarily ingested in the form of retinyl ester or provitamin A, which is then metabolized into active compounds. In the United States, the average daily intake of vitamin A is about 600 micrograms of retinol equivalents, with approximately 70-75% coming from retinol and retinyl esters[2]. Retinyl esters from the diet are hydrolyzed by enzymes in the pancreas and small intestine to form free retinol, which is then absorbed by intestinal mucosal cells.
Since retinol is insoluble in water, it binds to cellular retinol-binding protein (CRBP) II before being transported into the body[4] [5]. On the other hand, about 50% of provitamin A is fully absorbed by intestinal mucosal cells, while the other 50% is metabolized into retinal and then reduced back to retinol[6]. The absorbed retinyl esters, retinol, and carotenoids are esterified with long-chain fatty acids and transported in the body, bound to other lipids like cholesterol, cholesterol esters, and triglycerides[7].
The 4 Major Benefits of Vitamin A
1. Eye Health
Vitamin A is a precursor for 11-cis-retinal, which combines with opsin to form rhodopsin, an important photosensitive pigment in the eyes. Upon absorbing light energy, rhodopsin metabolizes into all-trans-retinal and opsin, transmitting visual signals to the central nervous system and producing a visual response[8]. A deficiency in vitamin A can lead to night blindness, dry eyes, conjunctival and corneal dryness, corneal ulcers, corneal melting (keratomalacia), and retinal degeneration[9]
A nutritional survey conducted among Indian schoolchildren revealed that vitamin A deficiency is the leading cause of blindness and severe visual impairment among Indian children. The survey also found significant differences in visual health between urban and rural areas, attributed to varying vitamin A intake among children from different regions, causing discrepancies in visual health between urban and rural children[10].
2. Wound Healing
Vitamin A helps regulate the growth and differentiation of various skin cells and aids in proper epithelialization. A deficiency can lead to abnormal keratinization of epithelial cells. In injured tissue, vitamin A stimulates epidermal cell turnover and aids in restoring epithelial structure. Besides its role in the inflammatory stage of wound healing, vitamin A also enhances the production of type I collagen and fibronectin[11].
Research suggests that local or systemic administration of vitamin A can improve delayed wound healing caused by steroids. Therefore, vitamin A supplementation has shown beneficial effects in wound healing among patients with diabetes, tumors, and acute or chronic wounds[12]. Pre-treatment with vitamin A for 2-4 weeks before facial cosmetic surgery or other skin wound procedures has been found to prevent scar formation or abnormal wound healing[13].
3. Enhanced Immune System
Vitamin A deficiency hampers the normal regeneration of mucosal barriers damaged by infections and reduces the functionality of neutrophils, macrophages, and natural killer cells, impairing innate immunity. Vitamin A aids in the development of T helper cells and B cells, crucial for immune function, and may be one of the reasons for increased infant, child, and maternal mortality rates due to vitamin A deficiency in many regions of the world[14].
Numerous studies have found an increased occurrence of Staphylococcus aureus skin infections with vitamin A deficiency. Vitamin A has been shown to enhance the skin’s production of antibacterial proteins, rapidly killing or inactivating microbes[15].
4. Reduced Memory Decline
An analytical review of 44 studies suggests that vitamin A could potentially be an effective supplement for treating and preventing Alzheimer’s disease, although more clinical trial data are needed for validation[16]. A study involving experimental animals subjected to a vitamin A-deficient diet for 12 weeks found a significant impairment in spatial learning and memory tasks.
However, after supplementation with vitamin A in subsequent diets, cognitive impairments were completely reversed. This suggests that vitamin A plays a critical role in the learning and memory processes associated with normal hippocampal function[17].
How to determine if you lack Vitamin A? What are the symptoms of Vitamin A deficiency?
Vitamin A deficiency remains a significant public health issue in developing countries, and supplementing Vitamin A can help improve vision and immune function. In contrast, Vitamin A deficiency is less common in developed countries [2]. In cases of severe Vitamin A deficiency, it may lead to symptoms such as night blindness, dry eyes, corneal softening, and retinal degeneration [9].
Recommended Intake of Vitamin A
| Recommended Intake (μg RE) | Males | Females |
| Under 9 years old | 400 | 400 |
| 10-12 years old | 500 | 500 |
| 13-15 years old | 600 | 500 |
| 16-18 years old | 700 | 500 |
| 19 years and older | 600 | 500 |
| During third trimester of pregnancy | / | 600 |
| During lactation | / | 900 |
| Upper Intake Limit (μg RE) | |
| Under 3 years old | 600 |
| 4-9 years old | 900 |
| 10-12 years old | 1700 |
| 13-18 years old | 2800 |
| 19 years and older | 3000 |
| During pregnancy | 3000 |
| During lactation | 3000 |
What foods contain Vitamin A?
Vitamin A is typically divided into two forms: Preformed Vitamin A and Provitamin A. Preformed Vitamin A can be obtained from foods like eggs, liver, butter, milk, and fortified cereals. Provitamin A is found in vegetables such as carrots, spinach, kale, pumpkin, and sweet potatoes [2] [3]. Regardless of the form, Vitamin A can be obtained through regular dietary intake.
How to consume Vitamin A?
Vitamin A is a fat-soluble vitamin [2], which means it is better absorbed in the presence of dietary fats in the intestines [4][19]. Therefore, it’s recommended to consume it after a meal to enhance absorption. However, when supplementing with Vitamin A, it’s advisable not to take high doses of supplements containing carotenoids (like beta-carotene) or lutein simultaneously, as they can compete for absorption with Vitamin A. If you intend to supplement, it’s suggested to space them out [20].
How to effectively supplement Vitamin A? Are there side effects?
Excessive intake of Vitamin A can be harmful. Single high doses might lead to nausea, vomiting, dizziness, and blurry vision. Research has shown that prolonged daily intake of Vitamin A exceeding 10,000 micrograms retinol equivalents (μg RE) could result in thinning of bones, liver damage, headaches, diarrhea, skin allergies, joint and bone pain.
If you’re pregnant or considering pregnancy, it’s advised to consult a doctor before taking Vitamin A supplements, as excessive Vitamin A intake during pregnancy could be linked to birth defects [21]. Therefore, for pregnant women, choosing to supplement with provitamin A sources like beta-carotene, which can be converted into active forms as needed by the body, is a safer approach.
Summary
Vitamin A plays a critical role in many metabolic processes within your body, including maintaining healthy vision, supporting the immune system, and normal skin function. However, even with its benefits, it’s important to consume it in moderation to avoid both deficiency and excess, as both can lead to serious side effects.
A healthy and balanced diet can provide a safe intake of Vitamin A. If you have additional Vitamin A needs, it’s recommended to consult with your physician or a nutritionist before deciding to supplement.
