Vitamin A - Benefits And Functions
Vitamin A is an essential, fat-soluble vitamin. Various compounds are included under the collective term vitamin A, including:
- Retinoic acid
- Retinyl esters (such as retinyl palmitate, the main stored form of vitamin A in the liver)
Retinol, retinal, and retinoic acid are the active forms in the body that contribute to the biological benefits and functions.
Vitamin A is absorbed in the duodenum (the upper part of the small intestine) and is transported to the liver, from where it is then carried to the body's cells. Approximately 80 to 90% of the vitamin A stored in the body is found in the liver (ranging from a minimum of 10 μg/g to a maximum of 1,400 μg/g of liver) 1. Additionally, carotenoid reserves are distributed in the body's adipose tissue 2 3.
In its pure form, vitamin A is yellow crystals or orange liquids 4. Vitamin A (Retinol) is sensitive to light, oxygen, and room temperature 5 6. Therefore, foods containing vitamin A should be stored cool and dark. Cooking or canning foods rich in vitamin A mostly destroys vitamin A 7 8. After cooking vegetables, only 33% of the original vitamin A content remains 9.
Functions and Benefits
Vitamin A is used in the formation of the protein rhodopsin, which arises from the binding between retinal and the protein opsin 10. Rhodopsin is a visual pigment of the retina that is incorporated into the cones and rods of the retina. It absorbs light in the retinal receptors and is necessary for light-dark adaptation, allowing the eye to adjust to different light conditions. Rhodopsin is constantly regenerated in a cycle after light absorption 11. Therefore, vitamin A enables normal vision.
Retinoic acid regulates the gene expression (formation of a gene product) of over 500 genes, which translate, among other things, the genetic information of structural proteins (such as keratin), enzymes (alcohol dehydrogenase), the extracellular matrix (laminin), as well as retinol-binding proteins and receptors. 12 13
In the body, vitamin A is also involved in cell division, cell growth, and the growth of bones and teeth, thus summarily contributing to the development and maintenance of body tissues (including the skin) 14 15 16 17 18.
In reproduction, vitamin A is also involved in the formation of sperm (spermatogenesis) 19. As retinoic acid, vitamin A plays a role in embryonic development, particularly in the development of the spinal cord, vertebrae, limbs, heart, lungs, eyes, and ears 20. Additionally, vitamin A also supports placenta development during pregnancy 21 22 23.
Vitamin A is involved in the development and maintenance of epithelial tissue 24. Epithelial tissue consists of closed cell arrangements that either cover and protect internal (such as mucous membranes) and external body surfaces (covering epithelium), produce and release secretions (glandular epithelium), or transmit stimuli (sensory epithelium). That is why vitamin A is also used in the treatment of skin disorders 25.
In addition, vitamin A is also involved in maintaining the functionality of the immune system (T-lymphocytes and phagocytosis) 26 27. Additionally, vitamin A stimulates the production of white blood cells (leukocytes), which play an important role in defending against pathogens 28.
Retinoids (related to vitamin A) and carotenoids like beta-carotene are good antioxidants 29 30. Antioxidants protect the body from free radicals that cause cell damage and are responsible for a range of diseases such as cancer, cardiovascular diseases, and neurodegenerative diseases 31. Vitamin A itself is not an antioxidant 32.
Vitamin A helps prevent dryness of the skin, protecting it from infectious diseases 33.
Results of a 2014 summary of studies suggest that higher intake levels of vitamin A and beta-carotene may reduce the risk of cataracts (clouding of the lens) 34.
Vitamin A Against Benefits
Possible higher intake levels of beta-carotene and vitamin A through daily food consumption may reduce the risk of lung cancer and gastric cancer 35 36. The results of a 2015 summary of studies indicate that higher intake levels of vitamin A significantly reduce gastric cancer risk 37. Compared to the lowest intake levels, the gastric cancer risk was 34% lower.
The risk of melanoma (skin cancer) is also reduced with higher intake levels of vitamin A (-14%), retinol (-20%), and beta-carotene (-13%) compared to lower intake levels 38.
A higher consumption of vitamin A is also associated with a reduction in bladder cancer (-18%) 39.
Carotenoids are converted into Vitamin A
In food, carotenoids are present as secondary plant compounds. Carotenoids are pigments that give foods a yellow to reddish color. The most well-known carotenoid is beta-carotene. The best example with a high content of beta-carotene is orange carrots, whose name is derived from carotene - see foods with beta-carotene. Carotenoids are precursors of vitamin A. Therefore, they are referred to as provitamin A. For example, beta-carotene is converted into two retinal molecules in the absorptive cells of the intestine through two enzymes (beta-carotene 15,15'-monooxygenase and beta-carotene 15,15'-dioxygenase) 40 41.
About 50 different carotenoids can be converted into vitamin A by the body 42. This includes alpha-carotene and beta-cryptoxanthin, for which scientists now believe to have higher bioavailability than beta-carotene 43. The term bioavailability refers to the amount of a nutrient (in this case, carotenoids) that can be used by the body for metabolism or storage under normal conditions. However, lycopene, lutein, and zeaxanthin do not have vitamin A activity and are therefore referred to as non-provitamin A carotenoids 44.
For scientists, it is a challenge to determine the exact conversion rates and differentiate the newly formed retinol from the body's own reserves. 45. In this context, the term retinol equivalents is also used because carotenoids have lower vitamin A activity in the body compared to pure vitamin A (retinol). The equivalents are used to calculate or standardize the vitamin A content of foods and daily requirements, taking into account the conversion rates of different carotenoids. That is why the vitamin A content of foods is often expressed in retinol equivalents (RE).
|Consumed Amount||Retinol Retained in the Body||RE Ratio|
|1 mg Retinol||1 mg||1:1|
|2 mg β-Carotene (supplement)||1 mg||2:1|
|12 mg beta-Carotene (from food)||1 mg||12:1|
|Alpha-Carotene or Beta-Cryptoxanthin (from food)||1 mg||24:1|
|1.15 mg all-trans-Retinylacetate (supplement)||1 mg||1.15:1|
|1.83 mg all-trans-Retinylpalmitate (supplement)||1 mg||1.83:1|
|3.33 IU Retinol||1 µg||3.33:1|
|1 IU Retinol||0.333 µg||3.33:1|
|1 IU beta-Carotene (from food)||0.05 μg||20:1|
|1 IU beta-Carotene (dietary supplement)||0.15 μg||6.66:1|
|1 IU alpha-Carotene or β-Cryptoxanthin (from food)||0.025 μg||40:1|
In order to produce 1 µg of retinol in the body, approximately 12 µg of beta-carotene or 24 µg of other carotenoids are needed. However, the conversion efficiency of beta-carotene to retinol varies depending on body weight, ranging from 3.6:1 to 28:1 52 53. According to a Vietnamese study on lactating women, the conversion rates for beta-carotene from green vegetables were found to be 26:1 and from orange fruits 12:1 54. Due to significant variations in these values, further research is required (RE ratio may change).
Furthermore, there is evidence that individuals with higher body fat have a lower ability to convert beta-carotene into vitamin A 55.
Improving Absorption of Vitamin A and Carotenoids
Retinol is highly efficiently absorbed by the body. The absorption rate ranges between 75 and 100%. 56. For beta-carotene, the absorption efficiency varies between 3% and 90%, but mostly falls within the range of 11.9% to 16% 57 58 59 60 61 62 63. Higher absorption rates are achieved only with the consumption of supplements along with oil.
Since vitamin A is a fat-soluble vitamin, it is absorbed in the intestine along with fat. Therefore, higher absorption rates are achieved only with the simultaneous consumption of fats.
In addition, the absorption of carotenoids from vegetables can also be enhanced. To achieve this, cook or homogenize the respective carotenoid-rich foods and serve them together with fats. 64 65 66 Furthermore, when consumed with fat or a small amount of oil such as canola oil or olive oil, beta-carotene can be more easily absorbed by the body, resulting in increased serum vitamin A levels. 67 68 69. Beta-Carotene can be absorbed twice as well when consumed with oil compared to without 70.
Scientists who have studied the absorption-enhancing benefit show that consuming provitamin A carotenoids with the fat-rich avocado improves carotenoid absorption in healthy individuals 71. Furthermore, simultaneous consumption of avocado with beta-carotene-rich tomato sauce and carrots resulted in 4.6-fold and 12.6-fold higher conversion efficiency of beta-carotene to vitamin A. Blood levels of vitamin A increased more significantly with avocado consumption compared to participants without avocado.
Fat also plays a crucial role in the absorption of carotenoids from a salad consisting of spinach, romaine lettuce, cherry tomatoes, and carrots 72. No carotenoid absorption was observed with a fat-free dressing. As the fat content of the dressing increased, absorption rates also increased.
Carotenoids in yellow, orange, and red plants or plant products are present in crystalline form and in the form of lipoproteins (fat-protein complexes), which automatically leads to higher bioavailability 73 74. Carotenoids in green leafy vegetables are located in the chloroplasts and are bound to pigment-protein complexes, resulting in low bioavailability 75.
Furthermore, even mild preparation techniques such as cooking at lower heat, brief cooking, soaking, and mincing enhance the bioavailability with only minimal carotenoid loss 76. On the other hand, other preparation methods such as prolonged cooking at high heat, frying, and deep-frying lead to a significant loss of carotenoids 77 78 79.
The more beta-carotene is consumed through food, the lower the efficiency at which the body can convert it into vitamin A 80.
Absorption Inhibiting Factors
Considerations for Vegans
Comparing study results on the intake levels of vitamin A in vegans is difficult as it is not clear how the vitamin A value is composed. Some results suggest higher intake levels of retinol in vegans (likely due to a carotenoid-rich diet) 84. Other results show the opposite. According to a large English study with over 65,000 participants, vegans (men: 74.2 µg, women: 76.6 µg) have the lowest daily intake levels of retinol compared to the general population 85. Additional studies also "reveal" that vegans have lower intake levels of vitamin A compared to the average population 86 87. However, this is not surprising, as retinol does not naturally occur in plant-based foods. Supplements with vitamin A are possible, for example in multivitamin juices. According to a Danish study, vegans consumed nearly twice as much beta-carotene as the average population 88. Nevertheless, half of the vegans did not reach the recommended intake of vitamin A through both diet and supplements.
Even though the amounts of retinol intake are relatively low compared to the general population, the frequency of diseases caused by vitamin A deficiency is considered low 89.
There are no plant-based foods with preformed vitamin A. Nevertheless, as mentioned earlier, the body can produce vitamin A from approximately 50 carotenoids. For vegans, provitamin A beta-carotene, a precursor of vitamin A, is of great importance. It is the most important source for vegans to obtain enough vitamin A naturally. By consuming highly colored fruits and vegetables, the daily requirement of vitamin A can be met through a vegan diet 91. In addition to a diet rich in carotenoids, the absorption-enhancing factors of carotenoids described in this article should also be considered. You can find the foods with vitamin A (retinol equivalents) here.