VITAMIN A & CAROTENOIDS
Written by Slawomir (“Swavak”) Gromadzki, MPH
Vitamin A is found in two primary types: First one is an active vitamin A also known as retinol, which can be converted to retinal essential for vision, or retinoate, which is involved in cell proliferation and cell differentiation via binding to the retinoic acid receptors.
The second and inactive form is beta-carotene, known as pro-vitamin A which in the body is converted to active vitamin A.
Vitamin A belongs to a family of active (directly usable by the body) substances called retinoids (often called preformed vitamin A) that include retinol, retinal, and retinoic acid. Retinol (vitamin A) which comes from animal-derived foods can be converted to both retinal and retinoic acid in our body.
Retinoids include the following:
- Retinol: is converted into retinal, retinoic acid, and retinyl esters
- Retinal: responsible for vision health & growth
- Retinoic acid: promotes kin health, tooth remineralization & bone development
- Retinyl esters: biologically inactive storage form
Beta-carotene is the most known carotenoid and the most often naturally occurring one and obtained from colourful vegetables and fruits. It is in the form of pro-vitamin A carotenoid which needs to be converted to retinol (active form of vitamin A) in small intestine and liver in order to be utilized by the body.
Also two other plant carotenoids, alpha-carotene and beta-cryptoxanthin can be converted to retinol but to a lesser extent than beta-carotene.
Other very important carotenoids (there are about 700 types known by now), including lycopene, lutein, and zeaxanthin, work in the body as antioxidants and provide various health benefits (including protecting eyes against oxidative damage), but cannot be converted to vitamin A.
Vitamin A deficiency is common in many developing countries, often because of the limited access to foods high in this vitamin.
People with autoimmune disorders, inflammatory bowel disease, leaky gut syndrome, celiac disease, pancreatic disorders or alcohol dependence are at a higher risk of deficiency.
It is rare in the developed countries to have a serious vitamin A deficiency of which symptoms usually include dry eyes, night blindness, skin problems, birth defects, dry lips, thick or scaly skin, impaired immunity, diarrhoea, etc.
Orlistat (Alli, Xenical), a weight-loss treatment, can decrease the absorption of vitamin A, other fat-soluble vitamins, and beta-carotene.
Vitamin A palmitate (retinyl palmitate or retinol palmitate), is found in liver, cod liver oil, fish, eggs, whole milk, cheese and butter. It is lost when the fat is removed from these sources.
Plant sources of pro-vitamin A (beta-carotene, alpha-carotene and beta-cryptoxanthin) as well as other carotenoids such as zeaxanthin, lycopene, and lutein are found in fruits and vegetables.
As naturally occurring colour, alpha- and beta-carotene and lycopene appear predominantly in red, orange, and yellow fruit and vegetables (the highest source is sweet potato, winter/butternut squash, kale, spinach & carrots), whereas lutein and zeaxanthin occur mainly in green-leafy vegetables.
Vitamin A palmitate is used in supplements and to fortified foods such as breakfast cereals or dairy products that have lost vitamin A palmitate during processing.
Supplements such as multivitamins usually contain 2,500–10,000 IU of vitamin A in the form of retinyl acetate or retinyl palmitate. They may also contain beta carotene alone (often combined with other carotenoids) or retinyls together with beta-carotene.
Retinyl acetate (retinol acetate) is a natural form of vitamin A. Retinyl palmitate (retinol palmitate) is a synthetic alternate for retinyl acetate often found in vitamin A supplements, and is available in oily or dry forms.
Vitamin A has several important functions in the body. It is essential for good vision, helps cells reproduce normally and is needed for the proper development of an embryo and fetus, and helps maintain normal neurological function, healthy skin and mucous membranes. It also plays important role in maintaining healthy immune system function, bone formation, reproduction, wound healing.
Major carotenoids, including beta-carotene, alpha-carotene, cryptoxanthin, zeaxanthin, lycopene, and lutein have important health benefits, including antioxidant and photo-protective.
Carotenoids have many positive benefits especially on age-related health problems. Lutein and zeaxanthin possess the most significant benefit with regard to age-related macular degeneration compared to other carotenoids. A study published in the Archives of Ophthalmology concluded that people at high risk for the disease who took a daily multivitamin that included vitamin A, vitamin C, vitamin E, zinc and copper had a 25 percent reduced risk of macular degeneration. (>)
IMMUNITY, INFECTIONS & CANCER
Vitamin A plays an integral role in promoting immune health and seems to be especially beneficial for warding off cancer and infections. According to one scientific review, a deficiency of vitamin A weakens immunity and can even alter the function of immune cells (>).
It’s thought that vitamin A deficiency makes mucosal barriers unable to regenerate, resulting in increased susceptibility to infections (>).
Carotenoids are linked with a reduced risk of prostate, colon, bladder, skin, cervical and other forms of cancer (>).
Lycopene (red carotenoid found in ripe tomatoes, grapefruits, and red watermelon) has anti-oxidant, anti-inflammatory, and chemopreventive properties against cancer (>,>,>,>). Its absorption is improved when consumed from heated and fat-containing dietary sources (>). It was shown that lycopene accumulates in the prostate and has anti-proliferative and pro-apoptotic activities against prostate cancer (>, >).
Apart from that, serum carotenoids (including beta-carotene and lutein) are positively associated with the activity of one of the most important antioxidants naturally found in our body and known as superoxide dismutase or SOD [>, >].
According to the relatively new discovery from 2010, Vitamin A plays critical role in energy production (>, >). Retinol seems to be very important in the synthesis of ATP in mitochondria, the power plant of the cells. When this vitamin is deficient the production of energy is reduced by 30 percent!
Vitamin A is a “crucial regulator of carbohydrate, protein, and lipid metabolism in all of the major metabolically active organs”. (>)
Vitamin A may help in the prevention of urinary stones as people with lower levels of vitamin A had greater risk of developing calcium oxalate crystals in the urine and a higher risk of urinary stones formation (>).
Higher levels of plasma carotenoids may reduce cholesterol and contribute to lower risk of cardiovascular disease.
RECOMMANDED DAILY INTAKE
Vitamin A RDA for adults is 700-900 mcg equivalent to 3,000 IU if the food or supplement source is an active vitamin A (retinol), and also equivalent to 5,000 – 6,000 IU of beta-carotene from supplements.
Safe upper intake for adults is not more than 10,000 IU (3,000mcg) of vitamin A per day.
Since the body converts all dietary sources of vitamin A into retinol, 1 mcg of physiologically available retinol is equivalent to 12 mcg of beta-carotene, 24 mcg of alpha-carotene and 24 mcg beta-cryptoxanthin.
From dietary supplements, the body converts 2 mcg of beta-carotene to 1 mcg of retinol. (>)
Vitamin A is usually listed on food and supplement labels in international units (IUs) and the conversion rates between mcg and IU are as follows (>):
- 1 IU retinol = 0.3 mcg of retinol activity equivalents (RAE)
- 1 IU beta-carotene from dietary supplements = 0.15 mcg
- 1 IU beta-carotene from food = 0.05 mcg
- 1 IU alpha-carotene or beta-cryptoxanthin = 0.025 mcg
It is important to be aware of the fact that RAE (retinol activity equivalents) should be converted into IU according to the source of vitamin A.
For instance, the RDA of 900 mcg RAE for adolescent and adult men is equivalent to 3,000 IU if the food or supplement source is an active vitamin A (retinol), which is also equivalent to 6,000 IU of beta-carotene from supplements, 18,000 IU of beta-carotene from food, or 36,000 IU of alpha-carotene or beta-cryptoxanthin from food.
Therefore, a mixed diet containing 900 mcg RAE can provide between 3,000 and 36,000 IU of vitamin A, depending on the foods consumed. (>)
Recommended Dietary Allowance (RDA) (>)
* Adequate intake equivalent to the mean intake of vitamin A in healthy, breastfed infants.
Since vitamin A palmitate is fat-soluble regular overdosing can accumulate it in the body and be toxic causing various symptoms including nausea or vomiting, headaches, birth defects, reduced bone mineral density or liver problems.
Chronic excessive intake vitamin A is usually a result of a combination of consuming too much high in vitamin A foods such as liver an active vitamin A from supplements. It may eventually lead to various toxicity symptoms including pain in joints and bones, increased intracranial pressure, headaches, skin irritation, dizziness, nausea, and even death.
In those who consume too much vitamin A for a longer period of time, it may take a long time to clear the excess after they discontinue the intake, and the resulting liver damage can be sometimes irreversible.
On the other hand, overdosing carotenoids (pro-vitamin A from plant sources) does not lead to toxicity. The only effect of long-term, excess beta-carotene is carotenodermia, a harmless condition in which the skin colour becomes more yellow or orange.
Tolerable (Safe) Upper Intake Levels for Preformed Vitamin A
The above ULs are expressed in mcg and in IUs (where 1 mcg = 3.33 IU), and they only apply to products from animal sources and supplements whose vitamin A comes entirely from retinol or ester forms, such as retinyl palmitate. However, many dietary supplements consist partly or entirely of beta-carotene (provitamin A) and require conversion.
BETA-CAROTENE AND INCREASED RISK OF LUNG CANCER IN HEAVY SMOKERS
In spite of numerous health benefits of beta-carotene, several studies suggest its possible negative effect in current and former heavy smokers. This effect does not originate from the antioxidative activity of beta-carotene, but is rather based on a molecular mechanism.
We know that lung cancer can be caused by chromosomal instability induced by a polycyclic aromatic hydrocarbon which is a dangerous component found in tobacco smoke. The smoker’s body tries to prevent cancer using isoform of glutathione S-transferase which is involved in the protective mechanism against polycyclic aromatic hydrocarbon metabolites.
Unfortunately it was found that this protective action can be weakened by beta-carotene in heavy smokers. In general, a maximum uptake of 20 mg of beta-carotene per day is regarded as safe. Therefore, those who smoke more than 20 cigarettes per day should not exceed this limit [>].
The CARET study researchers found that daily beta-carotene (30 mg) and retinyl palmitate (25,000 IU) supplements increased the risk of lung cancer and cardiovascular disease mortality [>].
EXAMPLE OF A GOOD AND SAFE BETA-CAROTENE SUPPLEMENT
The best and safe way to use beta-carotene supplement is in the form of Betatene (mixed carotenoids) and not more than 20mg. Betatene consists of beta-carotene combined with a small amount of other carotenoids which makes the formula more natural and holistic. Below I have included a good example of the product with the following ingredients per one capsule:
Betatene (mixed carotenoids) 16.5mg
Beta Carotene 15.5mg
Alpha Carotene 577µg
Cryptoxanthin, Zeaxanthin and Lutein 330µg
INTERACTIONS WITH MEDICATIONS
Vitamin A can interact with Orlistat (Alli, Xenical), a weight-loss treatment, which can decrease the absorption of vitamin A, other fat-soluble vitamins, and beta-carotene.
Synthetic retinoids derived from vitamin A, including the psoriasis drug acitretin (Soriatane) and bexarotene (Targretin), used to treat the skin effects of T-cell lymphoma, can increase the risk of hypervitaminosis A when taken in combination with vitamin A supplements.
INTERACTIONS WITH OTHER NUTRITIONAL SUPPLEMENTS
Vitamin A works synergistically with other vitamins and minerals, including vitamin D, K2, zinc, and magnesium, and without them it cannot perform its functions.
High supplemental intake of vitamin A may contribute to Vitamin D deficiency (>, >). A form of vitamin A, retinoic acid, can block the activity of vitamin D.
AVOID CREAMS WITH VITAMIN A
Since vitamin A is an antioxidant it is often added to skin products because is believed vitamin A slows skin ageing. Although it is true that oral ingestion of vitamin A can reduce the risk of carcinoma in people at high risk for skin cancer (>), but the according to the 2012 federal study when the same vitamin is used on the skin exposed to sunlight it may speed the growth of skin cancer.
– T.E. Moon et al., Effect of Retinol in Preventing Squamous Cell Skin Cancer in Moderate-Risk Subjects: A Randomized, Double-Blind, Controlled Trial. Southwest Skin Cancer Prevention Study Group. Cancer Epidemiology Biomarkers & Prevention, 1997, 6:949.
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