Vitamin A

Vitamin A is one of the most versatile vitamins due to the number of functions it supports in our body.  It is one of four fat soluble vitamins, which means that we are able to store the vitamin; principally in our liver.  It also means that small amounts of fat are required in the diet in order to absorb and utilise this vitamin.  Our diet can provide Vitamin A in two forms:

• Preformed Vitamin A (Retinol and Retinal): found in foods from animal sources, especially liver (animals as well as humans store vitamin A in their livers), but also eggs and dairy products.

• Pro-Vitamin A (Carotenoids): these are metabolised into retinal and retinoic acid, the active forms of vitamin A by our bodies.  The most important pro-vitamin carotenoid is beta-carotene, but Vitamin A can also be synthesised from alpha-carotene and beta-cryptoxanthin (ODS, 2012).  These carotenoids can be found in abundance in yellow, orange and green vegetables, although cooking will deplete between 15-35% of the available caroteniods. (Liska, 2004)

 

Key functions that rely on Vitamin A are:

• Vision: retinal binds to a protein called opsin to form the visual pigment: rhodopsin. Rod cells in the retina work with rhodopsin to detect very small amounts of light, making them important for night vision.  Inadequate retinal available to the retina results in impaired dark adaptation, known as "night blindness" (LPI-1, 2007).
• Regulation of gene expression: Vitamin A (along with the thyroid hormone and vitamin D) appears to influence gene transcription, through the stimulation and inhibition of transcription of specific genes, playing a major role in cellular differentiation.
• Differentiation of Cells: plays a critical role in the normal formation and maintenance of the cells of the heart, lungs, kidneys and other organs (ODS, 2012).  In addition, Vitamin A plays a central role in the development and differentiation of white blood cells, such as lymphocytes, which have critical roles in the immune response (LPI-1, 2007).
• Immune Support: Vitamin A is considered an anti-infective vitamin; the skin and mucosal cells that line our airways, digestive tract, and urinary tract function as a barrier and form the body's first line of defence against infection.  Retinol is required to maintain the integrity and function of these cells (LPI-1, 2007).

The UK RNIs for Vitamin A (retinol) are as follows:

UK RNIs:	INFANTS		CHILDREN		MALES		FEMALES		PREGNANCY & LACTATION
[Note: ug = micrograms (1/1,000,000 of a gram]	months	µg/d	years	µg/d	years	µg/d	years	µg/d	months	µg/d
	0-3 4-6 7-9 10-12	350 350 350 350	1-3 4-6 7-10	400 400 500	11-14 15-18 19-50 50+	300 700 700 700	11-14 15-18 19-50 50+	600 600 600 600	Pregnancy Lactation 0-4 4+	+10 +350 +350

 (BNF, 2009)

A couple of (sometimes) confusing notes about Vitamin A. Vitamin A values are often quoted in International Units (IU); essentially IU can be converted to preformed Vitamin A as follows: 1IU = 0.3 µg of retinol.  If the source of vitamin A is pro-vitamin A then retinol equivalents need to be calculated where 6µg beta-carotene equals 1µg retinol equivalent and  12µg of other carotenes equal 1µg retinol equivalent (Liska, 2004).

It is also important to understand that in excess preformed vitamin A (retinol) can become toxic, even deadly.  The Inuit and Artic explorers know that they cannot eat the liver of polar bears since the high levels of Vitamin A stored in the organ are so hazardous.  A treatment for severe acne, Roaccuatane, contains isotretinoin which is a retinoid which  chemically related to Vitamin A.  In the UK, it can only be prescribed by a dermatologist  and should not be taken for more than 6 months to ensure that levels of stored retinol do not become toxic.  High dose preformed vitamin A (retinol) (>2,800-3,300 µg/d) should be avoided in pregnancy since it has been linked to birth defects (BFN, 2009).  Carotenoids do not have the same impact, which is why pregnancy supplement formulas base their Vitamin A content on carotenoids rather than the retinol form of vitamin A.  

Like much in our bodies, Vitamin A does not function in isolation.  Two of the main micronutrients that work synegestically with Vitamin A are:

• Zinc is an essential component of more than 100 human enzymes.  One of these enzymes is the retinol binding protein (RBP) which is responsible for transporting vitamin from the liver to the tissues.  Zinc is also essential to the enzyme which converts retinol to retinal, which in turn form the visual pigment rhodopsin, consequently night blindness could be as a result of a Vitamin A or Zinc deficiency (LPI-2, 2008).
• Iron like zinc is an essential component of many proteins and enzymes, the most familiar being hemoglobin which is oxygen carrying component of red blood cells  (LPI-3, 2006).  There is increasing evidence that supplementing with Vitamin A and iron can reduce iron deficiency anemia in children and pregnancy (LPI-3, 2006).  This is believed to be because vitamin A is involved in the mobilisation of iron from its stores to the red blood cells (LPI-1, 2007).

Both alcohol consumption and the oral contraceptive pill are likely to have a negative impact on your vitamin A status.  Alcohol (ethanol) will compete for the same metabolic pathways as retinol in the liver, excessive alcohol consumption appears to increase the risk of toxic liver disease (hepatotoxicity).  Interestingly this risk is not isolated to retinol, but also beta carotenes (Leo et al, 1999).  Oral contraceptives that contain estrogen and progestin increase retinol binding protein (RBP) synthesis by the liver, increasing the export of retinol into the bloodstream and depleting vitamin A stores in the liver (LPI-1, 2007).

Recipies with Vitamin A in mind,...

• Veggie and Egg Tower from My New Roots
• Chorizo with New Potato (or Sweet Potato) and watercress from The Intolerance Gourmet
• Brussels sprout & kale salad with pomegrantes, hazelnuts and creamy avocado dressing from Good Things Grow
• Pork, Chicken Liver, Pistachio and Apricot Terrine from James Martin's Saturday Kitchen.  

Note when using liver I would recommend always to buy organic.  The liver is the organ whether animal or human is used to detoxify, by using organic you are ensuring there is not untoward accompanying your liver!

References:

BNF: British Nutrition Foundation. (2009). Vitamins. Available: http://www.nutrition.org.uk/nutritionscience/nutrients/vitamins. Last accessed 20 May 2013.

Liska, D et al (2004). Clinical Nutrition A Functional Approach. Washington: Institute of Functional Medicine. p97-187

Leo MA & Lieber CS. (1999). Alcohol, vitamin A, and β-carotene: adverse interactions, including hepatotoxicity and carcinogenicity1,2,3. The American Journal of Clinical Nutrition. 69 (6), pp.1071-1085.

LPI-1: Russell R.M. (2007). Vitamin A. Available: http://lpi.oregonstate.edu/infocenter/vitamins/vitaminA/. Last accessed 21st May 2013.

LPI-2: Ho E. (2008). Zinc. Available: http://lpi.oregonstate.edu/infocenter/minerals/zinc/. Last accessed 21st May 2013.

LPI-3: Higdon J. (2006). Iron. Available: http://lpi.oregonstate.edu/infocenter/minerals/iron/. Last accessed 21st May 2013.

ODS: Office of Dietary Supplements (2012). Dietary supplement fact sheet: vitamin A. Available at: http://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/#h1. Last accessed 21st May 2013.