Vitamin A

Vitamin A

What is vitamin A?

Vitamin A is an essential vitamin which means that we need to get if from our diet as it is not made in the body. It is a fat soluble vitamin and is available as preformed vitamin A from animal products and pro-vitamin A in fruits, vegetables and other plant based foods (1).

The levels of vitamin A in the body are dependent on levels of dietary intake; the capacity to absorb the micronutrient; the ability to convert the provitamin A to its bioavailable forms and how well it is taken up by the body’s tissues (2).

Retinol is the active form of vitamin A found in animal products and is easily accessible to the body. Due to its fat soluble properties, excess vitamin A is stored in the liver and adipose tissue (3). It is possible to consume too much vitamin A from animal sources and supplements.

Beta carotene is the most abundant carotenoid used by the body to synthesise vitamin A to its active forms retinol, retinal and retinoic acid. This is regulated by a homeostatic mechanism and is the safest way to consume vitamin A because toxic levels are not allowed to accumulate. Consuming large amounts can turn the skin orange but this is not harmful.

Western societies get more than 70% of their vitamin A from animal sources and less than 30% from plant sources. In contrast, populations in developing countries get more than 70% of their vitamin A intake from plants. 

Sources of vitamin A

Animal sources are liver, liver pate, milk, yogurt, oily fish (3).

Plant sources are pumpkins and squashes, sweet potato, carrots, parsley, green leafy vegetables including spinach and broccoli (3).

Health Benefits

Vitamin A is necessary to ensure the health of epithelial cells in the body. Epithelial cells cover all of the body’s surfaces. This includes maintenance of mucous membranes in the nasal passages, gastrointestinal tract, lungs, bladder, urinary tract and skin (5). Vitamin A is needed for growth and development in reproduction (5). Vitamin A is also necessary for maintaining bone, tooth health and eye health (5).

Immune health

A balanced immune response is also dependent on vitamin A which helps to promote and regulate the body’s innate and adaptive immune systems (5). This enhances the body’s ability to fight infectious diseases.

Recommended amounts

The recommended daily amount (RDA) of retinol is 700mcg for men and 600mcg for women in the 19-64 age range (4).

Toxicity

Supplements which contain more than 1500mcg of vitamin A from retinol should be avoided . High doses can cause birth defects in babies (4). For this reason, liver and liver products should be avoided during pregnancy as well as fish oils. If you are pregnant or trying for a baby do not exceed 1500mcg in total from food and supplements (4). If you are unsure what you should be taking, then consult your GP or nutrition professional. Some studies have shown an increased risk of bone damage and osteoporosis in menopausal women taking more than 1500mcg of vitamin A over several years (6). High dose supplementation is not recommended for smokers and non-smokers (7).

Interactions

Vitamin A can interact with certain medications. See you GP or nutrition professional for more advice.

How do we increase our absorption of vitamin A?

Fat is needed to increase the absorption of vitamin A (3). Foods containing vitamin A should be eaten with healthy fats such as olive oil, extra virgin olive oil, avocado and nuts. Saturated fats such as coconut oil and butter in small quantities are also useful sources of healthy fats as part of a nutritious, balanced diet. Some diseases can result in poor absorption of fat. These include Crohn’s disease, Celiac disease, lactose intolerance and ulcerative colitis. As a result, they may deplete levels of fat soluble vitamins in the body.

References:

(1). Albahrani, A.A. & Greaves, R.F. (2016) Fat-Soluble Vitamins: Clinical Indications and Current Challenges for Chromatographic Measurement. The Clinical biochemist. Reviews, 37 (1), pp.27–47. (2). Borel, P. & Desmarchelier, C. (2017) Genetic variations associated with vitamin a status and vitamin A bioavailability. Nutrients, 9. (3). Gilbert, C. (2013) What is vitamin A and why do we need it? Community Eye Health Journal, 26 (84), pp.65–65. (4). NHS.UK, 2020 Vitamin A [Online]. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-a/ [Accessed 01/10/2020]. (5). Huang, Z., Liu, Y., Qi, G., Brand, D. & Zheng, S. (2018) Role of Vitamin A in the Immune System. Journal of Clinical Medicine, 7 (9), p.258. (6). SACN, 2005. Review of Dietary Advice on Vitamin A [Online}. Available at: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/338853/SACN_Review_of_Dietary_Advice_on_Vitamin_A.pdf. [Accessed 01/10/2020]. (7). Satia, J.A., Littman, A., Slatore, C.G., Galanko, J.A. & White, E. (2009) Long-term use of β-carotene, retinol, lycopene, and lutein supplements and lung cancer risk: Results from the vitamins and lifestyle (vital) study. American Journal of Epidemiology, 169 (7), pp.815–828.

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