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Vitamin A

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Vitamin A is a fat soluble vitamin, this means that if we consume excess quantities it can be stored in parts of our body that also store fat. This is different to water soluble vitamins which we cannot store, excess quantities are excreted in our urine.

In most animals (including humans) the majority Vitamin A is stored in the liver. When other tissues and organs require Vitamin A it is transported from the liver to the target by a carrier protein, upon delivery it can be used immediately or tucked away for short term storage inside the cell.

Our bodies are not able to make Vitamin A, it is something we must consume in our diet. If you have ever heard the term ‘essential vitamin’ it means that not only is it required for our bodies to function, it is essential that we consume adequate amounts in our diet, because we are unable to make it ourselves.

In our diet, Vitamin A is present in two forms:
  • Carotenoids (these precursors to retinol and therefore often referred to as Provitamin A), found in fruits and vegetables.
  • Retinol, found in animal products.

If you think about it this, it makes sense. When animals eat carotenoids they convert it to Vitamin A in their intestines and it is stored in their tissues. When we eat the animal we are consuming their stores of Vitamin A.

Orange and yellow fruits and vegetables are good sources of β-carotene, also called provitamin A. Each molecule of β-carotene that we eat is cleaved by enzymes in our small intestine to produce two molecules of retinol. [2]

Our bodies will only convert as much β-carotene into Vitamin A as it needs to, so yellow/orange vegetables are safe to consume in abundance. [3] There is no upper limit for the amount of β-carotene you should consume through food as it cannot lead to Vitamin A toxicity, however if you find yourself drinking excessive amounts of carrot juice each day, you may find your skin adopts a yellowish hue! [1]

Common Food Sources of Retinol [4]
Food sources of β-carotene/100g
β-carotene (µg)
Vitamin A (RE)
(µg)
Liver, beef, raw
1660
13,877
Liver, chicken, raw
45
12,007
Cheese, cheddar
87
172
Whole eggs, white and yolk
2
157
Cows milk, whole
19
53

Common Food Sources of β-carotene [4]
Food sources of β-carotene/100g
β-carotene (µg)
Vitamin A (RE)
(µg)
Carrots, raw
7896
1316
Sweet potato, raw
6775
1129
Spinach, raw
2018
336
Capsicum, red
1292
215
Pumpkin, raw
757
126


How much Vitamin A do we need?

Vitamin A is important for our vision, bone growth, reproduction and the development of the embryo during early pregnancy. The recommend daily intake for pregnant women is only slightly higher than for non-pregnant women, in fact care must be taken not to consume too much Vitamin A as it can cause birth defects. For this reason medications that contain high amounts of retinol, such as acne creams, are not recommended for use immediately prior to or during pregnancy.

Recommended Dietary Intake (RDI) of Vitamin A as Retinol Equivalents [1]
Age
(years)
Boys
(µg/day)
Girls
(µg/day)
Men
(µg/day)
Women
(µg/day)
Pregnancy
(µg/day)
Lactation
(µg/day)
1-3
300
300




4-8
400
400




9-13
600
600




14-18
900
700


700
1,100
19-30


900
700
800
1,100
31-50


900
700
800
1,100
51-70


900
700


>70


900
700





Do carrots really help us see in the dark? The carotenoids in carrots are converted to retinol which is used by the light receptors in our eyes to transmit signals to the brain. Without retinol the signal cannot be sent and thus one of the tell tale signs of Vitamin A deficiency is night blindness! So mega-dosing on carrots won’t turn you into a ninja by improving your night vision but orange/red fruits and vegetables are essential for maintaining healthy vision.

[1] https://www.nrv.gov.au/nutrients/vitamin-a
[2] Biesalski HK, Chichili GR, Frank J, von Lintig J & Nohr D 2007, ‘Conversion of β-carotene to Retinal Pigment’, Vitamins & Hormones, Vol. 75, pp 117-130.

[5] http://www.intechopen.com/books/oxidative-stress-and-chronic-degenerative-diseases-a-role-for-antioxidants/the-exogenous-antioxidants

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