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Micronutrients part i: Vitamin A

Today I will be introducing micronutrients. The format will be similar to the post I did previously on their bigger and better known, but not necessarily more important (despite their undoubted belief that they are - bloody self centred macros) friends - macronutrients.  

So, micronutrients, where do we even start with these little guys. First off lets just underline their importance. Micronutrients - can be defined as 'chemical elements or substances required in trace amounts for the normal growth and development of living organisms'. They are more commonly, and potentially more accurately, known as vitamins and minerals, they are essential for health in a multitude of ways, and compared to macros they are typically consumed in much smaller quantities. Underconsumption of micros leads to malnourishment and micronutrient deficiency which can contribute to a plethora of negative health impacts and chronic disease - as well as death in the longer term. The WHO estimates that more than 2 billion people worldwide suffer from micronutrient deficiency. 

There are 27 micronutrients which are considered essential to proper human health and we will go through them all, starting at the very beginning (a very good place to start). 

Vitamin A

Where better place to start than with the Alpha of all the vitamins, not necessarily in terms of importance (though who am I to judge), but only because it happened to acquire the honour of attaining the first letter of the alphabet in its name!

Vitamin A is the common term used to describe a group of multiple fat soluble retinoids. Most vitamin A is stored in the liver. It is important for many things in the human body, including but not limited to: immune function, reproduction, cellular communication and cell growth - for example normal formulation and maintenance of the heart, lungs, eyes and other organs. Perhaps its best known importance is in its contribution to healthy vision. Vitamin A is a critical component of rhodopsin - a light-sensitive protein of primary importance in the retina - it also contributes to normal functioning of eye membranes and the cornea.  

Deficiency of vitamin A can cause severe eye-dryness leading to blindness, night blindness, reduced immune function and therefore increased susceptibility to diseases, thyroid dysfunction, dry skin/hair, fatigue and infertility.

Humans can get their vitamin A from two sources within their diet - preformed vitamin A and provitamin A. The preformed variety - retinol and retinyl esthers - can be acquired from animal food sources such as eggs, dairy, organs and fish. The provitamin variety - carotenoids - can be acquired from plant food sources such as (shockingly, given the name) carrots, sweet potatoes and pumpkin. 

Provitamin A carotenoids - which are actually plant pigments - once consumed are converted into a usable form of vitamin A within the human intestine. The preformed variety, as the name suggests, is already in a form which is ready to be used by the human body and requires no conversion.

Liver is by far the most abundant source of vitamin A in the human diet - 68g of beef liver contains 6,421μg of preformed vitamin A (seven times the recommended adult male daily intake - 900μg). The closest plant source of vitamin A is sweet potato which contains 961μg per 100g (baked with skin on) this being just over one times the adult male recommended intake - it is worth bearing in mind that this is in the provitamin form which requires conversion in the intestine - the conversion is not exact so not all is available, unlike the preformed vitamin A from animal sources from which all is available. 

If listed in terms of bioavailable (a term which I have defined in the link) vitamin A, the top 5 human food sources of vitamin A are:

1- liver (beef, lamb, goat, chicken, pork)

2- dark leafy greens 

3- cheese/butter 

4- small dried fish 

5- eggs 

Sources: 

Vitamin A Update: Forms, Sources, Kinetics, Detection, Function, Deficiency, Therapeutic Use and Toxicity, A.Carazo et al.

Dietary Reference Intakes for Vitamin A..., Institute of Medicine (US) Panel on Micronutrients

Priority Micronutrient Density in Foods, Ty Beal and Flaminia Ortenzi


I will continue to introduce the various micronutrients in further posts, this being because I don't want anyone to feel compelled to read a single post regarding every one of them, this would be quite the undertaking. So stay tuned for future posts regarding the others...

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