Trait: Vitamin A Requirement
Dr Haran Sivapalan
July 6, 2020
What is Vitamin A?
Vitamin A is a general term for a group of related fat-soluble micronutrients that includes retinol (found in animal products) and provitamin A carotenoids (found in fruit and vegetables).
The three forms of Vitamin A that are active in the body are: retinol, retinal and retinoic acid.
These compounds play important roles in immune function, skin health and maintaining healthy eyesight.
Much of the Vitamin A contained in our diet must first be converted into active forms of Vitamin A in order to be used by the body.
- Vitamin A is a group of fat-soluble compounds that are important in immune function, skin health and eyesight.
- The active forms of Vitamin A in the body are: retinol, retinal and retinoic acid.
- Vitamin A in our diet must be converted into these active forms before it can be used by the body.
Why is Vitamin A important?
Each of the three active forms of Vitamin A (retinol, retinal and retinoic acid) play different key roles in the body, including support of vision, immune function, skin health and red blood cell production.
Retinol and retinal are interchangeable and can be easily converted into one another by tissues in the body. By contrast, retinoic acid is formed from retinal and cannot be directly converted into other active forms of Vitamin A.
Retinol and retinal are used to make a photosensitive pigment called rhodopsin. This pigment is produced by rod cells in the retina of the eye, where it helps with vision in low-light conditions.
Interestingly, it is this function of Vitamin A that is the basis of the (exaggerated) claim that carrots (which are rich in beta-carotene, a source of Vitamin A) can help you see in the dark.
Retinoic acid regulates the function and development of various white blood cells that help fight infection. Much of this role relies upon the ability of retinoic acid to control how genes are switched on and off – a process called gene expression.
Cells in the outer two layers of skin (the epidermis and dermis) use active forms of Vitamin A for growth, development and regeneration. On this note, topical applications containing Vitamin A are sometimes used to treat skin conditions such as acne.
Red blood cell production
The active forms of Vitamin A likely play a role in the production of red blood cells by bone marrow. Furthermore, Vitamin A supports the metabolism of iron and the production of haemoglobin – the iron-containing protein that helps red blood cells transport oxygen around the body.
- The active forms of Vitamin A are involved in vision, immune function, skin health, red blood cell production and the control of gene expression.
- Retinol and retinal can be interconverted.
- Retinoic acid is formed from retinal and cannot be directly converted into other active forms of Vitamin A.
How do we obtain Vitamin A from foods?
There are two main sources of Vitamin A in food:
- Preformed Vitamin A in animal products – this includes retinol and retinyl esters.
- Provitamin A carotenoids in fruit and vegetables – this includes β carotene.
Preformed Vitamin A
As the name suggests, preformed Vitamin A is already formed, and can be readily used by the body. For example, retinol is used by cells in the eye and used to produce the photosensitive pigment rhodopsin.
Retinol found in animal products is also in the form of compounds called retinyl esters. Two important retinyl esters found in food are retinyl palmitate (also known as Vitamin A palmitate) and retinyl acetate.
These retinyl esters are easily broken down (hydrolysed) into retinol by enzymes in the intestines. Retinol can then be further metabolised and used by tissues in the body.
On this note, retinol can be converted into the other active Vitamin A forms: retinal and retinoic acid. Alternatively, retinol can be turned back into retinyl esters and stored in the liver or transported around the body in the bloodstream.
Foods that are rich sources of preformed vitamin A (retinol and retinyl esters), include:
- beef liver
- oily fish
Provitamin A carotenoids
Carotenoids are a class of naturally-occurring pigments that give plants their yellow, red and orange colours. For example, carrots are orange due to their high content of two carotenoids in particular – α-carotene and β carotene.
There are several hundreds of carotenoids produced by plants, but we can only make active forms of Vitamin A from a small group known as provitamin A carotenoids. The main provitamin A carotenoids are:
Rich sources of α-carotene and β-carotene include:
- Winter squash
Unlike preformed Vitamin A, pro-vitamin A carotenoids must first be chemically converted into active forms of Vitamin A (specifically retinal) before they can be effectively used by the body. This process is carried by enzymes in our liver and intestines.
- Vitamin A is obtained in two main forms: preformed Vitamin A and provitamin A carotenoids.
- Preformed Vitamin A includes retinol and retinyl esters and is found in animal products such as liver, eggs, and milk.
- Provitamin A carotenoids includes alpha and beta-carotene and is found in fruit and vegetables such as carrots, pumpkin and spinach.
- Provitamin A carotenoids must be converted into active forms of Vitamin A before they can be used.
What is BCMO1?
BCMO1, also known as BCO1, stands for beta-carotene oxidase 1.
It is an enzyme that helps convert provitamin A carotenoids into active forms of Vitamin A.
More specifically, BCMO1 converts provitamin A carotenoids (e.g. beta-carotene) into retinal, one of the active forms of Vitamin A.
Retinal can then be further made into the other active forms of Vitamin, retinol and retinoic acid, to carry out various processes in the body.
The activity of the BCMO1 enzyme influences how well you can make Vitamin A from pro-carotenoids in fruit and vegetables.
Low activity of the BCMO1 enzyme will result in reduced conversion of provitamin A carotenoids into Vitamin A and, consequently, a higher requirement for Vitamin A in the diet.
Since animal products (e.g. liver, milk, eggs) tend to contain preformed Vitamin A (retinol and retinyl esters), the BCMO1 enzyme does not directly affect how well you obtain Vitamin A from these foodstuffs.
- BCMO1 is the enzyme that converts provitamin A carotenoids into retinal, an active form of Vitamin A.
- Reduced BCMO1 activity results in poorer conversion of provitamin A carotenoids from fruits and vegetables into Vitamin A.
How do BCMO1 gene variants affect Vitamin A synthesis?
The BCMO1 enzyme is encoded by the BCMO1 gene.
Variants of this gene have been demonstrated to change the activity of the BCMO1 enzyme. This, in turn, affects how well the body makes Vitamin A from provitamin A carotenoids in fruit and vegetables.
For example, one SNP (Single Nucleotide Polymorphism), designated rs11645428, creates two BCMO1 gene variants (or alleles): the ‘A’ allele and the ‘G’ allele.
Studies suggest that carrying two copies of ‘G’ allele is associated with 51% reduced activity of the BCMO1 enzyme.
Similarly, another SNP (rs6564851) creates ‘T’ and ‘G’ alleles. People carrying two copies of the ‘G’ allele have been found to have 48% lower activity of the BCMO1 enzyme.
As a result of reduced enzyme activity, people carrying these alleles make Vitamin A from provitamin A carotenoids less effectively. Consequently, they will have a greater Vitamin A requirement.
Furthermore, people who exclusively rely on fruits and vegetables for Vitamin A (e.g. people adopting a vegan diet), may need to consider supplementation with preformed Vitamin A (e.g. retinol, retinyl palmitate).
Your Vit A requirement (BCMO1) trait analyses several different variants of your BCMO1 gene and accordingly classes you into one of three categories:
- Increased requirement – you carry BCMO1 gene variants associated with significantly reduced enzyme activity and poorer conversion of provitamin A carotenoids in fruit and vegetables.
- Moderately increased requirement – you carry BCMO1 gene variants associated with moderately reduced enzyme activity and moderately poorer conversion of provitamin A carotenoids in fruit and vegetables.
- Average requirement - you carry BCMO1 gene variants associated with normal enzyme activity and normal conversion of provitamin A carotenoids in fruit and vegetables.
- Variants of the BCMO1 gene affect the activity of the BCMO1 enzyme.
- BCMO1 gene variants associated with lower enzyme activity lead to poorer conversion of provitamin A carotenoids into Vitamin A.
- Individuals with poorer conversion of provitamin A carotenoids have a higher Vitamin A requirement.