Vitamin A is a fat-soluble, group of organic compounds that can be separated into two groups: the retinoids and carotenoids. Retinoids, also known as preformed vitamin A, include retinol, retinoic acid and retinaldehyde. The major carotenoids, also known as pro-vitamin A, are pre-cursors to preformed vitamin A, and include α-, β-, γ-carotenes, and β-cryptoxanthin. Retinol and retinoic acids can be found only from animal sources such as meat, fish, dairy, eggs, and milk with an adequate fat content. Carotenoids are red, yellow and orange pigments found in fruit and vegetables such as carrots, pumpkin, sweet potato, plums and enhance the deep green colour found in dark green leafy vegetables including kale and spinach.
Vitamin A Function
Retinol and retinaldehyde play important roles in vision, and deficiencies are associated with night blindness. With appropriate vitamin A supplementation night blindness can be reversed, but in severe cases keratinisation of the cornea can occur leading to irreversible damage and blindness. That’s why your mum said carrots will help you see in the dark!
Retinoic acids are important cell-signalling molecules that regulate a process called gene expression, which helps to regulate growth and turnover of the cells in our body. They are also important for embryonic development. Retinoic acid does this by interacting and modifying your DNA, without changing the code! This phenomenon is known as epigenetics, and when dealing with nutrients this is specifically referred to as nutrigenomics. It’s an exciting emerging field of study. The very food we eat can alter our DNA!
Conclusive evidence now also exists showing that retinoic acids play an important epigenetic role in the regulation and development of the immune system.
As a fat-soluble vitamin, vitamin A requires fat for its absorption, therefore it is important people also consume adequate amounts of dietary fat. Even if an individual meets their daily requirements of vitamin A, a consistent low-fat diet could lead to a functional vitamin A deficiency. Similarly, in our bodies we have a protein called retinol-binding protein (RBP) which is needed to aid delivery of vitamin A to target cells, and if we do not eat enough protein for our daily needs, then RBP production can drop which can also lead to a functional vitamin A deficiency. Vitamin A deficiency is not a public health concern in the UK, and if people eat a balanced diet they should avoid deficiency and toxicity.
Carotenoids are important precursors for preformed vitamin A, but they are not as biologically active, which means you need to consume more carotenoid-containing foods to form an equivalent amount preformed vitamin A. As such, the total amount of vitamin A in a food is expressed as micrograms of retinol equivalents (RE). 6µg β-carotene is equivalent to 1µg of preformed vitamin A and 12 µg of other carotenes is equivalent to 1 µg of preformed vitamin A. Therefore, it is important for people following a vegan diet to consume a variety of fruit and vegetables to ensure they get adequate vitamin A from their diet.
Carotenoids are potent anti-oxidants, however if you smoke, you would be incorrect in presuming that eating more of these free radical scavengers would be protective for lung cancer. Researchers hoped to show that β-carotene could improve prognoses in lung cancer patients in the 1990s, but the opposite was demonstrated. A large US randomised double blind placebo-controlled primary prevention trial observed an increase in mortality from lung cancer by 46% and had to be stopped 11 months early. Similar results had been observed in a Finland randomised double blind placebo-controlled prevention trial 2 years earlier with an increase for lung cancer incidence by 18% in male smokers receiving β-carotene. Conversely a Women’s Health Study over a two years period found no increased risk for lung cancer after a 2 year follow up. The reason for the disparity between studies for risk for lung cancer remains unknown. Therefore, whilst there are no known cases of vitamin A toxicity from β-carotene, except for skin discolouration (yellow), the Food Standards Agency advises that smokers do not take β-carotene supplements. Scientific Advisory Committee for Nutrition advises pregnant women also should not take Vitamin A supplements due to the teratogenic (can impair the development of an embryo or foetus) effects of retinol.
Reference Nutrient Intakes for Vitamin A (Public Health England 2016):
700 µgRE/day for men and 600 µgRE/day for women.
350 µgRE/day for 0-12m
400 µgRE/day for 1-6y
500 µgRE/day for 7-10y
600 µgRE/day for 11-14y
For girls 11y and above and boys 15y and above, the RNIs are the same as those for adults.
Sugar Free Science advises that vitamin A supplements should not be needed unless recommended by your GP. You can get your vitamin A from some meat sources but try to include some nutrient rich brightly coloured vegetables and/or leafy green vegetables with each meal. Liver or liver food products like pate are the best sources for vitamin A, and if you are partial to liver, try to have it no more than once a week, favouring oily fish as your meat source of vitamin A due to its host of associated health benefits.