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| Roasted Squash Salad with Chimichurri |
And you needn't worry, you'll still be getting just as many, if not more new recipes - this series will be a bonus, not an instead of! I'll try to keep it as clear as possible and will be posting links to sources at the bottom of the page - but as always, I'm happy to answer any questions you might have - simply comment at the bottom of the page, or send me an email and we can chat! Part of the standards I'm held to as a nutritionist is to only provide peer-reviewed, scientifically-backed evidence. So, at the time of publishing, this will be up to date - however, new findings are constantly emerging and our understanding of nutrition is always expanding so this is important to note. We're going to start at the beginning of the alphabet with vitamin A - though this journey won't necessarily go from A-Z and it will cover other nutrients than just vitamins and minerals too!
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| Kale Pesto Pasta |
Before we get started, if you're wondering how to identify a nutritionist - in the UK, a qualified nutritionist will have the letters ANutr or RNutr following their name - showing they've achieved a high standard of education and follow evidence-based practice - you can also find a list of nutritionists on the AfN register here. My current status is of a Registered Associate Nutritionist (ANutr), which with time and continued professional development (CPD) I will have demonstrated sufficient experience and will become a Registered Nutritionist (RNutr). Registered nutritionists have full autonomy to work independently with clients on a full-time basis - but both ANutrs and RNutrs are up to date on research, guidelines and adhere to the same standards of ethics, conduct and performance - the main difference is years of experience. You might actually find it a little confusing that the term nutritionist isn't a 'protected' term - i.e. anyone could, in theory, call themselves a nutritionist, which is why it's so important to keep an eye out for the above letters and to check the register to make sure. Dieticians are different from nutritionists - dietician is a protected title and they will typically work with patients through the NHS. The way I find easiest to explain it is that the people nutritionists advise would be considered clients and they will help them by promoting good health through food; however, dieticians are often consultants alongside doctors, to patients with chronic medical conditions that require specific diets.
If you're a bit of nutrition nerd like me, I hope that you'll find this article both interesting and informative. If you're looking for some recipes rich in vitamin A - simply click the text description at the bottom of each image and a new tab will open with that recipe! You can also use the contents guide below to jump to any particular section - happy reading!
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| Sweet Potato, Chickpea & Spinach Curry |
Vitamin A
If you're a bit of nutrition nerd like me, I hope that you'll find this article both interesting and informative. If you're looking for some recipes rich in vitamin A - simply click the text description at the bottom of each image and a new tab will open with that recipe! You can also use the contents guide below to jump to any particular section - happy reading!
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| Wasabi Ginger Tofu Poké Bowls |
We'll be covering:
- What is Vitamin A?
- Why do we need vitamin A?
- How much do we need?
- Where can you find vitamin A?
- Metabolism & bioavailability
- What happens if we have too little or too much?
- Vitamin A & life stages
- Vitamin A and medical conditions
- Summary
A note on abbreviations - I'll always explain what an abbreviation is before I solely use the short form and I'll include a handy list of all abbreviations at the bottom of the page, along with all the source links of information.
A quick disclaimer - it's also worth noting that the guidelines I'm talking through are UK based and recommendations can differ slightly from country to country. This is also intended as general information, for healthy individuals looking for information to be able to ensure their diets are giving them everything they need and learn more about nutrition. It's not intended to diagnose, treat or cure any disease - and please, always consult your doctor before changing your diet if you have any medical conditions - you may also be able to be referred to a dietician, who will be able to offer consultation alongside your treatment.
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| Cinnamon Whiskey Roasted Veg |
What is Vitamin A?
Vitamin A is an essential, fat soluble vitamin - vitamins are also under the category of micronutrients - organic molecules that we need in small amounts for essential metabolism and functioning. Including vitamin A there are 13 essential vitamins that we need to eat food sources of to obtain. There are actually a couple of different forms - preformed vitamin A i.e. retinol and provitamin A i.e. beta carotene - the term Vitamin A refers to all these compounds with the biological activity of retinol and once in their active form, these are known as retinoids [1].
Preformed vitamin A comes directly from animal products in the form of retinol or retinyl esters - it's present in foods like liver, eggs and dairy. This type of vitamin A is the most bioactive form in the diet - i.e. in a form that is easily utilised by the body [2].
Provitamin A comes from plants in a number of forms, known as carotenoids. There are 3 carotenoids that can be converted into retinoids; B-carotene, A-carotene and B-cryptoxanthin. These carotenoids aren't as bioavailable for the body and must undergo a chemical transition (more on this later). They're present in lots of plants; partly responsible for the yellow, orange, red and purple colours we see [2].
There are also other carotenoids significant in the human diet, the most prevalent being; lycopene, lutein and zeanxanthin, however, these don't have any vitamin A activity and are known as nonprovitamin A carotenoids [3].
There are also other carotenoids significant in the human diet, the most prevalent being; lycopene, lutein and zeanxanthin, however, these don't have any vitamin A activity and are known as nonprovitamin A carotenoids [3].
Why do we need vitamin A?
Carotenoids provide a range of functions, including being important for vision, the immune system, growth and reproduction. There's a little more later on exactly how carotenoids are metabolised within the body, but an important thing to note here is that carotenoids such as B-carotene are pre-cursors to retinoids - i.e. carotenoids are converted by the body into a form of Vitamin A that can be stored and utilised, known as retinoids.
Vision
In our eyes, retinoids establish visual pigments, these pigments allow us to translate light into a nervous signal, allowing us to see. It also allows night vision, by combining with a pigment in the retina called opsin, to form rhodopsin - a chemical that helps with night vision.
Nonprovitamin A carotenoids are also important for vision; zeaxanthin and lutein are present in large quantities in the retina of the eye; helping us to focus all colours of light to the same point, so our vision isn't distorted and they also protect the retina against oxidative stress caused from light. [4]
Immune system
Due to its role in enhancing immune function, Vitamin A is known as an anti-inflammatory vitamin. It is involved in the development of the immune system and contributes to regulation in both cellular and fluid immune responses.
Our front line defense comes from mucous membranes and epithelial cells. Mucous membranes are just as they sound; consisting of thick fluid that lines and protects our organs and other cavities in the body - they consist of layers of epithelial cells that overlay connective tissue. Epithelial cells act as a barrier and vitamin A is integral in the formation and functioning of these cells, providing the physical barrier that is the first point of our body's immune response - after the skin.
There are also other parts of the immune response where vitamin A is also integral - white blood cells fight infections from a number of pathways - i.e. once a pathogen has entered the body the white blood cells respond through a number of different mechanisms. Vitamin A is required for the development and differentiation (specialisation) of white blood cells, making its contribution to the immune system indispensable [5].
Reproduction & Growth
Vitamin A contributes to both male and female reproduction as well as embryonic development - supporting growth and tissue development. So, whilst high-dose preformed vitamin A sources and supplements are not recommended during pregnancy [6], sufficient amounts of carotenoids to meet the RNI are important [7]. Vitamin A also supports normal iron metabolism and the maintenance of the skin [8].
How much do we need?
Because of the different forms of vitamin A, the recommended amount you need each day, or Reference Nutrient Intake (RNI) is expressed as a retinol equivalent (RE). This is because you would need more carotenoids to have the same effect as a small amount of retinol (more on this later) [2].
The RNIs for vitamin A (as a retinol equivalent - RE) are [9]:
- 600ug for women (aged 19-75+)
- 700ug for men (aged 19-75+)
RNIs are calculated as the amount of intake sufficient for 97.5% of the population group, however, they are also based on a standard, 2000 calorie diet for women and 2500 calorie diet for men [10], so those who are more active are likely to require slightly different amounts [11]. You may also see the term NRV on food packaging, this stands for nutrient reference value, they are the EU equivalent of RNI values and may differ ever so slightly to the UK values. Because many foods are sold across Europe, food packaging adheres to EU legislation, including the use of NRVs.
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| Ultimate Charred Garlic Greens |
Where can you find vitamin A?
The amount of carotenoids in foods varies from season to season, with plant varieties, how long after harvesting the veg is eaten and how it's cooked or processed, too. The amounts below are taken from a book of the composition of foods, which takes many of these things into consideration and provides an average value, but it does mean that some amounts will be lower and some higher at times [12].
The list below shows REs of vitamin A in foods, many of these are for a 100g portion, but this may vary depending on typical portion size [12]. I've also linked a recipe on each of the sources for you to make should you wish to get a vitamin A boost! In brackets is the cooking method listed for the nutrient information - but the actual cooking method in the recipe I've linked to is likely to vary as not all cooking methods have been analysed and recorded.
- Carrots (raw) - 100g (~1 medium carrot) contains 1961ug (326% women's RNI)
- Carrots (boiled) -100g (~1 medium carrot) contains 1850ug (308% women's RNI)
- Spinach (frozen, boiled) - 100g contains 1101ug (184% RNI)
- Sweet potato (baked) - 100g (~1/2 medium sweet potato) contains 855ug (143% women's RNI)
- Pumpkin (boiled) - 200g (~1 munchkin pumpkin) contains 320ug (54% women's RNI)
- Butternut squash (baked) - 250g (~1/4 large squash) contains 310ug (52% women's RNI)
- Kale (boiled) - 50g (1-2 cups) contains 175ug (~29% women's RNI)
- Baby spinach (raw) - 50g contains 130ug (22% women's RNI)
- Tomatoes (lightly fried) - 125g contains 109ug (18% women's RNI)
- Red pepper (boiled) - 100g (~ red pepper) contains 103ug (17% women's RNI)
- Broccoli (steamed) - 100g contains 61ug (10% women's RNI)
- Mango (raw) - 50g (~1/4 large mango) contains 58ug (10% women's RNI)
Many plant based foods will surpass the RNI per serving, but exceeding this amount with veg isn't likely to be a cause for concern. With retinol, from animal products, the tolerable upper intake is 3000ug per day i.e. the amount humans can consume without severe adverse effects. There isn't enough data to establish a value for carotenoids, but they are processed in a different way by our bodies, so are, to a certain extent, considerably safer to have more of, should you eat lots of carotenoid-rich foods in one day [13].
Metabolism
Too little
Continuously having too much vitamin A can lead to acute or chronic toxicity, over a shorter and longer period of time, respectively - known as hypervitaminosis A. It is typically caused by a sustained intake above the tolerable upper limit (3000ug per day), predominantly from high dose sources of retinol like liver or supplements.
Too much vitamin A can lead to a number of health conditions, it can have significant effects on the development of a foetus as well as negatively affecting bone health. Sustained high intakes of vitamin A through retinol intake has been shown to cause bone resorption - where calcium is released from the bone into the blood, this can inhibit bone formation, contributing to conditions like osteoporosis and hip fractures [20]. Postmenopausal women are thought to be particularly vulnerable to this, due to the already increased risk of osteoporosis, due to lower oestrogen levels [21]. It is also likely to have effects on the central nervous system, causing headaches, nausea and vomiting and has been shown to cause hypothyroidism [20].
There are a number of studies that link both retinol and carotenoid intake to certain medical conditions and risk.
A condition called hypercarotenemia is a benign state whereby the skin develops an orange-yellow pigmentation. Whilst in rare cases this can be caused by excess carotenoid intake, it is more likely to be caused by high cholesterol or blood fats (hyperlipidaemia) or failure to convert carotenoids into vitamin A. Other conditions associated with hyperlipidaemia such as type 2 diabetes, hypothyroidism and a kidney disorder known as nephrotic syndrome may also cause this pigmentation [23].
Despite carotenoids showing a plethora of benefits in the average person, there are some studies that suggest that supplementation of beta carotene amongst smokers may increase risk of lung cancer. This risk appears to subside with former smokers [24]. However, this research has only established a correlational effect and is not yet accepted as causational.
There is also limited research into the preventative role carotenoids may play in the development and progression of cancer, however, no concrete evidence has been established thus far [25].
So, as you can see vitamin A is an essential nutrient - providing a whole range of functions, imperative to our vision, immunity and reproduction & growth. It's important not to get too much of it, nor too little - particularly for population groups like pregnant women, children and post-menopausal women.
Plant based sources are considered safer, particularly with at-risk groups, as it must first be converted to be utilised, which is done on an as-needed basis. Vitamin A-rich plant based sources includes dark leafy green veg like spinach or kale and bright orange and yellow fruits and veg like carrots, pumpkin, sweet potato or even mango.
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| Pan Roasted Tomatoes on Toast |
Metabolism and bioavailability
Metabolism
Provitamin A carotenoids are metabolised by the body into functional retinoids, used for many of the benefits listed above. Via a chemical process using oxygen, carotenoids are converted by the intestinal tissue or the liver to retinoids [14]. Being a fat-soluble vitamin means that fat globules traveling through the small intestines are required for absorption and that once converted, it can be stored in the liver in the form of retinyl esters, to be used as required [15]. The intestinal cell enzymes also determine whether the carotenoids are converted, or circulated into the bloodstream intact; as with the case for nonprovitamin A carotenoids like lycopene, lutein and zeaxanthin. Vitamin A metabolism is also influenced by zinc, which is required to help facilitate its absorption, metabolism and utlisation [26]. B-carotene - the most prevalent and most understood carotenoid - can be converted into 2 forms through this process; retinoic acid or retinol [14].
Carotenoids aren't as bioavailable as preformed vitamin A, but, because this conversion needs to occur for retinoids to be utilised, carotenoids are considered a safe source of vitamin A. As you consume more carotenoid-rich foods, the conversion to retinoids slows; showing that the body does not need to convert as much once the stores are already saturated [15].
Bioavailability
There are a number of factors which affect the absorption rate of vitamin A. The bioavailability of carotenoids, i.e. the amount that can be utilised as vitamin A is thought to be anywhere between 3 and 36% depending on the fruit or vegetable, how it is cooked, what it is eaten with etc [15]. This low bioavailability is why RNIs are expressed as retinol equivalents (RE), where the values have already been calculated and adjusted for you - but, like with any naturally occurring nutrients, this acts as a guide only and variation is to be expected.
In some veg, cooking seemingly increases the vitamin A content - i.e. the RE is higher because it becomes more bioavailable to your body. However, cooking vegetables is likely to lower the amount of other vitamins in the food - so cooking is not always best for total nutrient intake. Because carotenoids are fat soluble, ingestion of fats alongside vitamin A rich foods is also important for bioavailability [16]. This is particularly important for raw, salad vegetables with carotenoids, where adding even small amounts of an oil-based dressing has been shown to enhance the bioavailability [17].
Bioavailability
There are a number of factors which affect the absorption rate of vitamin A. The bioavailability of carotenoids, i.e. the amount that can be utilised as vitamin A is thought to be anywhere between 3 and 36% depending on the fruit or vegetable, how it is cooked, what it is eaten with etc [15]. This low bioavailability is why RNIs are expressed as retinol equivalents (RE), where the values have already been calculated and adjusted for you - but, like with any naturally occurring nutrients, this acts as a guide only and variation is to be expected.
In some veg, cooking seemingly increases the vitamin A content - i.e. the RE is higher because it becomes more bioavailable to your body. However, cooking vegetables is likely to lower the amount of other vitamins in the food - so cooking is not always best for total nutrient intake. Because carotenoids are fat soluble, ingestion of fats alongside vitamin A rich foods is also important for bioavailability [16]. This is particularly important for raw, salad vegetables with carotenoids, where adding even small amounts of an oil-based dressing has been shown to enhance the bioavailability [17].
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| Pumpkin Rosti with Chantarelle Mushrooms & Spinach |
What happens if we get too little or too much?
Too little
We discussed above the benefits of vitamin A, so it should come as no surprise that a deficiency shows the opposite effects. Because vitamin A is stored in the liver, it takes sustained, significant inadequacies (below the lower RNI) of dietary vitamin A intake to be considered a problem. Young children and pregnant women in low-income countries are considered most at risk of this deficiency; vitamin A deficiency is the leading cause of preventable blindness, including night blindness in children and also increases the risk of infections and pregnancy complications [18].
The national diet and nutrition survey (NDNS) shows that in the UK population, average vitamin A intake is meeting, or exceeding the RNI; except in the case of girls ages 11-18, where the mean intake is just short, at 96% of the RNI. However, this is an average intake value, meaning many individuals in each population group are well below this, failing to even meet the lower RNI. Furthermore, a substantial amount (16%) of children aged 11-18 have intakes below the lower RNI (the LNRI is the intake of a nutrient considered adequate for only 2.5% of the population) [19].
However, it is important to note that this data from the NDNS is based on data from food diaries completed over 4 days, requiring data projections to be made to establish estimations of longer term intakes. Therefore, the values we see for intakes of nutrients like vitamin A may not be representative, as food sources aren't as widely distributed as other nutrients [19].
Too much
Continuously having too much vitamin A can lead to acute or chronic toxicity, over a shorter and longer period of time, respectively - known as hypervitaminosis A. It is typically caused by a sustained intake above the tolerable upper limit (3000ug per day), predominantly from high dose sources of retinol like liver or supplements.
Too much vitamin A can lead to a number of health conditions, it can have significant effects on the development of a foetus as well as negatively affecting bone health. Sustained high intakes of vitamin A through retinol intake has been shown to cause bone resorption - where calcium is released from the bone into the blood, this can inhibit bone formation, contributing to conditions like osteoporosis and hip fractures [20]. Postmenopausal women are thought to be particularly vulnerable to this, due to the already increased risk of osteoporosis, due to lower oestrogen levels [21]. It is also likely to have effects on the central nervous system, causing headaches, nausea and vomiting and has been shown to cause hypothyroidism [20].
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| Roasted Red Pepper Pasta |
Vitamin A and life stages
The amount of vitamin A we need - like most other nutrients - changes throughout different stages of our lives.
Children
From the ages of 0-14, vitamin A requirements are the same for both males and females, after this, the RNI is equal to that of adults [9]:
- 0-12 months: 350ug
- 1-6 years: 400ug
- 7-10 years: 500ug
- 11-14 years: 600ug
- 15-18 years:
- Males: 700ug
- Females: 600ug
As per government advice, it is recommended that all children from 6 months to 5 years are given precautionary vitamin supplements containing vitamins A (233ug), C and D every day. This is only once they are consuming less than 500ml of infant formula per day, which is already fortified with these nutrients. More on this from the NHS and the safety of vitamins for children here [6].
Pregnancy and breastfeeding
An additional 100ug per day of vitamin A during pregnancy is recommended (RNI of 700ug) [10]. During pregnancy, rich sources of retinol like liver or vitamin A supplements are not recommended, unless advised otherwise by a health professional, as too much vitamin A can be harmful to the foetus [6]. No advice is currently given on the tolerable upper intakes of carotenoids, perhaps due to the conversion to retinoids occurring only as required, making toxicity less prevalent [16].
There's no doubt that above all else, fed is best when it comes to babies. But, for mothers who are breastfeeding, it's important to get the right nutrients to pass onto her baby. An additional 350ug of vitamin A daily is recommended (RNI of 950ug); on a plant based diet this can be obtained through the sources listed above as well as vegan margarine, which are required by law to be fortified with REs comparable to, or higher than butter [22]. As with pregnancy, high dose retinol supplements could be a cause for concern if exclusively breastfeeding [6].
Vitamin A and medical conditions
There are a number of studies that link both retinol and carotenoid intake to certain medical conditions and risk.
A condition called hypercarotenemia is a benign state whereby the skin develops an orange-yellow pigmentation. Whilst in rare cases this can be caused by excess carotenoid intake, it is more likely to be caused by high cholesterol or blood fats (hyperlipidaemia) or failure to convert carotenoids into vitamin A. Other conditions associated with hyperlipidaemia such as type 2 diabetes, hypothyroidism and a kidney disorder known as nephrotic syndrome may also cause this pigmentation [23].
Despite carotenoids showing a plethora of benefits in the average person, there are some studies that suggest that supplementation of beta carotene amongst smokers may increase risk of lung cancer. This risk appears to subside with former smokers [24]. However, this research has only established a correlational effect and is not yet accepted as causational.
There is also limited research into the preventative role carotenoids may play in the development and progression of cancer, however, no concrete evidence has been established thus far [25].
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| Spiced Veggie Fritters with Mango Salsa |
Summary
So, as you can see vitamin A is an essential nutrient - providing a whole range of functions, imperative to our vision, immunity and reproduction & growth. It's important not to get too much of it, nor too little - particularly for population groups like pregnant women, children and post-menopausal women.
Plant based sources are considered safer, particularly with at-risk groups, as it must first be converted to be utilised, which is done on an as-needed basis. Vitamin A-rich plant based sources includes dark leafy green veg like spinach or kale and bright orange and yellow fruits and veg like carrots, pumpkin, sweet potato or even mango.
I hope you found this useful and interesting, if you have any questions or would like me to write about a certain topic next, please leave a comment below or pop me an email!
Abbreviations
- AFN = Association for Nutrition
- ANutr = Registered Associate Nutritionist
- CPD = Continued Professional Development
- NDNS = National Diet and Nutrition Survey
- RE = Retinol Equivalent
- RNI = Reference Nutrient Intake
- RNutr = Registered Nutritionist
Sources
- Handbook of Vitamins, 4th Edition
- Handbook of Vitamins, 5th Edition
- B-Carotene and other Carotenoids
- Provitamin A Metabolism
- Role of Vitamin A & the Immune System
- Feeding in the First Year of Life
- Reproduction, Growth & Vitamin A
- EU Health Claims Register
- UK Government Dietary Recommendations
- Dietary Reference Values UK - explained
- Carotenoid Bioavailability and Fat
- McCance and Widdowson's Composition of Foods
- Tolerable Upper Intake Levels
- Conversion of Carotenoids to Retinoids
- Bioavailability of Carotenoids
- Bioavailability of Carotenoids 2
- Carotenoid Bioavailability and Fat
- Vitamin A Deficiency
- National Diet and Nutrition Survey
- Effects of Vitamin A Toxicity
- Vitamin A in Postmenopausal Women
- Breastfeeding Nutrient Requirements
- Vitamin A in Health and Disease
- Lung Cancer and B-Carotene
- Carotenoids and Cancer
- Vitamin A and Zinc
