What To Eat | Vitamin A

What it Vitamin A?

Vitamin A, shown in Figure 1, is a small, fat-soluble molecule that is essential for human health. Being fat-soluble means that vitamin A doesn't dissolve in water, preferring to mix with oils and fats. This is an important property of a handful of vitamins and should give you a clue as to which foods are good sources of vitamin A. As a fat-soluble vitamin, vitamin A can also be stored in body fat.

Figure 1. The chemical structure of retinol.

Vitamin A exists in two forms. The first form is called retinol, this is the functional vitamin that our bodies can actually use. Retinol is the form of vitamin A that animals use as well so animal products are the best source of this form. The other form is provitamin A. A provitamin is a molecule that our bodies can convert into a functional vitamin, in this case, provitamin A can be used to make retinol. Vitamin A has about 50 provitamins which are collectively called carotenoids. These include Beta-carotene, which is the most abundant dietary form of provitamin A and is found in many plants.

What does Vitamin A do?

Vitamin A is primarily required for vision, immune system function and normal growth and development. Vitamin A is therefore especially important for pregnant women as they will need to supply both themselves and their babies with all the vitamin A they need for healthy development.

What happens if we don't have enough?

A vitamin A deficiency can therefore cause problems with all the processes outlined above. A mild or moderate vitamin A deficiency can lead to an individual to having trouble seeing in low-light. In the developing world, vitamin A deficiency is the most important cause of childhood blindness. Deficiency can also cause slow growth, abnormal development, and can make it difficult for individuals to fight off infections, especially in children. Fortunately vitamin A deficiency is rare in the developed world as it is easily obtained through the diet and as a fat-soluble vitamin, can be stored in adipose tissue (body fat).

How much do we need?

According to the UK's Scientific Advisory Committee on Nutrition the reference nutrient intake (RNI) for vitamin A is 700 µg/d for men and 600 µg/d for women. The reference nutrient intake is defined as "the amount of a nutrient that is considered sufficient to meet the requirements of 97.5% of the population." This means the RNI is more than enough for most people, about right for some people and slightly less than enough for 2.5 percent of the population. RNIs are useful as they recommend an amount of a nutrient, such as vitamin A, that won't be too much for people who have low requirements but will be sufficient to prevent deficiency in almost everyone else.

Pregnant and lactating women have slightly increased RNIs of 700 µg/d and 950 µg/d respectively. This reflects their additional needs as they grow new tissue and supply their baby with all the vitamin A they need until weaning. However, too much vitamin A can be toxic, especially for developing foetuses. Most dietary sources of vitamin A won't provide too much, but vitamin A supplements, cod liver oil and foods like liver and liver pate should be avoided.

Unfortunately, the recommendations for vitamin A are further complicated by the inefficiency of converting provitamin A, e.g. beta-carotene, in to the active form of vitamin A: retinol. SACN advise that 6 µg of beta-carotene is equivalent to 1 µg of retinol, but the Institute of Medicine, now the National Academy of Medicine in the US, argue as much as 12 µg of beta-carotene contained in food is required to produce 1 µg of retinol. Therefore, a strict vegan may need between 6-12 times as much beta-carotene as a meat-eater will need of retinol. This discrepancy is large and is likely to narrow in the future. For the time being, those consuming all of their vitamin A as provitamin A from plant sources, should speak with their doctor or dietitian for further advise.

Which foods contain Vitamin A?

The best sources of vitamin A are liver, dairy and dark green, orange or red vegetables. Liver is such a good source of vitamin A that it shouldn't be eaten more than about once a week to avoid vitamin A toxicity (consuming too much vitamin A can be harmful). Just 2.5 g of beef liver is enough to supply 100% of a man's daily RNI. A 50 g serving of cheddar contains about 170 µg, and a single large egg contains around 100 µg. 30 g of spinach contains enough provitamin to supply 140 µg of vitamin A. Carrots, sweet potato and red pepper are also great sources. A simple three-egg omelette with spinach and cheese would likely contain enough vitamin A to meet a woman's daily RNI of 600 µg/d.

Summary

Vitamin A is a fat soluble molecule, meaning it can be stored in fat tissue. This means you don't need to worry too much about reaching your RNI for vitamin A every day. On average you need to eat around 700 µg/d if you're a man and 600 µg/d if you're a woman. Vitamin A can be found in animal products such as liver, dairy and eggs, it can also be made from the carotenoids found in vegetables such as sweet potato, red peppers, spinach and carrots. Too much vitamin A can be harmful so foods and supplements with lots of vitamin A should be limited. These include liver, liver pate and cod liver oil, as well as many multivitamin supplements. For pregnant or lactating women, while you do need slightly more vitamin A, it's best to avoid these products all-together. A balanced diet, containing some animal products and lots of fruit and veg should supply the right amount of vitamin A for most people.

Sources:

1. Department of Health (1991). DIETARY REFERENCE VALUES A GUIDE. London: Her Majesty's Stationary Office.
2. Institute of Medicine (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, D.C.: National Academy Press.
3. Ndb.nal.usda.gov. (2019). USDA Food Composition Databases. [online] Available at: https://ndb.nal.usda.gov/ndb/ [Accessed 12 Mar. 2019].
4. Ruel, M. (2001). Can food-based strategies help reduce vitamin A and iron deficiencies?. Washington, D.C: International Food Policy Research Institute.
5. Scientific Advisory Committee on Nutrition (2005). Review of Dietary Advice on Vitamin A. London: The Stationary Office.