Vitamin A and Carotenoids

Function

Vision, Immunity and Reproduction

Vitamin A is a fat-soluble vitamin that plays an important role in our vision, immune system and maintaining our reproductive system. Preformed Vitamin A is found in many forms, as retinol, retinal, retinoic acid and retinyl ester. In dietary terms vitamin A also refers to pro-vitamin A compounds. These are compounds that our body can convert to vitamin A and include the well known carotenoids (see below for more on carotenoids), that is, these compounds are not ‘preformed’ vitamin A rather they are converted to vitamin A inside our bodies.

Gene Expression

Retinoic acid regulates the expression of many important genes in our body cells. And so it plays an important role in embryonic development, particularly the development of our spinal cord, vertebrae, limbs, heart, eyes and ears. Another form of vitamin A, known as retinal, is needed by our eyes to convert light to neural signals. And it is the retinol version of vitamin A that supports our immune function through maintaining the integrity of our first line of defence – our skin and the tissues lining our airways, gut and urinary tract!

Carotenoids are Antioxidants

Many different carotenoids are found in plants but only a small proportion of them have pro-vitamin A activity, i.e. can be converted to vitamin A by our body. Examples are alpha-carotene, beta-carotene and beta-cryptoxanthin. For a long time scientists thought that only the carotenoids that could be converted to vitamin A were of any use to humans. We now know that many other carotenoids provide health benefits through their antioxidant action.

Carotenoids are Fat-Soluble

As the carotenoids are fat-soluble they tend to accumulate in the lipids contained within the walls of our body cells. Here they play an important part in scavenging free radicals and singlet oxygen molecules. They also help our body’s antioxidant systems by recycling vitamin E. The vitamin A precursor known as beta-carotene is a well known carotenoid with antioxidant action. Others include lycopene the red pigment found in tomatoes and the most potent of the carotenoid antioxidants and also lutein which is found in broccoli. But, these are just the tip of the iceberg; many more carotenoids are found in fruits and vegetables reinforcing why it’s so important to eat as many servings of fruit and vegetables as possible each day.

Role in Disease Prevention

Supports Immunity

Vitamin A plays a really important role in disease prevention and so is sometimes referred to as the ‘anti-infective’ vitamin. It helps to protect us from infectious diseases by maintaining the integrity of our first line of defence – our skin and the lining of our gut, airways and urinary tract (see above). Vitamin A and retinoic acid also play an important part in the development and differentiation of white blood cells such as lymphocytes that respond to foreign invaders in our body.

Cancer Fighter

The antioxidant benefits of vitamin A and its precursors such as beta-carotene have also been the subject of much research in relation to cancer risk. Although a number of observational studies have found a link between beta-carotene intake and a reduced risk of cancer, intervention trials have produced conflicting results. The Second Expert Report Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective, produced by the World Cancer Research Fund and American Institute of Cancer Research, states that it is plausible for carotenoids to have anti-cancer effects; as carotenoids are antioxidants that reduce oxidative damage which has been associated with tumour formation. The Second Expert Report states that there is limited evidence that retinol protects against certain forms of skin cancer and that non-starchy fruits and vegetables containing carotenoids probably protect against cancers of the mouth, pharynx, larynx and lung, that beta-carotene probably protects against oesophageal cancer; and that foods containing lycopene probably protect against prostate cancer.

Healthy Eyes

Low levels of carotenoids have been associated with cataracts. Studies also suggest that the carotenoids lutein and zeaxanthin may prevent macular degeneration, and possibly beta-carotene and lycopene may also have a protective role. More research is needed.

Food Sources

Preformed vitamin A is found only in foods derived from animals, for example liver and whole milk. Pro-vitamin A compounds such as the carotenoids are found in colourful vegetables and fruit, the brighter the colour the higher the carotenoid content. Vegetables containing carotenoids include broccoli, capsicums (bell peppers), carrots, green beans, leafy green vegetables, kumara (sweet potato), pumpkin/squash and of course the tomato and many tomato-based products such as sauce and soup. Fruits containing carotenoids include peaches, plums, apricots, mangoes, and some melons.

Food Processing Improves Carotenoids

Processing of food, such as cutting up and cooking, greatly increases the availability and absorption of carotenoids from food. So cooking, pureeing or finely chopping vegetables helps to release carotenoids. A great example of this is tomatoes; the availability of lycopene to our bodies is far greater from tomato paste than it is from fresh tomatoes.

Typical Contributions from Food

Food	Total Vitamin A Equivalents (mcg)	Beta-Carotene Equivalents (mcg)
Broccoli, raw (1/2 cup)	31	190
Broccoli, boiled (1/2 cup)	51	300
Brussel sprouts, boiled (1/2 cup)	55	325
Capsicum, red, raw (1/4 cup)	65	390
Carrot, raw (one)	1130	6760
Carrot, steamed (one)	711	4270
Tomato, raw (one)	117	697
Apricot, flesh and skin (one)	465	2790
Peach, flesh and skin (one)	110	658
Plum, flesh and skin (one)	34	204
Egg, boiled (size 7)	86	1
Pate, chicken liver (1 tbsp)	1510	1
Milk, whole (1 cup)	72	59
Yoghurt, (1/2 cup)	52	10

Source: The Concise New Zealand Food Composition Tables 5th Edition.

Recommended Dietary Intake (RDI)

When it comes to how much vitamin A our bodies need scientists talk about quantities of vitamin A as ‘retinol equivalents’ (RE). One RE is defined as “the biological activity associated with 1 mcg of all-trans-retinol. So, 1mcg RE = 1mcg all-trans-retinol. 1mcg RE = 6mcg beta-carotene. Alternatively you may hear people referring to one International Unit (IU) of retinol. One IU is equivalent to 0.3mcg of retinol equivalents. For simplicity we have presented all vitamin A recommendations in RE.

Vitamin A RDIs – New Zealand and Australia

Life Stage	Age	Males (RE mcg/day)	Females (RE mcg/day)
Children	1-3 yr	300	300
	4-8 yr	400	400
	9-13 yr	600	600
	14-18 yr	900	700
Adults	19+ yr	900	700
Pregnancy	14-18 yr	-	700
	19+ yr	-	800
Breastfeeding	14-18 yr	-	1,100
	19+ yr	-	1,100

Suggested Dietary Target for Reducing Chronic Disease Risk

The New Zealand and Australian governments have not set a recommended dietary intake (RDI) level for carotenoids. However, they do provide a ‘suggested dietary target’ to reduce chronic disease risk, for men 5800mcg/day and women 5000mcg/day. Remember this is a carotenoid recommendation, quite separate from the vitamin A RDI noted above.

A suggested dietary target (SDT) for vitamin A intake of 1500mcg/day for men and 1220mcg/day for women. There is some evidence to suggest that vitamin A intake at the SDT level, above the RDI level, may reduce the risk of various chronic diseases, in the opinion of the NZ and Australian health authorities. Hence, this higher SDT target is provided for the information purposes of individuals and is based on current scientific evidence relating diet to chronic disease risk.

Deficiency

Interactions between iron or zinc and vitamin A status have been found in both animal studies and within human groups in developing countries. The relevance of this finding to populations in developed countries is unclear. A vitamin A deficiency can result in lessened ability to adapt to dark conditions. This is occasionally followed by xerophthalmia, a condition where the eye becomes abnormally dry.

Toxicity

The highest level of average daily intake of vitamin A for adults (aged 19 years and over) that is believed to not cause any adverse health effects is 3000mcg/day. For those aged 14-18 years the safe upper limit is lower at 2800mcg/day. Lower limits apply to adolescents, children and infants.

Excess vitamin A is known to cause toxicity and birth defects. It is for this reason that medications such as roaccutane (a high dose form of vitamin A) are to be avoided during pregnancy.

References

Lister C. (2003). Antioxidants: a health revolution. Christchurch: New Zealand Institute for Crop and Food Research.

Ministry of Health. (2006). Nutrient reference values for Australia and New Zealand. Canberra: Commonwealth of Australia.

Last Updated: 9 October 2008

The material provided by Thinking Nutrition Ltd on this website is for information purposes only. It is not a substitute for appropriate health advice from a qualified medical practitioner.