Carotenoids are antioxidant yellow, red, and orange pigments made by plants, bacteria, and some kinds of algae. Over 700 natural carotenoids have been identified, of which around 50 are present in edible fruit and vegetables. Alpha-carotene, betacarotene, beta-cryptozanthin, lycopene, lutein and zeaxanthin account for more than 95% of carotenoids in the body. Carotenoids benefits are due to their antioxidant action, and their pro-vitamin A activity.
Carotenoids and vitamin A
Some carotenoids can be converted into vitamin A in the body, of which alphacarotene, betacarotene, gammacarotene and cryptoxanthin are most common in the diet. These supply up to 30% of vitamin A needs in those following a western style diet and in developing countries plant carotenoids supply up to 70% of vitamin A intakes.
Vitamin A is vital for switching on and off genes involved in cell growth and repair.
Originally, it was thought that 6 mg betacarotene produced 1 mg of retinol vitamin A. However, this conversion varies from between 10 to 1 and 28 to 1 depending on the form in which betacarotene is stored within the plant, how the food is prepared, and whether or not your diet already contains good amounts of retinol vitamin A.
One study found that taking 6mg ‘labelled’ betacarotene in oil had a conversion factor of 3.8 to 1 (ie the rate of conversion was 3.8mg betacarotene to produce 1mg of retinol vitamin A).
However, when the same person took a pharmacological dose of 126 mg betacarotene in oil, the rate of conversion was almost fifteen fold lower, so that 55mg betacarotene produced 1mg retinol. This is because conversion only occurs as needed to meet your vitamin A requirements.
Some people do not convert carotenoids to vitamin A as efficiently as others, so an average estimate of 12mg betacarotene to 1mg retinol vitamin A is now used until more data becomes available.
Because excess vitamin A can be harmful during pregnancy, supplements designed for pregnant women provide betacarotene rather than retinol vitamin A to support a healthy pregnancy.
Carotenoids are important antioxidants in the body, protecting cell membranes from free radical attack, reducing oxidation of circulating fats such as cholesterol, and maintaining antioxidant protection in the eye.
Carotenoids and macular degeneration
Those who eat the most carotenoids have at least a 32% lower risk of developing advanced age-related macular degeneration (AMD) of the eye than those with low intakes. These benefits are mainly related to the carotenoids, lutein and zeaxanthin.
You can make zeaxanthin from lutein, but you cannot make lutein so it is essential to obtain regular intakes of this key carotenoid pigment.
Lutein and zeaxanthin accumulate in the macula, a part of the retina responsible for fine vision. By increasing the level of pigment present, these carotenoids protect the macular by filtering out harmful blue light, and provide antioxidant protection against chemical reactions involved in light detection.
Carotenoids and cataracts
Good intakes of carotenoids, especially lutein and zeaxanthin, have been associated with a reduced risk of cataracts.
An Australian study involving 2322 people found that every 1mg increase in average intake of lutein and zeaxanthin was associated with a 33% reduced risk of nuclear cataract. A Finnish study also found those with the highest intakes of lutein and zeaxanthin were 41% less likely to develop age-related nuclear cataract than those with the lowest intakes.
Carotenoids and hip fracture
Increased intakes of total carotenoids may lower the risk of hip fracture according to a 17-year study involving almost 1000 adults with an average age of 75 years. When the effect of individual carotenoids was assessed, lycopene had the greatest protective effect. Carotenoids are believed to protect bone health through their antioxidant activity as oxidation reactions appear to increase bone breakdown and reabsorption.
Carotenoids and heart disease
Lycopene is a red carotenoid pigment found in tomatoes. People who regularly eat tomatoes are less likely to develop heart disease than those who eat them infrequently, and tomatoes are one of the reasons why a Mediterranean diet is so healthy. Good intakes of lycopene are associated with reduced thickness and improved dilation of artery walls. Lycopene also has beneficial effects on high blood pressure.
Carotenoids and diabetes
A study involving 133 people with diabetes, 155 people with glucose intolerance and 288 healthy controls found that those with the highest intake of carotenoids from carrots and pumpkins were 50% less likely to have poor glucose tolerance and diabetes than those with low intakes.
Blood levels of alpha- and beta-carotenes, lycopene, beta-cryptoxanthin, zeaxanthin and lutein all showed a beneficial effect suggesting that intake of vegetables and fruits rich in carotenoids might be a protective factor against developing raised blood glucose levels.
An Australian study involving 1634 people also found that those with the highest blood level of carotenoids were least likely to develop type 2 diabetes.
Carotenoids and diabetes complications
Diabetes is associated with complications that result from raised levels of glucose damaging blood vessel walls to hasten hardening and furring up of the arteries. These complications can also damage small blood vessels in the eye (diabetic retinopathy), kidneys (diabetic nephropathy) and nerves (diabetic neuropathy). Glucose damages blood vessels by interacting with proteins and triggering oxidative stress and inflammation.
A healthy diet can help to prevent these complication and good intakes of four antioxidant carotenoids – lutein, zeaxanthin, lycopene, and astaxanthin – may help to reduce the development and progression of diabetic microvascular complications.
Carotenoids and immunity
Vitamin A, and carotenoids that act as pro-vitamin C can improve immunity by stimulating and regulating the action of immune cells that fight infection, damp down inflammation and prune away abnormal cells.
Carotenoids and skin aging
Within plants, carotenoids provide antioxidant protection against ultra violet light. Lycopene, for example, protects tomatoes from burning. When eaten, carotenoids can enter the skin as they are fat soluble – high doses of carotenoids can even turn the skin slightly orange.
Within the skin, carotenoids provide some protection against ultraviolet light and a diet enriched with carotenoid supplements can reduce the level of inflammation caused by ultraviolet exposure, although this should not be used as a substitute for sunscreen, it can provide some increased protection against premature photo-ageing of skin.
Carotenoids and mouth leukoplakia
Oral leukoplakia is a condition in which white, thickened patches form on your gums and inner cheeks. These need treating as they may eventually develop into mouth cancer. In one study, 50 people with leukoplakia took high dose beta carotene (60mg per day) for 6 months. Of these 52% responded to treatment and, during the following year, only 18% relapsed.
In another study, taking 30mg beta carotene per day for 3 months had a complete or partial response rate of 71%.
NB The most common cause of leukoplakia is smoking. If you smoke, seek medical advice before taking betacarotene supplements (see cautions below).
Carotenoid rich foods
Carotenoids are found in yellow, orange, red and dark green fruits and vegetables, including sweetcorn, carrots, pumpkins, mangoes, oranges, guavas watermelons, tomatoes and leeks. Carotenoids are also present in high amounts in spinach, kale and other dark-green leafy vegetables, including broccoli, where their colour is masked by the green chlorophyll present. Even banana and avocados are sources of lutein, alphacarotene and betacarotene. Pistachio nuts are a good source of lutein.
Among animal-based foods, carotenoids are found in egg yolk and dairy products. One of the reasons why Channel Island milk, butter and cream produced from Guernsey cows looks so rich is because this breed do not convert carotenoids (from the grass they eat) to vitamin A very efficiently. More carotenoids remain in the milk and cream to give their butter a golden-yellow colour.
Some seafood also contains carotenoids, such as the red pigment, astaxanthin, found in salmon, shrimp, prawns, crab, lobster and in krill oil supplements.
These carotenoids are derived from the algae on which the fish and crustaceans have fed.
Edible algae are a good source of carotenoids for us, too. The betacarotene in Spirulina is particularly well absorbed due to its simple cell structure, which is easily digested to release the carotenoids.
Carotenoids are rapidly destroyed by heat and overcooking, so eat these carotenoid sources raw or only lightly cooked or steamed, as appropriate.
Fat boosts carotenoid absorption
Dietary fats and oils boost the absorption of betacarotene and its conversion to vitamin A. Meals providing carotenoid-rich yellow and green leafy vegetables need to provide at least 2.4 g fat to ensure optimal absorption of fat-soluble provitamin A carotenoids.
Adding chopped avocado or avocado oil to a salsa, for example, boosts the absorption of tomato carotenoids (lycopene and betacarotene) four fold compared with an avocado-free salsa.
How much carotenoid do you need?
Because carotenoids are not classed as vitamins, there is no recommended daily intake. Intakes range between 9mg and 22mg carotenoids per day.
For those at risk of age-related macular degeneration (AMD) intakes of 5mg to 10mg lutein per day are achievable from eating dark green leafy vegetables (especially cooked spinach, kale, romaine lettuce, endive, parsley, broccoli), asparagus, avocado, corn, egg yolk, orange peppers, pistachio nuts, pumpkin and eggs.
Supplements typically supply 6mg to 15mg mixed carotenoids per day.
Carotenoid side effects
High intakes of carotenoids such as betacarotene (from supplements or eating lots of carrots) can cause carotenodermia, in which the skin acquires an orange colour. This is harmless and will quickly resolve once intakes are reduced. In fact, this effect is deliberately sought to protect the skin in certain photosensitivity disorders, and high-dose carotenoids have not shown any significant toxicity.
Carotenoids and lung cancer
Supplements, but not foods, containing beta carotene may have a harmful effect on lung cancer risk in smokers. The results from four trials involving 109,394 people unexpectedly found a statistically significant increase in the risk of lung cancer among smokers who used dietary supplements containing beta-carotene, at high doses of 20mg to 30 mg per day. Smokers are therefore advised to avoid supplements containing betacarotene. The reason for the link remains unknown, as in healthy non-smokers carotenoids have beneficial effects.
When it comes to dietary betacarotene and other carotenoids naturally present in fruit and vegetables, evidence from 18 studies, involving 10,261 people who developed lung cancer, suggests that higher intakes of dietary betacarotene from fruit and vegetables can reduce lung cancer risk by around 23%.
Similarly, no association was found between dietary carotenoid intake and lung cancer risk in a study that involved 399,765 people, who were followed for 7 to 16 years. One dietary carotenoid, betacryptoxanthin appeared to protect against lung cancer. These findings were similar among never, past, or current smokers. The researchers concluded that, although smoking is the strongest risk factor for lung cancer, a greater intake of foods high in betacryptoxanthin, such as oranges, orange juice and tangerines, may modestly lower the risk.