Smokers and those with poor diets lacking fruits and vegetables have a higher risk of developing AMD. Lutein and zeaxanthin are two carotenoids with protective antioxidant effects that are found in the retina, the eye tissue that is damaged by AMD.
Studies have looked to see if supplements containing lutein and zeaxanthin, as well as beta-carotene, might be useful for preventing or treating this condition. Many breakfast cereals, juices, dairy products, and other foods are fortified with retinol preformed vitamin A.
Many fruits and vegetables and some supplements contain beta-carotene, lycopene, lutein, or zeaxanthin. Deficiency Vitamin A deficiency is rare in Western countries but may occur. Also at risk are adults and children who eat a very limited diet due to poverty or self-restriction.
Mild vitamin A deficiency may cause fatigue, susceptibility to infections, and infertility. The following are signs of a more serious deficiency. Toxicity Vitamin A toxicity may be more common in the U. Vitamin A is also fat-soluble, meaning that any amount not immediately needed by the body is absorbed and stored in fat tissue or the liver. If too much is stored, it can become toxic.
The tolerable upper intake of 3, mcg of preformed vitamin A, more than three times the current recommended daily level, is thought to be safe. However, there is some evidence that this much preformed vitamin A might increase the risk of bone loss, hip fracture [], or some birth defects. Signs of toxicity include the following.
In contrast to preformed vitamin A, beta-carotene is not toxic even at high levels of intake. The body can form vitamin A from beta-carotene as needed, and there is no need to monitor intake levels as with preformed vitamin A. Therefore, it is preferable to choose a multivitamin supplement that has all or the vast majority of its vitamin A in the form of beta-carotene; many multivitamin manufacturers have already reduced the amount of preformed vitamin A in their products.
However, there is no strong reason for most people to take individual high-dose beta-carotene supplements. Smokers in particular should avoid these, since some randomized trials in smokers have linked high-dose supplements with increased lung cancer risk. Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A.
The various forms of vitamin A are solubilized into micelles in the intestinal lumen and absorbed by duodenal mucosal cells [ 5 ]. Both retinyl esters and provitamin A carotenoids are converted to retinol, which is oxidized to retinal and then to retinoic acid [ 2 ].
Most of the body's vitamin A is stored in the liver in the form of retinyl esters. Retinol and carotenoid levels are typically measured in plasma, and plasma retinol levels are useful for assessing vitamin A inadequacy. However, their value for assessing marginal vitamin A status is limited because they do not decline until vitamin A levels in the liver are almost depleted [ 3 ]. Liver vitamin A reserves can be measured indirectly through the relative dose-response test, in which plasma retinol levels are measured before and after the administration of a small amount of vitamin A [ 5 ].
For clinical practice purposes, plasma retinol levels alone are sufficient for documenting significant deficiency. A plasma retinol concentration lower than 0. In some studies, high plasma or serum concentrations of some provitamin A carotenoids have been associated with a lower risk of various health outcomes, but these studies have not definitively demonstrated that this relationship is causal.
DRI is the general term for a set of reference values used for planning and assessing nutrient intakes of healthy people. These values, which vary by age and gender, include:. RDAs for vitamin A are given as retinol activity equivalents RAE to account for the different bioactivities of retinol and provitamin A carotenoids, all of which are converted by the body into retinol see Table 1. One mcg RAE is equivalent to 1 mcg retinol, 2 mcg supplemental beta-carotene, 12 mcg dietary beta-carotene, or 24 mcg dietary alpha-carotene or beta-cryptoxanthin [ 5 ].
For example, the RDA of mcg RAE for adolescent and adult men is equivalent to 3, IU if the food or supplement source is preformed vitamin A retinol or if the supplement source is beta-carotene. Therefore, a mixed diet containing mcg RAE provides between 3, and 36, IU vitamin A, depending on the foods consumed. Concentrations of preformed vitamin A are highest in liver and fish oils [ 2 ]. Other sources of preformed vitamin A are milk and eggs, which also include some provitamin A [ 2 ].
Most dietary provitamin A comes from leafy green vegetables, orange and yellow vegetables, tomato products, fruits, and some vegetable oils [ 2 ]. The top food sources of vitamin A in the U. Table 2 suggests many dietary sources of vitamin A. The foods from animal sources in Table 2 contain primarily preformed vitamin A, the plant-based foods have provitamin A, and the foods with a mixture of ingredients from animals and plants contain both preformed vitamin A and provitamin A.
FDA developed DVs to help consumers compare the nutrient contents of foods and dietary supplements within the context of a total diet. FDA does not require food labels to list vitamin A content unless vitamin A has been added to the food.
The U. Department of Agriculture's USDA's FoodData Central [ 9 ] lists the nutrient content of many foods and provides a comprehensive list of foods containing vitamin A in IUs arranged by nutrient content and by food name , and foods containing beta-carotene in mcg arranged by nutrient content and by food name.
Vitamin A is available in multivitamins and as a stand-alone supplement, often in the form of retinyl acetate or retinyl palmitate [ 2 ]. A portion of the vitamin A in some supplements is in the form of beta-carotene and the remainder is preformed vitamin A; others contain only preformed vitamin A or only beta-carotene. Supplement labels usually indicate the percentage of each form of the vitamin.
The amounts of vitamin A in stand-alone supplements range widely [ 2 ]. Multivitamin supplements typically contain —3, mcg RAE 2,—10, IU vitamin A, often in the form of both retinol and beta-carotene. Adults aged 71 years or older and children younger than 9 are more likely than members of other age groups to take supplements containing vitamin A.
Although these intakes are lower than the RDAs for individual men and women, these intake levels are considered to be adequate for population groups. The adequacy of vitamin A intake decreases with age in children [ 4 ]. Furthermore, girls and African-American children have a higher risk of consuming less than two-thirds of the vitamin A RDA than other children [ 4 ]. Frank vitamin A deficiency is rare in the United States.
However, vitamin A deficiency is common in many developing countries, often because residents have limited access to foods containing preformed vitamin A from animal-based food sources and they do not commonly consume available foods containing beta-carotene due to poverty [ 2 ].
According to the World Health Organization, million preschool-aged children and In these countries, low vitamin A intake is most strongly associated with health consequences during periods of high nutritional demand, such as during infancy, childhood, pregnancy, and lactation. In developing countries, vitamin A deficiency typically begins during infancy, when infants do not receive adequate supplies of colostrum or breast milk [ 12 ]. Chronic diarrhea also leads to excessive loss of vitamin A in young children, and vitamin A deficiency increases the risk of diarrhea [ 5 , 13 ].
The most common symptom of vitamin A deficiency in young children and pregnant women is xerophthalmia. One of the early signs of xerophthalmia is night blindness, or the inability to see in low light or darkness [ 2 , 14 ]. Vitamin A deficiency is one of the top causes of preventable blindness in children [ 12 ].
People with vitamin A deficiency and, often, xerophthalmia with its characteristic Bitot's spots tend to have low iron status, which can lead to anemia [ 3 , 12 ]. Vitamin A deficiency also increases the severity and mortality risk of infections particularly diarrhea and measles even before the onset of xerophthalmia [ 5 , 12 , 14 ].
In developed countries, clinical vitamin A deficiency is rare in infants and occurs only in those with malabsorption disorders [ 15 ]. However, preterm infants do not have adequate liver stores of vitamin A at birth and their plasma concentrations of retinol often remain low throughout the first year of life [ 15 , 16 ]. Preterm infants with vitamin A deficiency have an increased risk of eye, chronic lung, and gastrointestinal diseases [ 15 ]. In developed countries, the amounts of vitamin A in breast milk are sufficient to meet infants' needs for the first 6 months of life.
But in women with vitamin A deficiency, breast milk volume and vitamin A content are suboptimal and not sufficient to maintain adequate vitamin A stores in infants who are exclusively breastfed [ 17 ]. The prevalence of vitamin A deficiency in developing countries begins to increase in young children just after they stop breastfeeding [ 3 ]. The most common and readily recognized symptom of vitamin A deficiency in infants and children is xerophthalmia. Pregnant women need extra vitamin A for fetal growth and tissue maintenance and for supporting their own metabolism [ 18 ].
Dietary carotenoids : Because carotenoids are very soluble in fat and very insoluble in water, they circulate in lipoproteins , along with cholesterol and other fats. Evidence that low-density lipoprotein LDL oxidation plays a role in the development of atherosclerosis led scientists to investigate the role of antioxidant compounds like carotenoids in the prevention of cardiovascular disease The thickness of the inner layers of the carotid arteries can be measured noninvasively using ultrasound technology.
This measurement of carotid intima-media thickness is considered a reliable marker of atherosclerosis A number of case-control and cross-sectional studies have found higher blood concentrations of carotenoids to be associated with significantly lower measures of carotid artery intima-media thickness Additionally, higher plasma carotenoids at baseline have been associated with significant reductions in risk of cardiovascular disease in some prospective cohort studies but not in others More recently, an analysis of data from the US National Health And Nutrition Examination Survey NHANES in a sample of 2, adults found that serum total carotenoid concentration was inversely associated to blood concentrations of two cardiovascular risk factors, C-reactive protein CRP and total homocysteine While the results of several prospective studies indicate that people with higher intakes of carotenoid-rich fruit and vegetables are at lower risk of cardiovascular disease , , it is not yet clear whether this effect is a result of carotenoids or other factors associated with diets high in carotenoid-rich fruit and vegetables.
The year follow-up of participants in the Framingham Osteoporosis Study participants showed those in the highest tertile of total carotenoid intake median intake: No such association was reported with serum concentrations of other individual carotenoids. Whether carotenoid supplementation may help prevent bone loss and reduce the risk of osteoporosis in older individuals is currently unknown. Observational studies have suggested that dietary lutein may be of benefit in maintaining cognitive health , As stated above, among the carotenoids, lutein and its isomer zeaxanthin are the only two that cross the blood- retina barrier to form macular pigment in the eye.
Lutein also preferentially accumulates in the brain , Recent studies suggested that lutein and zeaxanthin concentrations in the macula were correlated with brain lutein and zeaxanthin status and might be used as a biomarker to assess cognitive health Additionally, in the Georgia Centenarian Study, the analysis of cross-sectional data from 47 centenarian decedents showed a positive association between post-mortem measures of brain lutein concentrations and pre-mortem measures of cognitive function Brain lutein concentrations were found to be significantly lower in individuals with mild cognitive impairment compared to those with normal cognitive function Most carotenoids in foods are found in the all-trans form see Figure 1 and Figure 2 above , although cooking may result in the formation of other isomers.
The relatively low bioavailability of carotenoids from most foods compared to supplements is partly due to the fact that they are associated with proteins in the plant matrix Chopping, homogenizing, and cooking disrupt the plant matrix, increasing the bioavailability of carotenoids 3. For example, the bioavailability of lycopene from tomatoes is substantially improved by heating tomatoes in oil , Lycopene gives tomatoes, pink grapefruit, watermelon, and guava their red color.
Lycopene is not a provitamin A carotenoid because it cannot be converted to retinol. Some foods that are good sources of lycopene are listed in Table 5 Although lutein and zeaxanthin are different compounds, they are both classified as xanthophylls and nonprovitamin A carotenoids see Figure 2 above.
Both pigments are present in a variety of fruit and vegetables. Dark green leafy vegetables like spinach and kale are particularly rich sources of lutein but poor sources of zeaxanthin Although relatively low in lutein, egg yolks and avocados are highly bioavailable sources of lutein. Good sources of dietary zeaxanthin include yellow corn, orange pepper, orange juice, honeydew melon , and mango Some foods containing lutein and zeaxanthin are listed in Table 6 Dietary supplements providing purified carotenoids and combinations of carotenoids are commercially available in the US without a prescription.
Carotenoids are best absorbed when taken with a meal containing fat. Lycopene has no provitamin A activity. Synthetic lycopene and lycopene from natural sources, mainly tomatoes, are available as nutritional supplements containing up to 15 mg of lycopene per softgel capsule Lutein and zeaxanthin are not provitamin A carotenoids. Lutein and zeaxanthin supplements are available as free carotenoids non-esterified or as esters esterified to fatty acids.
Both forms appear to have comparable bioavailability Many commercially available lutein and zeaxanthin supplements have much higher amounts of lutein than zeaxanthin Supplements containing only lutein or zeaxanthin are also available.
Lycopenodermia : High intakes of lycopene-rich food or supplements may result in a deep orange discoloration of the skin known as lycopenodermia. Because lycopene is more intensely colored than the carotenes, lycopenodermia may occur at lower doses than carotenodermia 8. Adverse effects of lutein and zeaxanthin have not been reported There is no reason to limit the consumption of carotenoid-rich fruit and vegetables during pregnancy The cholesterol -lowering agents, cholestyramine Questran and colestipol Colestid , can reduce absorption of fat- soluble vitamins and carotenoids, as can mineral oil and Orlistat Xenical , a drug used to treat obesity Colchicine, a drug used to treat gout , can cause intestinal malabsorption.
A three-year randomized controlled trial in patients with documented coronary heart disease CHD and low serum high density lipoprotein HDL concentrations found that a combination of simvastatin Zocor and niacin increased HDL2 levels, inhibited the progression of coronary artery stenosis , and decreased the frequency of cardiovascular events, including myocardial infarction and stroke These contradictory findings indicate that further research is needed on potential interactions between antioxidant supplements and cholesterol-lowering agents, such as niacin and statins.
One study in adults found that those who consumed more than 4. However, advising people who use plant sterol- or stanol-containing margarines to consume an extra serving of carotenoid-rich fruit or vegetables daily prevented decreases in plasma carotenoid concentrations , The relationships between alcohol consumption and carotenoid metabolism are not well understood. Updated in December by: Jane Higdon, Ph.
Updated in May by: Victoria J. Drake, Ph. Updated in July by: Barbara Delage, Ph. Reviewed in August by: Elizabeth J. Johnson, Ph.
Wang XD. Modern Nutrition in Health and Disease. Am J Clin Nutr. Dietary factors that affect the bioavailability of carotenoids. J Nutr. Priyadarshani AM. A review on factors influencing bioaccessibility and bioefficacy of carotenoids. Crit Rev Food Sci Nutr. Reboul E. Absorption of vitamin A and carotenoids by the enterocyte: focus on transport proteins.
Provitamin a carotenoid bioavailability:what really matters? Int J Vitam Nutr Res. Borel P. Genetic variations involved in interindividual variability in carotenoid status. Mol Nutr Food Res. Food and Nutrition Board, Institute of Medicine.
Dietary reference intakes for vitamin C, vitamin E, selenium, and carotenoids. Washington, D. National Academy Press. Weber D, Grune T. Free Radicals in Biology and Medicine. Lycopene as the most efficient biological carotenoid singlet oxygen quencher. Arch Biochem Biophys. Antioxidant and prooxidant properties of carotenoids. Kaulmann A, Bohn T. Carotenoids, inflammation, and oxidative stress--implications of cellular signaling pathways and relation to chronic disease prevention.
Nutr Res. Carotenoids activate the antioxidant response element transcription system. Mol Cancer Ther. Life Sci. Lian F, Wang XD. Int J Cancer. Lycopene inhibits cyclic strain-induced endothelin-1 expression through the suppression of reactive oxygen species generation and induction of heme oxygenase-1 in human umbilical vein endothelial cells.
Clin Exp Pharmacol Physiol. J Agric Food Chem. Biologic mechanisms of the protective role of lutein and zeaxanthin in the eye. Annu Rev Nutr. Supplementation with the carotenoids lutein or zeaxanthin improves human visual performance. Ophthalmic Physiol Opt. A week lutein supplementation improves visual function in Chinese people with long-term computer display light exposure. Retinol does not have enough antioxidant property. It is rather required for production of rhodopsin. It has high antioxidant property which neutralizes free radicals that damage the tissues.
Less is more: limited amount of retinol is also enough for body.
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