The human body converts beta carotene into vitamin A retinol — beta carotene is a precursor of vitamin A. We need vitamin A for healthy skin and mucus membranes, our immune system, and good eye health and vision.
Beta carotene in itself is not an essential nutrient, but vitamin A is. Vitamin A can be sourced from the food we eat, through beta carotene, for example, or in supplement form.
The advantage of dietary beta carotene is that the body only converts as much as it needs. Excess vitamin A is toxic. Toxic vitamin A levels can occur if you consume too many supplements. There are a number of ways that beta carotene can benefit human health.
Below, we give some examples:. Beta carotene, like all carotenoids, is an antioxidant. An antioxidant is a substance that inhibits the oxidation of other molecules; it protects the body from free radicals. Free radicals damage cells through oxidation. Eventually, the damage caused by free radicals can cause several chronic illnesses. Men who have been taking beta carotene supplements for 15 or more years are considerably less likely to experience cognitive decline than other males, researchers from Harvard Medical School reported in Archives of Internal Medicine November issue.
Oxidative stress is thought to be a key factor in cognitive decline, the researchers explained. Studies have shown that antioxidant supplements may help prevent the deterioration of cognition.
Their study, involving 4, men, compared those on beta carotene supplements for an average of 18 years to others who were given placebo. Schiedt et al. The authors did not administer BC in this study, but they suggested that it must be an intermediate in the pathway from xanthophylls to VA [ , ].
In summary, the literature demonstrates that several carotenoids are important precursors to both retinol and vitamin A 2 in fish. Which form of VA is produced from pro-VA carotenoids seems to be species-specific. Like in mammals and birds, pro-VA conversion efficiency appears to be regulated by VA status. However, fish may be distinct in having reductive pathways from xanthophylls to BC and lutein to retinoids.
The metabolism of BC has proven to be an endless field of study with metabolic variations being equal to the number of animal species studied. Many factors impact the bioavailability of BC and its conversion to VA, and estimation of the value of BC in mixed diets and under various conditions is difficult.
Nevertheless, some patterns emerge Tables 2 and 3. Several omnivores rat and chicken appear to be the most efficient converters of BC to VA. Perhaps this is due to the diversity of potential diets in these omnivores, requiring them to be the most flexible and able to readily utilize BC or preformed VA when available.
Herbivores also have high conversion efficiency, though they likely do not require maximal BC conversion efficiency since their diets generally have high concentrations of pro-VA carotenoids. It is not surprising that carnivores are less efficient at converting BC to VA, as they evolved consuming diets with abundant preformed VA.
In particular, only a few species of carnivores have been studied. Domestic cats are often assumed to be a good model species for all carnivores, but the example of dogs and ferrets demonstrates that there is likely a spectrum of BC conversion efficiencies even within carnivores.
In addition, there is no data on BC conversion on other faunivores, such as those that eat insects or other invertebrates, foods that have very different concentrations of preformed VA and carotenoids than vertebrate tissue. The variability between species in the ability to absorb BC intact and accumulate it in tissues is not understood. Early observations that some species had yellow adipose and others had white adipose led investigators to speculate that there was a relationship between conversion efficiency and tissue accumulation.
It was thought that white adipose species are perhaps so efficient at intestinal cleavage of BC that very little escapes into circulation. However, the story seems to be more complex. Some white adipose species are relatively poor convertors but do absorb BC into circulation e. Why are there species differences? Little research has been conducted in this area. It is often said that BC absorption occurs via a passive mechanism, but the selective absorption of carotenoids in many species indicates this may not be the case.
Some investigators have proposed that there are specific carrier proteins in the gut and other tissues that may regulate the absorption and accumulation of carotenoids [ — ]. Accumulation of carotenoids in the tissues of some species but not others may be related the lack of extraintestinal BCO activity, as has been proposed in cows [ ]. In chickens, the inhibition of the expression of BCO-2 in the skin allows for the accumulation of carotenoids in that tissue [ ].
Thus, species- and tissue-specific accumulation may be related to the expression and inhibition of BCO enzymes. Perhaps examining BCO activity beyond the gut would improve our understanding of species differences with regard to BC accumulation. Absorption and accumulation of intact BC in animal species determines if BC may have important functions i. Green was the primary author of the manuscript, and Andrea J.
Fascetti revised the manuscript. Green and Andrea J. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors. Read the winning articles. Journal overview. Special Issues. Green 1 and Andrea J. Academic Editor: A. Kate Shoveller.
Received 20 May Accepted 29 Aug Published 19 Oct Abstract Vitamin A is essential for life in all vertebrate animals. Introduction Vitamin A VA is an essential nutrient for all vertebrate animal species.
Vitamin A 2. Structure, Units, and Chemistry Vitamin A is actually a family of compounds that are structurally similar to and have the essential functions and biological activity of retinol. Figure 1. Table 1. Requirements listed are the minimum requirements or adequate intake values. Table 2. The value given is the total number of animals in the study.
Wild felids may absorb BC more efficiently than domestic cats No Yes but inefficient [ 18 , 27 , 47 , , , — ] Birds No, xanthophylls absorbed preferentially No Yes shown in chickens, cockatiels, canaries, quail, and ducks [ , , — ] Fish No, xanthophylls absorbed preferentially No Yes [ , , — ].
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View at: Google Scholar C. Ho, F. Kim, and A. Lemke, S. Dueker, J. Follett et al. Tang, J. Qin, G. Dolnikowski, R. Russell, and M. Dolnikowski, and R. West, Muhilal et al. Poor, T. Bierer, N. 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.
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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. It's found in dairy products, fish oils, eggs, and meat especially liver. But vitamin A from animal sources can. Vitamin A is available in multivitamins and as a stand-alone supplement. Vitamin A supplements can contain only beta-carotene, only preformed vitamin A, or a combination of both types of vitamin A.
If your body doesn't use all the beta-carotene you eat to make vitamin A, the beta-carotene that remains circulates in your body. Beta-carotene is also an antioxidant. It helps keep cells healthy. Beta-carotene and vitamin A play a vital part in the reproductive process. They also help keep skin, eyes, and the immune system healthy.
Beta-carotene and other carotenoids help reduce free radical damage in your body. Taking beta-carotene supplements can help you get enough vitamin A. These supplements are considered safe. Poor nutrition is a leading cause of beta-carotene and vitamin A deficiency. These problems can keep you from getting enough vitamin A:. Women who are pregnant or breastfeeding may need to take supplements.
Be sure to talk to your healthcare provider before doing this. Beta-carotene may reduce the risk of some types of cancer, such as prostate cancer.
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