Carotenoids

Carotenoids belong to the group of so-called secondary plant compounds, which are not considered essential for humans, but are considered beneficial to health. Carotenoids are lipophilic (fat-soluble) color pigments. They occur in the chromoplasts of plant organisms and give many plants and fruits their yellow to reddish color. Carotenoids can also be detected in the chloroplasts of green plants, whose color is masked by the green of chlorophyll. Carotenoids can be synthesized exclusively by plant organisms. There, during photosynthesis, they are involved in the absorption of light and the transfer of its energy to chlorophyll. They also broaden the absorption spectrum in the blue-green spectral range in photosynthetic organisms and serve as light protection factors. Furthermore, as antioxidants, carotenoids protect chlorophyll molecules of plants from photooxidative damage and protect animals consuming carotenoid-rich plant foods from the influence of aggressive oxygen species – “oxidative stress“. Today, 500-600 different carotenoids are known, of which about 10% can be converted into vitamin A (retinol) by human metabolism and thus have provitamin A properties. The best known representative with this property is beta-carotene. This carotenoid has the highest vitamin A activity. Vitamin A is found exclusively in the animal organism and, in addition to beta-carotene, can also be formed from other carotenoids, such as alpha-carotene and beta-cryptoxanthin. Under usual nutritional conditions, about 40 different carotenoids can be detected in human serum, with the following being the main carotenoids in the organism.

  • Alpha-carotene
  • Beta-carotene
  • Lycopene
  • Lutein
  • Zeaxanthin
  • Alpha-cryptoxanthin
  • Beta-cryptoxanthin

Beta-carotene accounts for 15-30% of total carotenoids in plasma.

Biochemistry

Chemically, carotenoids are composed of eight isoprenoid units and consist of a hydrocarbon chain with conjugated double bonds that can bear different substituents at both ends. They can be divided into carotenes, consisting of hydrogen and carbon, and xanthophylls, which also contain oxygen. The most important representatives of carotenes are alpha- and beta-carotene as well as lycopene and of xanthophylls lutein, zeaxanthin as well as beta-cryptoxanthin. While yellow, red, and orange fruits and vegetables contain mainly carotenes, 60-80% of xanthophylls are found in green vegetables. Beta-carotene represents the most abundant carotenoid, although the content of, for example, lutein in spinach and various cabbage varieties or lycopene in tomatoes is much higher.

Absorption

The overall absorption rate of carotenoids is very low, ranging from 1 to 50%. As dietary carotenoid intake increases, the absorption rate decreases. In addition, absorption is dependent on the following factors.

  • Type of food – dietary fiber, for example pectins, decreases absorption.
  • Form in which carotenoids are present in foods – as crystal size increases, the rate of absorption decreases
  • Combination with other food components, especially fat – to ensure optimal absorption, the presence of dietary lipids is essential
  • Type of processing – heat treatment, mechanical comminution promote absorption.

For example, beta-carotene from raw carrots is absorbed only about 1% because it is enclosed in a complex, indigestible matrix of proteins, lipids and carbohydrates in the plant cell. As the degree of processing increases – under the influence of heat and mechanical comminution, for example during cooking or in the production of ketchup – the absorption rate increases. The absorption of carotenoids follows the pathway of lipid resorption, which necessitates the presence of fats and bile acids. Carotenoids, together with other fat-soluble nutrients, are packaged into micelles after release from the food under the influence of bile acids and transported to the epithelial cells of the small intestinal mucosa.There, the aldehyde retinal is formed from the vitamin A-active carotenoids – beta- and alpha-carotene as well as beta-cryptoxanthin – as a result of oxidative cleavage by the enzyme dioxygenase – one to two molecules of retinal can be formed from beta-carotene. Retinal is converted into the actual vitamin A (retinol) by means of alcohol dehydrogenase. Subsequently, esterification of retinol molecules with palmitic, stearic, oleic, and linolenic acids, respectively, occurs, resulting in the synthesis of retinyl esters. The oxidative cleavage of carotenoids by dioxygenase and the formation of vitamin A take place mainly in the cells of the small intestinal mucosa. However, vitamin A-active carotenoids can also be converted into vitamin A in other tissue cells, such as liver, kidney and lung. Oxygen and a metal ion, presumably iron, are required to maintain dioxygenase activity. Finally, the extent of enzymatic cleavage and thus the amount of vitamin A synthesized depends on the level of carotenoid or protein intake, iron status, and simultaneous intake of fat and fat-soluble vitamins – vitamins A, D, E, K. Studies have shown that saturated fatty acids have a much more positive effect on carotenoid absorption than unsaturated fatty acids. The following causes are discussed.

  • Polyene fatty acids – PFS -, such as omega-3 and -6 fatty acids, increase micelle size, which decreases the diffusion rate
  • PFS alter the charge of the micelle surface, negatively affecting the affinity for the epithelial cell
  • PFS occupy more space in lipoproteins VLDL than saturated fats, limiting the space for other lipoids, such as carotenoids, retinol and vitamin E -tocopherol.
  • Omega-3 fatty acids inhibit VLDL synthesis. VLDL is important for carotenoid transport in the serum.
  • PFS increase the need for vitamin E, which is an antioxidant that protects carotenoids and vitamin A, respectively, from oxidation

Transport and storage

The resulting retinyl esters, unesterified retinol, carotenes as well as xanthophylls are stored in chylomicrons in the small intestinal mucosa. Chylomicrons belong to the group of lipoproteins and have the task of releasing fat-soluble substances from the epithelial cells of the small intestine into the lymph and transporting them in the serum to the liver or peripheral tissues. Only a small proportion of retinyl esters and carotenoids are taken up into extrahepatic tissues and converted to vitamin A. The larger proportion reaches the liver. The larger portion reaches the liver. On the way, the loaded chylomicrons are enzymatically degraded to “chylomicron remnants”, which are taken up by the parenchymal cells of the liver. In the liver, further conversion of carotenoids and retinyl esters to vitamin A occurs. The synthesized retinol is then transported to the stellate cells of the liver where it is re-esterified. More than 80% of the retinol formed is stored in the hepatic stellate cells. In contrast, the parenchymal cells of the liver have only low vitamin A contents. When needed, vitamin A is released from the liver, bound to retinol-binding protein (RBP) and transthyretin – thyroxine-binding prealbumin – and transported in the serum to target cells. Carotenoids released from the liver are distributed to all fractions of lipoproteins, especially VLDL, LDL and HDL, and transported in the blood plasma. The LDL fraction contains more than half of the total carotenoid concentration. Carotenoids are found in all organs of humans, although the levels in individual tissues vary. The highest concentrations can be found in liver – main storage organ – adrenal gland, testes (testicles) and corpus luteum (corpus luteum of the ovary). In contrast, kidney, lung, muscles, heart, brain or skin show lower carotenoid levels. If we consider the absolute concentration and the contribution of tissues to the total weight of the organism, about 65% of carotenoids are localized in adipose tissue.

Physiologically significant functions

Antioxidant activity As essential components of the antioxidant network of the human body, carotenoids are able to inactivate reactive oxygen compounds – quenching. These include, for example, peroxyl radicals, superoxide radical ions, singlet oxygen, hydrogen peroxide, and hydroxyl and nitrosyl radicals.These compounds can act on the organism either as exogenous noxae, in light-dependent reactions or endogenously through aerobic metabolic processes. Such reactive substances are also called free radicals and can react with lipids, especially polyunsaturated fatty acids and cholesterol, proteins, nucleic acids, carbohydrates as well as DNA and modify or destroy them. Carotenoids, especially beta-carotene, lycopene, lutein and canthaxanthin are particularly involved in the detoxification of singlet oxygen and peroxyl radicals. The process of “quenching” is a physical phenomenon. Carotenoids act as intermediate carriers of energy – when reacting with singlet oxygen, they release the energy in interaction with its environment in the form of heat. In this way, reactive singlet oxygen is rendered harmless. Carotenoids represent the most effective natural “singlet oxygen quenchers”. The deactivation of peroxyl radicals depends on the oxygen partial pressure. Carotenoids act as effective antioxidants only at low oxygen concentrations. At high oxygen partial pressure, on the other hand, carotenoids can develop prooxidant effects. As a result of the detoxification of singlet oxygen and peroxyl radicals, the formation of free radicals is prevented and the chain reaction of lipid peroxidation is interrupted. In this way, carotenoids protect against oxidation of LDL cholesterol, which is a risk factor in the development of atherosclerosis (atherosclerosis, hardening of the arteries). Since carotenoids are consumed during the deactivation process of prooxidants, care should be taken to ensure adequate dietary carotenoid intake. The antioxidant protection of carotenoids is more intense the higher their concentration in serum. If carotenoids are taken together with vitamin E (tocopherol) and glutathione – tripeptide of amino acids glutamic acid, glycine and cysteine – the antioxidant effect can also be enhanced. If the antioxidant protection system is weakened due to a deficiency of antioxidants, pro-oxidants predominate, oxidative stress may occur. By counteracting oxidative changes in biologically important molecules, increased carotenoid intake reduces the risk of certain diseases. These include

Anticarcinogenic effects According to numerous epidemiological studies, increased consumption of carotenoid-rich fruits and vegetables is associated with a reduced risk of tumors. This is particularly true for lung, esophageal, gastric, colorectal (colon and rectal), prostate, cervical/collum (cervical), mammary (breast), and skin cancers. Carotenoids exert their protective effects in the 3-stage model of carcinogenesis, particularly on the phase of promotion and progression

  • Inhibition of tumor cell proliferation and differentiation.
  • Prevention of oxidative DNA and cellular damage by detoxifying free radicals and preventing their development.
  • Enhancement of the immune response by promoting the body’s natural defense systems – this concerns in particular the proliferation of B and T cells, the number of T helper cells and the activity of natural killer cells.
  • Stimulation of cell communication via gap junctions.

Gap junctions are cell-cell channels or direct connections between two adjacent cells. Via these pore-forming protein complexes – Connexone – an exchange of low-molecular signaling and vital substances occurs, which regulate, among other things, growth and development processes. Such processes also play a role in carcinogenesis. Gap junctions maintain contact between cells and enable controlled cell growth through signal exchange. Tumor promoters inhibit intercellular communication via gap junctions. Finally, in contrast to normal cells, tumor cells exhibit little intercellular signaling, leading to uncontrolled cell growth.By enhancing cell communication via gap junctions, both vitamin A-active carotenoids and carotenoids without provitamin A property, such as canthaxanthin or lycopene, inhibit tumor cell growth and proliferation. In addition, the carotenoids astaxanthin and canthaxanthin can interfere with the initiation phase. They inhibit specific phase 1 enzymes, especially cytochrome P450-dependent monooxygenases, such as CYP1 A1 or CYPA2, which are thought to be responsible for the development of carcinogens. Similar effects of astaxanthin and canthaxanthin were also observed for some phase 2 enzymes. Age-related degeneration of the macula lutea The macula lutea (yellow spot) is part of the retina and the area of sharpest vision. There, in contrast to other tissues, the carotenoids lutein and zeaxanthin specifically accumulate. According to epidemiological studies, a sufficient intake of foods rich in lutein and zeaxanthin can reduce the risk of age-related macular degeneration (AMD). This effect is due to the physicochemical properties of carotenoids – they act as specific light filters and antioxidants. AMD is a common cause of serious visual impairment in the elderly and can be associated with blindness in old age. Sun protection effect – skin protection The skin protection effect of carotenoids can be attributed to their antioxidant properties. Increased intake of fruits and vegetables, especially those containing beta-carotene, is associated with an increase in skin carotenoid levels. Studies in which beta-carotene was used as an oral sunscreen agent showed a clear reduction in UV light-induced erythema (extensive reddening of the skin) when > 20 mg beta-carotene/day was administered for 12 weeks compared with the control group. Overall, beta-carotene can be used to increase the basic protection of the skin.

Bioavailability

Carotenes and xanthophylls differ in their heat stability. The oxygen-free carotenes are relatively heat stable. In contrast, most oxygenated xanthophylls are destroyed upon heating. This explains, for example, why heated vegetables have fewer health-promoting effects than unheated vegetables. In addition, the degree of processing of the food plays a significant role. Lycopene from processed tomato products, such as tomato juice, is significantly more available than from raw tomatoes, and the uptake of beta-carotene increases with the degree of comminution of the added carotenoid-containing food. Carotenoid content is highly dependent on, among other things, season, ripeness, growing, harvesting and storage conditions, and can vary considerably in different parts of the plant. For example, the outer leaves of cabbage have significantly higher amounts of lutein and beta-carotene than the inner leaves. Caution. According to the data available for the Federal Republic of Germany on the supply situation with carotenoids for men and women, the supply of beta-carotene is not optimal.