Beta-carotene belongs to the large group of carotenoids – lipophilic (fat-soluble) pigment dyes of plant origin – which are classified as secondary plant compounds (bioactive substances with health-promoting effects – “anutritive ingredients”). Beta-carotene is the best known and, in terms of quantity, the most important natural representative of the substance class of carotenoids, from which the collective name of the compounds is also derived. Structural feature of beta-carotene is the symmetrical, polyunsaturated polyene structure (organic compound with multiple carbon-carbon (C-C) double bonds), consisting of eight isoprenoid units and 11 conjugated double bonds (→ tetraterpene with 40 C atoms). A beta-ionone ring (unsubstituted, conjugated trimethylcyclohexene ring) is attached to each end of the isoprenoid chain-a structural element that also occurs in retinol (vitamin A) and is a prerequisite for vitamin A activity. The system of conjugated double bonds gives beta-carotene its orange-red to red color and is responsible for some physicochemical properties of the carotenoid that are directly related to its biological effects. The pronounced lipophilicity (fat solubility) of beta-carotene influences both intestinal (concerning the intestine) absorption and distribution in the organism.Beta-carotene can occur in different geometric forms (cis/trans isomers), which are convertible into each other. In plants, beta-carotene is predominantly present (~ 98 %) as a stable all-trans isomer. In the human organism, sometimes different isomeric forms can co-occur.In contrast to the xanthophylls, such as lutein, zeaxanthin and beta-cryptoxanthin, beta-carotene, like alpha-carotene and lycopene, does not contain an oxygen functional group. Of the approximately 700 carotenoids identified, about 60 are convertible to vitamin A (retinol) by human metabolism and thus exhibit provitamin A activity. Beta-carotene (all-trans and 13-cis isomer) is the most important representative with this property and has the highest vitamin A activity, followed by all-trans alpha-carotene, all-trans beta-cryptoxanthin and 8′-beta-apocarotenal. Thus, beta-carotene makes a crucial contribution to vitamin A supply, especially in individuals with low vitamin A intakes, such as vegetarians. Molecular requirements of carotenoids for vitamin A efficacy include:
- Beta-ionone ring (unsubstituted conjugated trimethylcyclohexene ring).
- Changes to the ring lead to reductions in activity
- Carotenoids with an oxygen (O)-carrying ring, such as lutein and zeaxanthin, or without a ring structure, such as lycopene, have no vitamin A activity
- Isoprenoid chain
- At least 15 C atoms plus 2 methyl groups.
- Cis isomers have lower biological activity than trans isomers
Light and heat or the presence of oxygen can decrease the vitamin A activity of beta-carotene through isomerization (conversion trans → cis configuration) and oxidative modification of the molecular structure, respectively.
Synthesis
Beta-carotene is synthesized by plants, algae, and bacteria capable of photosynthesis and is stored in the plant organism in the chromoplasts (plastids colored orange, yellow, and reddish by carotenoids in petals, fruits, or storage organs (carrots) of plants) and chloroplasts (organelles of cells of green algae and higher plants that perform photosynthesis)-incorporated into a complex matrix of proteins, lipids, and carbohydrates. There, beta-carotene, together with other carotenoids, provides protection against photooxidative damage by acting as a “quencher” (“detoxifier,” “inactivator”) of reactive oxygen compounds (1O2, singlet oxygen), i.e., directly absorbing radiant energy via the triplet state and deactivating it via heat release. Since the ability to quench increases with the number of double bonds, beta-carotene with its 11 double bonds has the strongest quenching activity compared to other carotenoids. Beta-carotene represents the most abundant carotenoid in nature.It is found in a wide variety of fruits (2-10 mg/kg) and vegetables (20-60 mg/kg), although the content can vary greatly depending on the variety, season, degree of ripeness, growth, harvesting and storage conditions, and in different parts of the plant. For example, the outer leaves of cabbage contain 200 times more beta-carotene than the inner leaves. Yellow/orange fruits and vegetables and dark green leafy vegetables, such as carrots, squash, kale, spinach, savoy cabbage, lamb’s lettuce, bell peppers, chicory, sweet potatoes, and melons, are particularly rich in beta-carotene. Due to its coloring properties, beta-carotene – extracted from plants or produced synthetically – is used as a colorant (E 160 and E 160a, respectively) in about 5% of all foods in Germany, including for coloring butter, margarine, dairy products, spreads, confectionery, or sodas, with an average of between 1-5 mg/kg and mg/l added to solid foods and beverages, respectively.
Absorption
Because of its lipophilic (fat-soluble) nature, beta-carotene is absorbed (taken up) in the upper small intestine during fat digestion. This necessitates the presence of dietary fats (3-5 g/meal) as transporters, bile acids to solubilize (increase solubility) and form micelles, and esterases (digestive enzymes) to cleave esterified beta-carotene. After release from the food matrix, beta-carotene combines in the small intestinal lumen with other lipophilic substances and bile acids to form mixed micelles (spherical structures 3-10 nm in diameter in which the lipid molecules are arranged in such a way that the water-soluble molecule portions are turned outward and the water-insoluble molecule portions are turned inward) – micellar phase for solubilization (increase in solubility) of lipids – which are absorbed into the enterocytes (cells of the small intestinal epithelium) of the duodenum (duodenum) and jejunum (jejunum) via a passive diffusion process. The absorption rate of beta-carotene from plant foods varies considerably between and within individuals, ranging from 30 to 60% depending on the proportion of fats consumed at the same time – on average 50% when approximately 1-3 mg of beta-carotene is consumed. In terms of their promoting influence on beta-carotene absorption, saturated fatty acids are far more effective than polyunsaturated fatty acids (polyene fatty acids, PFS), which can be justified as follows:
- PFS increase the size of mixed micelles, which decreases the diffusion rate
- PFS alter the charge of the micellar surface, decreasing the affinity (binding strength) to enterocytes (cells of the small intestinal epithelium)
- PFS (omega-3 and -6 fatty acids) occupy more space than saturated fatty acids in lipoproteins (aggregates of lipids and proteins – micelle-like particles – that serve to transport lipophilic substances in the blood), thus limiting the space for other lipophilic molecules, including beta-carotene
- PFS, especially omega-3 fatty acids, inhibit lipoprotein synthesis.
Beta-carotene bioavailability is dependent on the following endogenous and exogenous factors in addition to fat intake [3, 6, 7, 11-13, 16, 23, 24, 26, 30, 31, 33, 34, 37, 41, 42, 46]:
- Amount of alimentary (dietary) beta-carotene supplied – as the dose increases, the relative bioavailability of the carotenoid decreases
- Isomeric form – beta-carotene is better absorbed in its all-trans configuration than in its cis form.
- Food source – from supplements (isolated beta-carotene) the carotenoid is more available than from fruits and vegetables (native beta-carotene), which manifests itself in a significantly higher increase in serum beta-carotene levels after taking supplements compared to taking the same amounts from the usual diet
- Food matrix in which beta-carotene is incorporated – from processed vegetables (mechanical comminution, heat treatment) beta-carotene is absorbed significantly better (> 15%) than from raw foods (< 3%), because the carotenoid in raw vegetables is present in the cell crystalline and enclosed in a solid indigestible cellulose matrix
- Interactions with other food ingredients:
- Dietary fiber, such as pectins from fruits, decreases the bioavailability of beta-carotene by forming poorly soluble complexes with the carotenoid
- Olestra (synthetic fat substitute consisting of esters of fatty acids and sucrose (→ sucrose polyester), which cannot be cleaved by the body’s lipases (fat-cleaving enzymes) and is excreted unchanged) reduces beta-carotene absorption
- Phytosterols and stanols (chemical compounds from the class of sterols found in fatty plant parts, such as seeds, sprouts and seeds, which are very similar to the structure of cholesterol and competitively inhibit its absorption) impair the intestinal absorption of beta-carotene
- The intake of carotenoid mixtures, such as beta-carotene, lutein and lycopene, can both inhibit and promote intestinal beta-carotene absorption
- Proteins and vitamin E increase beta-carotene absorption.
- Individual digestive performance, such as mechanical comminution in the upper digestive tract, gastric pH, bile flow – thorough chewing and low gastric juice pH promote cell disruption and release of bound and esterified beta-carotene, respectively, which increases the bioavailability of the carotenoid; decreased bile flow decreases bioavailability due to impaired micelle formation
- Supply status of the organism
- Level of supply of vitamin A – with good vitamin A status, the absorption of beta-carotene is lowered
- Genetic factors
Biotransformation
In the cytosol of the cells of the jejunum (empty intestine), part of the beta-carotene is converted into retinol (vitamin A). For this purpose, the carotenoid is cleaved at either the central or an eccentric (decentralized) double bond by the cytosolic, non-membrane-bound enzyme 15,15′-dioxygenase – carotenase, with central cleavage being the predominant mechanism. While central (symmetric) cleavage of beta-carotene gives rise to two molecules of retinal, decentralized (asymmetric) cleavage of the carotenoid gives rise to 8′-, 10′-, and 12′-beta-apocarotene, respectively, depending on the site of degradation (decomposition), which is converted to one molecule of retinal by further degradation or chain shortening, respectively. This is followed by the reduction of retinal to the biologically active retinol by alcohol dehydrogenase – reversible process -, which binds to cellular retinol-binding protein II (CRBPII) and – at physiological concentrations – is esterified by lecithin-retinol acyltransferase (LRAT) or – at higher concentrations – by acyl-CoA-retinol acyltransferase (ARAT) with fatty acids, mainly palmitic acid (→ retinyl ester). Besides, retinal can be oxidized to retinoic acid-an irreversible process that occurs only to a small extent [1, 3-5, 13, 31, 36, 37]. The conversion (transformation) of beta-carotene to retinol in the cytosol of enterocytes (cells of the small intestinal epithelium) is estimated to be 17%. In addition to enterocytes, metabolization (metabolization) can also occur in the cytosol of liver, lung, kidney, and muscle cells. Both oxygen and a metal ion – presumably iron – are required to maintain the activity of 15,15′-dioxygenase. The conversion of beta-carotene to retinol depends on the following factors:
- Genetic factors
- Dietary characteristics that affect intestinal absorption, such as food matrix and fat content
- Amount of beta-carotene supplied
- Protein status
- Supply situation of the organism
- Supply level of vitamin A and vitamin E
- Alcohol consumption
When beta-carotene and retinol (vitamin A) are consumed simultaneously or when vitamin A status is good, the activity of 15,15′-dioxygenase in small intestinal cells decreases, reducing the conversion rate and increasing the amount of beta-carotene that is not cleaved. For this reason, there is no risk of hypervitaminosis A even at very high doses of beta-carotene. The influence of the type of food, the food matrix in which the beta-carotene is incorporated, and the amount of fat added at the same time on the enterocytic conversion of beta-carotene to retinol is shown in the following table.
| Approximately equivalent in effect to 1 µg of all-trans-retinol are. | 2 µg beta-carotene in milk | Conversion ratio 2:1 |
| 4 µg beta-carotene in fats | Conversion ratio 4:1 | |
| 8 µg beta-carotene in homogenized carrots prepared with fat or cooked green leafy vegetables, respectively. | Conversion ratio 8:1 | |
| 12 µg beta-carotene in cooked, strained carrots | Conversion ratio 12:1 | |
| 26 µg beta-carotene in cooked green-leaved vegetables | Conversion ratio 26:1 |
To achieve a vitamin A activity corresponding to the intake of 1 µg of all-trans-retinol, a beta-carotene intake of, for example, 2 µg from milk, 12 µg from cooked, strained carrots, or 26 µg from cooked green-leaf vegetables is required. This makes it clear that through targeted food selection, presence of dietary fats, and food-processing processes, such as cooking or mechanical grinding, respectively, less dietary beta-carotene needs to be supplied for conversion to retinol, which is due to their improved intestinal absorption. With increase in beta-carotene absorption, conversion of the carotenoid to retinol in enterocytes also increases.
Transport and distribution in the body
The portion of beta-carotene that has not been metabolized to retinol in the mucosal cells of the small intestine is incorporated, along with retinyl esters and other lipophilic substances, into chylomicrons (CM, lipid-rich lipoproteins), which are secreted (secreted) into the interstitial spaces of enterocytes by exocytosis (transport of substances out of the cell) and transported away via the lymph. Via the truncus intestinalis (unpaired lymphatic collecting trunk of the abdominal cavity) and ductus thoracicus (lymphatic collecting trunk of the thoracic cavity), the chylomicrons enter the subclavian vein (subclavian vein) and jugular vein (jugular vein), respectively, which converge to form the brachiocephalic vein (left side) – angulus venosus (venous angle). The venae brachiocephalicae of both sides unite to form the unpaired superior vena cava (superior vena cava), which opens into the right atrium of the heart. Chylomicrons are introduced into the peripheral circulation by the pumping force of the heart. Chylomicrons have a half-life (time in which a value that decreases exponentially with time is halved) of approximately 30 minutes and are degraded to chylomicron remnants (CM-R, low-fat chylomicron remnant particles) during transport to the liver. In this context, lipoprotein lipase (LPL) plays a crucial role, which is located on the surface of endothelial cells of blood capillaries and leads to the uptake of free fatty acids and small amounts of beta-carotene and retinyl esters into various tissues, for example muscle, adipose tissue and mammary gland, by lipid cleavage. However, the majority of beta-carotene and esterified retinol molecules remain in the CM-Rs, which bind to specific receptors in the liver and are taken up into the parenchymal cells of the liver by means of receptor-mediated endocytosis (invagination of the cell membrane → strangulation of CM-R-containing vesicles (cell organelles) into the cell interior). While retinyl esters follow the metabolic pathway of vitamin A, beta-carotene is partially metabolized (metabolized) to retinol and/or stored in liver cells. The other part is stored in VLDL (very low density lipoproteins; lipid-containing lipoproteins of very low density), through which the carotenoid travels via the bloodstream to extrahepatic (“outside the liver”) tissues. As VLDL circulating in the blood binds to peripheral cells, lipids are cleaved by action of LPL and the lipophilic substances released, including beta-carotene, are internalized (taken up internally) by passive diffusion. This results in the catabolism of VLDL to IDL (intermediate density lipoproteins). IDL particles can either be taken up by the liver in a receptor-mediated manner and degraded there or metabolized in the blood plasma by a triglyceride lipase (fat-splitting enzyme) to cholesterol-rich LDL (low density lipoproteins).Beta-carotene bound to LDL is, on the one hand, taken up into liver and extrahepatic tissues via receptor-mediated endocytosis and, on the other hand, transferred to HDL (high density lipoproteins; protein-rich lipoproteins of high density), which are involved in the transport of beta-carotene and other lipophilic molecules, especially cholesterol, from peripheral cells back to the liver. The total body content of beta-carotene is about 100-150 mg. Provitamin-A is found in all organs of humans, with the highest concentrations in the liver, adrenal glands, testes (testicles), and ovaries (ovaries), especially the corpus luteum (corpus luteum). Storage of the carotenoid is 80-85% in subcutaneous adipose tissue (subcutaneous fat) and 8-12% in the liver. In addition, beta-carotene is marginally stored in the lungs, brain, heart, skeletal muscle, skin, and other organs. There is a direct but not linear correlation between tissue storage and oral intake of the carotenoid. Thus, beta-carotene is released from tissue depots only very slowly over several weeks after cessation of intake. In the blood, beta-carotene is transported by lipoproteins, which are composed of lipophilic molecules and apolipoproteins (protein moiety, function as structural scaffold and/or recognition and docking molecule, for example for membrane receptors), such as Apo A-I, B-48, C-II, D, and E. The carotenoid is also transported by lipoproteins. The carotenoid is 58-73% bound to LDL, 17-26% bound to HDL, and 10-16% bound to VLDL [13, 23, 33, 36-38, 45]. In a normal mixed diet, serum beta-carotene concentrations range from 20-40 µg/dl (0.4-0.75 µmol/l), with women having an average 40% higher value than men. In addition to gender, biological age, health status, total body fat mass, and alcohol and cigarette consumption can also influence serum beta-carotene concentrations. While the carotenoid is optimally effective at a serum level of ≥ 0.4 µmol/l – in terms of health prophylaxis – serum concentrations < 0.3 µmol/l can be identified as beta-carotene deficiencies.Beta-carotene is placenta-permeable and passes into breast milk. In human serum and breast milk, 34 of the approximately 700 known carotenoids, including 13 geometric all-trans isomers, have been identified to date. Among these, beta-carotene has been detected most frequently along with lutein, cryptoxanthin, zeaxanthin, and alpha-carotene. Beta-carotene accounts for approximately 15-30% of total carotenoids in serum. While provitamin-A occurs primarily in its all-trans form in serum, the cis configuration (9-cis beta-carotene) is constantly present in tissue stores.
Excretion
Unabsorbed beta-carotene leaves the body in the feces (stool), whereas apocarotenals and other metabolites of beta-carotene are eliminated in the urine. In order to convert the metabolites into an excretable form, they undergo biotransformation, as do all lipophilic (fat-soluble) substances. Biotransformation occurs in many tissues, especially in the liver, and can be divided into two phases:
- In phase I, the metabolites of beta-carotene are hydroxylated (insertion of an OH group) to increase solubility by the cytochrome P-450 system
- In phase II, conjugation occurs with highly hydrophilic (water-soluble) substances – for this purpose, glucuronic acid is transferred to the previously inserted OH group of the metabolites with the help of glucuronyltransferase
Much of the metabolites of beta-carotene has not yet been elucidated. However, it can be assumed that the excretion products are predominantly glucuronidated metabolites. After a single administration, the residence time of carotenoids in the body is between 5-10 days.
